Federal Ministry of Water, Irrigation and Energy
Tana Bales Integrated Water Resources Development Project- TBIWRDP
MMitry
IrripUon * Energy
Design of Large Gully Treatment Works
in Five Woredas in Tana IBeles Project Areas, ETHIOPIA
Main Itcpori
MARCH 2014
ADDIS ABABA, ETHIOPIADesign of Large Gully Treatment Works: Tana Beles Integrated Water Resources Development Project
Acknowledgment
I  would  like  to  thank  His  Excellency  Ato  Alemyehu  Tegenu,  Minister,  Ministry  of  Water, Irrigation  and  Energy,  Ato  Fekahmed  Negash,  Director,  Trans  boundary  Rivers  Directorate  at the   Ministry   of   Water,   Irrigation   and   Energy   and   Ato   Asmamaw   Kume,   Director,   Basin Directorate,  at  the  Ministry  of  Water,  Irrigation  and  Energy  for  their  valuable  support  to  this consultancy assignment.
I  also  would  like  to  acknowledge  the  support  received  from  the  following  Tana  Beles  Project staffs:  Ato  Yeneshet  Tsemru,  Tana  Beles  Project  Coordinator  and  Ato  Damtew  Bekele  Soil Expert at the Ministry of water, Irrigation and Energy; Ato Mitiku Held Project Coordinator, Ato Lakew Desta, Ato Deres Fekadie and Ato Endager Getnet from Bahir Dar Project office at the Bureau  of  Agriculture  in  Amhara  Region  -  their  guidance  and  support  during  the  field  data collection  period  has  been  useful.  The  support  of  Ato  Derese  Fekadie  in  providing  a  well documented  reference  on  biological  measures  is  highly  appreciated  and  it  has  been  used effectively  in  this  study.  Special  thanks  go  to  very  many  kebele  project  staffs  and  woreda experts whom I cannot list their names here and I appreciate their contribution and support in working  with  us  during  the  field  work.  We  also  acknowledge  the  positive  contribution  of  the workshop  participants  who  participated  actively  at  the  inception  workshop  and  the  first  study findings presentation workshop in Bahir Dar and during the final workshop held in Addis Ababa.
This study on design of large gully treatment work is the first of its kind and we hope it will be a steeping ground for the future work in gully treatment and rehabilitation not only for Tana Beles Project areas but also to the wider users in the country. The challenge to this assignment was enormous. It was difficult to meet the needs of every manager, technical and high level experts' expectations  for gully  treatment, control and rehabilitation, simply  because there is  enormous challenge in the design of gully  treatment works  and a huge and demanding task  facing the engineers, planners and extension workers at present in the country following the GTP.
This  design manual is, therefore, prepared as  a tool to guide and assist field technicians, practicing engineers, land and water managers, and to provide technical support to the exiting government development efforts  in the rehabilitation and development of community watersheds  that are increasingly  being damaged and severely  affected by  gully  erosion. With the proper use and continuous  improvement of this  design materials  presented here in this report, the contribution and significance of this manual will be fully realized.
Getachew Alem
Principal Consultant Getachew Aiem & Associates Addis Ababa, Ethiopia
March 2014
Getachew Alum & Associates
I SECTION I: INTRODUCTION iiDesign of Large Gully Treatment Works: Tana Belcs Integrated Water Resources Development Project
%
BD
BoA
BTU
C
DEM FAO
G
GIS
GPS GTP
h
H
Ha
HH
km
L
LL
m.a.s.l
m3/sec mm
MoA MoARD MoWIE NRCS
Pl
PI
PL
Q
Qp
s
TBIWRDP TP
US SCS USDA UTM W
Acronym
Percentage
Bulk Density
Bureau of Agriculture
Basic Treatment Unit
Clay
Digital Elevation Model
Food and Agriculture Organization
Acceleration due to gravity
Geographic Information System
Geographic Positioning System
Growth and Transformation Plan
Crest height
Total height
Hectare
Household
Kilometer
Length
Liquid Limit
Meters above sea level
meter cube per second
millimeter
Ministry of Agriculture
Ministry of Agriculture and Rural Development
Ministry of Water, Irrigation and Energy
Natural Resources Conservation Services
Profile number
Plastic Index
Plastic Limit
Discharge
Peak discharge
Slope
Tana Beles Integrated Water Resources Development Project Total Porosity
United States Soil Conservation Services United States Department of Agriculture Universal Transverse Mercater
Width
Geiachcw Alcm & Associates
| SECTION 1: INTRODUCTION iiiDesign of Large Gully Treatment Works: Tana Beles lntegraled Waler Resources Development Project
Table of Contents
SECTION I: INTRODUCTION1
1.1     Background1
1.2     Tana-Beles Integrated Water Resources Development Programs2
1.2.1      Tana Basin3
1.2.2      Erosion and Sedimentation3
1.3     Objective and scope of the study6
1.3.1      Objective of the study6
1.3.2      Scope of the study6
1.4     Organization of the report6
SECTION II: METHODOLOGY8
2.1     Literature Review8
2.2     Description of the model gully watersheds8
2.2.1
Investigation and Evaluation of each model gully8
2.2.2
Production of maps8
2.2.3
Stream ordering of the gully network9
2.2.4
Determining gully geometry9
2.2.5
Classifications of gullies10
2.2.6
Study of geology and soils11
2.2.7
Identifying critical sections along the gully channel11
2.3     Estimating Peak flood12
2.4     Design specifications for check dam structures12
2.5     Engagement with the community13
SECTION III: LITERATURE REVIEW ON GULLIES AND GULLY EROSION14
3.1     Gully Erosion - The Basics
3.2     Gully Sections
3.3     Design for gully control
3.4     Design Peak Flood Estimation methods20
3.4.1    US Soil Conservation Service (SCS) Method:20
3.4.3    Adapted Cook's Method:25
SECTION IV: DESCRIPTION AND CHARACTERIZATION OF MODEL GULLY WASTERSHEDS .... 26
4.1 Introduction
26
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| SECTION I: INTRODUCTION ivDesign of Large Gully Treatment Works: Tana Belcs Integrated Water Resources Development Project
4.2     Megenagna Community Watershed - Misrak Eastie Woreda26 4.2.1      Geography26
4.2.2      Topography22
4.2.3      Climate27
4.2.4      Geology and Soils27
4.2.5      Socio Economics and Farming System28
4.2.6      The Drainage system and the gully watersheds29
4.2.7      Land Use31
4.2.8      Soil Erosion31
4.2.9      Hydrology of the community watershed31
4.2.10     Hydrogeology32
4.3     Enkulal Community Watershed - Dera Woreda33
4.3.1 Location33
4.3.2      Geography and climate33
4.3.3      Topography and vegetation cover34
4.3.4      Soils34
4.3.5      Land Use34
4.3.6 Hydrology of the Watershed34
4.3.7      Socio Economy35
4.3.8      The gully watershed35
4.4     Tikur Woha Community Watersheds - Farta Woreda36
4.4.1      Location of the Model Gully36
4.4.2      Geography and climate36
4.4.3      Relief, Topography and vegetation cover37
4.4.4      Land Use
4.4.5      Socio economy of the community in the watershed38 4.4.6      Soils
4.4.7      Hydrology of the Watershed39
4.4.8      Soil erosion
4.4.9      The gully watershed
4.5     Biret Afer Community Watersheds - Mecha Woreda41 4.5.1  Location and Climate of the Community watershed41
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| SECTION I: INTRODUCTION vDesign of Large Gully Treatment Works: Tana Bclcs Integrated Waler Resources Development Project
4.5.2      Topography
4.5.3      The sub watershed
4.6.1      Location of the Model Gully 4.6.2      Relief and climate
42
4.6 Washaw Community Watershed - Sekela Woreda44
44
44
4.6.3      Socio Economy and Farming System $
4
4.6.4      Geology and Soils
4.6.5      Land Use and land cover46
SECTION V: STUDY FINDINGS AND RESULTS47
5.1 Megenagna Community Watershed47
5.1.1      Conditions of the watershed47
5.1.2      Geology47
5.1.3      The Soils48
45
5.4 Characteristics of the Ephemeral Gullies in the Watershed52
5.4.1      Production of Maps for the gully52
5.4.2      Layout of the gully network and Geometry of the Gully52
5.4.3      Critical Section of the gully53
5.4.4      Gully Classifications54
5.4.5      Hydrology54
5.4.6      Soil loss from gullies54
5.2     Enkulal Community Watershed55
5.2.1 Characteristics of the Watershed55
5.3     Tikur Woha Community Watershed58
5.3.1      Characteristics of the Ephemeral Gullies in the Watershed58
5.3.2      Land use
5.4     Biret Afer Community Watershed
5.4.1      Conditions of the sub-watershed61
5.5     Washaw Community Watershed63
5.5.1      Topography and land cover63
5.5.2      The model gully watershed
5.5.3      Characteristics of the Ephemeral Gullies in the Model Watershed65 5.5.4      Soil loss from finger gullies in Washaw Community Watershed65
53
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| SECTION I: INTRODUCTION viDesign of Large Gully Treatment Works: Tana Beles Integrated Waler Resources Deseiopmeoi Project
5.6 Peak flood Values66
SECTION VI: PROPOSED DESIGN FOR LARGE GULLY TREATMENT WORKS69
6.1     Introduction
6.2      Basic Principles and standard norms in the design and construction of check dams... 69
6.3      Design and construction for gully stabilization and control - a Procedure74
6.3.1      Stabilization of the gully - the first step74
6.3.2      The use of Chute in steep slope gullies79
6.3.3      The use of Check Dams79
6.3.4       Check dam design in medium size gullies101
6.3.5      Design of check dams in large gullies102
6.3.6      Gully control using biological measures108
6.4. Bill of Quantities for Rock Check Dams115
SECTION VII: ENVIRONMENTAL EVALUATION...........................................................................117
7.1.1      Quarry for rock production117
7.1.2      Effects on the Flora and Fauna117
7.1.3      The effects on archeological and historical sites117
7.1.4       The effects on the Geology118
7.1.5      Effects of rock check dam construction118
7.1.6      Water quality118
7.1.7      Ground water potential118
7.2 Socio Economic Improvement to the community118
7.2.1      Economic Impact118
7.2.2      Health
7.2.3      Provide access
SECTION VIII: CONCLUSION AND RECOMMENDATIONS120
8.1     Conclusion......................................................................................................................... 120
8.2     Recommendations 121
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| SECTION 1: INTRODUCTION viiDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table of Figures
Figure 1.1:       One of the model gulley in Tana Belese Project areas1
Figure 1.2:       The sub basins that contribute runoff and sediment load to Lake Tana Basin3
Figure 1.3:       High sediment laden stream flowing to Gumera River 4
Figure 1.4:       Under-designed gabion rapped rock check dam5
Figure 2.1:       Gully stream ordering9
Figure 2.2:       Gully cross section measurements9
Figure 3.1:        Rill erosionI4
Pages
Figure 3.2:
Under cutting (Megenagna Watershed)15
Figure 3.3:
Collapsing/falling gully banks in unstable soilsI6
Figure 3.4:
A typical cross section of a check dam19
Figure 3.5:
Rainfall runoff relationship curve22
Figure 4.1:
location map of the study area26
Figure 4.2:
Land use map of the community watershed22
Figure 4.3:
Finger gullies and their drainage (watershed) areas29
Figure 4.4:
The soil is waterlogged and fluid several hours after rain in this ephemeral gully30
Figure 4.5
Ground water (base flow) interception by the gully32
Figure 4.6:
Enkulal Community Watershed .............................................................................................. 33
Figure 4.7:
Land use for model gully watershed36
Figure 4.8:
Tikur Woha Community watershed37
Figure 4.9:
Degraded landscape in Tikur Woha38
Figure 4.10:
Plowing up and down a steep terrain38
Figure 4.11:
Finger gullies and their drainage area40
Figure 4.12:
One of the finger gully in Tikur Woha41
Figure 4.13:
Map of the Biret Afer Community Watershed42
Figure 4.14:
The mouth of the Biret Afer Gully 44
Figure 4.15:
Continuous and discontinuous gullies in Washaw model gully watershed46
Figure 5.1:
The soil collapsing as its get wet in the first few rains49
Figure 5.2:
Color and structure of the soil profile in a watershed49
Figure 5.3:
Soil color along soil profile50
Figure 5.4:
Features of the gully network in Megenagna model gully53
Figure 5.5:
Meandering in Mengenagna gully...54
Figure 5.6:
Traditional cultivation in Tikur Woha...59
Figure 5.7:
Seasonal peak flow mark63
Figure 5.8:
Map of Washaw Watershed64
Figure 5.9:
Finger gullies and their drainage area66
Figure 5.10:
Watershed for each model gully in Washaw community watershed66
Figure 6.1:
Gully bank sliding and expansion processes 71
Figure 6.2:
Tree Lucen as live check dam72
Figure 6.3:
A stone rip rap chute for collecting runoff from adjacent sides77
Figure 6.4:
A chute with vertical wall used as a safe outlet77
Figure 6.5:
Cross sectional shape and possible dimensions of a grass waterway79
Figure 6.6:
Wood/brush check dam
Figure 6.7:
Construction of wood/brush
Figure 6.8:
Design parameters of a typical check dam for gully control....................................................83
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| SECTION I: INTRODUCTION viiiDesign of Large Gully Treatment Works: Tana Belcs Integrated Waler Resources Development Project
Figure 6.9:         Sectional view of a check dam................................................................................................ 85
Figure 6.10
Figure 6.11:
Figure 6.12:
Figure 6.13:
Figure 6.14:
Figure 6.15:
Figure 6.16:
Figure 6.17:
Loose rock check dam............................................................................................................86
Strengthened single gabion check dam ................................................................................ 102
Masonry (non-porous) check dam ........................................................................................ 103
A meandering river in Dera Woreda.......................................................................................104
Characteristics of a meandering gully at the different cross section ..................................... 105
Meandering gully section under stone rip rap........................................................................ 105
A poorly designed and constructed check dam......................................................................106
Long chute on steep gradient................................................................................................ 108
Figure 8.1:        Long Chute on steep gradient for finger gully # 1 in Farta Model Gully Watershed............... 125
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| SECTION I: INTRODUCTION ixDesign of Large Gully Treatment Works: Tana Beles Integrated Water Resources Development Project
SECTION I: INTRODUCTION
1.1     Background
Land and water resources management is key for sustainable livelihood in rural areas of Ethiopia. The success of any agricultural development program, which are (i) increased food production, (ii) increased production of goods and services, and (iii) protecting the agricultural resource base: land
and water resources, largely depends on how well the land and water resources are managed particularly the soil and water. These resources are essential inputs for crop production and food security and sustainable livelihood for rural communities particularly in rain-fed agricultural areas.
Soil and water conservation and continued land reclamation activities are, therefore, integral component of the land and water resources management. In Ethiopia it is also a vital
environmental protection activity.
Many  writers  have attributed the droughts  and associated food insecurity  problems  that have prevailed over the last five decades  to the low attention paid to land and water resources management, particularly for not addressing the problem of soil erosion [sheet and gully erosion] and the threat of land and environmental degradation. These challenges are more serious in the highlands which sustain the livelihood of approximately 80% to 85% of the country's population. In 1974, Leslie Brown in his book entitled "Soil Conservation for Survival - Ethiopia's choice* left a stem warning on the threat of the existing traditional farming and the subsequent soil erosion and land degradation in Ethiopia. Owing to the relief and the climate of the country, the writer predicted that soil erosion will be a serious  challenge to the country's  survival, and he has  indicated the eminent threat from enormous land degradation and associated environmental consequences.
Over the last five decades and continuing to the present, Ethiopia has been seriously and acutely affected both economically, socially and environmentally by the massive soil loss from erosion; both sheet and gully erosion. There is a magic figure always quoted in estimating soil loss by erosion- 1.5  billion metric tons soil loss per year. This is equivalent to approximately 231 Km2  (230,769 ha) productive land area at 50 cm soil depth or 115 Km2  (115,384 ha) as estimated at one meter soil
depth and at a soil bulk density of 1.65 and 1.55 gm/cm3, respectively. Along with the soil loss, billions of dollars worth fertilizer is also lost, and it is equivalent to 346,132 metric ton (3,461,520 quintals) of cereal at fanners' production level. The damage to infrastructures such as access roads in the rural areas, water resources infrastructures such as reservoirs, dams and irrigation canals, and the siltation of aquatic ecosystems such as lakes, streams banks and wetland areas are unaccounted. The displacement of the rural population as a result of the deteriorating land and environmental degradation is also substantially significant.
Most soil erosion research in Ethiopia focus on sheet available  on  gully  erosion.  Studies  in  other  countries 
in TBIWRDP
erosion, and there is very little information show gully erosion as a major treat and
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I SECTION 1: INTRODUCTION |Design of Large Gully Treatment Works: Tana Beles Inlegrated Water Resources Development Project
concern in soil erosion as  a result of the massive soil loss  by  this  process  (Capra et al., 2005). Mathys  and Poesen (2006) presented guly  erosion to account 70 to 90% of the overall sediment produced by erosion from gullies. Research in Spain (Poesen, 2005) also indicated that gullies alone contribute up to 70% of the total sediment loss by erosion.
Although reliable data on the magnitude of gully  erosion in Ethiopia is  scanty  and readily unavailable, people still argue that sheet and rill erosion contribute the largest soil loss in Ethiopia. But Taffa Tullu (2011) in his book on "soil and water conservation in Ethiopia" asserts that soil loss from gullies constitutes the lion share. While other observers attribute the severity of soil erosion in the eastern part of Ethiopia to the commonly  occurring deep and wide gullies (Shibru Daba et al 2003). A recent data on land affected by  gully  in Abay  River Basin1 is  estimated to reach about 9,838 square kilometers or 983,850 hectares which constitutes about 5% of the basin area. In the Amhara Region alone, it is estimated that approximately 3% (4,775 km2 area or 477, 521 ha)2 of the regional land is  affected by  gully. With the dramatic  population explosion in the country  by 2030, the threat of gully erosion is likely to become more serious affecting agricultural development and food security in the country. Available land for agriculture will shrink if the present gully erosion trend and its damage are allowed to continue.
Today than ever, gully erosion is well recognized by the government of Ethiopia as a major threat to agricultural lands  in many  parts  of the country. Land management and sustainable agriculture development program through well managed small watershed is taken as a solution to the problem of gully erosion and it is at the center of the Agriculture Development and Transformation Program in the Growth and Transformation Plan, GTP (2011-2015). Rehabilitating and restoring the productivity of degraded lands is an integral part of this program. The on-going five year GTP has identified about 19,560 small watersheds3 and they  are included in the rehabilitation and development program which involves  soil and water conservation activities, gully  reclamation, drainage channel (embankment) protection and stabilization and afforestation activities. According to the MoA, these activities are planned and implemented in an integrated approach. In the Amhara Regional state alone, hundreds and thousands of check dams have been constructed over the three years GTP period although very little is known on the quality and performance of these structures. In 2012/13, the regional government set aside a budget for investment on gully control programs and about 10, 628 ha of areas affected by gully has been put under treatment using both physical and biological measures  and the total volume of the check  dams  constructed was  estimated at about 1,026,189 m3. According to the BoA in Amhara Region, about 80% of the construction material for the check  dams  was  mobilized from community  participation; contributing free labor and locally available materials. During the same period about 154,059 m gully area was reshaped
2
and   353,363  m  check  dams  have  been  maintained  and  about  9000  field  technicians  and development agents have received trainings on gully control alone .
4
1.2    Tana-Beles Integrated Water Resources Development Programs
The government of Ethiopia with its development partners identified Tana and Beles River Basin as a potential economic  development corridor where investments  can stimulate integrated
' shared by three regions; Amhara, Oromia and Benshangul
’ Informal communication with Ato Mitiku of TB project. Bureau of Agriculture, 2013 4 Source: MoA, 2013 Small watershed has size of between 200 and 250 ha.
$ ourc Teshome (2013), Deputy Bureau Head for Agri culture
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| SECTION I: INTRODUCTION 2Design of Large Gully Trealmcnt Works. Tana Beles Integrated Water Resources Development Project
development and accelerate economic growth. The first phase indudes the set-up of
institutional framework for Integrated Water Resources Management and then plan development activities in key areas of natural resource management such as watershed and flood management.
Planned and ongoing water resources developments in the Tana-Beles Basin (SMEC, 2009) indude:
■   inter-basin transfer of water from Lake Tana to the Beles area,
■   hydropower development in the Beles Basin, to be supplied by water from Lake Tana,
■   construction of dams in the Tana-Beles area (Koga dams - completed, Megech and Ribb
which is ongoing, and other potential dam sites under study),
A wide range of watershed programs are also well under way in the Tana Beles Sub Basin. The focus is on protecting Lake Tana — the natural storage facility that sustains the natural flow for the
Blue Nile River. The design for large gully treatment works is  a  project  on  watershed  management  that  are  planned with  the  objective  to  protect  Lake  Tana  from  excessive siltation  ,  the  protection  of  the  river  banks  and  wet  land areas from massive sedimentation that is increasingly threatening the life of the lake and its aquatic ecosystem. The watershed programs targets three most denuded watersheds in the sub Basin, i.e. the Ribb, Gumara and Jemma Rivers that supply over 90% of the water
resources and sediments to the lake and its flood plains.
1.2.1  Tana Basin
Tana Basin covers an estimated area of about 15,083 km2 (figure 1.2). Lake Tana, which covers  about 20% of the Tana Basin is fed by a several smaller streams and three major rivers with watersheds in excess of 1000 km . The
“
“
2
2
Gilgel  [Bikolo]  Abbay  sub  basin  (4517  km2)  covers  the southern part of the lake, and the Ribb (2156 km ), which cover the eastern and north eastern part of the basin, and Gumara (1604 km ), which cover the south eastern part of
2
Figure 1.2 The sub basins that contributes runoff and sediment to Lake Tana (source: SMEC, 2008)
the basin, and the later two are draining through the fogera flood plain located north east of Lake Tana. The mean elevation of the Tana Basin is 2025 m.a.s.l, with the highest elevations far north at some 4100 m.a.s.l in the Simien Mountains (SMEC, 2009).
1.2.2   Erosion and Sedimentation
There are many small community watersheds that feed each of the three major sub basins- Rib Gumera and Gilgel Abay, discussed above in section 1.2.1 under Tana Basin. The small stream flows from small community watersheds constitute all the river flows that drain to these three *
5 ImpUon do.e.op^t |Ko«a «« 7000 ha unde. construct™.
SOOO ha. and 10000 ha aerad, app™^. and other po.rn.Ui a.ras
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I SECTION I: INTRODUCTION 3Design of Large Gully Treatment Works: Tnna Beles Integrated Water Resources Development Project
major rivers. These small community watersheds are also the source of massive sediment that is silting up Lake Tana and its surrounding wet lands and aquatic ecosystem.
Owing to the high and intense rainfall (hydrological characteristics) and steep land slopes  and rugged land feature, sheet and gully erosion are widespread, and they constitute among the major soil erosion and environmental degradation processes  confronting agricultural development in the
Tana Beles sub basin . The data below in table 1.1 on suspended sediment
8
loads from the three
major River Basin systems for Lake Tana demonstrate the severity of the soil erosion problems in the upstream areas and the potential siltation of the lake.
In this  study  report, a "Design of Large Gully 
Treatment Works" in five selected model gully 
watersheds; Megenanga in Misrak Estie Woreda,
Enkulal in Dera Woreda, Tlkur Woha in Faria Woreda, Biret Afer in Mecha Woreda and
Washaw in Sekela Woreda. Megenanga, Enkulal and Tikur Woha are located in South Gondar Zone, and Biret Afer and Washaw are in West Gojam Zone. The five model gully  watersheds are in the upstream end of the three major river sub basins; Gilgel Abbay, Ribb and Gumara. There are hundreds  and thousands  of small community  watersheds  like these ones contributing runoff and suspended soil sediment
----- :
to these three major rivers. This study  is  part of the watershed development and rehabilitation program of Tana Beles  Integrated Water Resources  Development Project with the objective of minimizing the sediment load entering into the tributaries.
Table 1.1
Fig 1.3 High sediment laden stream flowing into AysoaaeranfcW suspended sediment loads that
drains into Lake Tana
River
Catchment
Station of
measuremen
t
Sediment load
Sedimen
t load
(km2)
Like in most wet woina dega climatic zones, gully erosion is severe and widespread and alarmingly  serious in this part of the Tana Beles project areas. Gully erosion is increasingly becoming an enormous  challenge for many of the farmers in the study areas. It takes  away  productive agricultural lands, damage access rural roads  and even residential areas. It is a serious socio economic 6
('000
ton 'yr)
(ton/km /
Gilgel Abbay
Near Merawi
2
yr)
1664
2821
Koga
1695
Near Merawi
244
100
410
Ribb
Near Addis Zemen
1592
127
80
Gumara
Near Bahr
Dar
1394
1937
1390
Megech
At Azezo
462
Upper Ribb
263
569
Near Debre Tabor
844
47
56
(Source: BCEOM, 1999 in SMEC, 2008)
problem and a major challenge for the rural development programs particularly in the central
6 Gullies are recognized as a major sediment source threatening the aquatic ecosystem In the basin.
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I SECTION 1: INTRODUCTION 4□esign of l^rgc Gully Treatment Works: Tana Bcles Integrated Waler Resources Development Project
highlands where 65 to 85% of the incomes of the communities come from crops in
lands are the primary resource input. The figure 1.4 shows a case of oneof ^e elected m
[large] gully observed in one of the community watershed in Mecha Woreda In Lehulum Selam
Kebele. Protecting the farm lands from soil erosion and particularly from the damage by large
gullies is a priority and a legitimate concern for the planners, decision makers and extension
workers.
The soil sediment from these small community watersheds in the Tana Basin and elsewhere in the country are increasingly becoming, not only at the site where the gully is developed (on-si
effects), but it is also a treat to waters resources infrastructure downstream (off-site effects) such
as dams, reservoirs, water treatment plants and for the investment and development corridor and
the lake and its aquatic ecosystem in general.
Equally one can argue that gully control measures require heavy investment, design and physical
treatment works are challenging and expensive.   The   first   and   foremost challenge is that gullies occur largely on  communal  land  holdings,  along natural      drainage      courses      of watersheds. Most people including responsible development workers recognize  the  treat  after  it  has  left severely  damage  to  the  agricultural land and impede the socio economic activities of the communities. At this stage  of  the  gully  development,  its control requires large funding,
massive labor and other essential resources  for its  treatment. A household labor and resource is inadequate to treat large gullies. On the other hand the technical aspect is also a big challenge.
1.4
gabion rapped rock check dam
Constructing a check dam without proper assessment and evaluation, and without the right kind of design and structure, it is a risky business. As one can see the photo from figure 1.4 above, a check dam that is too small or too large could be a waste of money, time and labor, and ultimately does not achieve its  objective. The photo is  taken from the study  area in Biret Afer Community Watershed, and it shows Tana-Beles Project attempt to control the gully. It presents an example of improper design; undersized gully control structures [check dams]. When a check dam is too large, it is  more expensive and does  not necessarily  assure either stability  of the structure or effectiveness. More importantly a check dam that Is under designed can cause damage to all other check  dams  installed downstream making the erosion in the gully  more agressive. This  allows accelerated erosion to take place at a rate often greater than in an untreated gully (Heed 1960) and it can cause more damage to the watershed. Hence it is  imperative that first a detailed assessment and investigation of each gully is done prior to any physical or biological measures and even produce a design plan. The design based on proper investigation and evaluation of the gully would allow having the nght kind of design for gully control and treatment measures.
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| SECTION I: INTRODUCTION 5Design of Large Gully Treatment Works: Tana Beles Integrated Water Resources DevelopmerUR^
This gully treatment study is, therefore, planned to produce appropriate design that will improve
the quality of check dam structures for the control and rehabilitation of large gullies and reduce downstream siltation of infrastructures, pollution of streams, lakes and aquatic ecosystems 
general.
1.3 Objective and scope of the study
1.3.1   Objective of the study
The objective of this study is to prepare detailed designs for gully treatment in five selected gullies in five woredas  (Dera, Misrak  Estie, Farta, Mecha and Sekela) in Tana Beles  Integrated Water Resources Development Project (TBIWRDP) areas.
1.3.2   Scope of the study
The following activities represent the most significant aspects of the scope of work as stipulated in the terms of reference  of this study include:
7
•    Characterize, classify, and carryout detail survey (GPS, Tape, Clinometers, etc),
•    Using the survey results, select proper intervention options and present design,
•    Provide on the job knowledge transfer to the counterpart and other staffs to be assigned by 
the client.
1.4 Organization of the report
The report is  divided into eight sections-, and the first section contains  the introduction section and the present section is  part of the first section in the report. It is  focusing on background statements  that presents  the highlights  of the issues  associated with gully  erosion and the justification for the initiation of this  study  and the need for a design and construction of gully treatment works. It also discussed why gully is important in the overall development in the Tana Beles Integrated Water Resources development Project. The objective and scope of this study is also included in this section.
Section 2i This section presents and outlines the methodology of the study. It describes how the study  is  conducted, how each model gully  is  characterized, investigated, and as^sed It presents how soil and hydrological assessments are carried out.
Section  3. This  section reviews  the available literature relevant for the study  and presents  a summary of the experiences in Ethiopia and from other countries. The tools and instruments used in the study are also presented in this section.
Section 4:'This section contains the description of the study area; assessment and investigation of each model gully are presented in this section. It discussed the geography and climate, geology 
7 Please refer the terms of reference for this assignment at the end of this document.
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| SECTION I: INTRODUCTION 6Design of Large Gully Treatment Works: Tana Beles Integrated Water Resources Development Project
and soils, socio economic characteristics, land use, soil erosion, hydrogeology, drainage pattern and hydrological characteristics of the model gully watersheds.
Section  5: This  section presents  the study  findings  and results  on the assessment and investigation work  done on each model gully  including the total soil loss  from each gully. Soil analysis data are presented in this section. GPS readings taken in each model gully are analyzed and presented on how it is used for design.
Section  6: In this  section, details  on the proposed design of large gully  treatment works  are presented. It also include details  on what needs  to be done to treat each model gully  are presented.
Section Z  This  section presents  briefly  the environmental evaluation of gully  treatment works. It looks  at the direct and indirect impacts  of the gully  control work, which include environmental, social and economic impacts.
Section 8: presents conclusion and recommendations.
Annex-. It is an important part of this report. It contains attachments and explanatory notes to the materials presented in the main body of the report. The term of reference for this assignment is also included in annex.
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I SECTION I: INTRODUCTION 7Design of Large Gully Treatment Works: Tana Beles Inlcgratcd Water Resources Development Project-----------------------------
SECTION II: METHODOLOGY
The following study methodologies are followed for conducting detail survey of the gully and for the development of designs for large gully treatment works in the five selected model gullies in Tana Beles Project areas.
2.1     Literature Review
The literature review work covered the work in Ethiopia and research and development experiences from several other countries  including Nigeria, Spain, South Africa, Iran and Kenya. The experiences  and lessons  learned from these countries have been used in the preparation of the inception report and in the development of the methodology  of the study  particularly  in the characterization of the model gullies.
2.2     Description of the model gully watersheds
2.2.1   Investigation and Evaluation of each model gully
Design of a control structure for large gully  treatment works  requires, first and foremost, understanding the physical and socio economic environments under which the gully is developed, and the process  of the gully  development. This  entails  a careful evaluation of the gully  through characterizing the gully and geomorphology of the area around the gully. The general pattern of the surface flow is determined from the available hydrological information such as Digital Elevation Model (DEM) map of the project area, soils, slope of the land and rainfall characteristics of the watershed. The areas affected by gully erosion and its natural watershed areas are mapped and the general characteristics of surface water flow are determined.
2.2.2   Production of maps
For all drainage channels including gullies, there are well defined watershed that contributes runoff to its  stream flow, and within this  watershed there are well defined [land] area occupied with different land uses; cultivated area, forest and protected areas, grazing areas, stream channels and network  of gullies, residential areas, and wastelands  or barren lands. These different land uses have their own hydrological characteristics  and its  contribution to the gully  development is significant.
The first step in the planning, design and implementation of gully treatment works is to establish accurate ground control. This control is usually done in the form of maps to allow the investigation to locate, plot, delineate the runoff contributing units for each gully [units] and the land uses within the gully  watershed areas. For any  intervention the base stations  and control points  for these watersheds must be well established and the same for gully treatment works in a watershed and it should be done accurately. Hence geographic  and land use maps  are produced for all small watersheds  that contribute to each gully  and network  of gullies  under investigation. GIS based Digital Elevation Model (DEM) with a scale of 30 m resolution is produced to establish the land forms and shapes of the watershed including the gully using a recently taken Google Earth and Satellite Imagery. From the DEM, slopes, slope length, drainage patterns, etc. are established The watershed boundaries are defined and a marking of the proposed gully is also done using ground
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based GPS readings. Land use data is  also produced both at the watershed and sub-watershed level. The sub-watershed land use map targets  individual finger gully  with data and information including drainage area, slope and slope length, maximum flow length for each sub-watershed. All these maps are useful in the preparation of a detailed study and characterization of the gully.
2.2.3    Stream ordering of the gully network
Gully ordering is one
tool to establish relationship among
the different branches of gullies, and the
work  is  similar to stream ordering of drainage network  of a stream in a small watershed. It provides  a basis  by  which gully  drainage net work  can be characterized. The gully  considered under this  study  has  several finger gullies (branches of the main gully).
The finger gullies and main gully segment are properly  identified and numbered (as shown in figure 2.1). Their inter relationships  are investigated and such information could offer hydraulic  and morphological characteristics that are useful information for gully  control interventions and treatment. There are two methods  of stream/gully  ordering (i) Horton (1945), and (ii) Strahler. This study will follow the latter approach in stream ordering of gully branches as shown in figure 2.1.
2.2.4 Determining gully geometry
Measuring the geometry of the main gully and carried  out  in  this  study,  and  these  include taking  the  dimensions  of  the  gully  sections using  measurement  tapes,  ropes  and  GPS. The measurements made on the geometry of the  gully  cross  section  include  depth  (d), length  (L)  and  top  width  (TW)  and  bottom widths (W) as shown in figure 10 at selected and numbered points (pegs #) along the gully lengths.
Measurements are taken at each set of point [interval] at incremental lengths between five to twenty  meters  as  found appropriate to establish the shape of the gully  along the entire gully  length. These incremental dimensions measured along the whole gully
Figure 2.1 Gully stream ordering
finger gullies was one of the characterization work
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length gives the total length of the gully.
Data on the geometry of the gully provides key input to determine the volume of the total sediment removed from the gully over its life time. As there is a strong correlation between the length and volume of the gully, it is suggested that gully length is a significant and useful index to estimate t e volume of soil erosion in large scale gullies. Based on these data, drainage density is also estimated as the sum of stream lengths divided by the area of the basin (Horton et al) . Drainage density is a measure of the efficiency  with which a stream collects  and discharges  available water and its sediments in the drainage system within the watershed.
Hand held GPS is also used to record northing and easting locations of each peg point selected along the flow path and length of the gully and it is used to produce the volume of the whole gully and its fingers.
Gully  characterization provides  physical - morphological information about the gully, and this information helps  to understand the characteristic  features  of the gully. In so doing key data/parameters  have been collected for investigation, assessment and evaluation of the gully. Finally a digitized map of the gully and its runoff contributing watershed is produced.
2.2.5 Classifications of gullies
There are three types of gully classification methods widely employed in the characterization of the five model gullies.
(i) Based on size: As  shown in table 2.1, there are three broad categories  of sizes used for classifying gullies in the five model watersheds  under this  study, Gullies with depth (D) less than 1 meter are classified as  small and those with depths  greater than 3 meters  are large gullies. In between are medium gullies.
Table 2.1 Gullies defined by the size and drainage area
Gully classes
Depth (m)
Drainage area (Ha)
(a) Small gully
<1
less than 2
(b) Medium Gully
Ito 3
2 to 20
(c) Large gully
>3
more than 20
Medium and large gully depth modified after workshop
O')  Based  on  Shape:  Generally  there  are  three  major  and  distinct  gully  shapes  (i)  V-shaped, (ii) rectangular or U-shaped, and (iii) trapezoidal shape.
(iii)  Continuous  and  non  continuous  gullies:  The  gullies  under  investigation  in  this  study are largely continuous. A continuous gully generally gains depth rapidly from the head cut and then maintains a relatively constant gradient to the mouth. Continuous gullies consist of many branches of gullies. A continuous gully has a main gully channel and many mature or immature branch gullies (called  finger  gullies).  A  gully  network  (gully  system)  is  made  up  of  many  continuous  gullies. Discontinuous gullies also called independent gullies which may often develop on hillsides as the soil losses its cohesion as a result of the changes in the soil properties due to over grazinq loss of ground cover, shallow soil depth, etc.
“/Refer USDA, Agricultural Handbook No. 224.
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| SECTION II: METHODOLOGY 10Design of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
2.2.6   Study of geology and soils
Soil investigation is also conducted at selected gully sections along its length and in some selected points in the watershed. Data has been collected on a field note book. Representative soil samples based on a systematic sampling technique was carried in order to characterize the upper, about 20 cm thick, soil portion. The samples  were taken at each gully  bank  for all five model gullies  and send to Amhara Design and Supervision Works  Enterprise Laboratory. At Megenangna community watershed two undisturbed soil samples (depth of 0-50, 50-120 and 120-200cm depth interval) and 6 disturbed samples for Grain Size Analysis (sieve and hydrometer analysis) and soil classification (Atterberg limit analyses) were made. For each soil profile, a detailed fact sheet was  compiled. Moreover, soil samples  taken from the main diagnostic  horizons  were subjected to physical and chemical laboratory analyses to classify the sampled soils.
Particle size distribution was  determined with a bouyoucos  hydrometer after digesting soil organic matter with hydrogen peroxide and soil dispersion with sodium hexa metaphosphate (Okalebo et al., 2002). Structural stability  of soil aggregates  was  compared among the soil samples  in the laboratory. Air dried soil particles with the size between 0.5 and 2 mm were placed on a 0.5 mm sieve; excess amount of distilled water was added drop by drop to submerge the soils. Five minute after soil submergence, excess water was absorbed by tissue paper from the bottom of the screen. The soil structure was then compared with that of air dry soil samples.
These soil samples  were air dried, sieved through a 2-mm mesh and analyzed in triplicate as described below. Particle size distribution of the less than 2-mm fine earth fractions was measured by  the hydrometer method as  described by  Gee and Bauder (1986). Textural classification was carried out using the USDA classification system.
Undisturbed and bulk samples were collected from head scarps of each gully at depths midway between the ground surface and the basal slip plane. Samples were collected near the end of the
rainy season to minimize disturbance as soils were moist and readily sampled.
The bulk density (BD) was obtained by the gravimetric soil core method described by (Blake and Hartage, 1986) using a 5.0 cm long by  7.1cm diameter cylindrical metal core and the particle density (PD) was assumed to be 2.65 g cm .
3
The  total  porosity  (PT)  was  calculated  from  BD  and  PD  using  the  equation  and  relationship developed by Danielson and Sutherland (1986).
There was no soil observation or test pits that were dug and examined as the gully has deep and wide open and highly exposed gully bank surfaces up to four meters clearly showing the soil profile and hence the soil expert characterized the soil along its full depth. The visible soil materials at the gully appear to be appropriate for soil evaluation at each site.
2.2.7  Identifying critical sections along the gully channel
Identification of critical locations along the gully channels in each of the gully network system is done as part of the gully assessment and evaluation at the initial part of the study for desLnShe
Ino thX STnS.i re/K thLhGad CUt SeCti°n °f the gU"y'  ™  a sudden dr°P elevation along the floor of the gully, (m) sudden changes in the width and depth of the gully sections (iv) meandering section of the gully which often occur in low laying (plain) areas. °
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| SECTION II: METHODOLOGY 11Design of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
2.3      Estimating Peak flood
In Ethiopia, all community  watersheds  are un-gauged, and there is  very  little hydrological data (rainfall, rainfall intensity, stream flow) that can be used in the design and construction of hydraulic structures  for soil and water conservation and small scale irrigation. As  a result, peak  flood estimation becomes  the most challenging task  for a practicing engineer. So is  the case in this consultancy assignment.
To produce essential hydrological data for design of gully  treatment works, the methods  that are widely used elsewhere for estimating stream flow in small watersheds are: (i) US SCS method, (ii) Rational Method and (iii) Adapted Cook's  Method. This  study  in its  methodology  has  considered examining each of these methods, and selecting the one that is  easy  to use and more reliable, more appropriate to Ethiopia's situation and more importantly for the practicing engineer and field technicians working as extension agents.
2.4     Design specifications for check dam structures
To define the different dimensions for each check dam laid across and along each model gully and gully  fingers  or even the proposed chutes9 and grassed waterways, essential data have to be established for each gully watershed on which peak floods are to be estimated. These data are not only the hydrological ones, but also the gradient along each continuous gully considered for design and the length of the gully channel and the geometry of the gully upon which the peak flood and design specifications of each check dam structure is developed.
After the potential peak flood (Qp) for a ten year period is estimated, the height of the crest of the check dam (weir) is determined using equation (3) below. The design principle is that the spillway for the check dam structures must carry through the full peak discharge (Qp) safely. The standard norm established (Pathek, etal, 1983) suggest that the height of the peak flood above the crest of the spill way (H as shown in equation 3 or later on it will be "ymax") has to remain within 0.40 and 75 cm to ensure a safe flow and stable check dam structure. To achieve this minimum standard design, the spillway  length can be variable taking into consideration the measurement taken or dimension of the top width of the gully. The flow should fall on a well protected gully bank and apron. The height of the crest of the check dam of the spill way is determined in relation to the depth of the gully. The narrower the tolerances  (any  exceeding of the 40 to 75 cm peak  flow height), the greater must be the crest length.
The  following  adapted  version  of  a  broad  crested  weir  formula  is  used  to  determine  the  design specification of the check dams proposed in this study.
Q/L = C.|.y2i.H /2
3
Equation (3)
3
Where;
Q = Discharge in m  per second,
C = coefficient of the weir, the lowest of the range between 0.50 and 0.8 is taken L = effective crest length of the weir in m, and
9 Channels constructed along a gully or stream channel that has a very steep slope I
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H = height or depth of flow above the crest of the weir measured at a point four times the
height of H at back flow or upstream areas of the crest. In latter section H would be the
same as “yn^x",
The  design  specifications  for  each  check  dam  (identified  with  peg  number  on  the  GIS  map)  for every model gully in each woreda is presented in Annex A table.
2.5     Engagement with the community
A focus  group discussion is  held at each study  site, and the discussions  were with the riparian farmers10. The investigation focused on the history of the gully development and the causes for its initiation, occurrences, development, and its impacts on the livelihood of the farmers living around the gully. The socio economic  factors  that contribute to the development of the gullies  are also discussed. There has  been suggestion made on how to treat, rehabilitate and manage them to support the farmers livelihood. Inputs have been solicited on how to ensure the maintenance of the control measures for sustainable productivity of the land affected by the gully.
There has  also been discussion on how Tana Beles  Project support to the communities  on controlling gully, on how the gully project become a source of income for them and what that mean on the sustainability of the gully control effort on a long term basis.
The discussion also covered information on environmental issues associated with the gully damage. The [social, economic and environmental] impacts of the gully on their life and livelihood have been discussed and the information documented in the gully assessment section of the report for each gully site - an important step in the study, plan and preparation for gully control interventions.
The  communities  also  discussed  their  failure  to  respond  to  this  serious  threat  to  their  livelihood. The data collection was guided using a set of check list.
—  _fel
s
vin 9 agricultural lands or residential areas adjacent to the model gully
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| SECTION 11: METHODOLOGY 13SECTION III: LITERATURE REVIEW ON GULLIES AND GULLY EROSION
3.1       Gully Erosion - The Basics
Gully  erosion is  a serious  form of erosion by  concentrated running water. According to Morgan (1995)” gullies  are "relatively  permanent steep-sided water courses  which experience ephemeral flows which occur only during rainstorm". It is a global phenomenon and it exits in every part of the globe and Ethiopia is no exception. However, owing to Ethiopia's rugged terrain, relief and climate, soil, and more importantly  the diverse socio economic  characteristics, gully  is  much more widespread and the level of damage is severe. Most of the gullies are formed as a result of the human activities, and these activities include the following.
■ Destruction of vegetative cover of the land, along water courses and natural depressions,
•    Failure of badly designed soil and water conservation structures/works,
*    Animal trails and human foot paths,
*    Rural road construction and the high concentrated flows realized from their culverts,
*    Free grazing and high livestock population,
•    The use of conventional/traditional farming in steep slopes; up and down cultivation, and
•    Poor land use practices.
Discussing the basics of gully erosion is far beyond the scope of this study. However, it is important to discuss  certain basic  knowledge which are important and relevant to the occurrence and development of gullies and for the design, construction and management of gullies.
A gully is one significant feature of the soil erosion process and the most dominant phenomenon signifying the degree of severity of soil
erosion. The factors that are responsible for its occurrence are several and vary from location to location, but however, the dominant factors are the geology, soils, climate (rainfall), land uses, hydrological characteristics, agricultural and farming practices. There are several available research works elsewhere that have been conducted in many developed countries that helps to understand the process of gully development and control.
Several authors have tried to define gully, but there is one most common and widely used definition presented by the Soil
Science Society of America. 'Gully is a channel resulting from erosion and caused by concentrated but intermittent flow of water during and immediately following heavy rains. It was proposed to be deep enough usually not less than half meter and which interfere with the plowing, and cannot be obliterated by normal tillage operations" (Soil Science Glossary Terms Committee, 2008). Accordinq to U.S. Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS),
11   Reference made in Sirvio et al 2004Design of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
classical gullies are defined as gullies with concentrated flow and erosion features that are too large in its width or deep enough for normal farming implements to cross and may occur on all land uses.
Gullies  are generally  ephemeral where flow occurs  seasonally  (during the wet season), and it develops  in three distinct stages; waterfall erosion; channel erosion along the gully  bed; and landslide erosion on gully banks. Correct gully control measures shall address these different types of gullies  according to their development stages. The processes  that led to ephemeral gully development are relatively well understood.
Soil erosion - the removal of soil by running water follows two general erosion patterns. The first of
these is sheet erosion - a uniform detachment and removal of surface soil. The second pattern of
soil removal results from the channeling of runoff water (in rills) and depending upon the severity of the process, either rill or gully erosion occurs (Hussenbuiller, 1978). Rills and gully erosion are distinguished by the size of channel following erosion. Hence soil erosion by running water occurs in three distinct forms: (i) sheet erosion, (ii) rill erosion and (iii) gully erosion.
Generally  soil erosion and gully  formation and development depends  on a number of factors including soil type, vegetation, rainfall, concentrated flow, and human interference and disturbances  in the ecosystem. Soil erosion rates  are particularly  affected by  the kinetic  energy from raindrops and the amount of runoff developed from it that causes the removal of soil particles from its  point of origin. Soil erosion degrades  the soil environment, changes  the structure of the
soil,  reduce  the  water  holding  capacity and   as   a   result   prevent   vegetation growth  on  the  affected  areas  as  the availability of nutrients and water essential for plant growth is declining.
High population pressure, sever livestock grazing, deforestation and traditional tillage practices are some of the factors that cause gully erosion. Of all the many factors, the absence or poor land use practices  are the lead cause to gully erosion, which affects more than any
other factor the rainfall-runoff relationship, and as a result a network of gullies may be quickly formed. Most
gullies in the Ethiopian context are developed along human and livestock
trails and on cultivated and grazing lands (figure 3.2 and 3.3) and on unstable soils (figure 1.7). In most model gullies, rural road construction has contributed for the development of the gullies.
An existence of high density of gullies is a visible manifestation of poor land use practices in an area. Once the vegetation cover is  removed, potential energy  of the surface running water increases  and the overland running water continue to scour away  the soil. Then gullies  spread rapidly up and down the drainage lines until there is insufficient runoff to continue the erosion.
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When the gully cutting and widening stops, it is then said it has stabilized. Once they stabilize they lose very little soil despite their large sizes.
The most vulnerable soils  for gully  development are the ones  that are poorly  structured and dispersive soils; soil structure disperses  or collapses  when it gets  wet. In such soil types  gullies often start as  tunnel erosion underneath the soil surface. In a similar soil condition, fast flowing runoff hits a 'nick' point such as a fall on the ground surface or channel floor with a hole/tunnel created by  animals, plant or big tree roots  stock, trails  for pedestrian and livestock, new road construction in the area, etc.
Once an abrupt scar on the surface of the land occur as a result of one or a combination of the factors, gully erosion occurs and develops in two main direction both having the power to cut and break the land, and these include (i) down cutting and (ii) head cutting forces. In both cases, the side cutting is an essential feature in the process. Down cutting is the vertical lowering of the gully bottom that leads  to gully  deepening and widening (Ffolliott et al. 2003). Head cutting is  the upslope movement that extends  the gully  length (Ffolliott et al. 2003). Erosion is  focused at the gully  head, where overland flow erodes  the lip of the head as  the water flows  over it before plunging into the plunge pool at its base. This is where the deepening and undercutting of the gully take place. Subsurface flow moving towards the gully head can weaken the walls and result in the development of pipes or channels of aid within the soil that run along the side of the gully (Ffolliott et al. 2003). The collapse of pipes further contributes to gully head retreat (Charlton 2008). The presence of deep tension and desiccation cracks allow concentrated overland flow to penetrate the soil surface. In general the process  of gully  erosion and development is  complex, although it is 
recognized        as        a        hydrological phenomenon. What is critically needed is to manage the excess surface runoff
water. Global climate change is likely to increase rainfall [in amount and intensity] in the highlands of Ethiopia including
Tana Beles Project areas, and this implies more runoff in the future and this will increase and intensify the wide spread occurrence of gully erosion, land and environmental degradation.
Soil and water conservation works based on a community watershed approach should be the first task and it is a priority in gully control and rehabilitation. It provides control over surface runoff from rain particularly in high rainfall areas. The
Figure 3.3 Gully banks collapsing/falling and sliding in unstable soils
proper design of soil and water conservation structures is of great importance for sustainable land management for agricultural uses. Gully control and treatment is one aspect of this effort. To this end excess surface runoff water, the most active agent or cause has to be managed, so that it will not cause damage quickly to the agricultural lands, grazing areas and to rural infrastructures such as residential houses, water supply facilities (dams, reservoirs, wells and springs), irrigation canals and the environment and aquatic ecosystems, etc. at the downstream areas. Both the head cut and
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down cutting section of the gully have to be treated. The most effective and economical approach is always a preferred option.
It is essential to characterize gullies based on different features it exhibits. Gully erosion is usually distinguished from the broader term of watercourse erosion by the location where it exists and the path it follows. It occurs normally along an overland flow path which is less difficult to treat, and along the natural drainage channels of watersheds and this is always difficult because of the many finger gullies  and size of its  drainage area. Other gullies  are more often developing by  the concentrated flow released from newly constructed road culverts and such cases are quite common in Tana Beles Project areas. Gully erosion, however, can also occur within a natural watercourse, typically within the upper reaches, and typically resulting from an active head-cut rapidly migrating gully fingers up the valley. Gully erosion is best characterized as a scar resulting from a failure of the watershed system and morphology of the gully follow different shapes and types.
3.2       Gully Sections
a) Gully Head
• It is the upper part of the main gully where the gully starts. The upper part of finger gullies are also gully heads Gully erosion in these areas is very active with in the gully system,
• It is at these locations where most of the run off in concentrated form enters into the gully,
•  This part of the gully section in most of the cases is narrower in size (unless the gully is receiving storm water from culverts (in case of Enkulal Gully in Dera, Megenegna Gully in Estie). These gully heads are very much active for gully formation and expansion,
(b)        Gully Mouth
Gully mouth is the lowest part of the gully. It is the widest and deepest section of the gully often characterized as dead and inactive section of the gully system.
(c)        Intermediate section
These sections of the gully vary in size, sometimes a narrower section exists resulting from localized geological and soil and hydrological conditions.
(d)        Gully offset
• It is a part of gully area which is located away from the gully embankment and extended to the next land use type,
• It is a part which has to be considered in the gully treatment scheme since it is the gully sub watershed area and to avoid further expansion of the gully,
• In most of the cases these areas are characterized by medium soil depth, moderately wet in the rainy season and dry in the dry season, and with moderate slope,
(c) Gully sidewall
’ gully bed or floor'area^^
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•  It is characterized by high slope gradient, shallow soil depth, susceptible to erosion and mass movement, very dry in most of the time due to less water holding capacity,
•  Reshaping  and  hence  constructing  moisture  harvesting  structures  are  the  recommended measures to treat gully sidewalls
• As far as famners/land users are convinced to undertake reshaping, the gully offsets can be converted into potential areas for multiple purposes
(d) Gully bed/floor
•  It is a part of the gully on top of which the run off flows
•  It is the wetted perimeter of the channel or it is occupied with the flow of runoff throughout the rainy season
• This part of gully section is the area of attention in the gully treatment works, to build the
floor up to the desired elevation with silt through the designed physical and biological
measures.
• These areas are regarded as very wet in most of the year, with deep alluvial soil deposits.
3.3       Design for gully control
Check  dam is  used worldwide as  a preferred mode of gully  plugging technology  option. It is generally considered as engineering or a physical structure. The selection of check dam types and their design may  have to meet certain engineering and hydraulic  requirements, the selection of suitable construction materials and consideration of environmental factors.
A check dam is a barrier placed in an actively eroding gully to trap sediment that are carried away downstream during periodic flow events (Ffolliott
et al. 2003). Sediment filled up behind each check dam develops a new channel bottom with a gentler gradient than the original gully bottom
and thereby reduces the velocity of flow and the erosive power of the stream in the gully channel and increases sediment to fall behind the check dam. This process stabilizes the side slopes of the gully and to adjust their natural angle of repose, which reduces further erosion of the channel banks. This new situation in the gully channel supports and promotes the establishment of vegetation growth on the gully slopes and
Gully  control has  certain objectives  and requirements  that need to be met, and some of these requirements  may  include the following:
•   To trap as much as possible a large part of the sediment in a stream by the structure and other accompanying treatment works,
■ Bring the gully to its original situation (restoration),
•   Make the gully productive,
•   Increase ground water recharge.
____ ________________________
bottom; and increases soil water storage so that the underground water table can be raised, which further enhances vegetative growth even outside the gully (Ffolliott et al. 2003).
The cross section of a typical check dam can be seen in figure 3.4 below. The purpose of a check dams  is  to prevent further down cutting and subsequent head cutting and extension of the gully (Heed, 1966).
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Theoretically  each check  dam designed should be spaced upstream at the toe of the expected sediment wedge formed by  the dam below. The first dam should be constructed where down cutting does not occur. The spacing of subsequent dams constructed upstream from the base oam depends  on the gradient of the gully  floor, the gradient of the sediment wedges  deposited upstream of the dams, and the effective height of the dam as measured from the gully floor to the bottom of the spillway  (Ffolliott et al. 2003). Equations  have been developed to determine the spacing of these dams, but this alone is not sufficient to support a stable and reliable check dam
since this  equation does  not specify the effective height of the check dam. The Ministry  of Agriculture (MoA) guide uses  an equation for spacing of check dams which is 1.2 times  the effective height of the check dam divided by gradient of the gully  floor (in decimals). The challenge here is  how to get the effective height of the check dam at each point along the gully. Effective height of the check  dam is determined from estimated peak flood which is the practical challenge in the design of check dams for gully treatment and control.
Figure 3.4 cross section of a check dam (Ffolliott et al. 2003)
Therefore, field measurements (as discussed in the next section, methodology) for characterization of individual site such as  its  geology  and soils, land use, hydrology, etc. should be used to determine where check  dams  are installed. Nick  points  are areas  where an abrupt change of elevation and slope gradient occur and it is often found in sites where there is a lack of protective vegetative cover.
In gully sections where a vegetative cover can be established, channel gradients can sometimes be stabilized without using mechanical or engineering measures. However, in deep gullies vegetation alone can rarely  stabilize either head cuts  or main section of large gullies  because of the concentrated flow that occurs  at these locations  (Ffolliott et al. 2003). Selection of vegetation types that can grow rapidly and establish a high plant density and that have dense root systems are most effective at stabilizing gullies, and this  aspect of the study  will be presented in later sections of this report under biological measures.
To treat and rehabilitate large gullies, engineering structures  are usually  required at critical locations  on the gully  bed, head cuts, and gully  reaches  close to the gully  mouth. At the gully mouth, because the stream flow is large and concentrated, changes in flow cause frequent chanqes that are deepening, widening, and deposition process (Ffolliott et al. 2003). As expected different rain events cause different rates of erosion
An effective contrd structure design must help the survival and establishment of vegetation cover along the gully and it's environ. Once the required gully gradient is attained after the check dams are constructed, vegetation cover can be established on the gully  bottom, which leads  to the
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I SECTION 111: LITERA TURE REVIEW ON 19 GULLIES AND GULLY EROSIONDesign of Large Gully Treatment Works: Tana Bcles Integrated Water Resources Development Project^
stabilization of the banks  (Ffolliott et al. 2003). Ideally  vegetation cover integrated with engineering structures  can support quick  recovery  of the gully  affected area. The net effect of vegetation establishment in the channel and reduced channel gradient is  a decrease in peak  discharge (Ffolliott et al. 2003). Most often, mechanical treatments  are needed for the short term objective - to reduce erosion from gully  slopes  and floor and provide the stability  for the gully. Concurrently the effort made to control erosion at the head section of the gully through diversion of the flow at the upper section of the gully is one aspect of the gully treatment works. This is where design and construction of chutes for safe outlet of the head flow is very essential and mandatory for a reliable and stable gully control work.
In large gullies, the construction of large dams across the gullies to accumulate sufficient sediment is necessary to allow equipment to move across the gully. Large check dams may be uneconomical, but at certain circumstances as in this study area, it may be desirable. That is why such study is initiated.
All of the gully  control measures  discussed above are important. The correct implementation gully treatment works  across  the gully  affected landscape can help reduce runoff and increase ground water conservation and storage and reduce soil erosion. This  in turn enhances  the biomass  and provides  protective cover for the land against erosion. Maintenance of the vegetation across susceptible areas  is  of vital importance to achieve significant positive impacts  in the agriculture, water and generally the environment.
3.4      Design Peak Flood Estimation methods
In the design of gully control measures, surface runoff estimation is primary task. Surface runoff is the volume of excess waters that runs off a surface of a drainage area (watershed). Peak discharge is, therefore, the peak rate of runoff from such drainage areas for a given rainfall. In this study the drainage areas are the model gullies.
Three peak  flood estimation methods  are considered for review in this  study  and a better performing method is  further considered for use in the design check  dams  for the treatment of large gullies in the selected sites in Tana Beles Project areas. The methods include (i) US SCS method (SCS Engineering Field Manual, 1984), (ii) Rational Method (Chaw, 1985) and (iii) Cooks 
Method (Hudson, 1989).
3.4.1 US Soil Conservation Service (SCS) Method:
k'
United States  Soil Conservation Services  has  developed a method for estimating the peak discharge from a small rural watershed (SCS Engineering Field Hand Book, Chapter 2 1984) It is developed based on many years of recoded hydrological data from small agricultural watersheds in many parts of the United States. The method is established based on six major parameters- rainfall and hydrologic soil groups, cover type and land management practices, hydrologic conditions and topography (relief) of the watershed (see Annex D for detail on this).
Some characteristics and limitations to this method of design peak flood estimation
I.          The watershed drainage area must be greater than 0.40 ha and less than 810 ha.
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II.      The watershed should have only one main stream. If more than one exists, the branches must have nearly equal Tc’s,
III.      The watershed must be hydrologically similar; i.e., able to be represented by a weighted CN. Land use, soils, and cover are distributed uniformly throughout the watershed. The land use must be primarily rural,
IV.      If the computed Tc is less than 0.1 hour, use 0.1 hour. If the computed Tc is greater than 10 hours, which is a watershed having a stable system.
V.     The longest flow length is between 61 meters and 7900 meters,
VI.      When  the  average  watershed  slope  is  less  than  0.5  percent,  a  different  unit  hydrograph shape can be used.
VII.     When the weighted CN is less than 40 or more than 98, use another procedure to estimate peak discharge.
VIII.     When the average watershed slope is greater than 64 percent or less than 0.5 percent, use another procedure to estimate Tc.
The runoff Equation
The SCS runoff equation is:
Where:
(P-/g)2
(P-la)+ S
Q = Runoff in inches, P = rainfall in inches,
(Eq 3.1)
la = Initial abstraction in inches, and
S = potential maximum retention after runoff begins in inches,
Initial abstraction (la) includes all losses before runoff begins. Through studies of many small agricultural watersheds, la was found to be approximately equal to 0.2S. Removing la from above equation, the runoff relationship in eq above gives
(P-0.2S)2
Q (P+0.8S) ................................................................ 3 2) S value is related to runoff curve (CN) by the following relationship.
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1000
CN =
10+S
The runoff curve numbers for US watersheds are developed by examining rainfall runoff data given information of the soil cover type, soil and relief of the watersheds.
Rainfall
The 24-hour rainfall depths for a desired location and frequency can be obtained from the
appropriate, map developed for US. In the Ethiopian context a 24 hr rainfall curves do not exist. For this study, a 3 to 5 years rainfall data observed at the capital of each woreda several kms away 
from the project site. The maximum rainfall is used.
Estimating runoff
The runoff from a watershed may be expressed as the average depth of water that would cover the entire watershed. The depth is usually expressed in inches.
(Eq. 3.3)
Figure 3.5 Rainfall-runoff relationship curve developed based on field experiments in the US
using curve numbers (SCS Engineering Handbook, 1984)
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The volume of runoff is computed by converting depth over the drainage area to volume and it is usually expressed in acre feet since this methodology was developed using English Unit. When CN and rainfall (P) are determined for a watershed, runoff (Q) is determined using the above graph In estimating peak Q (flood), there is a need to know the estimate of the potential maximum retention or initial abstraction of the soil and this  is  often difficult for field works  of this  nature. However, experiments  in the US have revealed that the infiltration data for the different soil groups  under different soil texture as shown in table 3.1 below. These data are also used for different soil groups observed in the study areas.
Table 3.1 Hydrological soil group (SCS Engineering^ield Hand Book, Chapter 2, 1984)
Soil
Group
Description
Final
Infiltration Rate(mm/h)
Soil Texture
A
8-12
sand, loamy sand,sandy 
loam
B
4-8
silt loam,
loam
C
1-4
sandy clay 
loam
D
Lowest runoff potential. Includes deep sands with very little silt and clay, also deep, rapidly permeable loess
Moderately low runoff potential. Mostly sandy soils less deep than A, and loess less deep or less aggregated than A, but the group as a whole has above-average infiltration after thorough wetting
Moderately high runoff potential. Comprises shallow soils and soils containing considerable
clay and colloids, though less than those of group D. The group has below-average infiltration after Pre-saturation
Highest runoff potential. Includes mostly clays of high swelling percent, but the group also includes shallow soils with nearly impermeable sub -horizons near the surface.
0-1
clay loam, silty clay loam, sandy clay, silty clay,
clay
Time of concentration
Time of concentration (Tc) is the time it takes for runoff to travel from the hydraulically most distant point of the watershed to the outlet (SCS Engineering Field Hand Book Chapter 2 1984) T influences the peak discharge, and it can be estimated for small rural watersheds using the following empirical relationship:                                                                                                        y
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| SECTION 111: LITERATURE REVIEW ON 23 GULLIES AND GULLY EROSIONDesign of Large Gully Trcatmenl Works' Tana Beles Integrated W ater Resources Development Proved
T=
c
0 8 ,1000 ,
1 CN J
(Eq. 3.4)
ii4or05
Where:
Tc = time of concentration in hours,
1  = flow length in feet,
CN= runoff curve number, and
Y = Average watershed slope in percent.
After these parameters are determined, the worksheet as shown in Annex is used to determine the peak discharge of each small gully watershed using US Soil Conservation method
3.4.2   Rational Method:
This method is one of the methods often mentioned by hydrologists and hydraulic engineers in the prediction of peak runoff rate, and it is expressed by the equation
q = CiA................................................................... Equation (Eq..3.5)
where :
q =the design peak runoff rate in cubic feet per second (cfs),
i = Rainfall intensity in time Tq time of concentration in inches per hour
C = run of coefficients (it is dependent on catchment characteristics), C varies between 0.05
for flat sandy surface areas to 0.95 for impervious urban surfaces - 0.69 is used for this study
A = watershed area in hectares
In metric units the equation has the following relationship (chaw, 1985).
QP(m /s) = 0.278 C. i (mm h ). A (km )
For small natural watersheds, a formula developed by Kirpich (1940) for agricultural areas is used for determining rainfall intensity through the use of Tc for the watershed. TC is the time required for rain falling at the farthest point of the watershed to flow to the measuring point ( Shaw, 19S5) or downstream point of Interest - in this case at the outlet of the gully or gully mouth. Tc is the time where the runoff from all part of the watershed reaches the outlet point.
3
1                  2 12
T  = 0.00025(^=)°80
c
.................................................... Eq... 3.6)
Where
TC = Time of concentration
L = Length of the remotest point and longest rout or drainage to the outlet, meters 
S = Slope of the gully channel measures as a ratio of elevation difference between the remotest point of runoff and the outlet divided by the length of the channel
u 1 km2 = 100 hectares
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| SECTION III: LITERATURE REVIEW ON 24 GULLIES AND GULLY EROSIONDesign of Large Gully Treatment Works: Tana Belcs Integrated Water Resources Development Project
3.4.3   Adapted Cook's Method:
Adapted Cook's Method (Hudson, 1989) uses land use information and translates the land use data into runoff curve number (CN) as  described earlier in US SCS Method. In this  method unlike in temperate climates, the effect of surface storage (the fourth parameter) is  recognized as insignificant for African situation and it is  ignored, and three of the watershed characteristics: (i) Relief, (ii) soil infiltration and (iii) land cover are used. The weighted average values  of the relief soil infiltration characteristics and the land use give what is known as Runoff Curve (CC). The CC value with the area of the watershed is  translated into runoff using a table 3.3n below. This technique is  used only  for small watershed less  than 500 ha. The table Annex  CIO is  used for translating the CC value against the size of the gully watershed to get an estimate of the peak flood (Hudson, 1989). This method is widely used by the soil and water conservation technicians in the ministry of Agriculture in Ethiopia and it is one of the reasons included in this study.
Table 3.2 Watershed characteristics with designated values as adapted by Hudson, 1989).
Land Cover
Soil Infiltration and drainage
Relief
Heavy grass....................10
Deep, well drained soil........................ 10
Very flat to gentle 5
Scrub and medium grass. 15
Deep, moderately pervious soil............. 20
Moderate .............. 10
Cultivated land ................ 20
Fair permeability and depths................ 25
Rolling .................... 15
Bare or eroded ............... 25
Shallow soils w/impeded drainage....... 30
Hilly or steep........... 20
Medium heavy clay or rocky surface. 40
Mountainous........... 25
Impervious surfaces & waterlogged...50
H A uble o( runoff dmloped t» d on remteh conducted otter 200 >m.|l ^.tetthedt In th. Un.ted St.i.c
K
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SECTION IV: DESCRIPTION AND CHARACTERIZATION OF MODEL GULLY WASTERSHEDS
4.1      Introduction
In this  section, the description and characterization of each model gully  in the selected watersheds  are presented and discussed. The five model gullies  are located in five different woredas in the Amhara Regional State within the study areas of Tana Beles Integrated Water Resources  Development. The first two woredas  are (i) Mecha and (ii) Sekela and both are located in West Gojam Zone, and the three others (i) Farta, (il) Dera and (iii) Misrak Estie are in South Gondar Zone. The geographic location map of the five study woredas where the model gullies exist is shown in figure 4.1 below.
4.2      Megenagna Community Watershed - Misrak Eastie Woreda
Megenagna              Community Watershed is the selected
model gully in Misrak Estie woreda, and it is located in Alemaya or Shimagilew Giorgis Kebele. The UTM coordinates for the watershed are 3883993 to 391722 meter Easting and 1283013 to 1286079 meter Northing. The watershed is selected as one of the five
Tana Beles Integrated Water Resources Development Project sites for the Design of Large Gully Treatment Works. The total  area  of  the  watershed  is nearly 7000 ha (70 km ). The gully branches and main drainage channels constitute
2
about 28 ha (4% of the total watershed area). The watershed is the upstream and it is the source (origin) of Gumera River, which is one of the major rivers in the Tana Beles Sub Basin.
4.2.1 Geography
The model gully watershed is located approximately 8 km from Mekane Eyesus, the capital of Misrak Estie Woreda. The main gully section occupies the natural drainage channel and the active part of the gully  is  moving upstream towards  the elevated grounds, which are at the south eastern part reaching close to the watershed divide.
All weather access road connects the woreda with Bahir Dar, the capital of the region in two directions, and one of these roads crosses the watershed on the northern side. Part of the gully
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CHARACTERIZATION OF MODEL GULLY WASTERSHEDSDesign of Large Gully Treat men I Works: Tana Belcs Integrated Waler Resources Development Project
fingers occupies elevated grounds. One of the major finger gullies is created by the runoff from a road culvert. The altitude in the watershed varies between 2428 and 2603 meters above sea level. Figure 4.2 below shows gully and the location of the villages (settlement and other land uses) within the community watershed.
4.2.2     Topography
The watershed consists of a wide range of relief, mainly of very steep (with slope >50%) and
steeply  sloped hills  (between 30 and 50%) which cover around 0.1% and 5.6% of the total watershed area, respectively. About 40% have moderately steep slopes (15-30%) and areas having gentle slope (3-8%) and sloppy between 8-15% are 16% and 36%, respectively. Flat areas  (0- 3%) cover about 3%. Hillsides have scattered and dispersed trees mainly of eucalyptus species.
The   watershed   has   a   circular shape, and it is made from three sub-watersheds     with     intensive drainage network. The gully under this  study  is  part  of  the  main drainage      channel      for      the watershed  with  varying  size  and coverage of the watershed.
4.2.3 Climate
Available rainfall records from the woreda showed that the annual rainfall ranges between 1300 and 1800 mm with the main rainfall occurring in summer (June to
Figure 4.2 Land use map of the community watershed
Table 4.1 Land use of the community watershed
Land use
Coverage (ha)
Ha
%
1
Cultivated land
429
62.1%
2
Natural forest
19
2.8%
3
Woodlot
43
6.2%
4
Rocky or waste land
1
0.1%
5
Grazing and bush land
175
25.3%
6
Homestead
24
3.5%
Total
691       100%
Source: Mesrak Estie woreda Agriculture Office, 2010
September), and about over 60% cause erosive runoff. The highest rainfall observed over a five year period on a 24 hour basis is 74 mm and it occurred on the month of July and August.
4.2.4   Geology and Soils
The genesis  of the soils  In the Megenagna Model Gully  Watershed is  influenced by  the interaction of the soil forming factors  of climate, topography, organisms, parent material and time. There are two broad category of soils observed in the general soil map of the study area.
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Vertisol
Verisol is a black clay soil with distinct physical characteristics and chemical properties and it is the dominant soil types  in the low lying areas  of the community  watershed. The soil has  the properties  of expanding when wet and contract when dry  (as  dry  season starts) leaving large cracks; a typical characteristics  of a montimorilinte clay  and as  a result it is  unstable for structural work. Workability  is  poor particularly  during rainy  season (wet period) but it has  high nutrient for plant use. This  soil is  found in low lying areas  of the watershed that have slopes less  than 8%. The area occupied with this  soil type is  waterlogged and swampy  (wet land) throughout the rainy  season, and it is  not used for crop production as  well as  grazing. During the dry period it is used for grazing purposes and hence it has been kept for long as communal land. In recent years, as  the population pressure forces  communities  to start pushing and squeezing these wet land areas  for crop production. These situations  continue to increase the gully network particularly in this watershed.
4.2.5  Socio Economics and Farming System
The size of the population in Megenagna watershed is 1950 (390 households) and about 1050 are male and 900 are female. The communities  settle in four villages  widely  dispersed within the watershed and they  are located at elevated grounds  occupying the different parts  of the watershed. All weather-road provides  access  to the Megenagna watershed and it also contributes its own share to the gully problem in the area.
Agriculture is  the main stay  of the economy  for the people in the watershed and more than 95% of the households are farmers involved in activities related to crop production and herding. There is  sufficient rainfall1* to support farming in the study  area and hence agriculture is  the main stay  of the economy  and livelihood. Owing to the higher, long seasonal rainfall, the watershed is relatively rich in runoff water.
Like all other highland areas In the region, the farming system is mixed farming, which consists of crop production and livestock raising. Approximately 62% of the watershed is cultivated land indicating crop production as  the dominant farming system. The farming system refers  to the interrelationship of the different agricultural enterprises  and the farmers'  engagement in these enterprises. These enterprises14 15 include crop production, livestock production and other rural socio economic  activities  such as  off-farm business  - hand craft and pet trading. The major crops produced are tef, barley, potato, and pulses and the main cash crops are tef (ervgrastic tef), barley, pulses, millet (dagussa), maize, and potato. Crop productivity  is  low in the range between 20 and 38 quintals per ha except potatoes which is 160 quintals per ha. The livestock population in the watershed is  560 cattle, 1320 sheep, 112 goats  and 71 donkeys  and S7 horses. In terms of the level of intensity of cultivation, crop production is intensive and grazing areas are shrinking as more lands are brought under cultivation and gully is also takinq its own share out of production - 28 ha (4%) of the watershed.
14 Rainfall ranges between 1300 and 1800 mm per annum 15 Activities farmers do for their livelihood
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Cl1ARACTER1ZA HON OF MODEL GULLY WASTERSHEDSDesign of Large Gully Treatment Works Tana Belts Integrated Water Resources Development Project
The study  of the farming system is  important since it has  Influence on gully  formation and development and in the design gully treatment works. Equally, the decision to rehabilitate die gully  has  its  own long term benefit in terms  of improving the performance of the existing farming system and its  sustainable growth. The design Is  therefore targeting and supporting farming system through availing more reclaimed land for cultivation or grazing or browsing shrubs from the rehabilitated and recovered gully.
There are about three hand dug wells  equipped with hand pumps  and two of them are damaged and not functional. The wells appear to have a good yield. Water for domestic use Is generally free, but there is very limited Irrigation practice in the watershed and It is at the lower part of the watershed area and towards the mouth of the gully. The water is pumped and partly done with manual Irrigation.
The farming communities  are aware of the danger of the fast spreading gully  networks, but they have done very little in their own effort. The local communities are In full support of check dams since they recognize its serious threat to the farm land. These communities are willing to give away land without claiming compensation.
4.2.6  The Drainage system and the gully watersheds
As shown in figure 4 2. the shape of the main watershed is circular, and it is positioned to drain to the west with the valley bottom (low lying areas) stretching east-west.Design of Large Gully Treatment Works: Tana Boles Integrated Water Resources Development Project
Runoff originates  from the remote watershed divide and specifically  from north eastern and south eastern part of the watershed. As  one looks  at the layout of the finger gullies  in the context of the whole community watershed, it is difficult to try to differentiate the finger guilly from the natural watershed drainage system. In this  situation, the main gully  branches  fall along natural drainage of small watersheds. This  situation provides  us  to have independent gu y watersheds, and hence the community  watershed is  divided into five independent gully watersheds as shown in figure 4 3 above.
Run-off from each small gully watershed provides the seasonal stream flow in gully tributaries or finger gullies, and small gully tributaries feed the big gully located downstream (see figure 4.3 above) Therefore in this site, the community watershed is comprised of five major finger gullies with their own gully watershed in their own right and each drain to the main gully which falls at the natural drainage line at the downstream part of the watershed.
Owing to the dynamic nature of this gully system, the gully characteristics change from year to year as the stream flow patterns in the gully changes due to the process of erosion emanating from changes  in land use and climate (rainfall intensity). The writer of this  report observed a
significant  change  in  the  gully  characteristics  during  the  dry  and  wet  seasons  while  collecting
field data for this study (see figure 4.4).
Z\
‘
.V
---- ----
3
1
I
I
Figure 4.4 The soil along the gully banks is crumbling after it gets waterlogged following rain
During the wet season, it is difficult to take measurement of the gully fingers and main gully for design as  the bank  of the gully  collapse as  shown in figure 4.4 and measurements  change significantly  the next day. The dynamic  nature of the gully  expansion makes  the characterization; measuring the depth, width, length and stream flow (both the water and associated sediment load/discharge) difficult
The whole watershed has  comprised of five finger gullies as shown in figure 4.3 above This watershed is the origin and source of Gumera River that finally drains into Lake Tana and it is one of the major over networks in the Tana Beles Sub Basin. In general the catchment area is mountainous, relatively  degraded and with higher slopes  towards  the watershed divide The slope of the gully channel falls from about 2548 meters, a point at the furthest finger gully to 2454 meters at the mouth of the gully and the total length of flow is 3700 m or 3 7 Km the longest travel distance of the watershed.
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CHARACTERIZATION OF MODEL GULLY WASTERSHEDSDesign of Large Gully Treatment Works: Tana Belcs Integrated Water Resources Development Project
4.2.7   Land Use
Secondary data obtained from the woreda agriculture office presented a total of 691 ha area.
The statistics also showed
land occupied under
' ■ 4.2 Land use
cultivation is 62%, natural forest areas including areas occupied by a church and it's environ is 3%
Land use type
Area
Hectare
%
and wood lots 6%. While
the current land use
1
Cultivated
467.79
67.7
2
Grazinq land
69.53
10.1
3
Church Forest
25.67
3.7
4
prepared  using  recently  taken  satellite  imagery  for
this study shows a total land area of nearly 691 ha,
equal to the secondary data, although the individual
land use changes substantially, and this include the
cultivated land, grazing land dwindled by over 100%
and gully increased to 24 ha16. Perennial vegetative
cover reduced significantly. The impact of the such
low level and gradual loss of vegetation cover of the
land in the watershed area alters the balance of rainfall, infiltration, evapotranspiration, surface cover, root strength, and consequently  surface and ground waters  runoff. Under these conditions certain soils and landforms are especially susceptible to gully formation.
4.2.8   Soil Erosion
Soil erosion particularly gully erosion is severe in this community watershed. A substantial part of the community  watershed is  lost for large gully; about 28 ha out of nearly  700 ha of community watershed is occupied by large gully with substantial surface area vulnerable to soil erosion. Gullies are the main source of sediment for downstream areas. Over the last 25 years a lot of soil is lost from the gully surfaces and through this study soil loss from the entire network of gullies (including the main natural drainage channel)* 17 is estimated and it is 260,466 m ; and
Irrigated areas
4.74
0.7
5
Settlement
91.99
13.3
6
Barren Land
6.88
1.0
7
Gully
24.32
3.5
Total
690.92
100
3
this implies about 10, 418 m  per year soil
3
loss from the gully network in the watershed.
According to the woreda agriculture office report, currently  about 66% of the community watershed area is covered under some kind of soil and water conservation measures. A large portion (38%) is  protected under soil bund, while 16% is  protected under stone faced bund. Land use is the other factor to the sever soil erosion problem in the area. Only 5 ha (0.007%) is afforested indicating the watershed is highly deforested. Despite the presence of considerable soil conservation structures, soil erosion in the area continues mainly in form of gulling and the situation seems very severe. Drainage channels and high slope areas are major erosion sites and sediment sources in the watershed.
4.2.9  Hydrology of the watershed
Gully flow is ephemeral, i.e. there is only wet season flow and it occurs in the main rainy season which is  in the months  of June to September. The large size gully  at lower parts  of the watershed indicates the presence of huge volume of runoff from the uplands areas.
16/ A data collected by the woreda agriculture office two years ago reported the land area occupied under the gully as 15 ha 17 /Flow exists only during wet season (ephemeral gully).
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Owing to the high seasonal rainfall in the area and rapidly changing land use in the community watershed, the overland flow (runoff) and the seasonal stream flow both in the main drainage channel and the contributing finger gullies are substantially high. This is obvious as one travels along the gully banks  during the wet season. The low lying fields are extremely waterlogged, difficult to walk on foot, and the water pouring down from the surface of gully banks is quite visible as shown in figure 4.4. As a result, the soil erosion and damage caused along the bank of the gully  network  and the natural drainage channel is enormous. This hydrological process contributes high sediment and siltation at the downstream areas. This hydrological phenomenon continues to change the morphology of the gully. Stream flow and sediment transport are basic features to all channel changes (Heed, 1966).
Finally it is understandable that peak flood (expected once in ten years) in the gully watershed is generally  an expression of the hydrologic  performance of specific  watershed, and it is  the integrated result of various  hydrological factors  that functions  within the watershed. It is important to recognize the strong interdependency  between all hydraulic variables; the rainfall characteristics (amount and intensity), soil type and structure, land use and land cover in the targeted watershed.
4.2.10 Hydrogeology
The hydrogeology of the areas also influences  the gully development and expansion. As shown in figure 4.5 the rise in water table enhances  the development of gullies. Underlying wet conditions due to ground water rise or gully reaching the water table depth continue to widen the gully development.
Existing water supply facilities include hand dug wells equipped with hand pumps and this  situation indicates that the ground water in the low lying areas is within
Figure 4.5 Groundwater (base flow) intercepted by the gully
twenty meters depth. They are located close to the main natural drainage channel and in the swampy areas. The water table in existing shallow [dug] wells is in the range between 10 to 20 meters at the lower [plain] areas. This situation has been manifested more clearly by the base flow appearance along the gully sections and this base flow is used by the community as the source of water for domestic water supply as shown in figure 4.5.
Getachew Alan & Associates
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4.3     Enkulal Community Watershed - Dera Woreda
4.3.1   Location
Enkulal community watershed is located in Dera woreda, South Gondar Zone, Amhara Region. The UTM coordinates  of the watershed are 3662644.1 to 368844.6 meter Easting and 1284953.83 to 1286731 meter Northing. It is located approximately 27 km from Ambesame - the capital of Dera Woreda. An all weather rural road connects Dera woreda with Bahir Dar - the capital of the region, and Misrak Eastie woreda. This road crosses the watershed. The study area is  about 5 km from Arb Gebeya, a small pre urban center in Dera Woreda. This community watershed contributes runoff to the Gumera River, which is one of the major rivers that drain into Lake Tana.
The community watershed has a total area of 324 ha (3.24 km ).
2
It is selected as one of the
five woredas  in Tana Beles  Integrated Water Resources  Development Project sites. Enkulal gully, one of the major gullies in the watershed is selected as model gully for the study on the design of large gully treatment works.
4.3.2    Geography and
climate
The  gully  considered  for  this study is created by a runoff from two road culverts and the head of the two gully branches starts exactly at outlet of the two road culverts. The elevation ranges between 2428 and 2603 meters above sea level. Figure 4.6 shows the overall community watershed map, the model gully (brown color below the road), the
rural road and the location of
the settlement, grazing and farm areas.
Analysis of a five year rainfall record from the regional meteorological office suggests that the highest rainfall observed on a 24 hour period occurs on the month of August and it is 65.2 mm. The mean annual rainfall ranges between 1600 and 1800 mm (SMEC, 2008) with the main rainfall occurring in summer (May to September), and over 60% produce erosive runoff.
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4.3.3   Topography and vegetation cover
The main community watershed consists of a wide range of relief; 4% is very steep (with slope >50%) and about 4% is steep (between 30 and 50%) and about 37% of the total watershed area has  sloping terrain (8 to 15% slope). About 33% of the community  watershed is moderately sloping, while about 4% is flat or almost flat, and it is near the stream channel area. The stream channel meanders towards the mid and lower part of the watershed.
Hillsides  have scattered and dispersed trees mainly  of eucalyptus  species. The main community watershed is longitudinal in shape and drains  east to west, while the gully watershed is north to south.
4.3.4   Soils
Table 4.3 Land slope classes
classification
% slope
Area %
1
Flat or almost flat
0-3%
4.2
2
Gently sloping
3-8%
22.2
3
Sloping
8-15%
37
4
Moderately sloping
15-30%
32.6
5
Steep
30-50%
3.9
6
Very steep
>50%
0.1
Total
100
There   are   four   major   soil   types   in   the
watershed and these include Nitisol, Luvisols,
and Leptosols. On the upper parts of the
watershed, the major soil type is very shallow soil known as Lithic Leptosols. The soil where the gully is located is deep red clay soil or Nithosol. The parent material in the gully area is exposed
at the downstream areas along the gully floor. The soil in the area is relatively well drained.
4.3.5   Land Use
Based on this study, the total land area of the main watershed is 324 ha of which, 257 ha (79%) is  occupied under cultivation, followed by  grazing land (1%) and the forest areas constitute 5%.
Table 4.4 Land use of the main
The land use has shown a major shift from that of the land use taken few years ago which was 21% forest, and 69% cultivated land and 0.87% barren land. The land use of the watershed where the gully  is  located follows  the same pattern as  the main community  watershed land use; the land allotted for crop production is about 59% followed by  grazing land (22%) and forest (12%), respectively.
4.3.6  Hydrology of the Watershed
watershed
Land use type
Area
Hectare
%
1
Cultivated
257
79
2
Grazing land
37.5
12
3
Forest
15.0
4.6
3
Settlement areas
8.0
2.5
4
Barren Land
4.1
1.3
5
Gully
2.0
0.6
Total
298
100
The prevalence of the peak flood is generally an expression of the hydrologic performance of
the watershed characteristics. The runoff in the gully watershed originates in the north eastern
part of the community  watershed. The two finger gullies  downstream of the road joins  to
establish one main gully at downstream areas and the stream flow follows similar pattern. The
flow from the two joins and becomes a major stream to cause server erosion along the bank of
the gully downstream. Therefore, the hydrology of the gully watershed at the upstream of the
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road is collected and it passes through the culverts and the flow from the two finger gullies then joins downstream to form a larger stream in the lower part of the gully.
4.3.7  Socio Economy
There are 1310 people (682 male and 628 female), 528 households, in the community watershed. The communities  settle in widely  dispersed four villages  and they  are located at sloppy grounds occupying most of well drained area of the watershed.
Agriculture is the main stay of the household economy and about 96 percent of the households earn their livelihood from crop production and livestock  farming. The main cash crops  are tef (erograstic  tef), wheat, barley, maize, finger millet (dagussa), pulses  and potatoes, and the yield is 9, 17, 8, 19, 7, 7 and 65 quintals per hectare for the respective crops. While about 4% eke out their living from petty  trading businesses  and craft work. The daily  labor wage they earn from Tana Beles project when employed in gully treatment works is Birr 30 per day, and many recognized it as an important source of income.
The watershed has  sufficient rainfall to support farming, however, agricultural practices  are traditional and soil erosion is very severe. There are soil conservation practices but at a limited scale.
Farming system in the study  area refers  to the interrelationship of the different agricultural enterprises and the farmers' engagement in these enterprises. These enterprises’8 include crop production, livestock  production and other rural off farm activities  such as  petty  trading and crafting and socio economic activities. The study of the farming system in the targeted areas is important since it has influence on gully formation and development; however this aspect of the study is beyond the scope of this assignment. Equally, the decision to rehabilitate the gully has its own long term benefit in terms of improving the performance of the existing farming system and its  sustainable growth. The design is  therefore targeting and supporting farming system through availing more reclaimed land for cultivation or animal grazing and establishing browsing shrubs  from the rehabilitated and recovered gully. The present farming system in the study area is  crop and livestock  raising. Approximately  62% of the watershed is  cultivated land indicating crop production is the dominant farming system in the study area. The total livestock supported by the watershed is 603 cattle, 735 sheep, 425 goats and 206 equines. The land is intensely cultivated and grazing land is shrinking.
4.3.8  The gully watershed
There are two gullies and both are created by the road culverts and it is located at the lower end 9U JVthW J" ? resulting two gullies joins together to form the main gully and it is 
located at the downstream part of the gully watershed. The shape of the gully watershed is 
and i,s is 79 - " - * of
a
at L v5 x’zs 18
18 Activities farmers do for their livelihnnri GeuchcwAiem&Assoc. ^
a
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Cl IARAC 1 ERIZA DON OF MODEL GULLY WASTERSHEDSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Waler Resources Development Project
The highest points  or elevated grounds are located in the northern part of the Enkulal community  watershed and the slope falls  gently  towards  the road. Settlements  are located along these higher grounds. The Enkulal watershed drains into Lake Tana.
The sub watershed has  deep red clay soil to degraded shallow soil. The total length of runoff for the gully watershed is 1260 m or 12.6 Km, the longest travel distance.
4.4      Tikur Woha Community
Watersheds - Farta Woreda
4.4.1   Location of the Model Gully
The Tikur Woha community  watershed is  located in Awzet Kebele in Farta Woreda, South Gondar Zone, Amhara Region. It is  located approximately  15 km along the road connecting Debre Tabor; the capital of Farta woreda and Misrak  Eastie. It is  an all weather road and connects  the study  site and crosses  the watershed at the north east tip side. The UTM coordinates of the watershed are 405567 to 408217.6 meter Easting and 1298282.3 to 1301634 meter Northing. The watershed is also known by the name Tikur Woha Community watershed. It is the upstream of the Jemma River, which is one of the major rivers that drains into Lake Tana.
The community watershed is selected as one of the five Tana Beles Integrated Water Resources Development Project site and the projects cover several sectors; agricultural, health and water supply projects including this study "the Design of Large Gully Treatment Works".
The total area of the Tikur Woha Community  Watershed is  503 ha (5.03 km2) and the land occupied by a network of gully is about 2.2 ha. The gully watershed has an area of 57 ha. The gully  has  two finger gullies  and main gully  is  at the downstream part of the sub watershed where both branch gullies merge and overall the network of gully channels constitutes about
2.2 ha (3.9% of the total sub watershed area).
4.4.2   Geography and climate
The model gully  under this  study  is  a small part of the bigger Tikur Woha Community Watershed. The elevation ranges  between 2390 and 3065 m.a.s.l. Figure 3.8 shows  the watershed map, the gully and the location of the settlement, grazing and farm areas.
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The watershed possesses large size natural
drainage   network.   The   gully   under   this 
study is part of the main drainage channel
for   the   watershed   and   it   has   several
branches  of  significant  size  and  coverage
of the watershed. The watershed is located
in a narrow U-shaped valley.
Available  rainfall  records  for  the  woreda
indicate that the average total precipitation
received in the study area for every water
year (May to September) ranges between
1300 to 1800 mm. The highest rainfall
observed on a 24 hour period observed for
the month of August (highest rainfall
month) is 43 mm. The mean annual rainfall
ranges from 1600 and 1800 mm with the
main rainfall occurring in summer (April to
September), and over 80% cause erosive runoff.
4.4.3 Relief, Topography and vegetation cover
The relief of the community  watershed is  very  steep and rugged for farming. The main watershed consists of a wide range of relief, mainly of very steep and steep slope which cover 4% and 22% of the total watershed area,
respectively. While moderately steep slopes cover the largest, 44% of the total watershed
area, slopes between 8% and 15% cover only 20% of the total watershed and flat
(4%).
Hillsides  have scattered and dispersed trees mainly of eucalyptus species. The watershed has an elongated shape, and it is made from several sub watersheds  and it is  in one of these sub watersheds the model gully exists.
4.4.4   Land Use
Table 4.5 Land slope classes
classification
% slope
Area
%
1
Flat or almost fiat
0-3%
4.1
2
Gently slopinq
3-8%
6.9
3
Sloping
8-15%
20
4
Moderately sloping
15-30%
44
5
Steep
30-50%
22
6
Very steep
>50%
3.8
Total
100
Based on the woreda report, of the total 482 ha of land area in the community watershed 407 ha is occupied by cultivated land (84%), followed by grazing land (8%) and natural forest areas 
(3%). The settlement areas are located along the higher and middle grounds in the watershed.
The land use in the sub watershed did follow the same pattern as the main watershed land use- 0      f  r Cr°P prOduCtion is 59% followed by grazing land (22%) and forest (12%/
respectively ^^ °
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4.4.5   Socio economy of the community in the watershed
The community watershed is densely populated and the land is under pressure. Agriculture is the main stay of the household economy as more than 98% of the households are farmers involved in activities related to agriculture and herding. There is sufficient rainfall to support farming in the study area. Agricultural development is not hindered by a water shortage in the catchment; rainfall range between 900 and 1400 mm per year, and temperature ranges between 7 and 25°C and hence the watershed has relatively adequate waters resources. The main cash crops are tef (erograstic tef), potato, barley, millet (dagussa), maize, nug and to limited extent chickpea (TB, 2010).
Although the watershed has high rainfall, relatively rich in runoff water
and has a long length of growing period (LGP) for most crops, the topography is rugged and has steep slope, and less suitable for farm operation and sustaining water balance. The soil is eroded and highly degraded (see photo in figure 18). Such an environment has a bearing in the livelihood and socio economy of the farmers. Crop productivity is low and farmers' income is low. Communities  settle in scattered and widely  dispersed villages  and they  are located at elevated grounds. The scattered settlement pattern has  its  own challenge in terms  of developing infrastructure such as water supply facilities and close by school, health and market services.
The  project  site  is  about  15  km  from  Debre  Tabor  town,  the  capital  of  the  woreda and  it  is accessed by all weather road.
4.4.6  Soils
Secondary sources suggest that the major soil types in the area are Leptosol and vertisol The large portion of the community watershed is very degraded and the subsoil is exposed and it is shown in figure 4.9 above,
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Leptosols
Leptosols  mainly  occur in the upper part (on higher grounds) of the watershed. The area at higher elevation of the watershed is  characterized by  deep, acid, well-drained, red and yellow ferrisolic and ferrallitic soils, often with stone lines. These soils are generally very shallow and are formed directly over basalt parent rock. Their use is very limited. The Leptosols in the study area are classified as lithic Leptosols according to the FAO-Unesco-ISRIC revised legend of the soil Map of the World (1998) and as lithic Ustorthent.
4.4.7   Hydrology of the Watershed
The watershed is part of the Rib River sub basin that drains to Lake Tana - from the north east side. The annual rainfall for the woreda is estimated to range between 1200 and 1800 mm per annum and stream flow is expected to be higher. The prevalence of peak flood in the watershed under investigation is  generally  an expression of the hydrologic  performance of that specific watershed, and it is the integrated result of various hydrological factors that functions within the watershed. It is  important to recognize the strong interdependency  between all hydraulic variables such as land use, such as cultivated land, gazing land, settlement pattern and land slope in the runoff contributing areas  within the sub watershed that contributes  runoff to the gullies.
4.4.8   Soil erosion
Soil and water conservation works are very limited and available information from the woreda agriculture office indicates  that only  less  than 30% of the watershed area is  covered under some kind of soil and water conservation measures. On higher grounds and as one moves to all directions towards the watershed divide areas, and where the land is either homestead areas or cultivated, soil erosion and land degradation is widely observed. Large gully is visible at lower parts of the watershed and it is located along the natural drainage channel. A network of finger gullies  of varying sizes  start off from the large gully  and spread out in all directions  of the Watershed. A large gully, which is in this case the main drainage channel, collects all ephemeral flows from the finger gullies from all direction of the drainage area. Gully flow is ephemeral, seasonal flow and it occurs in the main rainy season which occurs in the months of June to September.
Based on the above characterization of the watershed, soil erosion in the area is very severe Drainage channels and high slope areas are eroded, Ttiere are very little conservation work to arrest soil erosion and gully formation in the study site. There some work done to control the gully using check dams, and this limited to the downstream part of the gully in relatively low
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| SECTION IV: DESCRIPTION AND OF MODEL GULLY WASTERSHEDS
39Design of Large Gully Treatmcnl Works: Tana Bclcs Intcgraled Waler Resources Dcvclopmenl Project
4.4.9    The gully watershed
As  the relief and other watershed characteristics  in Tikur Woha vary significantly, development plan is done based on development zones developed for the community watershed, and the watershed is divided into six  development zones (woreda report). The model gully under study is located in Development Zone One and Four.
4.4.9.1   The shape and area of the
sub watershed
Figure 4,11 Finger gullies and their drainage area
The sub watershed is longitudinal in
shape, the area is about 57 ha19 and it is located in the north eastern part of the Tikur Woha community watershed. The slope falls rapidly at the water divide area, and down in the mid of the sub watershed, the slope is moderate (15 to 30%). The longest drainage distance for the sub watershed is estimated at about 1640 meters as a result the peak runoff is expected to be lower due to its shape and long runoff travel time.
4.4.9.2  Slope of the sub watershed
The sub watershed is  mountainous  having very  steep slope and the soil is  degraded by continuous  traditional cultivation. The land is  very  vulnerable to active soil erosion including gully development. The slope of channel floor for one of the figure gullies in the study area falls from about 3133 m to 2981 m.a.s.l. as it joins the main drainage channel of the watershed. The upper part of the gully has a slope more than 100%.
4.4.9.3  Land use
The land use report from the woreda suggest that nearly 90% of the sub watershed is used for crop production as seen in figure 4.11 and this is done at heavy environmental cost. While our study indicates that the land used for crop production is about 67% and much of the area is in slopes greater than 30%.
19 The report by the woreda put the area as 73 hectare.
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4.4.9.4   Gully
The gully  under this  study  has three finger gullies  and the upper part of the gully  is  located at a steep slope having a slope of more than 100%. The other one is to the south side having a small branching gully  with deep cut reaching the bed rock as shown in figure 4.12.
Figure 4.12 One of the figure gullies in Tikur Woha
4.5 Biret Afer Community Watersheds - Mecha Woreda
4.5.1   Location and Climate of the Community watershed
Biret Afer community  watershed is  located in Lehulum Selam Kebele in Mecha Woreda, West Gojam Zone about 18 km from Bikolo Abay town near Merawi, a town located on the Asphalt road between Debre Markos and Bahir Dar. It is about 5 km from Gerchech; a small pre urban area located about 18 kms  as  one drives  on a gravel road east of Bikolo Abay. The UTM coordinates  of the watershed are 300393.05 to 302240.2 meter Easting and 1240973.97 to 1243424 meter Northing
This watershed is  one of the five model community watersheds selected for the study on the Design of Large Gully Treatment Works. It is located at the upstream area of Gilgel Gibe River Basin, which drains into Lake Tana and it is one of the many hundreds and thousands of small community watersheds that constitute the head stream of the Blue Nile Basin.
Figure 4.13 below shows the watershed map and the location of the gully and the settlement, grazing and farm areas. The watershed is part of Gilgel Abay River sub basin which drains into Lake Tana, and it is  one of the major river networks  that contribute to the siltation of Lake Tana. The watershed has relatively gentle slope to moderately sloping relief with deep red clay soil.
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SMEC (2009) report the annual rainfall in the range between 1400 and 2000 mm per annum. While a three year rainfall record at Merawi showed an annual rainfall in the range between 1300 and 1800 mm per year. The average total precipitation received for every water year (April to October) ranges between 1400 to 1600 mm with the main rainfall occurring in summer (June to September), and about over 60% cause erosive runoff. The highest rainfall observed on a 24 hour period for the month of August (highest rainfall month) is 95.4 mm.
4.5.2  Topography
1U
Rirst Afer Community Gully Slope Classification Map (^0
1
1
** r ’t> ' J
________________
✓ 
./tj f
Vr
"V*-. > sj
.
----- ---------------------
'ft..;
ST
iB>- • LT»»» *
it -
GA*
Figure 4.13 Map of the Biret Afer Community Watershed
The watershed consists of a wide range of relief. The highest point of the community watershed is located in the north eastern part and the slope falls gently from all sides towards the natural drainage channel located at the center of the main community watershed. The elevation ranges between 2428 and 2603 meters above sea level. The relief is mainly of very steep (with slope >50%) and steeply sloped hills (between 30 and 50%) which cover around 0.1% and 4% of the total watershed area, respectively. While moderate, rolling, gently sloping and flat are 33%, 37%, 22% and 4%, respectively. Hillsides  have scattered and dispersed trees  mainly  of eucalyptus species. The watershed has an elongated shape, and it is made from several sub watersheds. The watershed possesses  an intensive drainage network. The gully  under this study is part of the main drainage channel for the watershed and it has several branches of significant size and coverage of the watershed. The watershed is located in a narrow U-shaped valley.
4.5.3  The sub watershed
The sub watershed, where the gully under investigation exists, is longitudinal in shape, and its area is about 79 ha and the peak runoff is expected to be lower due to its shape. The gully is located at the center of a suitable farm land and near the homesteads. The gully channel falls from about 2120 m.a.s.l. a point at the furthest finger gully to 2042 m.a.s.l at the mouth of the gully downstream. A major part of the gully falls on areas with gentle to moderately sloppy - 8 to 15%. The deep incision is located in the midst of the farm lands and residential areas.
The total length of drainage in the sub watershed is 1300 m or 1.30 Km, the lonqest travel distance in the sub watershed.
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4.5.3.1   Geology and Soils
The soil at the gully  site is  deep red clay  soil and it is  known as  Nitisol. The name of the watershed, Biret Afer may have reference to these physical and chemical characteristics of the soil which is a deep dark clay soil and this soil dominates the soil in the watershed.
4.5.3.2   Land Use
Based on this study, of the total 323 ha of land area in the main watershed, 257 ha is occupied by  cultivated land (79%), followed by  grazing land (12%) and natural forest areas  (5%), and gully accounts about 0.6%. The remaining are barren land and settlement areas.
4.5.3.3  Socio Economics and Farming System
The watershed under study is a rural area where agricultural activities are carried out and the following are the major produces; tef, barley, and pulses. Crop productivity  is  low in the watershed and yield ranges between 15 and 22 quintals per ha, respectively. Major parts of the catchment area have been developed in limited soil conservation works while a large part of the main watershed is  untreated. The farming communities  are aware of the danger of the spreading gullies but they have done very little in their own effort. In addition to the farmland, rehabilitation work is also observed on both sides of the catchment area. The lower part of the catchment area in the watershed is acting as a natural drainage channel. There are few poorly constructed check  dams  constructed with support from Tana Beles  project and the local community groups.
4.5.3.4   Hydrology of the Watershed
The annual rainfall for the woreda is  estimated to range between 1300 and 1800 mm per annum. The prevalence of peak  flood in the watershed under investigation is  generally  an expression of the hydrologic  performance of that specific  watershed, and it is  the integrated result of various  hydrological factors  that functions  within the watershed. It is  important to recognize the strong interdependency between all hydraulic variables such as land use, such as cultivated land, gazing land, settlement pattern and land slope in the runoff contributing areas within the model gully watershed that contributes runoff to the gully.
4.5.3.5  Soil and water conservation in the area
Soil and water conservation works in the study area are very limited and more work is needed to cover the watershed fully. The present gully was a trail for the people and livestock 25 years ago when the people were villagized by the previous government and the villages were where the present head of the gully is located. Later the villagers left their new village back to their former sites. Over the last 25 years  when the gully  developed, there was  not body  took responsibility and therefore, there was no work done to stop the expansion of the gully Even today, it is  with Tana Beles  project support that such program starts. Hence soil and waters conservation in the area is weak.
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4.5.3.6    The gully
The name of the gully is
Zimac. It is quite a large gully particularly  at  the  mouth  or downstream end (figure 4.14). The gully site once was a foot trail for human and livestock. About 25 years ago, the communities used to live in scattered hamlets, and the villageization program by the Military Government in the 1980s brought the rural hpouseholds to live in clustered villages requiring foot   and   animal   trails.   The present Biret Afer gully site was then a trail for the
Figure 4.14 The mouth
households to the market, to their farm and livestock to their grazing areas. Over such a long period the present gully developed into such big size gully. Today the gully depth reaches to
about seven meters as shown in figure 4.14 above.
4.6       Washaw Community Watershed - Sekela Woreda
4.6.1    Location of the Model Gully
The Washaw Community  Watershed is  located in Abseken Kebele in Sekela Woreda, West Gojam Zone in Amhara Region. The Washaw sub watershed is located approximately 16 km to the east of a small town called Gishe Abay - the capital of Sekela woreda. All weather access road crosses the sub watershed connecting the site with Rebu Gebeya and Adet which are in the north east of the site. It is situated about 46 km from Tilili..The UTM coordinates of the watershed are 314916.53 to 315303.3 meter Easting and 1222245.82 to 12222578 meter Northing. The watershed is selected as one of the five watersheds considered for gully design study by the Tana Beles Integrated Water Resources Development Project.
The total area of the Washaw sub watershed designated for this study is 9.55 ha (0.09 km ). The area is a sloppy ground that was once used as a communal grazing area. Now the gully covers  a large part of the community watershed. The gully has two finger gullies (branches) and a main where both branch gullies merge into one big gully and the total area of the gullies is about 1.2 ha (12.3% of the total watershed area).
4.6.2    Relief and climate
The topography of the community watershed is mountainous and mostly sloppy and its has an average altitude of about 2600 m above sea level (masl).
2
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Available rainfall records for the woreda ranges between 1300 to 1800 mm, and over half of the precipitation cause erosive runoff. The highest rainfall observed on a 24 hour period for the month of August (highest rainfall month) is 42 mm.
4.6.3    Socio Economy and Farming System
Farming system in the study  area refers  to the interrelationship of the different agricultural enterprises  and the farmers'  engagement in these enterprises. These enterprises  include crop production, livestock  production and other rural socio economic  activities  such as  off farm business  such as  pet trading. The study  of the farming system in the targeted areas  is important since it has  influence on gully  formation and development. Equally, the decision to rehabilitate the gully  has  its  own long term benefit in terms  of improving the performance of the existing farming system and its  sustainable growth. The design is  therefore targeting and supporting farming system through availing more reclaimed land for cultivation or grazing or browsing shrubs from the rehabilitated and recovered gully.
The farming system in washaw sub watershed is  mostly  mixed farming system in which the farmers  are working both animal husbandry  and crop production. Few crops  are produced in the area such as tef, barley, and pulses. The total number of livestock is estimated at about 534 cattle, 690 sheep, 14 goats  and 94 equines. In addition farmers  practice beekeeping by  few innovative farmers  in the cultural beehive production system. While most landless  farmers especially youngsters are wage laborers in the area and outside In large agricultural farms to generate additional income to their family. Soil erosion in the area is very severe and drainage channels and high slope areas are eroded. These and other external environment problems lead the communities into distress and poverty and hunger.
The woreda is accessed by all weather roads that connect the woreda to Bahir Dar - the capital of the Region in two directions - through Adet and through Tilili. Two of the three finger gullies are created by the runoff from two road culverts.
There is sever soil erosion in the washaw watershed due to ragged topography and human interference; plowing steep slope land, deforestation of natural vegetation, over grazing and
low level soil and water conservation activities. Rill erosion and gully erosion are, therefore, common in all land features. According to the woreda office report, soil conservation activities started in washaw watershed at about 1993/94 through Local Level Participatory Planning Appraisal which focus mainly on natural resource, and activities include 950 km of soil bund, 50
km stone bund and 30 km grass strip. There are few activities on two finger gullies to control erosion and these are loose rock check dams. These check dams have been constructed with support from Tana Beles and the local community groups.
4.6.4 Geology and Soils
The underlying geology  of the site is  basalt rock, which has  shallow soils  that overlays  the
parent rock material.
20 Activities farmers do for their livelihood
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4.6.5   The gully
There are two types of gully observed in this watershed. The first is the continuous and the second one is  discontinuous gully. Continuous gully is caused by the road side drainage and they  are continuous. Soil profile from gullies along this  gullies  show deep red clay  soil (Nitosol) of up to 2 meters.
Discontinuous gully has a unique geology and soils. As  shown in figure 4.15, the underlying soil is  exposed not in continuous  but in patches  to form the gully. This occurs particularly at the upper part of hill side. The lower part of the watershed areas (3% to 8%) have grass cover and it is used now for grazing.
Figure 4.15 Continuous and discontinuous gully in Washaw model gully watershed
4.6.6   Soil fertility
The study area is highly degraded soil due to overgrazing, which changes the soil properties; structure and hydraulic properties. Owing to the high rainfall and poor soil conditions, the area is  affected by  soil erosion including gully. The production potential of the soil is  therefore declining from time to time. This  problem is  due to loss  of nutrient, soil mass  movement, inadequate water holding due to shallow soil and poor structure.
4.6.7   Land use and land cover and fanning system
A large part of the land in the gully watershed has been put under grazing for quite a long time until Tana Beles Project intervened. Most of the sub watershed is presently put under closure. Some check dam structures are already in place. Soil and water conservation activities such as on site water harvesting structure (half moon structure) cover a large part of the study area. Outside of the sub watershed, the major crops grown are barely, maize, teff, and wheat.
Based on a report from the woreda agriculture office, nearly  15% of the main Washaw watershed was covered by grazing land. Free grazing is a traditional practice which led to sever land degradation in the area. After the closure of the present study  area particularly  the upstream part, the soil and vegetation is rehabilitating. The presently grazing area is on lower and low lying part of the watershed. There is also communal and individual grazing land in the studied watershed area. Communal land has a great role for holding all community animals, although it has  an effect on soil erosion and land degradation. There are individual grazing lands owned by some members of the community in the watershed.
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SECTION V: STUDY FINDINGS AND RESULTS
5.1      Megenagna Community Watershed
5.1.1   Conditions of the watershed
Land degradation is a severe and most prevalent situation in large part of the study area. Soil erosion and particularly gully erosion is the most aggressive phenomenon that contributes to the worsening land degradation and it is found as a threat to the agricultural lands and with catastrophic impacts on fragile agricultural and natural ecosystems in the area. The effects of soil erosion include on-site damage to the land and destructive siltation downstream or it is known as off-site consequences.
A  large  portion  of  the  gully  in  Megenagna  and  Washaw  Community  Watersheds  exists  on communal lands that have long been used for grazing and the grass species are grazed to the ground. While in the other three study sites, the gullies are on farm lands. This situation has detrimental impacts on reducing the cultivable land size, decline in agricultural productivity and on ecological function that ultimately affect livelihood and quality of life in the area.
The gullies are large in size with an age of over twenty years, and land use and land cover has changed significantly, and such changes have influence soil erosion and gully development and consequently land degradation processes. In all sites under this study there has not been any land use work  done by  the government or local administration although there are land use policy in the region. Land use changes are the primary causes of increased soil erosion, gully development and land degradation in rural areas and its absence directly impact ecosystem services that support human needs in the area.
5.1.2   Geology
The general geology of the community watersheds is dominantly covered by tertiary plateau volcanic, which are shield groups  of Miocene alkali-olivine basalt, tuff, agglomerates  with basaltic flows and related spattered cones (SMEC, 2008). Most part of the study areas and its vicinity are covered by volcanic rocks of aphanitic and trachy basalt, tuff and consolidated red ash rock formations. Geographically the five model study areas are found in the north western Ethiopian highlands and it is bounded by blue Nile (Abbay) river in the south part and the area is free from and tectonic and seismic activity or risk (SMEC, 2008).
The genesis of the soils in the five model gully treatment sites are influenced by the interaction of the soil forming factors  of climate, topography, organisms, parent material and time. According to the FAO soil classification of 1996, there are six major types of soil in the five model watershed and sub-watersheds, and these include Chromic Luvisols, Cambisols, Eutric Nitosols, Pellic Vertisols, Regosols, and Leptosols. They are common soil units in general. On the upper parts  of the watershed and sub watersheds, the soils  are very  shallow (Lithic Leptosols), especially this is common at Tikur-wuah and Washaw Acheken community sub watershed areas.
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5.1.3    The Soils
The soil at the lower and plain land areas  (slopes  less  than 8%) has  a vertic  property  which
cracks when dry and swells when wet. The clay fraction of these soils is dominated by expandable 2:1 lattice clays, mainly montmorillonite. They have clay texture throughout the
profile and in most cases the
proportion of clay fraction is very high. Changes in soil properties as a result
of its geologic nature and soil
structure and its intensive use have increased risk for gulling, especially if left exposed or subjected to concentrated flow for extended
periods.   It   is   in   this   part   of   the watershed gully networks. The
Vertisols in the study area are
classified as eutric Vertisols according
to FAO-Unesco-Isric revised legend of the soil map of the world (1998) and as ustic Epiaquerts according to keys to soil Taxomomy (1992).
Figure 5.2 Color and structure of the soil profile in the watershed
The soils of the study area crack widely during the dry season, and swells when wet and closes the crack. This phenomenon makes the soil less suitable for structures including check dams.
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Vertisols are extremely hard to plow when dry making farmers land preparation at its early part more difficult. While it is equally extremely plastic when wet, and almost at liquid under certain conditions. This is what is observed at lower part of the study site in Megenagna Watershed during the 2013 wet season for physical observation of the stream flow in the gully. A picture taken shows the characteristics of the gully during the rainy season. In the figure you observe fresh gully bank collapsing and walking in the field was difficult, waterlogged and slippery.
The thickness of the solum ranges from 75 to 120cm. The bedrock is generally deeper than 3 m in most areas investigated. The expansion and contraction property of the soil is responsible for the formation of grooved shiny face (slicken sides) and wedge shape structures at a depth 25 to 100 cm. During the dry season animals, as well as wind cause the fine surface material to fall in the widely opened cracks (pedoturbation) causing soil heaving as the soil expands during the rainy season. Such soils have more risk for gully erosion. Failure to use preventive measures may allow gullies to form. Although gully formation and enlargement are typically episodic, they are not instantaneous.
The general influence of lithology  on soil erosion processes  is  manifested directly  by  the resistance of the denuded bed rocks exposed to the flow of the stream in the gully and it is more affected by the character of parent materials (the bed rock). Some geological materials are vulnerable than others to aggressive energy of the rainfall and runoff. High erosion risks match with units of weak unconsolidated geological formations. Hence geologic structure is one of the major factors influencing gully development in the study area.
Soil Color
Soil  color  of  surface  horizons  reflects  a  strong  imprint  of  biological  processes,  notably  those
influences by the ecological origin of the soil organic matter. Soil organic matter imparts a dark brown to black color   to   the   soil.   Generally,   the higher the organic matter contents of the   soil,   the   darker   the   soil.   As indicated  in  the  figure  16,  the  soil color   becomes   lighter   down   with depth.
The soil color at the surface becomes dark brown color and subsoil become dark  colored. Parent material, such as  basalt, accumulation of manganese can imprint a black color to the subsoil horizons.
Soil texture
Figure 5.3 Soil color along soil profile
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of the soil profile. The texture changes with increased clay and silt content in the 50 to 100 cm depth. While most soil in the low laying areas show weak to moderate structures. The structure shapes are described as being prismatic, and angular.
As shown in table 4.1, clay is dominating the soil textural classes in most of the study sites. It showed minor textural variations (sandy to clay) in Tikur woha and Washaw sub watershed.
Three out six samples, clay is the major texture at upper and lower zone of the soil profile, and
two out six sites, both silt and clay are the major ones. This minor variation in soil texture was caused by parent materials, topography, in situ weathering and translocation of clay.
The fine and medium textured soils (e.g. clay loam, silt clay loam, sandy silt loam) are favorable for agricultural uses  because of their high available retention of water and exchangeable nutrients. In fine pores  as  in the case of clay, water is  strongly  adsorbed in pores  but not available for plants, i.e. cohesion and adhesion water occupy the macrospore spaces and they are retained in the soil by forces that exceed gravity. At the same time contain more moisture than other soils. In this soil types infiltration rates is lower than other soil types; that means the initial absorption of water could be less and more runoff occurs this situation is more significant in august when the soil is having enough moisture. Peak runoff in this type of soils is very high. Other factors like compaction of the soil, management practices, vegetation, and saturation of the soils have also a significant impact on infiltration and have to be considered.
Table 5.1.Soil texture of the lithological layers in the study watershed
Topsoil
(0.50 m deep)
Subsoil
(0.50 to 2.0 m
Sand (%)
36
61
Silt (%)
31
20
Clay (%)
33
19
Bulk density (g cm"3)
1.65
Cohesion (kPa)
1.55
4.5-9.0
Saturated hydraulic
4.5-9.0
0.5-1.8
Conductivity (cm day"l)
Soil structure
Sand, silt and clay particles were the building blocks from which soil is constructed The manner which these building blocks are arranged together is called soil structure. Soil structure is just as  important as  soil texture in governing, how water and air moves  in soils. Structure
fundamentally influences the suitability of the soils for the growth of plant roots.
0.5-1.0
Table 5.2 Soil
Site
#
GPS location
depth
(cm)
Bulk density
QI IV1 IL-5 YVO moisture content
uei itru
Shear Parameter
Northing
Easting
Elev.m
Pl
g/cm3
%
Cu (KN/m2)
• •
1285012
391198
2510
0-50
1.65
31.6
2
13
50-120
1.55
32.7
120-200
1.5
1.51
5
P2 | 1285629 T"389489         2475
30.2
0-50
4
1.59
10
34.2
5
7
50-120
1.33
35.4
3
7
V: STUDY FINDINGS AND 50 RESULTSDesign of Large Gully Treatment Works: Tana Bcles Integrated Water Resources Development Project
The structure of soils in Megenagna watershed varied from fine, weak and sub-angular blocky to medium to coarse, moderate to strong and sub-angular blocky  to angular blocky  structure (see picture 4.2 B.). Table 4.2 gives  a summary  of relevant data concerning ephemeral gully erosion in each woreda.
Soil structure forms  due to the action of forces  that push soil particles  together. Subsurface structure tends to be composed of larger structural units than the surface structure (see figure 14 above). Subsurface structure also tends  to have the binding agents  on ped surface than mixed throughout the ped. Physico-chemical processes  (e.g., wet-dry, clay  translocation, formation/ removal of pedogenic  weathering products) influence soil structure formation throughout their profile. However, the nature and intensity of these processes varies with depth below ground surface. The structure and hydrological function of the plant communities, texture, mineralogy, surface manipulation and topography  all serve to modify  local climatic effects through their influence on infiltration, storage and evapotranspiration of the water. Plant roots  exert compactive stresses  on surrounding soil material, which promotes  structure formation. Soil-dwelling animals (e.g., earth warms, gophers) also exert compactive forces, and in some cases  (e.g., earth warms) further contribute to structure formation via ingestion/excretion of soil material that includes incorporated organic secretions influencing the density of the soil.
The bulk density of the top 50 cm soil is about 1.65 and this decreases slightly to 1.55 as depth increases.
The permeability of the soil is slow to moderately slow. The internal soil drainage depends on the soil texture, structure and other characteristics  of the soil profile and hence it is  a major factor that influences  surface runoff and a major cause to soil erosion (including gully). The infiltration rate study carried out in few selected sites along the gully indicates that the internal drainage is very slow to slow.
The soils in the low lying (0 to 8% slope) areas of the watershed, dark clay soils (vertisol) is dominant (figure - a soil having a vertic  property  - crack  when dry  and swells  and loss  its cohesion when wet. The clay  fraction of these soils  is  dominated by  expandable 2:1 lattice clays, mainly  montimorillonite clay. They  have clay  texture throughout the profile (up to 2 meters and above) and in most cases the proportion of clay fraction is very high. The soils crack widely  during the dry  season, but the cracks  close up again on re-wetting. The resulting expansion and contraction is responsible for the formation of grooved shiny face (slickensides) and wedge shape structures at a depth 25 to 100cm.
During the dry season animals, as well as wind, cause the fine surface material to fall in the widely opened cracks (pedoturbation) causing soil heaving as the soil expands during the rainv season, developing characteristics hollow and mounds micro-relief known as gilgi.
The main constraint is  the narrow range of moisture conditions  within which the soil characteristics  change. It is  extremely  hard when dry, but extremely  plastic, almost at liquid stage when wet and hence easily move. The thickness of the solum ranges from 75 to 120 cm.
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5.4      Characteristics of the Ephemeral Gullies in the Watershed
5.4.1   Production of Maps for the gully
Maps of the gully in Megenagna Watershed are produced using GIS tools. Finger gullies and their stream orders were developed and used in this study. As shown in map figure 4.4, the gully has several branches (finger gullies) and it is falling along the natural drainage channel. It is  aggressive with fast spreading head cut. Two of the branches  in the gully  network  are developed by the runoff flow from two road culverts constructed along the gravel all weather road that crosses the community watershed.
5.4.2   Layout of the gully network and Geometry of the Gully
Megenangna Gully has a network of finger gullies as shown in figure 25. Based on the field measurements made,  gully  dimensions  for  each finger  gully  are  determined.  The shape     of     the     gully     varies significantly,  and  over  65%  of  the observed  section  of  the  gully  is trapezoidal, the most common gully form observed in this study site. Less than 10% is V- shaped while the  remaining  section  of  the  gully cross section is U shaped. In U- Shaped   gullies,   the   topsoil   and subsoil erode at the same resistance.  Because  the  subsoil  is eroded as easily as the topsoil, nearly vertical walls are developed
on each side of the gully. This is where the gully banks are collapsing.
In the case of V-shaped gullies, the subsoil has more resistance against the erosion force than the topsoil and it mostly occurs at the head cut in small sized gullies.
The large part of the gully formation process at low elevation areas in the watershed is a plane failure within the soil profile. There exists  a layer of weak  material at the lower layer approximately 60 to 100 cm depth. The soil layer above and below is a black clay soil (with vertic properties) very much wet and becomes fluid during the rainy season that causes the soil to move in mass and as a result one sees a massive fall of the wall of the gully, and hence qullv expansion is very fast.
The total length of the gully network in the small community watersheds is nearly 8000 meters (8 km), and it is a very huge burden to the land and environment. There are distinctly existinq five mam gully branches with several small finger gullies. The total surface area of the qullv [occupied] is estimated at about 23.77 ha (0.24 km ). The longest gully is in sub watershed # 2
2
having a total gully length of about 3 km (3000 m). The depth ranges nearly between 1 and 7
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meters and the width to depth ration to a large extent are fairly consistent. These data shows that the gully and its fingers are large gullies of significant magnitude.
The gullies  having V-shaped, trapezoidal and rectangular (U-shaped) were calculated and the areas  vary  substantially  even within each finger gully. All gullies  - the main and finger gullies are located along the natural drainage channels  or paths  of the watershed except finger gully #3 and # 4 which are created primarily by runoff from road culverts.
Table 5.3 Gully characteristics
Finger Gully
Location
Sample
observations
GPS Location
Easting
Northing
Avg.
top
width
(m) -w
Avg.
bottom
width
(m)
Avg.
depth
(m)-
d
W/d
Avg. X-
sectional
area
(m )
2
#1
Peg #1
390351
1284547
14.70
5.30
3.30
4.45
33.00
Peg #2
390342
1284528
25.70
20.00
3.20
8.03
73.12
Peg #3
390372
1284518
20
20.20
3.20
6.25
64.30
Peg #4
390333
1284448
19.40
10.00
4.10
4.73
60.27
Peg #5
390409
1284300
25.50
5.00
5.30
4.81
80.83
Peg #6
390424
1284221
12.40
5
6.70
1.85
53.27
Peg #7
390439
1284970
14.90
3.50
6.50
2.29
59.80
Peg #8
390430
1284862
12 00
3.50
6.50
1.85
50.38
5.4.3 Critical Section of the gully
A   meandering   section   of   significant importance  is  identified  at  low  laying areas  of  the  gullies;  a  part  of  the section  is  shown  in  figure  26.  The thwaleg section is properly marked and the high and low flow velocities sides of  the  gully  bank  are  mapped  for  its special  treatment  as  required  in  the design  of  gully  treatment  works.  It  is important    to    clearly    analyze    the hydraulic characteristics of the stream flow during the wet season and identify which  sections  of  the  meandering  or bending  sections  of  the  gully  has  a high velocity (erosive sections) and low velocity (depositing sections). Based on this information a design for treatment is proposed.
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5.4.4   Gully Classifications
Based on the geometry of each gully [finger], the gullies in Megenagna Community Watershed under investigation are classified as small, medium and large size gullies, whose depths are in the range of less than one meter as small, gullies In the range between 1 and 3 meters as medium and greater than 5 meters as large. Over 75% of the model gullies are medium and large and about 98% of the gullies are continuous.
5.4.5   Hydrology
The estimated peak flood for each finger gully using Adapted Cook's Method is presented in Annex. A summary of the estimated peak flood is presented in table 4.4 below.
Table 5.4 Estimated peak flood for finger gullies
s.
n
Finger
Gully
Area of sub
watershed
Estimated
Q
(m /sec)
Used Q (m /sec)
3
for design
3
1
#1
62
11.05
11
2
#2
117
20.72
20
3
#3
64
12.23
12
4
#4
59
8.81
9
5
#5
68
7.60
7
5.4.6   Soil loss from gullies
The soil removed and lost to the stream from the network  of gullies  in the watershed is calculated based on the measured geometry of the gully and the total soil loss from the whole
3                  is equivalent to 26 hectare of land at one meters depth over a period of about 25 years.
Table 5.5 Estimated soil loss from Megenagna Model Gully
community watershed is estimated at about 260,466.47 m   which
Gully
Soil loss from the gully m3
Finger gully # 1
76,909.64
Finger gully# 2
91,330.40
Finger qully# 3
53,499.97
Finger qully# 4
11 029 71
Finger gully# 5
,.
27,696.75
Total
260,466.47“
Soil Lost from the gully sections over the life of the gully which is estimated at about 25 years for branrh o
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5.2      Enkulal Community Watershed
5.2.1   Characteristics of the Watershed
5.2.1.1  Geology and Soils
The major soil parent materials within the study area are basalt. Leptosols and Luvisols occur mainly  on steep side slopes; mountains  and hills  and they  are prominent in the whole Dera woreda including Enkulal community watershed. The soils are generally young and are lim'ted by their topsoil horizon or directly over altered parent rocks from which they have developed.
5.2.1.2  Soil Color
Soil color at the study  area is  red clay  soil. The surface horizons  reflect a strong imprint of biological processes, notably those influences by the ecological origin of the soil organic matter. Soil organic  matter imparts  a dark  brown to black  color to the soil. The higher the organic matter contents of the soil, the darker the soil.
5.2.1.3  Soil texture
As  shown in table 5.6, clay  is  dominating the soil textural classes. It showed minor textural variations (sandy to clay) along the soil profile. Clay is the major texture at upper and lower zone of the soil profile both silt and clay are major ones. This minor variation in soil texture was caused by parent materials, topography, in situ weathering and translocation of clay.
Tab e 5.6 Soil characteristics for the             area
Soil
depth(c
m)
pH
(H20)
Particle Size Analysis
(%)
Hydraulic
conductivit
y
infiltration
rate
Sand
Silt
Clay
USDA
Textu
re
class
cm/hr
cm/hr
Pl
0-15
5.3
12
38
50
C
2.3*10'5
2.2
15-55
5.4
14
30
57
C
55-90
6
12
24
64
c
90-135
6.5
13
23
64
C
135-200
6.6
10
28
62
c
The fine and medium textured soils (e.g. clay loam, silt clay loam, sandy silt loam) are favorable for agricultural uses  because of their high available retention of water and exchangeable nutrients. In fine pores  as  in the case of clay, water is  strongly  adsorbed in pores  but not available for plants, i.e. cohesion and adhesion water occupy the macrospore spaces and they are retained in the soil by forces that exceed gravity. At the same time contain more moisture than other soils. In this soil types infiltration rates is lower than other soil types; that means the initial absorption of water could be less and more runoff occurs this situation is more significant in august when the soil is having enough moisture. Peak runoff in this type of soils is very high. Other factors like compaction of the soil, management practices, vegetation, and saturation of the soils have also a significant impact on infiltration and have to be considered.
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5.2.1.4  Soil structure
Soil structure forms  due to the action of forces  that push soil particles  together. Subsurface structure tends to be composed of larger structural units than the surface structure. Subsurface structure also tends to have the binding agents on ped surface than mixed throughout the ped. Physico-chemical processes  (e.g., wet-dry, clay  translocation, formation/ removal of pedogenic weathering products) influence soil structure formation throughout their profile. However, the nature and intensity of these processes varies with depth below ground surface. The structure and hydrological function of the plant communities, texture, mineralogy, surface manipulation and topography  all serve to modify  local climatic  effects  through their influence on infiltration, storage and evapo-transpiration of the water, plant roots  exert compactive stresses  on surrounding soil material, which promotes structure formation. Soil-dwelling animals  (e.g., earth warms, gophers) also exert compactive forces, and in some cases  (e.g., earth warms) further contribute to structure formation via ingestion/excretion of soil material that includes incorporated organic secretions.
The bulk density of the top 50 cm soil is about 1.65 and this decreases slightly to 1.55 as depth increases, and the decrease in bulk density indicated compaction of the soil.
5.2.1.5  Gully Geometry
As shown in table 5.7 the gully has two branches and it is located on a farm land away from the natural drainage channel of the community watershed.
The two gully branches are created by the runoff from the newly constructed road that passes through the watershed. Concentrated runoff from two of the road culverts  created the two branch gullies. At downstream the two branch gullies  meet to form the main gully. The road construction through steep lands, without adequate provision for safety  to the downstream areas  in the drainage systems, is  a major cause of this  gully  erosion. Inadequate energy dissipating structures  at the outlet of the road culverts  and insufficient attention given to the potential damage for the downstream drainage area are major causes of gulling.
The damage has been very aggressive with fast spreading head cut until it is stopped at the two culverts of the road.
Based on the data collected and analyzed from the field, the gully  under investigation is classified as medium size gullies which have a depth in the range of 2 and 4 meters depth The gully is continuous.
When U-Shaped gullies are formed with the topsoil and subsoil eroded at the same resistance. BeCKUSj th< lubso'u iS eroded as easi|y  as the topsoil, nearly  vertical walls  are developed on each side of the gully. In the case of V-shaped gullies, the subsoil has more resistance against the erosion force than the topsoil. This is the most common gully form observed in this study
The soil is a stable day soil with no plane failure but cracks at the gully edge as the stream
Sp oXS deep at 9U"y "“r
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| SECTION V: STUDY FINDINGS AND 56 RESULTSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table 5.7 Gully Characteristics for the study area
Finger
Gully
Location
Sample
observations
GPS Location
Easting
Northing
Avg.
top
width
(m) -w
Avg.
bottom
width
(m)
Avg.
depth
(m)-
d
W/d
Cross
sectional
area
m2
Main
(1)
(2)
(3)
P#01
11.80
4.00
2.46
19.43
19.43
P#02
16.40
9.00
2.90
36.83
36.83
P#03
17.40
9.00
3.46
45.67
45.67
#1
Branchinq
#1
P#01
10.60
0.60
4.05
22.68
P#02
8.40
0.60
2.4
10.80
P#03
6.90
4.30
2.14
11.98
11.98
Road culvert #1
#2
P#01
11.0
0.60
3.90
4.31
22.62
P#02
19.60
0.60
5.00
4.64
50.50
P#03
18.00
4.50
4.42
8.92
49.73
P#04
17.60
4.20
3.5
6.05
38.15
P#05
9.00
0.60
2.40
6.05
11.52
P#06
10.00
6.00
1.58
9.71
12.64
P#07
10.00
6.00
1.05
3.74
8.40
Road culvert #2
The gully formation process in Dera is caused by runoff from the two road culverts. The field
measurement of ephemeral gullies has been conducted in May 2013. The total length of the
gully network in the small community watershed is about 0.87 km (870 m). There is one main
gully and two branch gullies. The total surface area of the gully [occupied] is estimated at about
2.0 ha (0.02 km ). The depth ranges between 2 and 4.4 meters and the width to depth ratio
2
were fairly consistent. These data shows that the gully and its fingers are medium sized gullies of considerable magnitude.
5.2.1.6  Peak Flood
For this  study, a graduated gauge was  installed at the lower part of the gully  and measurements (depth of flow) was observed during august 2013 season, and the flood in the gully did not exceed 1.35 meters, which is 13m /sec. The depth of flow of the stream in the
3
gully  is  much  lower  than  the existing  gully  depth  which  more than five meters. However the stream continues to cut deep into the gully each season as we do not intervene.
The Cook's Method is used to estimate peak flood for the design. This method is based on evaluating the watershed characteristics of the gully and it is found more rational in terms of evaluating the peak flow. Based on this method, peak flow is found to be 10 m /sec and with a
3
o.75 adjustment 8 m3/sec is used for check dam design.
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5.2.1.7    Soil loss from gullies
The soil removed and lost to the stream from the gullies in the watershed is calculated based on the measured geometry  of the gully  and it is  estimated at about 21,000 m . The o a volume and total sediment from both rectangular and trapezoidal section of the channel for each finger gully is given in table 5.8 below.
Table 5.8 Total volume of soil lost by gully 
erosion
Finger Gully I.D Number
Bulk density 
g/cm3
Total soil lost
from the gully 
(m )
3
#1
1.35
11,874
#2
1.35
9,794
Total
21,668”
5.3 Tikur Woha Community Watershed
5.3.1   Characteristics of the Ephemeral Gullies in the Watershed
The gully is located on a steep slope (over 100% at the top end as shown in figure 5.7 below)
and the gully is developed as a result
of inappropriate land use. As shown in
figure 5.6, the land cultivation practice
is traditional; tilling the land up and
down making the soil vulnerable to
sever soil erosion and gully
development. The damage has been
very aggressive with fast spreading
head  cut  until  it  is  stopped  at  the
culverts of the road.
Based on the data collected and
analyzed from the field, the gully
under  investigation  is  classified  as 
medium size gullies but on a steep
slope. The gully is continuous. The
shape of the gully varies significantly
along the slope. The second gully
located  to  the  south  has  very  wide
and steep gully.
The problem of land degradation in Tikurwuha watershed area is strongly linked with number of factors, and these factors are complex; rugged terrain and intensive cultivation on these
“ Soil lost from the gully section over the last 25 years
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terrains  (no land use), population density, shortage of land (Average holding 0.5 ha per HH) and free access  to common pool resources  like livestock  grazing land. According to the information obtained from the Wereda agricultural office, more than 45% of the land area in Tikur wuha watershed is  under intensive cultivation. This  includes  steep slope and marginal lands which can accelerate the erosion process.
Furthermore, a review of studies  by  the woreda indicates  that farmers  in the study  area are food insecure due to high soil degradation and consequently  low productivity  of the land. For example the agronomic  report of this  study  suggests  that the food produced by  many households  can last the family  for 6 months  only. Although there is  abundant rainfall in the watershed, there is  acute food production problem. To address  this  intricate problem, integrated natural resources  management through watershed management approach is indispensable which makes  systematic  efforts  to improve soil and land productivity. Gully treatment is one proposed alternative in the integrated water resources management plan.
Table 5.9 Land use for the main
5.3.2   Land use
The land use (table 5.9) has shown a major shift from that of the land use taken few years ago by the agriculture office. The data obtained from this study  has  shown new information in terms  of introducing land area occupied by  barren land. Perennial vegetative cover amounts to only 5 % of the watershed.
The impact of such low level and gradual loss of vegetation cover in the watershed area alters the balance of rainfall, infiltration, evapotranspiration, surface cover, root strength, and consequently 
watershed
Land use
Hectares
wored
a
This study
1
Cultivated
land
407
307.0
2
Grazinq land
40
114.2
3
Forest
15
11.0
4
Settlement
12
20.6
5
Gully
?
2.21
6
Barren land
?
70.7
7
Others
9
Total
483
511
surface and ground waters runoff. Under these especially susceptible to gully formation.
5.3.3   Geology and Soils
conditions certain soils and landforms are
Leptosols  occur mainly  on steep side slopes; mountains  and hills  and these soil types  are prominent in the woreda including Tikur Wuha community  watershed. The soils  are generally young and are limited by their topsoil horizon or directly over altered parent rocks from which they have developed. The soil physical and engineering characteristics are presented below.
Soil Color
Soil color of surface horizons reflects a strong imprint of biological processes, notably those influences by the ecological origin of the soil organic matter. Soil organic matter imparts a dark 
b ^Ck C0i°At0 tile SOlL General|y' the higher the organic matter contents of the soil
the darker the soil. As indicated in figure 4.7 above, the soil color becomes lighter with depth' This soil is shallow (20-50 cm), excessively drained, sandy loam to clay. They are commonly 
with common rockiness and stoniness.
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Apart from offering limited grazing resources, they  have little or no agricultural potential. In spite of their low agricultural potential, they  are widely  cultivated. The major limitation of the soils is a shallow effective soil depth.
Soil texture
The textural class for the top 50 centimeter is silt clay as shown in table 4.9.
Table 5.10 Textural classification of soils of Tikur Wuha community watershed
Location
GPS location
Depth
(cm)
Particle Size (%)
Texture class
Northing
Easting
Elevation(m)
Sand
Silt
Clay
Pl
1300132
407699
3051
0-50
50
20
30
SCL
Soil structure
Soil structure is just as important as soil texture in governing, how water and air moves in soils. Structure fundamentally  influences  the suitability  of the soils  for any  structure to support and and for the growth of plant roots. The bulk density is an indicator of how good the soil structure is. The top 50 cm soil has a bulk density of about 1.36 gm/cm and porosity of 0.49 cm /cm .
3
33
Table 5.10
of soils of Tlkurwuha
watershed
GPS location
soil
depth
(cm)
Bulk 
density
Porosity
Pd
Hydraulic conductivit y
Northin
g
Easting
Elevatio
n
(g/cm3)
(cm3/cm3
)
g/cm
3)
cm/hr
P
1
130013
2
40769
9
3051
1.47
0.45
2.65
0-50
1.13
P
2
0-50
1.36
0.49
2.65
0.71
Table 5.11
Location
GPS location
soil depth(cm)
Atterbergs Limit
Northing Easting
Elevation(m)
LL
PL
Pl
PI
1300132 | 407699
3051
0-50         47
35.47
11.53
LL= liquid limit PL= plastic limit PI=plastic index pl( profile numbe rl)
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5.4 Biret Afer Community Watershed
5.4.1   Conditions of the sub-watershed
5.4.1.1  Geology and Soils
The soil in the large part of the watershed affected by deep gully is Nitisols. These soils are classified as Haplic Nitisols and Rhodic Nitisols. They are developed in well-drained areas with good permeability  and a favorable structure. They  are found dominantly  on flat to rolling topography. They  are characterized by  their very  thick, (greater than 150 cm) more or less uniform clay  distribution. The soils  are reddish in colour, derived from different parent materials.
Soil color and texture
The color of the soil is  deep soil. Soil color of surface horizons  reflects a strong imprint of biological processes, notably those influences by the ecological origin of the soil organic matter. Soil organic matter imparts a dark brown to black color to the soil.
As  shown  in  table  5.12,  clay  is  the  dominant  soil  texture  class.  It  shows  major  textural variations in clay content with depth.
Table 5.12 Textural classification of soils of Biret Afer community watershed
GPS location
Depth
(cm)
Particle Size (%)
USDA Texture class
Northing
Easting
Elev. (m)
Sand
Silt
Clay
Pl
1241661
300977
2084
0-50
12
26
62
C
50-100
11.2
20.4
68.4
C
Soil structure
Sand, silt and clay particles were the building blocks from which soil is constructed. The manner which these building blocks are arranged together is called soil structure. Soil structure is just as  important as  soil texture in governing, how water and air moves  in soils. Structure fundamentally influences the suitability of the soils for the growth of plant roots.
Table 5.13 Properties of soils of Biret Afer community watershed
GPS location
soil
depth
(cm)
Bulk 
densit
y
Porosity
Pd
Northing
Hydraulic conductivi ty
Easting
Elev .m
g/cm3
cm’/cm5^
g/cm3
cm/hr
Pl
1285687
389411
2434
0-50
1.36
0.49
2.65
P2
1285628
389473
0.0017
2476
0-50
1.31
0.51
2.65
0.00025
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Table 5.14 Selected Geotechnical properties of the studied soils
GPS location
Soil
depth
(cm)
Atterberg's Limit
Bulk 
density 
g/cm3
Moistur
e
content
(%)
Shear
parameter
s
Northing
Easting
Elev.
m
LL
PL
PI
Cu(K N/m2
J
• •
Pl
1241661
300977
2084
0-50
45
23.4
21.6
1.49
34.7
5
1
7
50-100
50
28.7
21.3
1.39
30.25
3
9
LL= liquid limit PL= plastic limit PI=plastic index Pl( profile numberl)
5.4.1.2            Characteristics of the Ephemeral Gully
Based on the data collected and analyzed from the field, a large section of the gully is classified as medium size gullies of considerable magnitude which have a depth in the range of 2 and less than 3 meters depth, with a very deep and wide gully mouth and the width to depth ratio were fairly consistent. The gully is continuous.
The shape of the gully varies significantly, and
about 45% of the observed section of the gully 
is trapezoidal, while the remaining section of
the gully cross section is V shaped as shown in
Figure 5.7.
The soil is a stable deep clay soil with no plane
failure  within  the  soil  profile  around  the  gully 
channels. The gully formation is caused by the
erosive energy of the runoff from the upstream
areas of the watershed. These data shows that
the  gully  and  its  fingers  are  medium  size  to
large gully towards the downstream areas.
In figure 5.7, the seasonal flood mark for 2013
season is shown light red and it is measured at
about  1  meter  depth.  However  because  of  it
concentrated  flow  and  channel  gradient,  its 
erosive  power  is  very  high.  The  erosion  has 
been   very   aggressive   and   the   damage   is 
visible.
5.4.1.3            Soil loss from the gully networks
The estimated soil loss from the model gully is 94,410.24 m  or 9.44 ha at one meters depth.
3
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5.4.1.4             Peak Flood
As shown in the photo, peak flood left a light red color in 2013 season as shown in figure 4.8.
The depth of the peak flood observed during the 2013 season is far lower than the gully depth. However, the stream continues to cut deep into the gully and every season it continues to do so. As seen in picture, the soil is deep red clay soil (Nitisol) and moderately permeable and it is susceptible to erosion. The estimated peak  flood at the gully  mouth using Cook's  Method is estimated at about 5 m3/sec; as measured hundred meters downstream of this site.
5.5      Washaw Community Watershed
5.5.1   Topography and land cover
The topography  of the watershed is  sloppy  and with gentle slope in some parts  of the watershed. The average altitude is 2600 m above sea level. The watershed has a wide range of relief, mainly of very steep (with slope >50%) and steeply sloped hills (between 30 and 50%) which cover around 4.3% and 34.8% of the total watershed, respectively. While moderate 33%, rolling 4.2% and flat 6%.
Hillsides have scattered and dispersed trees mainly of eucalyptus species. The sub watershed has a longitudinal shape and it has three branches of ephemeral gullies of significant size and coverage in the sub watershed. The sub watersheds are located on the side of a hill and it does not have a classical watershed shape.
5.5.2   The model gully watershed
5.5.2.1   The Gully
The model gully watershed has a discontinuous gully as seen as a patched surface. Slope failure at the upper part of the watershed appears to be the cause for the unusual form of gully. A slip on the surface depth may be geotechnical properties of the soil and sloping profile characteristics i.e. slope and depth. As
Figure 5.8 Map of Washaw Watershed
observed  during  field  work,  slope  failure
would only take place when the soil becomes saturated from the terrain surface to the depth of the potential slip surface. This saturation depth may be function of the soil profile, initial soil moisture distribution, and rainfall amount and intensity. The model gully  watershed is  now under closure. Hence it has  limited agricultural services  at present. The gullies  occur in the watershed with the head water starting from road side runoff. Major part of the main watershed has been developed in limited soil conservation works while a large part of the main watershed is untreated.
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5.5.2.2   Soils
Leptosols are the most prevalent soil type in the study area. These soils in general are shallow
(20-50 cm), excessively drained, sandy loam to clay. They occur mainly on steep side slopes; mountains and hills. The soils are generally young and are limited by their topsoil horizon or
directly over altered parent rocks from which they have developed.
Soil texture
The textural class analysis shows that clay dominates the surface soil in the study area (table 4.16). The soil texture is  loamy  and silt loam but it is  highly  degraded and large gullies  are created and in most parts of the watershed soil depth is very shallow and has low to medium infiltration that increases the occurrence of gully due to the movement of the soil in mass along the slope.
A soil sample analysis showed that the sand, silt and clay particles of the soil constitute 48%, 18% and 34% of the soil texture, respectively. This minor variation in soil texture was caused by parent materials, topography, in situ weathering and translocation of clay.
Soil Color
The color of the soil in large part of the watershed is red. Soil color reflects a strong imprint of biological processes, notably those influences by the ecological origin of the soil organic matter. Soil organic matter imparts a dark brown to black color to the soil. Generally, the higher the organic matter contents of the soil, the darker the soil.
Soil structure
The manner which these building blocks  are arranged together is  called soil structure. Soil structure is just as important as soil texture in governing, how water and air moves in soils. Structure fundamentally influences the suitability of the soils for the growth of plant roots.
The structure of the soil in the study area is friable, and varied from fine, weak and sub-angular blocky to medium to coarse, moderate.
Table 5.15 soil engineering properties
Location
GPS location
Atterberg's Limit
Northing        Easting
Elevation(m)
Soil depth
(cm)
LL
PL
PI
Pl
1222642
315307
2682
0-50
47
35.47
11.53
Table 5.16 Textural classification of the soils of the experimental site
Location
GPS location
Particle Size Analysis (%)
Northing Easting
Elevation(m)
Soil depth(cm)
Sand
Silt
Clay
Texture
class
Pl
1222642 | 315307
2682
0-50
48
18
34
SCL
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5.5.3 Characteristics of the Ephemeral Gullies in the Model Watershed
There are three finger gullies, two originating from the road side and the third from the watershed.
The shape of the finger gullies is
V-shaped and continuous. The gullies  start at shallow depth at upstream and it gets  deeper at the downstream areas.
However, there are discontinuous gullies  on  the  slope  of  the  sub watershed as shown in figure 5.9, and they are shown as patches. The soils move in mass as the soil
Figure 5.9 finger gullies and their drainage area
saturates during the wet season since the soil is shallow with underlying impermeable layer of basalt origin.
The gullies are medium size gullies which have a depth ranging between 1 and 5 meters. In the case of V-shaped gullies, the subsoil has more resistance against the erosion force than the
5.5.4   Soil loss from finger gullies in Washaw Community Watershed
The soil removed and lost to the stream from the gullies in the watershed is calculated based on the measured geometry of the gully and the aggregate total is estimated at about 13,005.84
3          1.3 ha by one meter depth. The total volume and total sediment from both rectangular m   or
and trapezoidal section of the channel for each finger gully is given in table below.
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Table 5.17 Soil loose from finger gullies in Washaw Model Gully
Finger gullies
Soil loss from each finger gully in mJ
#1
5,280.45
#2
2,852.10
#3
2.619.20
#4
2.254.09
Total
13,005.84
5.6      Peak flood Values
As  shown in table 4.18 below and based on the procedures  followed in each of the three methods  in computing the peak  flood (Qp) for each of the small model gully  watershed, the following observations and recommendation are made.
US SCS Method:
(i)
The estimated values of Qp are very low compared to other two method,
(ii)
It requires well collected and reliable hydrological data.
(iii)
The procedure requires the use of tables, figures, exhibits and worksheets. This 
tables and figured developed for use of this method are for US only. To use this 
tables and figures, several hydrological data have to be measured including soil
types, land use and ground cover conditions, the curves are developed for US.
(iv)       Rainfall intensity is the primary data needed to estimate the design peak flood for small rural watershed, however it is estimated indirectly.
(v)       This method is rigorous, and it is more appropriate to well gauged watersheds.
(vi)      The writer of this report has not come across any work on Ethiopia using this 
method. May be this exercise may be the first of its kind in small watershed of this magnitude.
(vii)     The method is very difficult for use by extension workers in Ethiopia unless improvements and simplifications are made such as curved are developed for
specific areas for its use,
(viii) The worksheets are attached as annex to follow though how the values in table 5.18 are produced.
This method  produces peak  flood  that  are very small, and less reliable, and the use of  this method needs further adaptation on its use for the design of check dams.
The Rational Method:
The rational method is  another most recognized method for un-gauged watersheds. As indicated in many of the references, the rational method tends to give slightly higher or lower values in many of the estimated flood values. Based on well conducted research by academia in developing countries, rational method often recommended for urban storm water estimates
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since the watershed characteristics are considered similar. This study recognizes the usefulness of this method in Ethiopia, although higher estimates of the peak fold.
Adapted Cook's Method
Cook's method is  the method widely  used by  the Ministry  of Agriculture over the last thirty years. The soil and water conservation extension services  have been  using it widely. The initiation of its  use for this  study  arises  from this  strong justification. This  assignment also introduced the use of GIS and GIS generated data and information in the use of the method. With very few field data, and good judgment and experience, a stable and more reliable data can be obtained for design purposes. None of the design peak flood exceeds the bank of each model gully, and this  is  demonstrable evidence that suggests  the use of this  method. More importantly a direct measurement made during the wet season in one of the model gullies gave a good match with the estimate.
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I SECTION V: STUDY FINDINGS AND 6?
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Table 5.18 Design Peak Flood (DPF) estimation for the model gullies in the study areas
23  Time of concentration based on Kirpich formula TC = 0.00025( ^)080 (after Elzabeth Shaw, 1983) 24    Time of concentration based on US SCS method
25
The direct measurement observed in 2013 season gave 13 m /sec
3
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| SECTION V: STUDY FINDINGS AND RESULTS
68Design of Large Gully Treatment Works: Tana Deles Integrated Water Resources Development Project
SECTION VI: PROPOSED DESIGN FOR LARGE GULLY TREATMENT WORKS
6.1      Introduction
Gully treatment is more appropriate and less expensive if there is early action as it appears and while it is in its young stage or it is at a manageable size. Once we let the gully develop fully, its rehabilitation, restoration and control is difficult and expensive.
Gully treatment and rehabilitation is, therefore, a process of creating the stabilization of active gullies  by  vegetative or structural measures  [physical/engineering] or a combination of both which is most recommended. Many of the gully treatment methodologies are site specific and widely structural, and one of the widely used technologies is the use of check dams. Design adjustments and special consideration may be required for differences in soil types, rainfall, and land use and climatic conditions. However the prescription for gully treatment needs to address the severity of the gully condition for each specific site. It should exercise all means for specific ways to take advantage of existing conditions to produce a stable gully.
Check dams can be constructed from a variety of construction materials such as wood, loose rock, gabion rapped stone, plastic  rapped sand or soil mud, concrete, live trees, etc. These structures are used to control the flow of water to its minimum effects and ensure adequate and efficient trapping of sediment in gully channels. The use of rock check dam has age old history  in many  parts  of the world and in Ethiopia - the case of Konso, SNNP region. The existing practices worldwide and local socio economic factors and conditions demand that rock check dams are the most widespread and principal construction material for gully control.
Check dams that are constructed from rock, concrete and other solid material are engineering structures often requiring proper designs for a stable, effective and successful gully treatment and rehabilitation. Technically the set of check dams constructed all the full length of the gully is considered as fixed overflow weirs with no movable control device. They dissipate energy of runoff flow so that it's  potential damage along the bank  and floor of the gully  and at downstream areas is reduced. It obstructs the flow and raises the upstream water level at the check  dam sufficiently  high above its  natural flow level during the rainy  (wet) period and thereby reducing the velocity of flow substantially, and during this process sediments and silts are dropped and deposited immediately  above the structure all along the gully  where check dams are constructed. The great advantage of fixed weir crests is their simplicity in construction and easy  maintenance service and their reliability  in operation. When they  are designed properly, they have the ability to trap silt very effectively and efficiently.
6.2       Basic Principles and standard norms in the design and construction of check dams
There are basic principles to follow in the design, construction and treatment of gullies to fully control and minimize the damage of gullies  to agricultural lands  in all small community watersheds where they exist. The following are the basic principles but not limited to:
• A system approach in the overall design and management: Understanding the mechanics involved in the process of gully development and recognizing that gully is one
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element in the watershed system. Any  attempt to address  gully  should think  about its interrelations  with other aspects  (elements) of the watershed system such as  the people and their activities, livestock management, foot trail and road networks, etc. In so doing, it is  one aspect of the Integrated Watershed Management Approach. It is  now well recognizing that network of gully existences in an area is one indicator recognized as a sign of failure of the watershed system. Ideally, the system approach recognizes that all system elements in the watershed (land water, forest, humans and livestock, micro-organisms, etc.) operate in balance. When an imbalance in the system occurs, changes are observed; there appears a network of gully system.
Any attempt to restore the watershed must look on the broader aspect of the interventions recognizing the role of each element in the system; the different actors  act on the watershed system differently  for that specific  situation. Isolated intervention such as  gully plugging alone may  not be a long lasting and sustainable approach and solution, but a better result could be achieved by  identifying the role of the different factors  that govern soil erosion process and eventually gully development in the planning process. To this effect an integrated [holistic] approach is always recommended. The essence of this study is to promote integrated watershed management,
Mobilizing and Engaging Stakeholders: Who are the stakeholders and these need to be properly  mapped, identified, organized and mobilized for planning, implementation and management (protection and maintenance) in the gully  control and stabilization process. The participation of the community  in the planning, implementation and management is, therefore, critical in the sustainability of the check dam structures and its services,
Soil  improvement  through  soil  conservation:  Increasing  the  absorptive  capacity  of the soil, reducing the surface runoff after
rain, protecting the land surface and
natural drainage ways from erosion, and
safely removing surface runoff water out of
fields at a non-erosive velocity through
properly designed, prepared and
maintained waterways are preventive
measures in the gully control effort. These
activities are basically soil and water
conservation activities, and they are priority
action plans in the land management and
specifically in the design and construction
of gully control measures,
Selecting a continuous gully as a Basic
Treatinent Unit (BTU): In the use of
physical structures, selecting the whole continuous  gully  as  a basic  treatment unit is important. Each continuous gully in a gully system should be regarded as a basic treatment unit for planning, designing, control and management and all the activities  and control measures in that single unit should be finished before the rainy season,
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Investigation  and assessment for a unique and special gully development: When a gully bank slide, crumbles as observed in this study in megenagna model gully in MisraK Estie (figure 6.1), it is  necessary  to fully  understand the causes  for the gully  formation, failure of the banks, etc., identify  and determine exactly  the major interventions, so mat proper remedial measures can be taken to correct it. The detection of the causes of fai ure may require continuous observations and time. Excess runoff water and water in the soil is at the center of the problem in all soil erosion and gully  formation. The initial remedia measures  should be in capturing and controlling surface runoff and draining all the soil water at the head of the gully,
The use of biological measures: The land surface cleared of its  vegetative cover is vulnerable for erosion of different forms; sheet, rill and gully  erosion. Gully  treatment through biological means (the use of live plants as a hedge or check dam and vegetating the banks) need to be considered as an integral part of the overall design and construction for the treatment of all gully types; large, medium and small gullies.
Gully treatments that depend largely on vegetative measure are easier and less expensive, but lasts for very short period and it has to continuously be maintained on regular basis. Such a practice is more feasible in moist and humid climate areas with productive soils and where there is adequate soil moisture and deep soil. This situation is typical for many of the study site presented in this report. However, the recommended size of the gully is small; less  than one meter. In large gullies  as  it is  observed in this  study  areas, the biological measure need to be integrated with the engineering measures  such as  the use of rock check dams.
Wherever it is suitable under all cases, planting trees and herbaceous plants for stability of
gully banks is desirable. After large gullies are reshaped and rock check dams are put in place, grass species should be sown to establish plan material and increase the organic matter and improve the structure of the soil. Tef (erogrestis tefT) is a fast growing crop with sufficient plant material to cover the inside steep surfaces of the gully banks and floor. It can use the seasonal
rain to grow and leave its biomass on the gully surface adding biomass level to the soil at the surface that will support the growth of plant species. A professional (agro-forester and agronomist)   support   is   needed   in   terms   of
Figure 6.2 Tree Lucen as live check dam
identifying type of plant materials and their cultural practices and their species to use in the treatment of the gullies. The gullies  having a depth less  than one meter are more appropriate for live check dam use. A detail presentation on the biological measures is Given in the later part of this section of the report.
The use of layers of planting trees across the gully section is one of the inputs as part of the integrated approach to gully treatment activities in the proposed gully sites. Some of the
recommended plants for use in gullies as live check dams are elephant grass, bana grasL Spanish reed, popular, willows (MoA, 2006). A trees lucen plant (figure 6.2) has been found
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well adapted at the Enkulal Model Gully, and its use as a live check dam material is w demonstrated as shown in figure above. Along with the live check dam, the use of organic matter (e.g. straw, tree branches, etc.) should be encouraged
The middle and  downstream part of the gully sections require large and  s^a^le structural works for effective gully treatment. In this  part of the gully  network, the gullies are very large in size and the damages are so great that vegetation establishment alone cannot bring the gully  back  to a stable and productive state of condition. The recommended practice is that always vegetative measures are needed even after structures are used,
Improving  the soil and  water conditions in  arid  and  semi arid  areas: Establishing vegetative cover may be a challenge in relatively degraded arid and semi arid areas since it is moisture and nutrient deficient. The main purpose of the check dam structure in the gully channel is to trap the sediment and improve soil moisture that can support plant growth on short term basis  that can be useful for the stabilization of the check  dam structures  in moisture stress areas,
Integrating  both  biological and  physical measures: Like it has  long been the case, this study has also recognized the need for both biological and engineering structures as an integrated approach in the proposed treatment works of large gullies. In large gullies as in this  study  sites, focusing exclusively  either on biological or on physical structures (engineering structures) in gully  treatment works  offer only  a partial explanation of the causes  of gully  development and land degradation. It is  important also to recognize and appreciate the role played by  plant or crop/vegetation cover and the density  which also matters in controlling erosion and gully development,
Hydrology and  Hydraulic requirements for design: Design of check  dams  requires peak  flood estimation of the gully  watershed, and this  becomes  the most difficult task particularly for un-gauged watersheds which is often the case in Ethiopia. For a practicing engineer, this is the most challenging one since there is no readily available hydrological data for the site under consideration. High water marks with a return period of ten years have to be recorded and experience based judgment have to be made to establish the conditions of peak flow. In this study, adapted Cook's method is found to be useful and realistic than the Rational and the US SCS methods for Ethiopia's rural watershed conditions as discussed and well explained earlier,
Use an  easy to  use guide (Annex B) for making  preliminary decision  on  the type of measures for gully treatment: After the gully characterization is completed, the check dam design work  can start by  conducting preliminary  identifications  of what type of structures to use. This is a simple and general rule of thumb for the selection of control measures. Basic data such as drainage area of the watershed for the gully and slope of the gully channel is required to determine whether the gully requires temporary or permanent structures. Once the gully  structure is  determined for permanent structure, then the different engineering structures are considered, and details are presented in the sections to follow.
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■  Identification  of  critical  location:  Identification  of  critical  locations  in  the  gully  network system is crucial in the initial study for design. This is done while conducting transect walk through the gully area and gully watershed. The critical locations widely recognized but not limited to are (i) the head cut section of the gully, (ii) a sudden drop in elevation along the longitudinal section (floor) of the gully, (iii) sudden changes in the width [and depth] of the gully sections, (iv) sections of the gully that are meandering which often occur in low laying (flat) areas.
• The use of check dams: They are constructed at specified points along the gully in order to  catch  sufficient  sediment  to  fill  up  the  gully,  arrest  erosion  from  the  gully  surface  and provide the desired stability.
It is constructed and anchored at a designed height and depth great enough to provide the desired control and sediment trapping so that it provides  suitable rooting space for plant growth to take hold and support re-vegetation of the entire gully. Without a proper design and construction of check  dams, it is  difficult to achieve this  objective. Check  dam is  a stable structure to withhold the sediment deposit in a gully. Instability  from hydrostatic pressure is always during the first season, and the next and later sediments will fully build upstream side of the check dam.
The standard norms in the design and construction of rock check dams are the following.
•    Sediment is allowed to settle and deposit filling the [upstream side of] the structure,
■ The sediment deposition behind the check dam will build up to the spillway crest height (h)  and  this  elevation  should  fall  at  the  toe  of  the  next  check  dam  immediately upstream. The acceptable slope variation can be as much as 2% to 3% - a standard practice,
•     In addition to reduced siltation to downstream areas, the designed hydraulic structure should allow recharging the ground water from seepage from the water temporarily or contained and stored for long period behind the check dam,
•    Site selection for the gully control structures is a critical decision. Narrow sections (neck points) should be avoided. Narrow necks of the gully are not appropriate sites for a gully control.  This  may  be  true  and  appropriate  for  sitting  a  dam  (reservoir)  which  has  a purpose of more water storage for water supply or hydropower because of stability and cost  reason.  A  wider  gully site  is selected  for  check  dam  since  this  provides  shallow depth and allows less pressure head over the crest of the spillway of the check dam particularly,
•     Excavation of foundation and a side trench for anchoring the structure, and lay out of the check dam must be at right angles to the direction of flow and the structure will be extended at least by 0.70 and 1 meter [key] into the gulley banks,
•     The size and shape of the gully, soil type and anticipated peak flood will influence the dimension and type of structure to select,
-     Dimensions (depth, width and slope) of the structures will be defined after each gully is surveyed and investigated,
-    In any hydraulic design of any structure, it is necessary to first determine the location of the water surface control. In the case of check dams for gully control, the hydraulic
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control is the crest of the check dam which influences the upstream sediment level that must rise to its full height, and this must meet the set of objectives26 agreed upon,
* Unless or otherwise circumstances dictate, fairly inexpensive structures with stable
design are used,
6.3 Design and construction for gully stabilization and control — a Procedure
There are several technological options available for the treatment of existing model gullies in each watershed. Broadly they are classified as biological and physical (engineering) methods. The biological methods  include the use of plants  that can very  well adapt to the gully environment and support the trapping of silt from the water flowing through and also protect the gully bank and from further erosion. These different technologies and approaches are used in an integrated fashion to improve the success  rate of the interventions  for gully  treatment works. The following step by  step approach should be pursued in the implementation of this design guide for the treatment of the model gullies.
6.3.1  Stabilization of the gully - the first step
Sustainable stabilization of gullies can only be done through a program of management of the watershed as a whole, along with the treatment of the gully itself. Although the primary task in this  consultancy  assignment is  to produce designs  for gully  treatment; i.e. engineering or structural measure, this option of a gully treatment work is possible when the watershed within which the gully exists is fully put under rehabilitation. The action plan should include closure of sensitive and critical sections of the gully, vegetative planting and seeding and sustaining the protection of the gully watershed from further erosion. This is considered as part of the overall solution of gulling problem and associated erosion and high sedimentation load and pollution that exist in each of the study area and in most rural areas in country at large.
A full gully treatment works shall have the following.
(i) Preparation
Gully  site and its  boundary  may  have to be marked using aerial photos  (if available), topographic map, the use of Google Earth and GIS for producing a working paper on each gully. Data may have to be collected on geometry of the gully; the length of the gully and area its occupies, and the soil of the watershed and gully may have to be investigated.
(ii)      Closure or fencing the entire area around gullies
Closure of the gully from human and animal contacts and divert the head stream flow away from existing channel to a safe outlet (grassed water way or stone rip raped chute) is the first activity  in the rehabilitation and treatment of gullies. This  action is  intended to keep the affected areas  from access  by  domestic  animals  and people until the vegetation and other physical measures are well established and even beyond. Fencing is also necessary from the protection of wild animals that can browse in the areas. Fencing is prescribed along with both
Any or a combination of the following (i) To fully recover or restore the gully or (ii) stabilise and
biomass production, etc.
(lii) use the stabilized gully for grazing and
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the vegetative and structural option to help gullies reclaimed quickly and effectively. This study will consider in the study the feasibility of this option in the gully treatment work.
Gully  control  measures  give  a  priority  to  the  treatment  of  the  flow  where  it  is.  Treatment  of gullies by planting the waterways takes the water out of the gully by changing the topography. This implies the establishment of well adapted perennial vegetation using some combination of grasses,  trees,  shrubs  and  legume  crops.  Rehabilitating  waterway  projects  strive  to  esta  is  a vegetation  cover,  and  sometimes  this  is  done  after  the  land  reshaping  is  finished, intervention in itself take long time to recover the gully, however it is often planned along with other option such as engineering options to be discussed later in this section.
Closure of the affected area in a watershed is  the simplest and most impactful step in the rehabilitation of an active gully section of the watershed. The environment does its own healing and rehabilitation as  long as  it is  not disturbed by  or brought in contact with humans  and livestock. There is always a challenge to this plan from the community side since the land and pasture has been providing some socio economic benefits to the local people. The community will be reluctant to welcome this idea at the initial stage. It is for this reason that consultation with the local community on the proposed plan is imperative. The plan should get the assurance of the local community will respect to their support to close the area where the gully exists and continue to protect the closure or protected area.
Stabilization of gullies through closure27 halts or slows down the expansion of gully erosion and gully  networks, reducing sediment yield, and improving water quality. Closure not only  stops the erosion and gully expansion but also improves the vegetative growth of the gully area which can be of economic  and environmental significance. Teff is  one of the crops  for the quick establishment of biomass on a gully surface, while other suitable tree species could be planted. The success of such an effort depends on the availability of sufficient moisture to support the growth of the plants and the study areas seem to have no problem. However, the high rainfall and flash floods could be a problem in the establishment of vegetative covers of the gully. Such impacts could be reduced by laying stone riprap along the banks and floor of the gully.
(in)      Diversion of the runoff from the head of the gully area to a safe outlet
A diversion is a channel structure with a supporting ridge on the lower side if the channel is sited along a graded elevation and constructed across the slope or could be designed as a chute where both sides have a wall. A diversion structure has many purposes and one of these is to divert water away from active gullies or critically eroding head gully areas.
For establishing a well designed diversion, a survey may be required to identify the presence and properly locate an area that can be a safe outlet for the runoff entering into the gully head. The potential safe outlet could be (i) a properly designed, well established and stable grassed channel, (ii) a natural drainage channel and (iii) a stone rip rapped channel which is referred as chute (figure 6.3 and 6.4).
Surface runoffs from small corrugations (channels) constructed in tef fields to drain excess soil water and diversion channels for soil conservation structures such bunds and terraces produce
27/ Avoidance of human and animal movements and interference in the gully area
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massive runoff in many  of the sites  visited, and unless  properly  managed they  are the major causes  to gully  development. An innovative and a local approach to this  problem has  been observed in Farta Woreda - stone rip rap (figure 6.3) and a traditional chute for collecting runoff from crop fields and draining it to a safe outlet is one alternative solution to controlling gully  in crop lands. One shortcoming in this  demonstration of a chute is  that there is  no sufficient height to the wall in both sides and when the flow from the terraces and crop fields increases it could overflow to one of the sides and this can cause server erosion and this has to be improved.
Figure 6.3 A stone rip rap chute for Figure 6.4 A chute with vertical wall used as a safe
collecting runoff from adjacent crop outlet for runoff diverted at the head of the gully 
fields
(iv) The grass waterway is one common safe out let
In an attempt to prevent gulling by  concentrated flow from agricultural lands; terraces, hill slopes, etc., the runoff flow is intercepted and it is diverted [often from the head of the gully] to a safe outlet such as  grass  waterway  and chute using rock  riprap. Farmers  construct [waterway] channels (structures) using [grass] planted or chute (rock rip rapped channel) down the slope.
A  grassed  waterway  channel  has  usually  been  shaped  or  graded,  and  a  vegetative  cover  is established on it and this channel is called a grassed waterway.
Shape: The cross sectional shape of the grass waterway may be either a (i) trapezoidal, (ii) parabolic, or (iii) triangular (V shaped). See figure 6.5 below. The trapezoidal and parabolic shapes are most widely used. The parabolic shape is commonly used for water ways located in natural depressions, and the trapezoidal shape for terrace outlets  located on ridges  or side slopes (Beasley, 1972). In the trapezoidal water ways the flow is distributed evenly over the full width.
Location: It is always important that an overall water management plan be produced for the agricultural area where such waterways are needed before the grass water way is considered
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for construction. A local expert in water management is responsible for developing such plans. Factors for locating and developing grassed waterways:
« Waterways (including grassed) should be located where it will not interfere unduly with the farming operations, should not be used as road ways or livestock lanes or grazing
place,
* Waterways to carry runoff from contoured fields are usually constructed in natural
depressions
Design: A [grass] waterway should normally be designed for the peak flood expected in ten years period as is done for check dam design for gully treatment. The waterway should be of such a width that the resulting velocity will not erode the channel (permissible velocities) when the grass is short and of such depth that it will carry the peak rate of runoff when the grass is long.
Table 6.1 Permissible mean velocities for different soil types
Permissible mean velocity, V (m/sec)
Type of soil
Clean water
Water
containing
colloids
m/sec
m/sec
Very fine sand
0.45
0.75
Sandy loam
0.55
0.75
Silty loam
0.60
0.90
Alluvial silt, without colloids
0.60
1.00
Dense clay
0.75
1.00
Hard clay, colloidal
1.10
1.50
Very hard clay
1.80
1.80
Fine gravel
0.75
1.50
Medium and coarse gravel
1.20
1.80
Stones
1.5
1.80
Source:  Herman  J.  Finkel,  1982.  Handbook  of  Irrigation  Technology  CRC
Press.
The carrying capacity of grass waterway channel can be computed by the formula Q=AV, where Q is the peak flood, A is the cross sectional area and V is the velocity. The Manning formula is used in place of V.
Depth:The depth of flow is determined by the design procedure. If the waterway is used as a terrace outlet, it is usually necessary to construct it deeper than the design depth.
MfaftfrThe width of the [grass] waterway is also determined by the design procedure given an estimated peak flood that has to pass safely through the grass waterway. This width will result in a velocity in the waterway equal the maximum permissible velocity of the soil when the grass 
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is short. It should be noted that, however, the grass waterway can be made ™der
design width to make it safer if there is sufficient available space that facilitate the construction or maintenance of the waterway. If the design waterway is found to be very wide (>2U meters) division into smaller waterways should be considered.
The different shapes of the [grassed] waterways widely used are presented in figure 6.5 below.
—. j.
30 cm min
____ __L
_____ <r
l FreeboardJ_^ cm min
T
_______ b-----------------—
4 min
Cross section of a trapeioidal-shaped [trass] waterway showing permanent side dikes (After Beasley. 1972)
30 cm min
Cross section of a parabolic-shaped [grass) waterway showing temporary side dikes [After Beasley, 1972)
Figure 6.5 Cross sectional shape and possible dimensions of a grassed waterway
(v)      Reinforcing the effort on soil and water conservation activities
For effective gully erosion control, soil and water conservation activities at watershed level are priority tasks. It has significant role in the reduction of gully occurrence and development. For its  effectiveness, the soil and water conservation works  require the proper planning and implementation at community watershed level. This is a plan that has to be set from the outset with a clear long term vision for a sustainable land management and investment. All small community  watersheds  should ensure there is  a full coverage with appropriate type and properly designed and constructed soil conservation measures. Agricultural lands experience a great deal of soil erosion and this is often visible with the presence of a network of gullies. The most widely  used soil and water conservation practices  include terracing, use of contour farming, cover crops  along the bunds, buffer strips  and stream bank  fencing and protection. There are various  soil and water conservation activities  widely  used for land management practices  in Ethiopia, and they  are effective and economical. These techniques  are well documented and available in the guidelines  and manuals28 prepared by  the Ministry  of Agriculture (MoA) and its development partners, and it is beyond the scope of this assignment to deal with on this in detail. However it is important to recognize the significance and
n GudtUmi .nd minuaU m .vitUbto; Sol ind w.t.r com.nr.tion fchr^u., - . |uid. to d'v.loo'mnt fudtlinc by MoA (2005) 4nd unpubiuhed handout manu.b pr«p.rvd by th. MoA.
by Hum. (IMS). Comm^ty wtofy w.frVwd
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importance of soil and water conservation activities in the whole watershed and gully treatment works.
The causes to soil erosion and particularly gully erosion and development in all the study sites is the use of traditional farming practices; up and down plowing, framers  use small furrows  or rills/micro drainage ditches to drain water from the [tef] crop field where excess water prevails following each rain. Runoff collected in these micro surface drains converge to a common outlet often falling in between two land owners, and both farmers see the site as a common land. Gullies develop along these common land areas where deep incision appears that will become a starting point for a network of gullies. It is important that the excess runoff water from the farm lands are redirected to a safe outlet with well established channels as shown in figure 5.3 and 5.4 above. The structures that may be considered for a safe channel outlet could be (i) a well established grass waterway, or (ii) a channel with stone riprap along the floor of the channel and a vertical or sloping stone wall in both sides of the channel. These structures allow the removal of excess water safely to an outlet with minimal erosion of the channel. These activities should be done with skilled and trained personnel. The available data at the community watersheds visited for this study suggested that there are few soil water conservation works and there is a need to bring to a full coverage of the model gully watersheds and the main community watershed at large.
6.3.2   The use of Chute in steep slope gullies
Gullies developed in steep slopes as in Farta (which has more than 100% slopes), chute is the recommended design for this site. Check dams are unstable under this situation. The design procedures for chutes are presented in Annex E.
6.3.3   The use of Check Dams
A [rock] check dam structure is a designed device, constructed or manufactured and used in a soil and water conservation or management system (gully control fall under this category) and it is intended to regulate or control the flow of water. The establishment of a hedge of live vegetative structures and properly covering the gully along with proper land use is one of the measures  recommended along with structural/engineering measures. However check  dam is the most effective gully  control measure to trap sediment above it and gradually but slowly restore the gully. These check dams could be filled with loose rock, brush wood, debris or trashes and even household waste to establish a strong control to the water flow in the gully thereby trap the sediment behind these check dams. There are different types of check dams depending upon the size of the gully; depth and width [geometry] of the gully.
6.3.3.1 Check Dams Design for Small Size Gullies
Wood or brush check dams are temporary structures and they are used in small size gullies which have sizes less than 1 meter depth and a drainage area of less than 2 hectares, and such gullies appear in early gully formation process and during the early stage of the gully. These gullies have widths and depths in the range between 1 to 3 meters and less than one meter depths. The other ones are the finger gullies or gullies with stream order I and II.
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Figure 6.6 Layout of wooden poles for the construction of check dam
(i) The use of Wooden Fence Check Dams in finger gullies:
These are check dams that are constructed using wooden pools and brushes of trees in many parts of the country.
However these wood or brush check dams are temporary and they are constructed to support the development and establishment of other alternative options such as tree planting, sodding, plastic  rapped check  dams, rock  check  dams, etc. simultaneously  or integrated in the gully treatment techniques. The check dam traps limited silt behind the structure and it provides the tree planting and sodding the potential to strive and grow and develop to function as effective barrier against gully erosion. These structures are used as rills (small channel line gullies) are developing into gullies, and it can also be used in young gullies  at head cut areas  in the upstream areas of large gullies. They have to be maintained every year since they do not serve more than one season. However, they  are effective in stopping/trapping sediment to help establish tree and grasses on the deposits that can serve as live check dam structures, and in the mean time they check the advancement of the gully further up stream.
Gctachcw Alcm & Associates
| SECTION VI: PROPOSED DESIGN FOR 80 LARGE GULLY TREATMENT WORKSDesign of Large Gully Treatment Works: Tana Bcles Integrated Water Resources Development Project
The head of the gully is where ^diversion of the runoff to a safe
outlet should be sought
Wood
Check
Dam
so to 40 cm deep
below the
original gully
floor
A wooden pole
driven into the
ground
io to 40 cm
deep into the wall
of the gully
Wood poles (size • to toern
diamter) or wood splits are properly cut to the dimentions of
the gully depth to the bottom minus the free board plus the so
cm depth below the floor level
of the gully as shown in figure are driven into the soil at i to i.ao meters spacing across the gully.
Figure 6.7 Construction of wood/brush check dam and their spacing
The following are the procedures to follow on the construction of wooden fences check dams in small gullies and or their finger gullies.
« Use treated poles of not less than 100 mm in diameter, and a maximum of 1.83 meters for single and double fenced check dams.
•     Cut the wooden poles into the required lengths and if available paint the ends with tar or used car engine oil,
•    Plant the anchor poles in less than 1 meters spacing,
■   Bind the horizontal poles onto the anchors on the upstream side, using 3-mm galvanized wire,
•     Put or lay down plant material or debris onto the upstream face of the structure and monitor the situation until sediment covers fully,
•    Ensure a concave spillway as shown in figure 5.5 and 5.6 above,
•     Construct an apron using durable rock overlaid with strong wire netting securely wired and fixed to the main body of the structure. The apron should be 0.5 m wider than the overflow  width.  The  length  of  the  apron  should  be  twice  the  overflow  height  of  the crest,
■   Treat the site and immediate surrounds with ant poison.
(ii) Earth check dams enveloped with plastic or bamboo net
Earth check  dams  are used under exceptional cases  although workshop participants  for reviewing this report dismissed its significance and effectiveness. Basically it is the soil rapped or enveloped with plastic or bamboo nest which in this case is considered as a construction material for the check dam. Hence it is recommended for small but stable gullies with very little
Geudiew Alon & Associates 
| SECTION VI: PROPOSED DESIGN FOR 81 LARGE GULLY TREATMENT WORKS___________ Design of Large Gully Treatment Works: Tana Bcles Inlegrated Waler Resources Development Project
flow. There is experience in the use of such check dam in Tana Beles Project areas although
there life time seems to be rather short. Earth check dam is a feasible option except at the hea
cut areas/sections of the gully where the erosion and gully development is active. This may 
have to be seen and experimented at a limited scale and when rock/stone is not available.
6.3.3.2  Check dams in small, medium and large size gullies
The check dams  are the two types; (i) temporary and (ii) permanent structures used in small, medium and large size gullies. Rock check dam is a permanent structure constructed at one or more locations  along the gully. Regardless  of the size and material of construction, the gully plug provides grade control and impounds water and retains sediment and in so doing impacts ground water storage.
There are two types  of rock  check  dams  :(i) nonporous  dams  (without weep holes) and (ii) porous check dams. The later releases part of the flow through the structure. A weep hole is a porous  material located in an otherwise impermeable structure that allows  water to seep through and drain the structure (Ffolliott et al. 2003). Nonporous  check  dams, such as  those built from concrete, sheet metal, wet masonry, or earth, receive heavy  impact from the hydrostatic forces of gully flow. Earthen dams are less used and in exceptional cases, but at times it can be used in sites where large gullies at their mouth can have proper site conditions for earth dam. Wet masonry check dams have sites in Megenagna Model Gully Watershed # 1. Feasible sites for such structure are rare but where ever possible it has both a water storage and environmental services. A more detailed study has to be conducted.
6.3.3.3  Design specifications for check dams
Design parameters used for the model gullies are shown in figure 6.8 and they are determined based on the existing gully geometry and the estimated peak flood (assumed to serve over a ten year return period). These parameters  include the height of flow of the maximum flood (ymax), the spillway crest height (h) and the freeboard (200 mm) is considered for this study.
(i) Design peak flow
depth (yTOX)
The design peak flow depth (ymax) over the spillway crest is determined by a hydraulic formula that is  commonly used for broad crested weir.
The following empirical equation is used to estimate flow (Q) for a broad crested weir such as  concrete and rock check dams.
--------------- ■ Gully bottom width--------------------- -----
Figure6,8_Dgsi n parameterjfoj^^pjcaiTh^k dam for gully controT
a
Gctachew Alem & Associates
I SECTION VI: PROPOSED DESIGN FOR 8"> LARGE GOLLY TREATMENT WORKSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Flow over a typical rock check dam is shown in figure 6.8 below.
Q = CLH3'2
This  relationship  is  developed  in  English  unit  and  the  following  relationship  is  developed  in metric (SI) units taking into account the kinetic energy of the flow.
Equation (5)
Where:
Q = Peak flood in m3/sec,
C = Coefficient of flow (dimensionless),
L = Spillway crest length, in meters,
H = Flow depth over the spillway crest in meters (y™ Zs used instead of H here after and
in the design specification tables below and see also figure 6.8 above and 6.9 below)
For broad crested weir, the coefficient C ranges between 0.5 and 0.9 (Vennard and Street,
1976) and it is true when a free outfall is assumed - no submerged flow occurs. Equation (5) is 
reduced  to  equation  (6)  taking  C  value  of  rock  check  dam  as  0.5  and  using  flow  per  unit
[spillway crest] length (q),
q = lASymax ^
32
Equation (6)
The H in equation (4 and 5) is replaced by ymax. The spillway length is adjustable so as the optimize flow over the crest to within a range of 0.40 to 0.75 meters - an acceptable value for a stable flow over the check dam (Pathak P, Wani SP and Sudi R. 2005). The height of the spill way wall is 1.00 meter using the standard Gabion. It is important to recognize that the values of "ymax" (in above equation) change with the change in the selected value of the length of the spill way crest, "L" (see figure 6.8 above) and the selected value depends on the observed or actual top width (TW) of the gully at each measurement point.
Excel Model is used to optimize these values and adjust these parameters in order to produce stable design for the structure. Based on this process of determining the maximum flow and optimization of "L" on the basis of standard design requirements, the spillway crest height is finally set. It requires repeated iterative process using excel model until a reasonable and a stable design values are obtained. The spill way dimensions and design specifications of the proposed check dams at each point along the gully channel for each model gully in the study areas are presented in table 6.1 to 6.11 below.
Gctachcw Alcm & Associates
| SECTION VI: PROPOSED DESIGN FOR 83 LARGE GULLY TREATMENT WORKSDesign of Large Gully Treatment Works: I ona Bcles Integrated Water Resources Development Projert
(ii) Design spillway crest height of the rock check dam
One objective of a check dam is to trap as much sediment as possible to rehabilitated large gullies. A typical check dam design is presented in figure 6.10 below.
However based on feed backs received from workshop participants, the maximum spillway crest height (h) from the floor of the gully is 4 meters even if there are gullies more than five meters. It is  also agreed that after the structure is  stable and gully  is  rehabilitated, incremental heightening of the crest could be tried. Hence the spacing and locations of check dams in all model gullies  have been changed to new values. The new spillway  crest height (h) for the proposed check dams in the different model gullies are presented in tables 6.2 to 6.12.
Gcuchew Alcm & Associates
| SECTION VI: PROPOSED DESIGN FOR 84 LARGE GULLY TREATMENT WORKSDesign of Large Gully Treatment Works: Tana Bcles Integrated Water Resources Development Project
Apron
Figure 6.10 Typical gabion rapped rock check dam
Gciachcu Aicm & Associates
I SECTION VI: PROPOSED DESIGN FOR 85 LARGE GULLY TREATMENT WORKSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table 6.2 Design specifications for Megenagna Model Gully in watershed # 1
1 u.■
GPS Locations
Gully Geometry, m
Spillway Design Parameters
SE
b
Peg
X
y
TW
BW
D
Length,
L
Q
Q/L=q
Ymax
h
Free
Board
Key
Remark
Recommended Check 
dam type
This is the head area of the gully
1163
390535
1283611
2520
3
0.6
1
1.5
1
0.67
0.59
0.21
0.2
1.00
Disregard
Loose Rock check dam
1103
548
660
2519
5
1
2
3
1
0.33
0.37
1.43
0.2
1.00
Loose Rock check dam
1003
504
746
2518
7
2
3
4
2
0.50
0.48
2.32
0.2
1.00
Gabion Check dam
963
484
776
2516
7
2
3
4
3
0.75
0.63
2.17
0.2
1.00
Gabion Check dam
863
430
862
2505
12
3
6
8
3
0.38
0.40
4.00
0.2
1.00
Maximum, h
Strengthened Gabion (
713
390426
1283999
2497
20.4
3.5
6.6
15
4
0.27
0.32
4.00
0.2
1.00
Maximum, h
Strengthened Gabion
521
420
184
2492
30
8
6.1
20
6
0.30
0.34
4.00
0.2
1.00
Maximum, h
Strengthened Gabion |
461
424
241
2488
24.7
6
6.3
18
6
0.33
0.37
4.00
0.2
1.00
Maximum, h
Strengthened Gabion ||
356
385
332
2485
19
6
5.3
13
7
0.54
0.51
4.00
0.2
1.00
Maximum, h
Strengthened Gabion
There is a w et masc>nry structure site at thi s area and furth er investigated for soil stability and its appropriateness needed
92
355
502
2483
12.2
0.6
3.5
9
9
1.00
0.77
2.53
0.2
1.00
Gabion Check dam ||
50
355
521
2478
21.1
0.6
3.4
15
10
0.67
0.59
2.61
0.2
1.00
Gabion Check dam l|
0
351
547
2479
14.7
0.6
3.3
10
10
1.00
0.77
2.33
0.2
1.00
Gabion Check dam ||
This is the mouth of the finger gully in watershed # 1, and Measurement starts hear at the mouth of the gully an
It joins with the gully in watershed #2. d continues to the head of the gully
Gelacheu Alcm & Associates
| SEC TION VI: PROPOSED DESIGN FOR 86 LARGE GULLY TREA TMENT WORKSDesign of Large Gully T reatment Works: Tana Belts Integrated Water Resources Development Project
Table 6.3 Check dam design specifications for Megenagna Model Gully Watershed # 2
GPS Locations
Gully Geometry
Check dam spill way dimensions
X
y
Spillway 
crest
Length
TW
BW
D
Q
Q/L = q
Crest
height,
h
Key
Remark
Recommended
Check dam Design
35
13
391067
1283472
13.1
2
2.6
8.00
1.0
0.125
0.191
0.2
2.21
1.0
Loose Check dam
75
15
087
502
4.7
1.5
2.5
2.00
1.0
0.5
0.483
0.2
1.82
1.0
Loose Check dam
95
16
1103
515
9.3
1.7
4.8
5.00
1.0
0.2
0.262
0.2
4.00
1.0
Strengthened Gabion
115
17
1109
533
12.4
2.2
4.6
8.00
1.0
0.125
0.191
0.2
4.00
1.0
Maximum
strengthened Gabion
135
18
1121
550
8.5
3
3.5
5.00
1.0
0.2
0.262
0.2
3.04
1.0
Strengthened Gabion
155
19
1132
566
8.1
3
2.9
5.00
1.5
0.30
0.343
0.2
2.36
1.0
Gabion Check dam
315
26
1216
668
17
8
3.5
12.00
1.5
0.13
0.191
0.2
3.11
1.0
Gabion Check dam
455
33
1254
775
18.3
2.5
5.4
15.00
2.0
0.13
0.199
0.2
4.00
1.0
Maximum
strengthened Gabion
495
35
1287
799
31.3
2.5
4.6
20.00
2.0
0.10
0.164
0.2
4.00
1.0
Maximum
strengthened Gabion
535
37
1313
828
27.5
3
4.2
20.00
2.0
0.10
0.164
0.2
3.84
1.0
strengthened Gabion
615
41
1367
892
19.3
6
5.5
15.00
3.0
0.20
0.262
0.2
4.00
1.0
Maximum
strengthened Gabion
815
51
1291
4053
14.7
3
4.8
10.00
3.5
0.35
0.381
0.2
4.00
1.0
Maximum
strengthened Gabion
Gctachcw Alem & Associates
| SECTION VI: PROPOSED DESIGN FOR 87 LARGE GULLY TREATMENT WORKS
Distance, m
Peg #
Free
boardDesign of Large Gully Treatment Works: Tana Deles Integrated Water Resources Development Project
Table 6.3 (continue)
GPS Locations
Gully Geometry
Check dam spill way dimensions
X
y
TW
BW
D
Spillway
crest
Length
"L
Q
Q/L
=q
Yn»x
Crest
height,
h
Key
Remark
Recommended
Check dam Design
975
59
391186
1284123
15.5
5.5
4.3
10.00
3.50
0.35
0.381
0.2
3.72
1.0
strengthened Gabion
1015
61
1154
4146
22.2
3.5
2.8
15.00
4.00
0.27
0.317
0.2
2.28
1.0
Gabion Check dam
1075
64
1111
4185
30
12
2.5
20.00
13.00
0.65
0.576
0.2
1.72
1.0
Gabion Check dam
1235
72
390992
4272
10.6
4
4.2
6.00
14.00
2.33
1.357
0.2
2.64
1.0
Gabion Check dam
1575
89
0740
4416
11.7
6
4.2
8.00
17.00
2.13
1.274
0.2
2.73
1.0
Gabion Check dam
1675
94
0694
4448
18.4
8
4.5
15.00
17.00
1.13
0.836
0.2
3.46
1.0
strengthened Gabion
1755
98
0625
4475
18.3
6
5
15.00
18.00
1.20
0.869
0.2
3.93
1.0
strengthened Gabion
1815
92
0620
4534
36.3
15
3.4
20.00
18.00
0.90
0.717
0.2
2.48
1.0
strengthened Gabion
1915
98
0512
4572
13.1
5
2.8
8.00
19.00
2.38
1.373
0.2
1.23
1.0
strengthened Gabion
2135
109
0354
4559        15.4
3
2.5
10.00       20.00
2.00
1.224
0.2
1.08
1.0
strengthened Gabion
Gcuchcw Alcm & Associates
| SECTION VI: PROPOSED DESIGN FOR 88 LARGE GULLY TREATMENT WORKS
Distance, m Peg #
Free
boardDesign of Large Gully Treatment Works: Tana Deles Integrated Water Resources Development Project
Table 6.4 Design specifications for Megenagna Model Gully in Watershed # 3
GPS Locations
Check dam and Spillway Dimension Specifications
Gully Geometry
X
Y
Spill
way 
length,
L
Estimated
peak 
flood, Q,
m3/sec
Q/L=q
Depth
of
flow,
(Vcnix),
m
Spill
way
crest
height,
h
Recommended Check dam type
Free
Board
0
Head of the
gully
1.8
20
0
14.9
0.6
2.7
10
2
0.20
0.26
2.24
0.2
Single Line Gabion Check dam
240
1
7.4
1.8
2.85
5
2
0.40
0.42
2.23
0.2
Single Line Gabion Check dam
300
15
6.3
2.5
10
3
0.30
0.34
1.96
0.2
Single Line Gabion Check dam
459
16
10.5
3
3.6
6
4
0.67
0.59
2.81
0.2
Single Line Gabion Check dam
747
31
23.1
2.3
2.4
18
6
0.33
0.37
1.83
0.2
Single Line Gabion Check dam
849
38
26.6
3.1
1.9
20
7
0.35
0.38
1.32
0.2
Single Line Gabion Check dam
1064
51
20.4
3.1
2.7
15
8
0.53
0.50
2.00
0.2
Single Line Gabion Check dam
1101
53
15.3
1.5
2.8
12
9
0.75
0.63
1.97
0.2
Single Line Gabion Check dam
1136
54
10.9
2.35
2.45
8
9
1.13
0.83
1.42
0.2
Single Line Gabion Check dam
1155
55
16
3.4
2.2
12
9
0.75
0.63
1.37
0.2
Single Line Gabion Check dam
1174
56
9.7
4.8
2.25
6
9
1.50
1.01
1.04
0.2
Single Line Gabion Check dam
1222
57
19
3.3
2
10
10
1.00
0.77
1.03
0.2
Single Line Gabion Check dam
1375
58
12
2.7
2
10
11
1.10
0.82
0.98
0.2
Single Line Gabion Check dam
1504
60
20.2
7.6
2.7
15
12
0.80
0.66
1.84
0.2
Single Line Gabion Check dam
1534
69
10.5
3.8
3
8
12
1.50
1.01
1.79
0.2
Single Line Gabion Check dam
1602
72
12.8
7.3
2.75
8
12
1.50
1.01
1.54
0.2
Single Line Gabion Check dam
Gelachcw Alcm & Associates
| SECTION VI: PROPOSED DESIGN FOR 89 LARGE GULLY TREATMENT WORKS
Distance in
meters
Peg in the Atlas
Top Width
Bottom width
DepthDesign of Large Gully Treatment Works: Tana Deles Integrated Water Resources Development Project
Table 6.5 Design specifications for Megenagna Model Gully in Watershed # 4 (1)
GPS Locations
Gully geometry
Spillway design specifications in meters
ZZZJ
X
y
tr
it
a
5
E
>
Free
board
Recommended Check dam
Type
WT
BW
D
0
Head of the gully
1 15
389804
1284703
21.0
9.6
2
10
5
0.5
0.48
1.32
0.20
Single line Gabion check dam
37
821
718
23.1
4.2
1.95
15
5
0.33
0.37
1.38
0.20
Single line Gabion check dam
89
861
747
22.9
4.1
2.35
15
5
0.33
0.37
1.78
0.20
Single line Gabion check dam
143
908
767
24.3
7.8
1.85
18
4
0.22
0.28
1.37
0.20
Single line Gabion check dam
1 282
044
053
12.4
2.1
1.9
8
3
0.38
0.4
1.3
0.20
Single line Gabion check dam
| 338
061
077
11.0
2.5
1.9
8
3
0.38
0.4
1.3
0.20
Single line Gabion check dam
377
065
100
13.3
2.7
1.9
8
3
0.38
0.4
1.3
0.20
Single line Gabion check dam
462
095
155
19.8
5
2.2
15
2
0.13
0.2
1.8
0.20
Single line Gabion check dam ||
492
117
173
2.3
0.6
1.2
1.5
2
1.33
0.93
0.07
0.20
Disregard and use the next ||
1 512
169
185
14.0
0.6
2
10
2
0.2
0.26
1.54
0.20
Single line Gabion check dam 1
|580
153
198
1.9 |        0.6
1.65
1
1
1
0.77
0.68
0.20
Loose rock check dam
Crest heights for check dam are low less than 60 ems No special prescription is needed for the rest of the gully Use loose rock check dam based on experiences
Getachcw Alcm A Associates
| SECTION VI: PROPOSED DESIGN FOR 90 LARGE GULLY TREATMENT WORKS
Distance (m)
Crest length
L
Peak floow,
Q
Crest height,
hDesign of Large Gully Treatment Works: Tana Boles Integrated Water Resources Development Project
Table 6.6 Design specifications for Megenagna Model Gully in Watershed # 5
Gully Geometry
GPS Locations
Spillway Dimension
Specification
X
Y
L
Q
Q/L=q
Ymax
Crest
height,
h
Free
Board
Recommended Check 
dam type
5
6
0.6
1.83
389316
1285484
4
0.5
0.13
0.19
1.44
0.2
Gabion check dam
30
10.9
4
1.5
343
479
8
0.5
0.06
0.12
1.18
0.2
Gabion check dam
60
8.4
3
1.25
368
456
6
0.5
0.08
0.15
0.90
0.2
Gabion check dam
150
28.9
1.5
3
417
409
20
1.0
0.05
0.10
2.70
0.2
Gabion check dam
195
11.10
3.0
2.8
432
361
8
1.0
0.13
0.19
2.41
0.2
Gabion check dam
274
16.50
4
1.7
489
319
12
1.5
0.13
0.19
1.31
0.2
Gabion check dam
300
22.70
6
1.7
514
313
16
1.5
0.09
0.16
1.34
0.2
Gabion check dam
360
21.80
5
1.6
516
286
15
1
0.07
0.13
1.27
0.2
Gabion check dam
388
25.50
7
2.3
578
264
20
1
0.05
0.10
2.00
0.2
Gabion check dam
418
25.70
7
1.85
586
235
20
1
0.05
0.10
1.55
0.2
Gabion check dam
468
17.60
6
2.9
613
196
12
2
0.17
0.23
2.47
0.2
Gabion check dam
528
15
2
2.57
639
145
10
2
0.20
0.26
2.11
0.2
Gabion check dam
558
34.7
5
2
662
127
25
2
0.08
0.14
1.66
0.2
Gabion check dam
585
17.3
5
2.5
688
118
10
3
0.30
0.34
1.96
0.2
Gabion check dam
635
19.4
6
2
689
073
15
3
0.20
0.26
1.54
0.2
Gabion check dam
685
18
4
1.8
689
026
10
4
0.40
0.42
1.18
0.2
Gabion check dam
725
21.3
11
1.4
723
990
15
4
0.27
0.32
0.88
0.2
Loose check dam
815
35.2
17
0.9
389698
1284935
25
5
0.20
0.26
0.44
0.2
Disregard
1020
28.5
5
0.95
728
763
20
3.5
0.18
0.24
0.51
0.2
Loose check dam
1110
23.5
8
1.3
691
709
15
3.5
0.23
0.29
0.64
0.2
Loose check dam
GcUchew Alan &. Atwciatcs
| SECTION VI: PROPOSED DESIGN FOR 91 LARGE GULLY TREATMENT WORKS
Top Width Bottom width
DepthDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table 6.7 Design specifications for Tikur Woha Model Gully Watershed for finger gully # 1
I
Location of the structure
Gully Geometry
E,
C
GPS Locations
Top
Widt
h
Bottom
Width
Depth
Chute
Floor
Length
Recommended
Design
s.
no
1
2
3
4
5
6
7
8
9
X
y
"WT"
"w"
"D”
0
NA
NA
1.6
1.2
1.0
1
Chute construction
20
NA
NA
12.3
3.0
5.0
10
Chute construction
40
NA
NA
6.7
2.5
3.2
4
Chute construction
60
NA
NA
4.0
2.3
2.1
2
Chute construction
80
NA
NA
4.7
2.0
2.6
2
Chute construction
100
NA
NA
2.1
1.0
1.6
1
Chute construction
120
NA
NA
5.4
5.0
0.9
3
Chute construction
140
NA
NA
12.5
5.0
2.1
8
Chute construction
160
NA
NA
6.8
1.5
1.8
5
Chute construction
10
180
NA
NA
11.4
1.0
4.2
8
Chute construction
11
200
NA
NA
6.5
1.5
2.5
4
Chute construction
12
220
NA
NA
2.65
0.5
2.1
1.5
Chute construction
13
240
NA
NA
6.6
2
1.6
5
Chute construction
14
260
NA
NA
7.22
5
2.2
5
Loose rock check dam
280
NA
NA
3.32
2
2
2
Loose rock check dam
Gctachcw AI cm &. Associates
| SECTION VI: PROPOSED DESIGN FOR 92 LARGE GULLY TREATMENT WORKS
------------ 1 SpaciDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table 6. 8 Design specifications for Tikur Woha Model Gully Watersheds finger # 2
Location of the structure
Gully Geometry
Sill way dimensions
Dimensions of
E,
bo
GPS Readings
Top
Width
Hydraulic Parameters
Recommended check
dam with concave spill
way for the site
S.
no
Peg
Easting
Northing
Elev
(m)
H WT"
"w"
"D"
x:
±■
Q
Q/L
=q
Vmax
h
1
18
17
365
407900
1299397
3087
7.0
0.60
3.4
5
4
0.8
0.7
0.2
2.5
1.0
Loose Rock Check dam
19
18
385
407877
1299399
3081
10.0
0.60
5.3
7
4
0.6
0.5
0.2
4.6
1.0
Loose Rock Check dam
20
19
405
407862
1299412
3083
11.6
0.60
5.8
5
4
0.8
0.7
0.2
4.9
1.0
Loose Rock Check dam
21
20
425
407846
1299419
3076
10.1
0.60
4.7
5
4
0.8
0.7
0.2
3.8
1.0
Loose Rock Check dam
22
21
445
407828
1299433
3073
11.0
0.60
4.6
7
4
0.6
0.5
0.2
3.9
1.0
Loose Rock Check dam
I23
22
465
407801
1299435
3067
15.8
0.60
3.4
10
4
0.4
0.4
0.2
2.8
1.0
Loose Rock Check dam
24
23
485
407789
1299445
3070
12.0
0.60
2.6
7
4
0.6
0.5
0.2
1.9
1.0
Loose Rock Check dam
25
24
505
407774
1299465
3062
7.7
0.60
4.0
5
4
0.8
0.7
0.2
3.1
1.0
Loose Rock Check dam
26
25
525
407746
1299465
3055
14.0
0.60
5.7
10
5
0.5
0.5
0.2
5.0
1.0
Loose Rock Check dam
27
26
545
407720
1299469
3051
13.6
0.60
6.8
10
5
0.5
0.5
0.2
6.1
1.0
Loose Rock Check dam
28
27
565
407692
1299474
3050
23.0
3.0
4.8
15
6
0.4
0.4
0.2
4.2
1.0
Loose Rock Check dam
29
28
585
407674
1299482
3052
20.0
4.0
5.0
12
6
0.5
0.5
0.2
4.3
1.0
Loose Rock Check dam
30
29
605
407653
1299494
3041
9.4
4.0
1.6
7
6
0.9
0.7
0.2
0.7
1.0
Single line Gabion
-
31
30
625
407626
1299492
3039
18.1
3.0
3.8
10
7.5
0.8
0.6
0.2
3.0
1.0
Loose Rock Check dam
32
31
652
407596
1299505
3042
20.0
0.60
8.2
12
7.5
0.6
0.6
0.2
7.4
1.0
Loose Rock Check dam
33
32
672
407547
1299519
3032
20.0
0.60
6.2
12
7.5
0.6
0.6
0.2
5.4
1.0
Loose Rock Check dam
34
33
692
407538
1299529
3030
20.0
0.60
4.2
12
7.5
0.6
0.6
0.2
3.4
1.0
Loose Rock Check dam
35
20.0 0.60
4.7
12
7.5
0.6
0.6
0.2
1.0
Loose Rock Check dam
1
36
1
34
35
712
732
407530
407514
1299543
1299556
3026
3024
20.0 0.60
6.7
12
7.5
0.6
0.6
0.2
3.9
5.9
1.0
Loose Rock Check dam
Gclachcw Alem & Associates
| SECTION VI: PROPOSED DESIGN I OR 93 LARGE GULLY TREATMENT WORKS
--------------- 1 Spacir
Bottom
Width
Depth
Spillway
crest
length
Free
board
KeyDesign of Large Gully Treatment Works: Tana Deles Integrated Water Resources Development Project
Table 6.9 Design specifications for Tikur Woha Model Gully Watershed finger # 2 branch gully
Gclachcw AI cm & Associates
| SECTION VI: PROPOSED DESIGN FOR 94 LARGE GULLY TREATMENT WORKSDesign of Large Gully Treatment  Works: Tana Bclcs Integrated Water Resources Development Project Table 6.10 Design specifications for Enkulal Model Gully, Dera Woreda
Location (GPS readings)
Gully Geometry
Check dam hydraulic Parameters
1
tt
M
s
Q.
X
Y
Elev (m
TW
BW
D
O
—i
X
M
E
>
Recommended Check dam type
1
0
Mouth of gully
14.50
1.50
2.04
10.00
13.00
1.30
0.92
0.20
0.92
0.70
Single Gabion Check dam
2
20
6.90
0.60
3.05
5.00
13.00
2.60
1.46
0.20
1.39
0.70
Single Gabion Check dam
3
40
8.60
0.60
2.90
6.00
13.00
2.17
1.29
0.20
1.41
0.70
Single Gabion Check dam
4
60
8.40
0.60
2.90
6.00
13.00
2.17
1.29
0.20
1.41
0.70
Single Gabion Check dam
—
9
160
14.40
4.70
3.50
10.00
13.00
1.30
0.92
0.20
2.38
0.70
Single Gabion Check dam
12
220
21.40
0.70
3.80
15.00
13.00
0.87
0.70
0.20
2.90
0.70
Single Gabion Check dam
13
240
10.30
0.70
3.83
7.00
13.00
1.86
1.16
0.20
2.47
0.70
Single Gabion Check dam
16
300
11.70
0.70
3.50
8.00
13.00
1.63
1.06
0.20
2.24
0.70
Single Gabion Check dam
17
320
17.50
0.70
2.90
12.00
12.00
1.00
0.77
0.20
1.93
0.70
Single Gabion Check dam
| 18
340
10.30
3.00
2.25
8.00
12.00
1.50
1.01
0.20
1.04
0.70
Single Gabion Check dam
19
360
11.80
4.00
2.46
8.00
12.00
1.50
1.01
0.20
1.25
0.70
Single Gabion Check dam
20
380
16.00
9.00
2.96
12.00
12.00
1.00
0.77
0.20
1.99
0.70
Single Gabion Check dam
22
420
10.00
0.60
4.30
8.00
6.00
0.75
0.63
0.20
3.47
0.70
Single Gabion Check dam
26
500
6.90
4.30
2.14
4.00
6.00
1.50
1.01
0.20
0.93
0.70
Single Gabion Check dam
Road culvert
22
560
11.00
0.60
3.90
8.00
6.00
0.75
0.63
0.20
3.07
0.70
Single Gabion Check dam
25
620
14.00
0.60
4.24
10.00
6.00
0.60
0.55
0.20
3.49
0.70
Single Gabion Check dam
28
680
30.00
0.60
4.50
20.00
6.00
0.30
0.34
0.20
3.96
0.70
Single Gabion Check dam
32
760
12.00
0.60
3.57
10.00
6.00
0.60
■ 39
0.55
0.20
2.82
0.70
Single Gabion Check dam
900
6.60
0.60
2.25
4.00
6.00
1.50
1.01
0.20
1.04
0.70
Single Gabion Check dam
40
.
920
Road culvert
===L
4.10
0.60
1.10
Gctachcw Alcm &. Associates
| SECTION VI: PROPOSED DESIGN FOR 95 LARGE GULLY TREATMENT WORKS
Spacing, m
Length
L
free
board
crest
height
KeyDesign of Large Gully Treatment Works: Tana Boles Integrated Water Resources Development Project
Table 6.11 Design specifications for Washaw Model Gully in Sekela Woreda
GPS Readings
txfi
•1
CL
Gully Geometry
Spillway Design parameters
Key
Recommended Check Dam Type
X
y
TW
BW
D
Length
Ymax
Crest
height, h
0
315002
1222472
P01
20
467
2707
7.5
6.5
2.2
5.0
0.3
1.7
0.70
Loose Rock check dam
P02
40
314037
2463
6.3
0.5
3.2
3.0
0.4
2.6
0.70
Single Line Gabion
P03
60
053
461
12.3
4.0
2.1
10
0.2
1.0
0.70
Loose Rock check dam
P04
80
075
462
12.5
0.5
3.6
10
0.2
1.7
0.70
Loose Rock check dam
P05
100
096
466
8.4
1.0
3.5
5.0
0.3
3
0.70
Reinforced Single Line Gabion
P06
120
116
469
14.8
2.0
3.5
10
0.6
2.2
0.70
Single gabion
P07
140
134
477
15.8
1.0
5.5
10
0.5
4.8
0.70
Reinforced Single Line Gabion
P08
160
151
488
16.3
1.0
3.2
10
0.6
2.3
0.70
Single gabion
P09
180
173
496
10.5
1.5
4.5
8
0.4
3.9
0.70
Reinforced Single Line Gabion
PIO
200
191
501
9.4
1.5
3.0
6.0
0.5
2.3
0.70
Single gabion
Pll
220
209
504
31.2
2.0
4.6
20
0.2
4.2
0.70
Reinforced Single Line Gabion
P12
0
314132
488
2.1
0.5
1.8
1.0
0.3
0.8
0.70
Loose Rock check dam
P13
20
165
498
12
0.5
1.0
7.0
0.3
0.7
0.70
Loose Rock check dam
P14
40
186
499
7.6
0.5
1.8
7.0
0.3
1.3
0.70
Loose Rock check dam
P15
60
204
513
13
2.5
3.4
10
0.2
3.0
0.70
Reinforced Single Line Gabion
P16
80
224
525
12.2
1.2
3.4
7.0
0.4
2.8
0.70
Reinforced Single Line Gabion
P17
100
233
535
12.5
0.5
4.6
8.0
0.5
3.9
0.70
Reinforced Single Line Gabion
P18
120
252
552
14.2
0.5
4.5
10
0.3
4.0
0.70
Reinforced Single Line Gabion
P19
140
247
572
14.2
0.5
4.8
10
0.5
4.1
0.70
Reinforced Single Line Gabion
P20
160
246
598
15.8
0.5
4.8
15
0.3
4.3
0.70
Reinforced Single Line Gabion
P21
180
269
609
18.0
0.5
3.8
12
0.4
3.2
0.70
Reinforced Single Line Gabion
P22
200
287
614
2.2
0.5
1.6
7.0
0.6
0.8
0.70
Loose Rock check dam
P23
220
304
621
8.0
0.5
3.2
10
0.5
2.5
0.70
Single Line Gabion
Gctachcw Alcm & Associates
| SECTION VI: PROPOSED DESIGN FOR 96 LARGE GULLY TREATMENT WORKS
SpacinDesign of Large Gully Treatment Works: Tana Deles Integrated Water Resources Development Project
Table 6.10 (continues)
P24
240
306
637
13.8
0.5
3.2
12
0.4
2.6
0.70
Single Line Gabion
P25
260
309
656
4.4
0.5
2.0
2.0
1.3
0.5
0.70
Loose Rock check dam
P26
280
312
675
12.0
0.5
2.0
2.0
1.3
0.5
0.70
Loose Rock check dam
P27
300
305
693
8.4
0.5
1.8
5.0
0.6
1.0
0.70
Loose Rock check dam
Gctichcw Alcm A Associates
| SECTION VI: PROPOSED DESIGN FOR 97 LARGE GULLY TREATMENT WORKSDesign oI Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table 6.12 Design specifications for Biret Afer Model Gully in Mecha Woreda
-2
k<
TW
BW
D
X
y
Elev
length,!.
Q
Q/L=q
ymax
h
Free board
Key
Aprone
Recommended
Check dam Design
0
301251
1241766
Head of the long finger gully
20
8.6
0.6
2.7
229
763
2120
4
0.5
0.13
0.19
2.31
0.2
0.7
4.0
Single Gabion
85
11.4
0.6
2.3
163
769
2110
8
0.5
0.06
0.12
1.98
0.2
0.7
3.5
Single Gabion
145
8.7
0.6
3.6
106
769
2106
6
0.5
0.08
0.15
3.25
0.2
0.7
5.7
Single Gabion
205
8.1
3.2
3.1
55
755
2093
6
1
0.17
0.23
2.67
0.2
0.7
4.7
Single Gabion
265
19.8
4
5.8
789
2083
15
1
0.07
0.13
4.00
0.2
0.7
9.6
Single Gabion
265
Big and wide gully requiring
aedal treatment
305
14.1
5
8.8
969
813
2083
8
1
0.13
0.19
4.00
0.2
0.7
14.7
Single Gabion
365
7.4
2.1
3.5
903
813
2081
5
2
0.40
0.42
2.88
0.2
0.7
5.0
Single Gabion
23
425
13.1
3
4.4
850
826
2078
8
2
0.25
0.30
3.90
0.2
0.7
6.8
Single Gabion
Finger
gully (small length finger gully)
445
15.7
7
5.5
300957
1241665
2085
10
2
0.20
0.26
4.00
0.2
0.7
8.8
Single Gabion
503
21.7
3.5
8
915
706
2084
15
2
0.13
0.20
4.00
0.2
07
13.3
Single Gabion
612
13.2
3
5.5
841
786
2077
8
3
0.38
0.40
4.00
0.2
0.7
8.6
Single Gabion
692
10.5
3.5
4.2
811
840
2073
6
3
0.50
0.48
3.52
0.2
0.7
6.2
Single Gabion
—
The twi3 finger gullies meet here
24
732
12.7
3.5
4.6
798
874
2070
6
4
0.67
0.59
3.81
0.2
0.7
6.7
Single Gabion
28
812
11.2
4
3.1
744
926
2067
6
4
0.67
0.59
2.31
0.2
0.7
4.0
Single Gabion
34
932
7.76
2
3.7
624
952
2060
5
4
0.80
0.66
2.84
0.2
0.7
5.0
Single Gabion
37
992
8.2
2.5
2.4
577
970
2060
5
5
1.00
0.77
1.43
0.2
0.7
2.5
Single Gabion
40
1052
9.5
0.5
4.5
524
989
2055
6
5
0.83
0.68
3.62
0.2
0.7
6.3
Single Gabion
43
1112
10.1
1
5
479
30
2054
6
5
0.83
0.68
4.12
0.2
0.7
7.2
Single Gabion
51
1172
15
0.5
7.41
426
67
2052
10
5
0.50
0.48
4.00
0.2
0.7
11.8
Single Gabion
53
1212
28.7
7
6.5
382
77
2046
20
5
0.25
0.30
4.00
0.2
0.7
10.5
Single Gabion
54 1
1232
29.7
10
3.4
357
73
2042
20
5
0.25
0.30
2.90
0.2
0.7
5.1
Single Gabion
Gclachcw Alcm & Associates
| SECTION VI: PROPOSED DESIGN FOR 98 LARGE GULLY TREATMENT WORKS
longitudinal
distance__________ Design of Large Gully Treatment Works: Tana Belcs Integrated Water Resources Development Project
(iii)      Spacing (S) of the check dam structures
Based on this study, the sitting for the proposed check dam starts at the mouth or lower end of the model gully. The location of the next check dam will be determined based on the elevation of the gully floor along its channel course and the check dam crest height. Requirements for spacing depends on the gradient of the sediment deposits expected to accumulate above the dam, the effective height of the check dam crest (h) and also the available fund (Heed, 1966). A wooden check dam, a loose or gabion rapped rock check and a concrete check dam differ in their design of the spacing between two successive ones. Wooden check dams do not have enough strength and hence it's used in rills or young gullies less than one meter depth and they are located close enough within 5 to 10 meters spacing.
For practical purposes, and as rule of thumb, the spacing of check dams is a distance between the upstream point on the original gully floor where the next [second] check dam upstream will be located and where the elevation of the toe of this check dam is equal to the elevation of the crest height (h) of the [first] check dam downstream. Or in other terms, the most economical spacing is obtained if a check dam is placed at the upstream toe of the final sediment deposits of the next dam downstream. The spacing of check  dams  is  great enough to allow the full utilization of the sediment-holding capacity of the structures (Heed, 1966). GPS, Engineers level or local soil conservation spirit level and a rope is needed to determine the spacing between the first check dam and the next check dam up stream once the crest height of the check dam to be constructed is determined. In this way, a sequence of check dams is spaced properly all along the gully length.
(iv)      Key
A section of the check dam will have to get into the wall of the gully (which is called keying) and down the original floor level of the gully (the foundation) in order to give stability to the check dam structure, and how much deep it has to extend into the wall and the floor of the gully cross section greatly enhances the stability of the structure. Such keying is significantly important in structures that accommodate large peak flow through its spill way. The purpose of keying and gully floor trenching is to prevent any undermining of the structure by under flow and flows  of water around the dam structure. Allowing such flow around the structure will eventually scour and remove the soil and create gaps which could undermine the structure or lead to a collapse of the structure as observed in many instants during the visit at the study sites.
A general design of the keys is usually recommended as 60 cm for stability for non dispersive soils, and in sites where large amounts of loose soils exist on the floor and wall of the gulley, the keying and gully floor trenching has to increase to about 70 to 100 cm.
(v)       Total height of the Check dam (h)
The total height of the check dam (H) is the sum of the spill way crest height (h) plus the maximum flow (ymax) and the allowable free board (20 cm is  taken as  a standard). The effective height (h) of the check dam structure is the height of the crest of the spill way above the original gully floor. The height not only influences structural spacing between the successive check  dams  but also the volume of sediment deposits  (Heed, 1966). But at last it's  the
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situation on the ground and the experience and judgment of the engineer or technician that matters most in order to have a sound estimate of the spacing (S) between the check dams in each model gully. The judgment is largely based on planned objective of the structure, (i)i to rehabilitate the gully, (ii) restore the gully  and trap as  much sediment as  possible or (hi) enhancing ground water, etc.
Under  all  conditions  three  important  criteria  determines  the  height  (h)  of  the  structure  and these include (i) cost, (ii) stability and (iii) channel geometry.
Based on these experiences a maximum of 2.20 meters for loose rock check dam and these implies its use is restricted to medium to small sized gullies. Wire bound check dams are also under this category where the soil is less stable. Based on the feedback and decision made at the workshop held for this  study, the maximum height of the check  dam crest height is restricted to be up to 4 meters.
(vi)     Apron
Apron is a structure built to provide stability for the check dam through the protection of the gully bottom and adjacent wall of the gully banks. The apron structure when designed properly, it creates a steeling pool where the energy of the water is allowed to dissipate. It is a protective and safety measure that cannot be ignored. In most of the check dams visited in the Tana Beles Project areas, it is not observed. As a result underflow seepages are observed in many structures  and this  undermines  the structure and gradually  the check  dam will collapse. Standard specifications are also made and the following rule of thumb is adopted for field use (Heed, 1966).
(i)   The length of the apron should be 1.5 times the crest height of the check dam structure in channel, where the gradient does not exceed 15%,
(ii)  The  length  of  the  apron  should  be  1.75  times  the  crest  height  of  the  structure  in channels where the gradient is steeper than 15%,
(iii) The  apron  should  be  embedded  on  the  floor  of  the  gully  at  about  50  cm  below  the original floor level of the gully. Our suggested foundation depth for unstable soil is 60 cm.
Aprons  should also be constructed at the upstream side of the gully  if the underlying soil material at the gully  bottom is  dispersive. This  provides  sufficient margin of safety  to the structure and prevent the water fall from leaving damage to downstream side of the weir and adjacent sides  of the gully  wall (banks). The client experts  in the workshop insist that the upstream apron be limited to 40 to 60 cm, as opposed to the consultant proposal of one meter, and the clients proposal is accepted for this assignment.
(iv)    Side-slope protection
Loose rock  is  used for lining the sides  of the bank  of the gully  both the upstream and downstream side of the check dam. A single layer rock lining with 20 to 30 cm depth excavation (depending on the size of the rock) is sufficient for laying the loose rock on both sides of the wall and floor of the bank both at upstream and downstream areas of the check dams.
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For U-shaped channel with narrow width, where the water fall from the check dam will strike against  the  bank  of  the  channel,  the  height  of  the  bank  protection  is  suggested  to  same elevation as the spill way crest height; i.e. it should have the same vertical height of the spi way crest.  During reshaping of  the gully,  both  U  and  V-shaped  gullies will  have a shape o trapezoidal channel and all sections in this study are considered trapezoidal in shape. As a result check dams are all designed to meet a trapezoidal channel. In situation where the bottom width of the gully is very wide and the designed spillway length is lesser than the bottom width of the gully,  i.e.  the  designed  spill  way  will  allow  the  fall  of  the  water  directly  on  the  floor  of  the channel  (rock lined  apron),  and  in this situation the height of the side-slope protection could substantially be lowered than the crest height of the check dam structure so that cost could be reduced.
6.3.4    Check dam design in medium size gullies
Medium gully sizes are possibly those in the middle stem of the gully and stream orders II and III or even IV. As discussed earlier in the methodology, this aspect of the gully classification will assist in facilitating preliminary screening of the gully types by size and by type. The size of the gully  will be based on the available filed data obtained during the gully  characterization work.
6.3.4.1  The use of loose and porous check dam
Loose rock check dam is most widely used engineering measures in gully plugging. The crest of the spillway is a concave shape at the center of the check dam as shown in figure 6.11 below. The check  dam is  a porous  check  dam in which it releases  part of the flow through the structure, and thereby decreases/reduces the head of flow over the spillway. The design of the rock  check  dam is  suitable in gullies  that are small or medium in size or preferably  in the
upstream part (tributary  or
branch gully) of a large (big)
gully  where the gully  size is 
small. Once the catch basin is 
filled by  sediment deposits,
structural stability is less critical
because   the   dam   crest become a new level of upstream gully floor.
has 
the
From the design sketches, it is 
possible to calculate the volume
of construction materials 
.11 Loose rock check dam (Head, 1979)
required. The excavation and placement of the loose rock should be well placed in such a way that the check dam is at right angle to the direction of flow. One could notice how well the check dam in figure 6.11 is placed; it is well done in terms of placing the check dam across the direction of flow. Please notice also the protection made against the downstream bank of the gully with rock rip rap. Since loose - rock dams are not reinforced, the angle of rest of the rock (this is similar to angle of repose for soil) should determine the slopes of the dam sides This angle depends ori the type of rock, the weight, size and shape of the individual rocks and their size distribution. For the design of the dam, the following design criteria are used
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Height of the crest: The consultant recommends up to a height of 2.50 is acceptable for loss rock check dam and the designs are produced for this document as per this norm and standard However in the workshop, the client experts strongly insist that the maximum allowable height be limited to 1.5 meters. The client can take whatever is best suiting its need.
The angle of rest: Based on experience the following information is used as a rule of thumb. For angular rock, a slope ratio of 1.25 to 1.00 is recommended, and for round rock, a slope ratio of 1.50 to 1.00 may  be considered and the latter is  recommended for eroded stones collected from farm lands.
6.3.4.2          Gabion Check Dam
Loose rocks are rapped with mesh wire (gabion) and it is done in 2 meters by one meter or four meter by one meter and by one meter height at site level. The check dam shown in figure is properly  constructed and well placed except the designing of the height of the crest is  not properly determined.
The wire mesh should be (i) resistant to corrosion, (ii) of sufficient strength to withstand the pressure exerted by flow and rock, and (iii) have openings not larger than the average stone size in the dam.
A Gabion Check Dam is one of the most feasible options in the treatment of gullies in this study site and it can be used both in medium and large size gullies. One aspect on the use of gabion check dam in medium size gullies is shown in figure 37 above. Gabion wire is manufactured locally and its availability makes its use on a sustainable basis possible.
6.3.5    Design of check dams in large gullies
6.3.5.1   Reshaping the gully sides - the first step
The first action for the treatment of large gullies will be reshaping the sides of unstable banks.
Gully sections that require reshaping have to be identified and done manually.
6.3.5.2    Strengthened gabion check
dam
work will have to be
For situations  as  in the case of Misrak Estie, where gully size is large and banks are collapsing, (see photos  in section three), the gabion check dam is designed to have another gabion rapped rock placed along both sides of the gully banks at downstream side and at right angle to the gabion check dam (as shown in figure 6.12). This work is needed for check dam
Megenagna Model Gully where the soil is unstable
Figure 6.12 Strengthened gabion check dam
having a crest height of more than 2.5 meters i
>
b meters ln
(0-8%).          The          effective          height
of               the          sJp'^^S^('2S               ~r‘h3          S
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and the length is the same length as the apron. This structure is needed to strengthen and stabilize the check dam structure that is put in place across the gully. This design is more cost effective and has  higher stability  than a double gabion to resist put across  the gully  and perpendicular to direction of flow.
6.3.5.3             Non-Porous (Masonry or sometimes concrete) Check Dams
Non-Porous (masonry or concrete) check dams similar to the one shown in figure 6.13 below are also other options that can be used in big gullies. They are built from concrete and wet masonry structures. These structures receive strong impact from the dynamic and hydro static forces of the flow. These structures require strong anchoring of the dam into the gully banks to which most of the pressure is transmitted.
This study will examine the local situation and characteristics of the gully for the feasibility of using such design and construction material for a gully treatment work. This type of check dam is expensive than all other types of check dams. However depending on the situation and need at the site under consideration, this  intervention option may  be considered as  one for assessment and use in the proposed study sites.
The value for its high cost is the potential for such structures to impound large volume of water that can recharge ground water and other environmental, esthetic and socio economic value of the water made available in the area.
Precast (prefabricated, pre stressed) concrete check dams are widely used in many developed countries as a very effective technology option in reclaiming and restoring large gullies. In some cases shortage of land for production and grazing purposes may justify the reclamation of land at high investment costs. The future in using precast concrete  check  dam  cannot  rule  out  as 
Figure  6.13  Masonry  (non-porous)  check  dam (source: Pathak et al, 2005)
cement production in the country increases. Based on the experience of other countries, the use of precast concrete check  dams  have been found very  dependable, stable, and most feasible option in restoring big gullies. In this study it may be possible to test it, and design may be developed for future use in Ethiopia for large gully treatment work where it may appear feasible.
6.3.5.4  Treating meandering gullies
Of all the study sites, the Megenagna Model Gully has meandering sections that require special treatment based of the study of the stream hydraulics. However, the main drainage channel of Enkulal community watershed has a meandering feature, and since this part of the watershed has not been in the assignment and because it is also the main drainage channel, it has not been treated in this work. The technical guide and advice presented here could be of assistance
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8
Water Rcsourccs Dcvdopmcn( Pn)jcc(
to  the  technicians  working  in  the  project  area  to  treat  it  as  presented  here.  Hence  it  is presented here as a reference and a guide for action.
A land manager should be aware and understand the general concepts of the [hydraulics] behavior of the stream. When stream flow is disturbed or a change course is introduced on its 
course,     it     will     result"    in     ch‘ anges    in
velocity which could either make is
aggressive/destructive/erosive or slow its flow resulting in sediment deposition. It is  important to be aware of how management actions affect stream flows and behavior, since such actions  can influence streams characteristics such as velocity of flow, sediment load, etc.
For example, in a similar assignment that was made in a gully in a small community watershed in Tikur Woha, known as Alekte Wonze, the meandering section of the gully has been trained (straightened) as a measure to threat
the expanding gully  at the meandering
Point bar or deposits of sediment
at low velocity area at the bend
section. It is important to understand the Figure 6.14 A meandering river in Dera Woreda behavior  of  the  stream  flow  before  intervening.  What  happens  in  Alket  wonze  is  that straightening increases the velocity of flow in the gully and makes the stream flow aggressive and gully erosion becomes very active and erosive along the gully banks and scour the bed of the  channel  increasing  the  sediment  load  of  the  stream  flow.  Such  kinds  of  activities  are inappropriate for gully treatment. Such activities are done usually for harvesting more water for downstream use which otherwise would have been lost in the meandering channel.
Thru ic-ni
Section a-a above is where the
cross section is uniform, where
the high and low stage thalweg
intersect. While in b-b the
outer section is erosive and the
floor is deep. At b-b both
thalweg at maximum distance
apart.
Figure 6.15 Characteristics of a meandering gully at the different cross sections
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A meandering gully is one that consists of alternating bends, giving an S shape  PP
a
eara^c^!
shown in photo figure 6.14 and figure 6.15. It is a common phenomenon to obseive and identify in small community watershed where its natural drainage channel is ep em . most community watersheds, the main section of the natural drainage channe mean some point. A meandering gully has erosive [degrading] side and depositing [aggra in9J $ shown in photo above, and these behaviors of a stream flow have to be identified care y marked.
After the erosive bank of the gully is marked, gully reshaping needs to be done to line the surface with stone rip rap. The eroding section of the gully bank should be treated with single line stone rip rap of 25 to 30 cm thickness as shown in figure 6.16.
Streams are dynamic systems that are prone to change even without human interference     (Heede     1975).     For
Bank protection
, using rocks against bank erosion
Sediment is---------     r *        o? depositing in
this side of the
gully and does
not need
protection
?(
Figure 6.16 Menderinq qully section under stone rip rap
instance several research findings  have suggested that vegetation cover manipulation on a watershed can increase water yield (Hibbert 1971), or streamside forest removal can activate bed load transport (Heede 1977) and training [straightening] a river or gully  increases  water downstream. These research experiences suggest that understanding the gully stream behavior is essential for its management, land and water resources in a watershed system. The sketch shown in figure 6.14 shows which section of the meandering gully is to be treated. The wall of the gully at the erosive side has to be lined with rocks to protect the bank of the gully from erosion.
6.3.S.5           Check dam Construction
(i)       Locating the check dam construction site
The location of a check dam is determined primarily from the required spacing of the structures. Requirements  for spacing depend on the gradient of the sediment deposits  expected to accumulate above the check dams, the effective height of the dam (h), the available fund, and the objective of the gully treatment (Heed, 1966).
The approach followed in this study in locating the check dam sites is based on the peak flood estimation at the mouth (lower part) of each model gully, and the work starts here and moves upstream. For instance the effective height of the check dam (hl) at the mouth of the gully is identified as peg #1, and the crest height (hl) is added to the elevation reading at the floor of the gully at Peg #1, and this new elevation is identified at the upstream areas along the qully and it will be the next point (peg #2). The crest height (h2) is  added on to the elevation
ra
third (peg #3) is identified and this process continues up to the gully head point. Elevation
! ?/9 °n
P ?n3 (P<19 #2) and the new elevation readin9 is Seated upstream and a
readings are taken using GPS along with the readings of the gully geometry top width CTWj floor width (BW), and gully depth (D) at certain distance intervals during the field work as part of the characterization of the gully,y
as part
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It  should  be  noted  here  that  at  certain  sites  (i)  the  desired  elevation  readings  may  not  fa exactly as it is recorded by the GPS, and (ii) the GPS readings have a precision level of 2 to 3 meters. The use of GPS is more efficient but there is a compromise in the precision compared to  the  use  of  engineer  level  or  total  station  to  undertake  a  differential  leveling  wo  ,  a  mos common approach for profile elevation readings. As a result field technicians may have to ma e field test and improve the sitting of the check dams during the project implementation.
The second method for determining the spacing (S) of the check dams is the method most used by  the soil conservation technicians  of the Ministry  of Agriculture (MoA). This  method is proposed by the participants of the workshop and it is based on an empirical relationship or formula as  shown in equation (3) below. Reference is  made to the community  watershed guideline prepared by the MoA (2005). Unlike the consultant method discussed earlier, locating the check dams start at the head of the gully and moves down stream to the mouth of the gully. The reason given for starting at head of the gully is that the structure will be safe if rain occurs before finishing all check dams in the gully.
The conventional formula is presented as follows.
S=
1 2 x Effective height of the check dam
Gully floor gradient in Decimal number
Equation (3)
Where, S is spacing between successive check dams in meters
It is very important to note that under both approach the effective height of the spill way crest (h) has  to be determined based on the estimated peak  flood. This  part of the task  in this assignment is the most challenging task and it has always been a difficult one particularly for a practicing engineer and most agricultural extension workers in Ethiopia.
Both methods have been exercised in this study and the second method is difficult since it is possible that the gully floor changes rapidly, estimating the peak flow at that specific point is also difficult unless the crest height is in priory set fixed and using such approach will locate the check dams very close and makes the project expensive.
However, the technicians are presented with the choice here for selecting whichever approach is sound and appropriate for their situation. The sitting of the check dam presented in this study is based on the first approach. If the spacing seem far apart, it is possible to put check dams where there exists a gap at a later time.
Summarized note on the sitting of the check dams at the gully, maximum height and apron.
• Start the sitting of the check dam at the mouth of the gully (at downstream end), It is important to note that sitting as per the specification [given in tables 6.2 to 6.121 may difficult since the initial field data and the data on the Atlas maps are GPS and GIS  based,  respectively,  is  different.  However,  while  locating  it  is  important  to  accept the use of ± 1 or 2 meters up and down from the specified peg #. To avoid serious difficulties is to use the distance set in the table and the distance set on the Atlas,
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■    All specifications made have a maximum spillway crest height of 4.00 meter, except for Biret Afer. The design height of the check dam at the mouth of the Biret Afer Model Gully is more than 4 meters Gabion Check dam (it is a small dam with Gabion) with a three meter and two meter height double line gabion check dam placed downstream to support the largest check dam and provide stability.
• Apron should have 1.5 times the height of the spillway crest unless the gradient is more that 15% which otherwise may require 1.75 times.
■   The strengthened Gabion should have a full length of the apron (1 meter in both sides).
(ii)      Construction Specifications
Check dam construction starts by marking the dimensions of the foundation at the floor of the gully. The standard dimensions for the depth of the foundation are 60 cm for vertisol and 50 cm for deep clay soil (Nitosol). The width of the key (the depth the check dam should be allowed to protrude into the wall of the gully in both sides) is 1 to 1.20 meters for vertisol and 0.70 meter for Nitosol. The width of all rock check dams is one meter and the length vary depending on gully width. The total length of each check dam includes the length of the key. The foundation is marked and pegged and after that excavation starts to the required depth, and the bottom 15 to 20 cm depth is filled with rock aggregates to prepare for laying the gabion wire cage. The gabion check dam is erected over the rock aggregate fill.
The check dam should rest perpendicular to the direction of the stream flow. In the event of any deviation, a pool of water is created with a pressure in one side of the gully which has a potential for causing erosion of the bank as shown in figure 6.17 below.
The photo is taken in summer 2013 at road site about a km near Enkulal Model Gully, and it is  a typical situation for most [gabion] check dam failures in the project area. The structure left standing in the middle while the stream finds its new way at the newly eroded channel at one side of the gully. It is  important to note that rock  size distribution in the construction of the key section of the check  dam is  important. Too much voids  in the key  area equally increases the velocity of flow and this could lead to wash out of the soil from the sides of the gully (bank).
In  the  use  of  prefabricated  gabion wire cages filled with loose rock, the
Figure  6.17  A  poorly  designed  and  constructed  check dam
hT PlaC<? °rde^ maintainin9 ^e vertical height and angles and this structure should extend to the designed key and height dimensions.
structure
The height of each gabion is either half or one meter and the length varies between two widely used standard srzes; 2 and 4 meters. Therefore, gabion rapped rack check dams will have” size
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of 2 meters long by 1 meters height and by 1 meter width, and it Is the most widely used size of gabion rapped rock check dam in the country. It is also recommended to use 4 meter long gabion check dam (Hudson, 1989) if the width of the gully is larger - say more that 10 meters - a large gully. To obtain the dam shape, individual cages of gabion wire with the above standard sizes are filled with loose rock and placed beside and on to each other. Normally this dam is more esthetically pleasing, but it is more costly than loose rock check dam.
The upstream and downstream part of the check  dams  (the gully  floor and banks) require protective surfaces  such as  stone rip rapped apron and gully  banks. Standard design specifications (discussed earlier) are used in the design of these structures.
Selecting or choosing the right time to do the gully treatment work is a key factor to the gully stabilization. The ideal practice in check dam construction is that it is planned during the wet season and construction is  done and completed during the dry  period only. The study  on characterizing of the gully is done during the rainy season, while the actual construction work of gully treatment should start between Oct to January - during the dry period.
(iii)    Construction materials and labor
The most widely available material for check dam construction in all sites is stone collected from the nearby farms or rocks from a quarry. In all model gully sites, there are potential quarry sites where rocks could be mined and collected. The rocks from quarries are angular and preferred construction material. While weathered and round stones are also abundant in the farm plots around each model gully, and selection and use of these stones require care.
Material and labor force for check dam construction would be provided at the dam sites by the adjacent communities and most preferred are riparian farmers. These communities are willing to provide manpower for labor and avail themselves for training. It should be noted that the riparian farmers have a stake and they can be effective in providing sustainable maintenance to the structures.
All of the model community watersheds are accessible by vehicle, and transportation of construction materials to drop them along the roads is possible. However the sites where construction is to take place require travel by foot or animal. Communities can offer service to drop stones, gabion and other construction materials and equipments. From there on animals 
and humans may have to be used to transport construction materials and equipments to the check dam sites (peg #). The distance from the closest and farthest sites from the access road
is in the range between half to one and half km.
6.3.6 Gully control using biological measures
After gully reshaping and check dams are constructed in their appropriate locations, vegetative planting are essential wherever it is possible in order to stabilize the engineering structures and the environ surrounding the gully banks.
There are two reasons for the use of plants in degraded lands such as gully. First properly established plants  provide the soil the protection against direct raindrop that further exacerbates soil loss through erosion. The plant material, when used as a barrier against the
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direct flow of surface runoff or stream flow in the gully channel, reduced the veloaty o e
runoff and stream flow allowing silt in suspension to fall or deposit along the ground surface or
gully channel. Secondly plants established in the gully coupled with the engineering structures provide stability and at the same time provide economic and environmental benefit throug
crop and biomass production.
Well established grasses  and shrubs  are good obstruction structures  against the veloaty  of runoff water and can become an effective trap of silt. Generally the assessment of the practice suggest that all vegetation barriers  established as  a biological treatment perform well in controlling, rehabilitating and restoring eroded or degraded areas such as gullies. For small size and early  gully  development, and in head cut areas  in gullies, planting suitable trees  and grasses  along affected areas  is  a common practice. However, integrating vegetation with engineering structures  are the most proposed measures  for the treatment of big and large gullies.
The selection of suitable plant materials  and the use of special planting techniques  are discussed below based on the experience of local technicians in the study areas are discussed. Plan materials  which have proven record both in Ethiopia and outside are presented and discussed in this report for use in the treatment of large gullies in the study areas.
6.3.6.1   Plant materials and planting techniques for gully control
The use of vegetative material in gully  control and rehabilitation offers  an inexpensive and permanent protection. Vegetation will protect the gully  floor and banks  from scouring. Certain plant materials are more suitable for a particular situations; climate type, soil type, etc. A book by  Azene Bekele et al (1984) provides  a check  list of the plant species  suitable and well identified and recognized local plant materials for different agro-ecologies in Ethiopia. The book also provides the method of planting and management for its establishment. Using such guides, the vegetative material can be established in a gully  by  natural/existing or use of new and adapted planting materials.
6.3.6.2   Selection of suitable species
For Farta, Estie and Sekela Wereda: Acacia decurrens, Acacia saligna, Eucalyptus  globulus, Gravillea robusta, Accasia melanoxilon, Acacia abyssinica, Acacia angustissima .Spanish reed,' elderberry, poplar, and willow, Tephrosia vogelli Croton macrostachyus  are considered adaptable and useable for gully treatment.
Forage Species. Tree luceren, Teline spp, Setaria grass, elephant grass,green gold, bana qrass Phalaris aquatic, kikuyu and crown vetch are recommended species.
For Dera and Mecha Wereda: Tree species such as Acacia saligna, Eucalyptus cameldulensis, Spanish reed, elderberry, poplar, and willow Croton macrostachyus and Gravillea robusta Acacia abyssinica and tree luccen are recommended. Among forage species: Piqeon oeas' leucaenea leucocephala and Sesbania spp, kikuyu are proposed for use. Rhodes grass, Guinea grass, Elephant grass, bana grass, green gold grass and Siratro are appropriate species for usT
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Plant spacing techniques
Spacing varies  in the strip and in the dispersed tree planting patterns. Although spacing is determined by  a number of factors  such as  the availability  of enough moisture, type of tree species and community needs are the most important factors in deciding tree spacing for the rehabilitation of degraded gully lands. When the need for grasses for forage use is very high, wider tree spacing is  preferred to allow free growth of grasses  and avoid negative shading effects to harvest more per unit area. Therefore, where dispersed tree planting pattern is used and the whole gully  area is  planted with trees, minimum tree population is  needed. Woreda Agriculture office and agro-forestry  experts  should be consulted on the characteristics  of the plant species and their cultural practices before deciding on the use of these suggested planting techniques.
The introduction of improved legume trees/shrubs  or herbaceous  legumes  that have rizobium improves the chemical and physical conditions of the soil. These species fix nitrogen and add nutrient to the soil allowing more plants and grasses to establish in the gully sites and off sites. Accordingly, the quality and quantity of grasses from closed gully areas considerably increases because of the improved soil conditions. But, still the level of grass production depends on the natural or planted tree/shrub population. In the early  part of the gully  treatment works, right after the gully bank reshaping work, teff (Erogratis tef) is more appropriate if sown properly for three reasons (i) it is a grass species itself having a tiny seed (size of a pin head) that can easily establish itself on the wall of the gully banks, and it is a well adapted plant species to all study areas, (ii) it can establish itself with little moisture and grow faster and protect the surface from soil erosion during the first season, and (iii) when it dries  and fall on the gully  surface it increase the organic matter content of the soil from the biomass it leaves and allow other grass species to establish.
6.3.6.3  Planting along the different gully sections
(a)        Gully Head
The most commonly accepted measures for this spot are physical structures, such as paving with loss stone, diverting water using cutoff drains and reshaping.
• Nevertheless integrated treatment with biological measures will also help in stabilizing the
gully head.
• Some creeping plant species can be used for reinforcing the structures constructed/serdo in Amharic/ and kukuy grass
(b)       Gully offset
• Micro basin construction, trench and sub soiling are recommended for better performances of crops planted in the area
’   ?e/,lant     SPede^   recommended     for     the   treatment   of   this   area   are   those   with moderate tolerance to dryness and wetness.
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(c)       Gully sidewall
•  Reshaping and hence constructing moisture harvesting structures are the recommended measures to treat gully sidewalls
• As far as farmers/land users are convinced to undertake reshaping, the gully onsets can be converted into potential areas for multiple purposes
• Biological measures can play a pivotal role in rehabilitating this section of the gully
• The species to be selected should have invading characteristics, with light foliage and steam biomass and high tolerance to drought
(d)      Gully bed/floor
•  The biological material recommended for this part of the gully should be tolerant to
water logging, with high root biomass and, resistant to soil sedimentation and high flow
of water
• A lot of biological material which can fit to this condition can be found at the local condition in consultation with the farmers.
(e)       Reinforced bundling (or wattle)
Bundling or wattle is a technique where fresh stems of plants (e.g. elephant and bana grass, green gold, Spanish reed, elderberry, poplar, and willow) are bound together, then horizontally planted (across the gully bed or along the sidewall), and covered by soil. To economize planting material the bundles  (also wattles) may  be supplemented with other bulky  organic  matter (filling material) such as dry grass or straw, or the dry stems of plants such as Acacia saligna, Sesbania, etc.
Reinforced bundling requires that mature poplar or willow stems or truncheons (branches), 70 cm in length, are inserted 20 cm deep next to each other across the gully floor in two rows, spaced 50cm apart. Downstream buttressing (support) should be provided for stability. The space between the two rows  of truncheons  should then be filled with bundles  consisting of approximately  30% vegetative (fresh) bundling material, and 70% filling material. To prevent the drying out of the bundling material, it should be lightly covered with moist soil.
As  soon as  the poplar or willow stems  have exhibited adequate vertical growth, the space between the stems can again be filled with bundling material. There will thus be an incremental growth in the height of the structure, which will in turn add to the level and amount of siltation. This technique can only be applied when the rainy season has commenced. Through time the bundles will grow and serve as live check-dam. Bundles put across the gully bed and supported with popular and willow stems
(f)       Layering of vegetative material
Layering is the horizontal planting of fresh stems of plants (e.g. elephant and bana grass, green gold, Spanish reed elderberry, poplar, willow) across the gully floor and or reshaped sidewall or  at  the  base  of  gully  walls.  This  technique  is  applied  when  a  satisfactory  level  of
growth initiated
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Sedimentation occurs and increases as the shoot growth forms a dense barrier that disturbs and breaks the velocity of the water. This leads to a gradual build-up of the gully floor. Planting of elephant grass by layering to rehabilitate gully bed
(g) Gully bed plantation
The planting of water-loving or moist tolerant trees, shrubs and grasses such as Paraserianthes lophantha, Salix spp., Acacia meianoxyion, Phaiaris aquatica, and Pennisetum ciandestinum, Pennisetum riparium, Pennisetum purpureum and green gold grass on the gully bed, adequately spaced, breaks the flow and velocity of water run-off, traps the sediment, and protects the gully bed from erosion. Biomass production already during the second rainy season will be
astonishing.
Gully bed and sidewall planted with various tree, bush/shrub and grass species
(h)    Retaining wall with bamboo-mat
In gully rehabilitation scheme, the difficult part is to control the lateral flows which are coming from farm fields, footpaths, degraded grazing areas and other miscellaneous land use types. To protect the lateral flows  and hence mass  movement and soil sliding/melting from fragile sidewalls of a gully, retaining walls made out of reed/bamboo mat can be installed along the foot of the sidewall. The mat can be strengthened on the lower side by wooden sticks, possibly using vegetative propagating species  like popular, willoo and indigenous  species. Truncheons/branches  of various  species  can be layered on the upper part of the mat. This technology  facilitates  self reshaping of gully  and hence rehabilitation. "Mini" bundles, with or without pegging, are also employed in small soil and moisture pockets. Arundo donax  and Hyparrhenia stems  consisting of three to four nodes  each proved particularly  useful for this purpose. Retaining, bundling and pegging commence with the onset of the rainy season..
(e)       Planting of trees, shrubs and grasses on gully sidewalls
The long-term stabilization of gully sidewalls also requires the establishment of woody perennial plant. Multipurpose trees  like Dodonea angustifoiia, Acacia saiigna, and Teiine species  are recommended for treating reshaped gully  sidewalls. Excellent results  have been achieved by planting Bana Grass and Green Gold Grass in rows across the gully slope. Planting of kikuyu around the lower sidewall and crown vetch even on steep side walls have shown interesting results.
(f)        Direct sowing or broadcasting
Direct sowing (broadcasting) on gully beds, and into cracks on sidewalls during the rainy season results in an almost immediate cover of these fragile areas. Seeds of Sesbania sesban, Accacia saiigna, Pegeon pea, common vetch and other can be mixed together and sown on different sections of the gully. To assure an economical use, and to avoid over-planting, seeds should be mixed with dry sand during sowing.
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Successes in protecting gully sidewall
Gully can be aggravated due to the lateral flows coming from cultivated fields located left and right of the fragile gully sidewall. Therefore in the beginning agreement has to made with the respective land owners to manage the excess flood coming from their plots either by diverting to the nearby  natural streams  or by  reducing the speed of flow via constructing bunds  and planting vetiver and other vegetative materials along the contour of the farm fields. Then after the actual gully has been treated by installing wire gabions to control the head; constructing simple check-dams and preventing soil mass movement from the gully embankment area can be done by  fixing bamboo/reed mats  along the lower part of the sidewall. The physical structures  have been integrated with biological measures  by  planting various  species  like popular, Spanish red, Green gold, Bana grass, Elephant grass, Kikuyu, etc. on appropriate locations of the gully.
(g)     Off-Set Plantations
The gully offset is the area that extends from the top edge of the gully wall up to five meters away.
Adequate stabilization is  inevitable to prevent the sideway  extension of the gully and further encroachment of arable land. Generally, gully  offsets  are moisture deficient. Thus  drought tolerant multi-purpose species of trees, shrubs, grasses, and fodder legumes are suggested for stabilizing this fragile area. Recommended species include: Medicago sativa, Teline canadensis, Acacia saiigna, Acacia abyssinica, Acacia angustissima, , Grevillia robusta, Sesbania sesban, Lupinus arboreus, Tephrosia vogelli, ieucaena leucocephala
6.3.6.4           Reforestation/re-vegetation of the gully
Reforestation/Revegetation is  one of the essential measures  to rehabilitate gully  lands  and ensure its stability. However, success of the activities largely depends on the quality of works employed to harness the physical and chemical conditions of the soil.
Species selection
Quality  of the species  is  determined by  their merits  to stabilize and protect the degraded ecology, and by  their efficacy  to meet community  needs. The restoration of productivity increases the carrying capacity of the land, and it simultaneously helps to address community needs of fuel wood and fodder production.
Therefore, appropriate species selection should be based on ecological and economic needs. To meet these requirements, community needs and ecological demands should be critically assessed. Information most commonly required in relation to environmental conditions includes climate (mainly rainfall amount and distribution), soil (depth, texture and structure) and altitude or elevation.
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Species composition
Mono-plantation is not technically sound for the rehabilitation of degraded gullies because it
may not guarantee effective control of soil erosion. Trees in pure plantation do no a ow
formation of under story/multi-story canopy structure and hence expose the groun °
erosion. Therefore, to ensure effective control of soil erosion a mixture of tree speaes, w i
can form multistory, should be established.
Moreover, multiple species do exploit more physical resources (nutrients, moisture, solar
energy, etc) than pure plantation with ultimate benefits of higher biomass production. ix 
plantation also can address more comprehensively community needs of fuel and construction wood, fodder, medicine, food, etc. Besides, different tree or shrub speaes are not efiua V
affected by adverse environmental factors including drought, disease and pests, and hence mixed plantation avoids risk of total failure (loss).
Critical factors to consider while planting the gully section after check dams are
constructed
The selection of plant species should be done with the support of local agriculture office. It is not only  the type of species  but time of planting also influences  the success  in establishing vegetative cover along the banks  and floor of the gullies. This  is  directly  related to the availability  of moisture and the amount of sediment deposited at the different sections of the gully. As  a result, the general recommendation is  to undertake planting on the offset and sidewall immediately  when the rain starts. But the gully  bed should be planted when the seasonal stream flow reduces  and the main rainy  season ceases. Otherwise, the planted seedlings and seeds sown will be easily washed away by the running water in the gully.
Having the aforementioned suggestions in mind, the following biological measures can be taken as potential alternatives for gully rehabilitation. They can be combined with physical measures to avoid wash away and or be used alone for gully rehabilitation depending on the amount of peak discharge coming from the upper catchment and nature of the gully.
Vegetative Management and Utilization
Rehabilitated gully  areas  should be carefully  managed to guarantee their stability  and productivity  on a sustainable basis. Due to the extreme reduction of soil depth and poor physical and chemical properties  of the soil in the gully, situations  are fragile and can easily return to their original condition or even get worse. Conservationists  and farmers  should properly assess the soil and moisture conditions in the gully head, gully floor/bed, gully sidewall and gully  offset/gully  buffer zone. Practically  speaking, these different locations  of a gully  do have different soil and hydrological characteristics  which determines  the type of species  of grass, shrubs/bushes and trees to be planted.
The   following   are   well   recognized   management   strategies,   recommended   practices   and techniques to keep gullies and their environment stable and productive:
(i)    The gully areas and it's environ should be free from animals and human interference and it should be put under control.
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(ii)     Maintaining good soil moisture through soil moisture conservation measures is essential to cope with moisture loss through evaporation and to optimize plant material esta is men . If   possible   mulching   should   be   considered   as   an   important   practice   an   app accordingly  especially  on  gully  sides  and  gully  head  planted  seedlings.  Thus,  part  o  e grasses  harvested  the  gully  areas  should  be  returned  and  applied  to  the  ground  area surrounding the seedlings as much around the tree seedlings planed within a radius of 50 cm  for  at  least  the  first  1  to  2  years.  The  grasses  in  between  trees  can  be  clearly harvested  after  every rainy season  providing  opportunity of  both pasture  production  and control of fire hazards.
(iii)    Judicious  utilization  of  the  products  could  be  achieved  through  continues  soil  and  plant manipulation.   Plant   population   density   should   be   maintained   at   optimum   level   by replanting when some species are missing or harvested.
(iv)    Fertility of the soil can be maintained by manipulating soil-conserving plants in the gully area and by avoiding over utilization of the biomass. Harvesting of the products should be carried  out  systematically  to  avoid  damage  to  the  ecosystem.  When  the  trees  reach optimum  stage  of  production  they  can  be  harvested  and  used  by  the  beneficiaries. Technical people determine the optimum stage of harvest of trees/ shrubs grown for fuel wood or construction purposes. Fodder trees can be harvested at any time of the year when  they  following  moisture  content  of  the  soil  and  produce  enough  biomass  for livestock  feed,  except  during  first  year  of  establishment  when  cutting  is  not  allowed  to ensure  better  establishment  and  survival.  However,  some  leaves  should  be  left  on  the plants   guarantee   continuity   of   photosynthesis   when   harvesting   the   biomass.   Clear harvesting the leaves and too frequent harvesting may cause die back of the plants.
(v)     Harvesting  and  grazing  methods:  -  conservation  sound  cutting  techniques  should  be employed to avoid erosion hazards. For instance, clear cutting could be dangerous to the ecology. Therefore, rotational, strip, patch or selective cutting can be used instead of clear cutting.  Grasses  can  be  harvested  at  any  time  of  the  year  provided  there  is  adequate biomass  for  harvest.  The  optimum  time  of  harvesting  grasses  is  at  heading,  before flowering, when the balance of the biomass and nutritive value are at optimum level. Zero grazing  is  highly  recommended  to  avoid  overgrazing,  if  grazing  is  allowed  during  wet season, damage to forage grasses and legumes could be very high and productivity of the pasture may be impaired.
(vi)    A  gully  will  re-vegetate  naturally  if  the  water  causing  erosion  is  conserved  or  diverted before it reaches the gully, and if livestock are kept away. Costs are minimal but recovery will be slow if the soil is poor. Furthermore, if the gully sides are steep, vegetation mav not establish itself.
(vii)   Where establishment of natural vegetation is too slow to cope with the erosion or where a particular  species  is  desired,  planting  should  be  done.  The  establishment  of  vegetation either  naturally  or  artificially  has  to  contend  with  a  hostile  environment.  The  type  of planting material to be used should be seriously considered based on the specific situation of the gully.
6.4. Bill of Quantities for Rock Check Dams
Bill of quantities (BQ) is prepared based on the design dimensions of each in each model gully and a separate report is produced.
check dam  planned
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I SECTION VI: PROPOSED DESIGN FOR H5
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Excel is used for determining the volume of rock required for each check dam in gully. A procedure followed and a summary of the BQ is presented in table Annex I.
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SECTION VII: ENVIRONMENTAL EVALUATION
This section is not an environmental impact assessment (EIA) for the proposed project (gully treatment works) since design study for treatment of gullies itself is considered as a mitiga on measure against gully erosion. However, this environmental assessment or evaluation presents the potential environmental consideration that needs  to be taken along side with the gu y treatment works in each model gully.
7.1     Physical impacts as a result of check dam construction
The likely  effects  of the check  dam constructions  are both site specific  and the areas downstream (to be discussed below). Excavation of the foundation for each check dam is up to 60 cm depth across the gully, and such excavation works are done on not more than 25 sites at most within the gully considered for this study. The nature of the construction is temporary and the potential negative impacts  could be also temporary  until the gully  control and treatment work is fully developed.
7.1.1   Quarry for rock production
In certain sites, the absence of stones close to the site may demand the establishment of a new quarry for the production of rocks needed for the construction of design check dams. Hence, quarries may leave damage to the landscape and to natural vegetation at the site level and also increases soil erosion and sediment pollution to the downstream.
7.1.2   Effects on the Flora and Fauna
The project recommended a minimal use of wooden check dams and this is allowed if wooden poles can be spared for such services. To a large extent rock/stone check dams are of no significant impact envisaged on deforestation of the area. It is also believed that the use of plants as live check dams [biological measures] along with the rock check dams across the gully section do not impose any enveloping or invasive impact to the indigenous vegetation that exist in the study area. Most species selected for this purpose are local and well adapted to each locality.
The plant materials proposed as biological measures are local species that are very well identified, tested and studied by the local agro foresters and researchers. The selection of these species for this task is made based of the available species recommended by the local experts29
It should be noted that such selection of plant species can be evaluated as the performance of
the check dams are assessed each season. Maintenance of both the plan and rock check dams is a regular function of the woreda agriculture office staffs, and adjustments could be made as needed.
7.1.3  The effects on archeological and historical sites
There are no archeological or historical sites impacted by the excavation work and construction of check dams in the study areas.
”These plant speciesarescreened in consultation with Ato Derese, the Tana Beles Advisor on SWC
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7.1.4    The effects on the Geology
There are no anticipated impacts on the geology of the gully watershed as a result of the check dam construction.
7.1.5   Effects of rock check dam construction
The  impacts  for  the  proposed  check  dams  are  largely  positive,  and  here  are  some  of  these positive impacts.
•     Check  dams  are  traps  for  silts  that  are  transported  in  ephemeral  gullies  and  thereby reduce   potential   damage   by   sedimentation   (i)   to   the   low   laying   and   productive agricultural  lands,  (ii)  Water  resources  infrastructure  such  as  water  supply  pipes, irrigation canals, dams and reservoirs, and (iii) Aquatic ecosystems such as lakes, rivers and their banks and flood plains, wet lands areas.
•     It increases agricultural productivity through the provision of additional land for crop or fodder production,
•    It helps controls flood water down stream
•     As a result of the check dams constructed, there is better environment and increased productivity on the land around the treated gully site, there will be improved social well being,
7.1.6   Water quality
The water quality downstream is impacted as a result of the check dams and plantation of the gullies. Suspended solids are trapped at the upstream side of the check dams, and this allows silts  to settle at site level where check  dams  are constructed. As  a result water resources infrastructures  (reservoirs, irrigation canals  and water supply  facilities) downstream gets relatively less sediment load, siltation and improved water quality.
7.1.7   Ground water potential
The construction of check dams and plantations along the gully banks increases infiltration into the soil and promotes ground water recharge from the water impounded behind by a series of rock check dam constructed along the gully section.
7.2     Socio Economic Improvement to the community
7.2.1 Economic Impact
•     The  gully  treatment  project  provides  employment  opportunities  and  increase  cash earnings for the local communities,
"  Increases  agricultural  productivity  which  in  turn  increases  the  income  of  the  riparian
households 
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■   Avails more land for grazing and can even be used for crop production and forage
species for the livestock In communal holdings,
■   Provide additional harvest of biomass for animal feed, fuel wood and construction
material from the plantation
7.2.2   Health
Since all the five model gullies are located outside the range of malaria prevalence, there Is very little negative impact from the construction of check dams in the study area.
7.2.3  Provide access
The construction of check dams particularly in medium and large gullies provide access for the communities to cross the gully and also transport farm equipments and agricultural produces, access to market place, etc.
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SECTION VIII: CONCLUSION AND RECOMMENDATIONS
8.1 Conclusion
Gullies  in all small community  watersheds  in general are major source of sediment and the sediment from these gullies  is  an important pollutant to the aquatic  ecosystems  and a rea o the water resources  infrastructures  downstream; reservoirs, dams  and irrigation cana s. e total estimated soils removed from all the five model gullies studied is over 400,000 m and it is quite significant and a big concern for environmentalist, land managers and engineers, planners and decision makers who recognizes the severity of the damage to the land resources and the impacts in the downstream areas. This study, although conducted in few sites, clearly indicates the magnitude and severity of gully erosion in the country at large.
The level of accuracy required for peak flood estimation for each model gully, is dependent on the site specific  conditions  and characteristics  of the watershed. Therefore, it is  important to understand not only what each design flow represents, but also the underlying assumptions and the limitations of the techniques used to estimate the flow.
The selected method employed to estimate the design flood in each model gully in this study is the adapted Cook's Method, and the verification, although limited in scale, showed adequacy of the proposed method and techniques. It is selected for the following reasons:
•    It is based on the characteristics of the gully watershed,
•    GIS can be used to generate essential data based on limited field data collection,
•    It can be done by extension workers and practicing engineers in a reasonably short
period
The GIS technique employed in this study proved to have a useful service in the design and construction of gully treatment works. It provided data and information such as land use of the gully watershed, longest drainage length for the runoff, gully length and area, etc. Therefore, GIS techniques  has  proved to be a useful tool in generating data for gully  design which otherwise would have required substantial cost and time for employing field workers to collect very massive data and information to do this study at this scale.
The design dimensions for check dams are developed using a a broad crested weir equation (equation 3). In order to facilitate the computation of the dimensions, excel spreadsheet is used quickly, efficiently  and adequately. The use of excel model and the GIS is  of significant importance in developing design for gully control check dams in a very short time The spacing between the check dams in meters and the corresponding location x and y of GPS readings are given to facilitate the sitting and construction work by the project owners. Locating the sites for each check dam based of the GPS readings has experienced difficulty in exactly locating it on the atlas  because of variations  in the precision of the GPS readings  taken at field level.
^T^Or!JhrlNAPVietnfi9iSnn^GISfreadl^9S         are/elated         bY      the
meters (see table 6.1 to 6.11). Therefore, the methodology that is developed in this study for
spacing      (distances)      given
in
the design of [physical] gul y  control measures  allows  us  to develop several hundredths  of
gullies in one budget year. It has avoided carrying engineer's level over several km of gully 
length, guess work on the estimate of peak flood, etc.
yY
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Based on the advices made at verification workshop, maximum design crest height (h) is limited to 4 meters. For depths  much higher than 4 meters, it Is  suggested that further incremental heightening of the crest of the check dam could be made as it appears imperative at a la er time. For check dams less than 2 meters, loose rock check dam is recommended. Gabion use s recommended for crest height of more than 2 meters. In gullies  with unstable soils  such as vertisol, the gabion check  dam is  to be supported by  another gabion check  dams  placed downstream at both sides of the gully bank to retain the wall from collapsing and support the stability of the check dam that is placed across the gully.
As indicated in the design principles, gully control has to be envisioned as a system; all gullies in a community  watershed treated together at the same time. The attempt to treat only  one gully section in a watershed where there is net work of gullies is of little success in terms of achieving the goal for gully treatment. As assigned in this consultancy assignment, one gully in a community watershed cannot be singled out for treatment and the rest of the gully network neglected. Research (Heede, 1977) has demonstrated that there is a strong relationship among the gullies in a network in a given community watershed. Yet, it should be very well recognized that the individual hydraulic  [check  dam] structure installed along the gully  section represents only  one component of the treatment system; and the quality  of the treatment as  a whole determines  the success  or failure. Furthermore, this  study  also recognizes  the need for a combination of the vegetative and engineering measures that can promote effective restoration of a deteriorated and damaged watershed.
The environmental impact of such activities is enormous and the social and economic impacts are substantial, although the study focus is not on these areas. However in the last section a list of environment, social and economic  impacts  of the project when implemented in briefly presented.
8.2     Recommendations
(i) The need for assessment and evaluation of gully erosion
Gully erosion has now reached to a level of being a major threat to the agricultural lands which directly  affects  the agricultural productivity  and GDP of the country. There is  qeneral understanding now that gully erosion has incurred a huge damage into the national economy through impacting the agricultural productivity of the country. The estimated soil loss from the five model gu lies <s viv.d ev.dence of the seriousness of these damages and it is important to
take gully control work as a priority. To this end,
National level assessment:
First the country needs to have a national survey of gullies (it or ecological zones) and assess the level of damage attributed and this would allow setting priorities for intervention
important to
can  be  disaggregated  by  regions under each agro ecological zones
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Analysis of gully network at local level:
Before designing a gully  treatment and restoration work, the network  of gullies  in the whole community  watershed must be analyzed for the following reasons: (i) to understan t e relationships among each gully in the network within the whole watershed and (ii) to determine the gully types.
(ii) The use of continuous gully as a basic treatment unit
Gully control is one of the most important restoration methods used in watershed management, and timing is  an essential element. The field work  in all structural and vegetative control measures selected should be completed during the dry and early rainy season. This is the most important aspect of gully  control - especially  in tropical and subtropical countries. Otherwise, the incomplete structural work can easily be destroyed during the first rainy season. In addition, vegetative measures such as the planting of tree seedlings and shrub and grass cuttings cannot begin until structural work  is  completed. Each continuous  gully  in a gully  system should be regarded as a basic treatment unit, and all the control measures in that unit should be finished before the rainy season.
(iv) The implementation of vegetative and engineering measures combined
In all the model gully sites, it is recommended that both the biological and physical measures are designed in an integrated fashion. Rock check dam has to be integrated with tree planting. Based  on  local  experiences,  some  of  the  species  recommended  are  elephant  grass,  Vetiver grass  (vetivaria  zizanoides),  switch  grass  (panicucium  virgatum),  teff  (Erograstis,  teff),  etc. These grass species are reported to have both ecological and economic merits. It retains soil moisture, enables water storing, prevents soil erosion and helps in mitigating climate change. At the same time, it can be used as fodder, hand craft/weaving, pest control, and house roofing. It is deep rooted and therefore not competitive with crops. Being dense, it becomes an effective barrier  of  siltation.  Being  fast  growing,  it  can  fortify  terraces  without  further  intervention.  For Megenagna model gully specifically, tree and grass species that have higher use of soil water. The selection of the tree species need to consider economic importance of the plant as well. Woreda  Forestry  experts  at  woreda  level  may  have  to  be  consulted  in  selecting  types  most adapted  and  developing  the  cultural  practices  needed  for  the  management  of  these  plant species and in providing additional planting materials for maintenance over the project period.
In some tributary gullies, it is possible to control by vegetation management alone if there base levels are controlled by gullies that are structurally treated. Heed (1978) estimate generally only one third of the gully network length required structures. This way it is possible that one can save considerable money.
(v) Specific recommendations for each model gully
Megenagna Community Watershed
The present level of soil and water conservation activities  is  inadequate There is  a need to cover the watershed fully so that the watershed areas are able to absorb optimally as much as possible the excess runoff water in the area.
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I SECTION VIH: CONCLUSION AND 122 RECOMMENDATIONS__________ Design of Large Gully Treatment Works: Tnnn Belcs Intcgralcd Water Resources Desclopinciil I rojccl^-------------------------
The first attempt in the treatment of gullies is the stabilization of the gully at the head' 
be done by the use of loose rock check dam and filling up with more stone o s p
the crumbling gully wall at the head section of each finger gully. Wood or brus c ec .
very little value in terms of addressing the problem fully in this site specifically in areas w
the soil at the gully head is vertisol, and it should be avoided. At gully watershed # 1 an
partly in #5, brush check dam may be tried but the result of some work already done in these
sites do not encourage Its use given the high level of deforestation that already exist in e are
and low level of performance of the wooden structure.
The banks of large and medium gullies have to be reshaped and banks are stabilized before check dams are constructed. Most gully control work in this watershed should use loose rocks and gabion as recommended In this study and when appropriate as well in small and medium size gullies. Three types of check dams are designed are designed and recommended for the sites in this model gully (i) loose rock check dam for medium size gully sections, (ii) single line gabion check dam for medium and large gullies depending on local circumstance, and (iii) re enforced single gabion check dam for large size gullies  where the site is suitable (flat). The recommended type of check  dams  are given in table on design dimensions  for each model gully.
Enkulal Community Watershed - Dera Woreda
Because of the relatively  stable soil (Nitosol), the recommended gully  design is  single line gabion check dam for small and medium gullies. However loose rock check dam is also feasible but limited to small gully sizes near the culverts. At the outlet of the culvert downstream, there a need to have energy dissipater constructed to kill the energy of the stream flow before it is released to a safe outlet or to the field. The structure has to be designed and constructed by the road authority. It is the authority that has the mandate to do so.
Standard design for energy dissipater for all road culverts may have to be developed to reduce the environmental impacts  of storm water released to small watershed. Given the kind of damage roads have on the rural areas and the farms, relevant authorities may have to take this problem and address the issue for good.
Tiis  study  recognizes  the need for the construction of energy  dissipaters  immediately downstream at road culverts and the released flow should be redirected to a safe outlet (chute)
sites' drain39e SyStem °f the WaterShed- ™S problem is a wide sP ^
r
d concern in all
Farta Woreda
The steep land feature at the gully site made the gully plugging design verv difficult „t i^ • 
one of the finger gullies (finger gully #1) InaDDronriatL land 9 erY difficult at least in
conservation activities coupled wi^tradi bonVSTandI nd
worse in this  model gully  watershed. The recommended desion fJ  u|tlvatl0n made the gully chute constructed from stone rip rap at both side of the walls  and fl nger ,9^lly # 1 ,s a lon9 proposed designs  annex). Check  dams  construction should be avoid^ c^ 9h Y (See als°
unstable under such steep slope of this magnitude.
avoided since such structure is
Geiachcw Alem & Associates
I SECTION vill: CONCLUSION AND 123 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Boles Integrated Water Resources Development Project
In  the  lower  part  of  the  gully where the land and gully slope is fairly      stable,      designs      are recommended.   For   finger   #2 practicing       engineers      and technicians are advised to make the   necessary   adjustments   as needed since slope is still higher in this model gully as well. Given the  large  size  gully,  anticipated high  stream  flow,  the  height  of the gabion and loose check dams may  have  to  be  adjusted  as needed.
Biret Afer Community
Watershed
At the upstream part where the gully is small in size, loose rock check dams are recommended. In the middle and lower section of the gully, single line gabion
-.^■r -
Figure 8.1 Long chute on steep gradient for finger gully # 1 in Farta Model Gully Watershed
check dam as per the proposed design dimensions should be implemented. It is important to ensure that the check dams are put in the proper design standards.
Washaw Community Watershed
A mix of loose rock and gabion check dam is recommended for the continuous gullies. However, the areas affected by discontinuous soil movement needs to be put under closure and planting with trees and grasses. The presently existing practices to conserve the soil in a moon crest type structure to harvest rainwater and support the establishment of the planted material should be strengthen and maintenance work has to be continued.
(V) Training
Training will be essential for the implementation of this project. The training will allow project staff upgrades their capacity for the effective implementation of the project and for the proper construction and management of the check dams.
Gdachcw Alcm & Associuics
I SECTION VIII: CONCLUSION AND 124 RECOMMENDA T1ONSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Bibliography and References
Assefa D. Zegeyel, Tigist Y. Tebebul, Anteneh Z. Abiyl, Helen E Dahlke2, Eric D. White, Amy S. Collick, Selemyihun Kidnau, Farzad Dadgari, Matthew McCartney and Tammo S. Steenhuis, 2009. Assessment of Hydrological and Landscape Controls  on Gully Formation and Upland Erosion near Lake Tana. CP 19 Project workshop Proceedings
Billi, P., Dramis, F., 2002. Geomorphological investigation on gully erosion in the Rift Valley and the northern highlands of Ethiopia.
Blake. GR Bulk density, in Black, CA (ed) 1965, Methods of Soil Analysis, part 1, American Society of Agronomy.
Capra, A , Mazzara, L.M., Scicolone, B., 2005. Application of EGEM Model to predict ephemeral gully erosion in Siciliy, Italy . Catena 59, 133-146.
Daba, S., Rieger, W., Strauss, P., 2003. Assessment of gully erosion in eastern Ethiopia using photogrammetric techniques. 273-291.
FAO,Working paper NO. 16. 2003. Integrated natural resources management to enhance Food security.
Frederick R.Troeh, J. Arthur Hobbs and Roy L. Donahue, 1980. Soil and Water Conservation for productivity and Environmental Protection. Prentice Hall International Inc. London.
United States  Department of Agriculture, 1979. Field Manual For Research in Agricultural Hydrology. Agricultural Handbook No. 224. Library of congress catalog card No. 77-600031. Washington DC
Henrique Momm, Ronald Bingner, Robert Wells  and Seth Dabney. Methods  for Gully Characterization in Agricultural Croplands Using Ground-Based Light Detection and Ranging USDA-ARS-National Sedimentation Laboratory United States of America, Journal on Sediment Transport - Flow Processes and Morphology
Herman J. Finkel, 1982. Handbook of Irrigation Technology. CRC Press.
Hicks, R. W. (2007). Soil engineering properties. In 'Soils - their properties and management' 3rd edn. (Eds P. E. V. Charman and B. W. Murphy.) pp. 192-205. (Oxford University Press Melbourne.)
Hudson, Norman, 1989. Soil Conservation. B T Batsford Limited. London ISSN 1996-0786 © 2007 Academic Journals
Lesschen, J.P., Kok, K., Verburg, P.H. and Cammeraat, L.H. 2007 Identification of vulnerable areas  for gully  erosion under different scenarios  of land abandonment in Southeast Spain. Catena 71: 110-121.
Mathys,  N.,  Poesen,  J„  2006.  Gully  erosion  in  mountain  areas:  processes,  measurement modeling and regionalization. Earth Surface Processes and Landforms 31, 133-134.
GeudKW Alcm & Msoc.alcs
| SECTION Vlll: CONCLUSION AND 125 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Mitiku , H., Herweg, K., Stillhardt, B., 2006. Sustainable land management - A new approach to soil and water conservation in Ethiopia. Mekelle, Ethiopia: Land Resource Management and Environmental Protection Department, Mekelle university  ; Bern, Switzerland: Center for Development and Environment (CDE). University  of Bern and Swiss  National Center of Competence in Research (NCCR) North-South, 269pp.
MoARD,2005. Community Based Participatory Watershed Development. Volume 1 and 2 .Addis Ababa, Ethiopia, near lake tana, northern highlands  of Ethiopia. MSc  thesis. Bahir Dar University.
Nyssen, J., Poesen, J., Moeyersons, J., Luyten, C., Veyerety Picot, M., Deckers, J., Mitiku, H., Govers, G., 2002. Impact of road building on gully erosion risk: a case study from the northern Ethiopia highlands. Earth Surface Processes and Landforms 27, 1267-1283.
Nyssen, J., Poesen, J., Veyret-Picot, M., Moeyersons, J., Haile , M , Deckers J., Dewit, J., Naudts, J., Teka, K., Govers, G., 2006. Assessment of gully erosion rates through interviews and measurements: a case study  from northern Ethiopia. Earth Surface Processes  and Landforms 31, 167-185.
Pathak P, Wani SP and Sudi R. 2005. Gully control in SAT watersheds. Agroecosystems Report no. 15. Patancheru 502 324, Andhra Pradesh, India: Source: SAT eJournal | ejoumal.icrisat.org August 2006 | Volume 2 | Issue 1
PCF,  2012  Proposal  on  Abbay  River  Basin  Watershed  Development  Project.  Addis  Ababa, Ethiopia, March, 2012.
Peter F Ffolliott, Kenneth N. Brooks, Roberto Pizarro Tapia and Pablo Garcia Chevesich, 2013 Soil Erosion and Sediment Production on Watershed Landscapes: Processes and Control UNESCO 2013
Poesen,  J.  W„  Nachtergaele,  J.,  Verstraeten,  G.,  &  Valentin,  C.  (2003).  Gully  erosion  and environmental change: importance and research needs. Catena Vol. 50, pp 91-
Poesen, J. W., Vandaele, K., & Wesemael, V. (1996) Contribution of gully erosion to sediment production on cultivated lands  and rangelands. Erosion and Sediment Yield: Global and Regional Perspectives, IAHS Publ. Vol. 236, pp. 251-266.
Schwab, Glenn O,Richard K.Fervert, Talcott W Edminister and Kenneth K.Barnes, 1966 Soil and Water Conservation Engineering. Second Edition. The Ferguson Foundation Agricultural Engineering Series. John Wiley & Sons, New York.
SCS Engineering Field Hand Book, Chapter 2, Fourth Edition 1984.
Shibru Daba, Wolfgang Rieger, and Peter Strauss, 2003. Assessment of gully  erosion in eastern Ethiopia using photogrammetric Techniques. ELSEVIER Publication, Vol 50, Issue 2-4 1 January 2003.
Gctachcw Alem & Associates
| SECTION VIII: CONCLUSION AND 126 RECOMMENDATIONS__________ Design of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
SirviO T„ A. Rebeiro-Hargrave & P Pellikka, 2004. Geoinformation in gully  erosion studies  in taita hills, SE-Kenya, preliminary  results. In the Proceedings  of the 5th African Association ot Remote Sensing of Environment Conference, 17-22 Oct. 2004, Nairobi, Kenya.
Soil Science Society of America ,1996. Glossary of soils science terms. Soil Science society of America, Madison, Wl, USA, and 134 pp. Tamene, L., Vlek, P.L.G., 2007. Assessing  the potential of changing landuse for reducing soil erosion and sediment yield of catchments, a case study in the highlands of northern Ethiopia. Soil Use and Management 23, 82-91.
Tigist Yazie Tebebu, 2009. assessment of hydrological controls on gully formation
Valentin, C , Posen, J., Li, Y , 2005 Gully erosion. Impacts, factors and control. Catena 63, 132-
Vennard,  John  K.  and  Robert  L.  Street,  1976.  Elementary  Fluid  Mechanics.  Fifth  Edition.  SI version. John Wiley & Sons, Inc. New York.
Western Kenya. African Journal of Environmental Science and Technology Vol. 1 (3), pp. 049- 052 October, 2007 Available online at http://www.academiciournals.org/AJEST
Getachcw AJcm &. Associates
| SECTION VIII: CONCLUSION AND RECOMMENDATIONSANNEX
Gctachew Akm & Associates
| SECTION VIII: CONCLUSION AND RECOMMENDATIONS
128Design of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Annex A:
Terms of reference for the design of gully treatment works - TBIWRDP 14/2012
1.  Background and Justification
Gully  erosion is  major threat in Tana Beles  Integrated Water Resources  Development Project areas. In the project area which is  covering 85,026ha about 2% or 1370ha is affected by gully erosion. Gully erosion apart from snatching fertile agricultural fields is a major sediment contributor to reservoirs, lakes and infrastructures. It also obstructs field operation and movement. The subsoil and gravel mined by erosion is a major threat on lower lying fertile agricultural fields  by  inundating and burying them under. A lot of farmers are presently affected and complaining that their plots taken away by sediments and debris  which is  difficult for them to remove. Many  low-lying areas  and public infrastructures facilities have been overburdened / overlaid by subsoil which is not fertile. At times, the subsoil is composed of course sand, gravel, cobbles and boulders.
Although there are many on-going efforts to rehabilitate gullies the scale at which it is expanding has  not been adequately  coped up with the existing level of treatment. In many  places  the quality  of implemented gully rehabilitation measures  are found below the standard and often seen as  run off concentrations  and further gully  initiation and aggravation reasons. It is established fact that gully should be treated as a subset activity of the overall community  / micro-watershed development approach. The catchment contributing to the gully formation should be treated first in a ridge to valley approach. Gully treatment follows a principle of “Prevention is better than cure". Otherwise it will be treating the symptom rather than the cause. Having known this basic principle, due to its severity, magnitude and the damage it is presently causing the need of standard, quality and intensive approach that includes  engineering and non-engineering measures  has been identified.
Therefore, intervening on model and focused micro-watersheds of TBIWRDP has been considered as  a top priority. Demonstrating the quality  approach on selected five catchments then scaling up will take place to others. In the model watersheds selected gullies  will be characterized, surveyed, mapped, classified and treatment options  are specified. In the 5 woredas of TBIWRDP 5 model sites have tentatively been identified (i.e Megenagna in East Estie, Toma in Mecha, Enkulal in Dera, Aiekt Wenz in Faria and Mewalel in Sekela). The gully characterization, classification, design and implementation approach implemented in these models  would be used for expanding (scaling up) for other areas of the project and beyond. The model approach is considered because it is difficult to treat the entire gully affected areas at one go in the project area. These are also the community watersheds where catchment treatment has already started and gully treatment would fit in the top-sequence. The knowledge and experience gained at the model watersheds will be used as a demonstration site for the respective Woredas to apply and scale it up in neighboring and other catchments.
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SECTION VIII: CONCLUSION AND 129 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Beles Integrated Water Resources Development Project
This consultancy is therefore, needed to overcome the lack of proper characterization, quality design and implementation of gully affected areas.
2.  Objective
Prepare  detail  design  for  gully  treatment  in  5  selected  gullies  in  five  woredas (East Estie, Farta, Dera, Mecha and Sekela) of the TBIWRDP
3.  Scope of the service
-    Within the length of 1 to 2.5km gully in each woreda the consultant characterize, classify, and carryout detail survey (GPS, tape meter, clinometers, etc.).
Using these survey results select proper intervention options and present design.
-    Provide on job knowledge transfer to the counterpart and other staffs to be
assigned by the client.
4.  Detail tasks of the service provider
Considering the different land forms, soil types  and bearing capacity, land uses  and cover, farming systems and sediment characteristics (size of particles) and hydrological parameters (peak flow, meandering points) and their interaction in the process of gully initiation and expansion.
-    Characterize, survey and classify the gully type under investigation
-    Identify the key causal factor/s for the initiation and expansion
-      Identify  intervention  options  (engineering  and  non-engineering)  and  select  the most viable intervention for  plugging the specified gully (selection criteria to be developed   considering   simplicity,   affordability,   replicability,   and   possibility   of enhanced productivity)
Prepare and provide an engineering design of gully 
plugging/treatment/rehabilitation for the specified gullied areas with detail design, drawing, specifications sand materials required (the design should be based on the appropriate flood discharge rate taking in to consideration the inflow and outflow points)
-    Prepare gully atlas
-    Build capacity of technicians (Demonstrate/show-how/train) on the above by enabling client staff participation in all phases of the execution of the design
-    Contribute to the preparation of the training manual by documenting the design process and phases
-    Participate/contribute to the implementation of planned applied training on gully mapping and rehabilitation
Gctachcw Alum & Associates
| SECTION Vlll: CONCLUSION AND 130 RECOMMENDATIONS______Design of Large Gully  Treatment Works: Tana Bcles  Integrated Water Resources  Development Project
>. Output of the consultancy work
Gully atlas with its detail description on soil compaction and flood discharge and critical hydrological pressure points (analysis of landforms, soil types and bearing capacity, land uses and cover, farming systems)
Database  on  inflow,  seasonal  flood  discharge  rates  and  peak  carrying  capacity created
Design   maps   for   selected   intervention   options   (e.g.   gabion,   earthen,   RCC, wooden, biological, etc.)
-    Site specific interventions outlining all technical specifications and cost estimate
Built capacity of the resource people at Woreda and Kebele level through a consultative process
6.  Timeframe
-    1 June-30 Oct 2012
7.  Qualification and experience
-       M.Sc.   in   Civil   Engineering   /   Agricultural   Engineering   /   Water   Resources Engineering  and  related  qualifications  with  not  less  than  10  years  of  related experience
8.   Implementation Arrangement
The Regional Project Coordination Unit / BoWRD will be responsible to manage the consultancy
-    The client (BoA) may provide counterpart staffs to work with the consultant for knowledge transfer
-    The consultant will conduct the assignment in consultation with BoA and BoWRD
9.  Deliverables
Inception  report  within  15  days  of  the  signing  of  contract  indicating  the  detail methodology and work plan
-    Draft design report after 90 days of signing
Final report after 120 days of signing of the contract
Gctachcw AI rm & Associates
I SECTION VIII: CONCLUSION AND 131 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Bcles Integrated Water Resources Development Project
Annex B.
A guide for preliminary selection of a gully control measures Preliminary identification of gully control measures:
Drainage Area in Acres
Slope of
1-    3-      6-       11-          16        26-      41-        81-      Cher
Waterway in %        1 5            10       15           25-       40       80         150       150
0.5-2
3-4
5-6
7-8
9-10
11-14
15-19
20-25
Above solid line., gullies can ordinarily be controlled by blading-in and sodding 
or seeding. 
s
| 
below solid line, gullies will ordinarily require permanent structures
J
for reducing grades to provide safe velocities.
26-35_____________________________________________________________________________________________ Figure B.1 Selection of control measures  - The first step in the design of gully  control measures (source: www)__________________________________________________________ Note: 1 acre is equivalent to 0.405 hectare area or multiply the area in hectare by 0.405 to get the watershed or drainage area in acres
Gclachcw Alem & Associates
I SECTION VIII: CONCLUSION AND 13-? recommendationsDesign of Large Gully Treatment Works: Tana Belos Integrated Waler Resources Development Project
Annex C: Peak flood Estimation using Cook's method
| SECTION VIII: CONCLUSION AND 133 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development F’rojcct
Table C.l Peak flood (Qp) for Megenagna Model Gully Watershed # 1, Misrak Estie Woreda
Land
use
Cultivated Land        (LUI)
Church
(LU 2)
Grazing Land (LU
3)
Gully
(LU 4)
Settlement Area
(LU 5)
, Total
Average
CC
0-5%
3.17
15.9
0.94
4.7
0.05
0.25
0.33
1.65
0
0
5-10%
31.6
316.0
7.93
79.3
0.16
1.60
2.04
20.4
0.66
6.6
10-30%
5
10
15
7.37
110.6
2.51
37.65
0.1
1.50
0.83
12.45
0.01
0.15
>30%
22
3.61
79.4
1.00
22
0
0.00
0.02
0.44
0.05
1.1
Sum
45.75
521.8
12.4
143.7
0.3
3.4
3.2
34.9
0.7
7.9
Avag
11.4
0.0
11.6
10.8
10.9
10.9
55.6
11.119
0-5%
50
3.17
158.5
0.94
47
0.05
2.5
0.33
16.5
0
0
5-10%
25
31.6
790.0
7.93
198.25
0.16
4
2.04
51
0.66
16.5
10-30%
20
7.37
147.4
2.51
50.2
0.1
2
0.83
16.6
0.01
0.2
>30%
10
3.61
36.1
1.00
10
0
0
0.02
0.2
0.05
0.5
Sum
45.75
1,132.0
12.38
305.45
0.31
8.5
3.22
84.3
0.72
17.2
Average
24.7
24.7
27.4
26.2
23.9
126.9
25.38
V.
s
O
0-5%
20
3.17
63.4
25
0.94
23.5
15
0.05
0.75
25
0.33
8.25
25
0
0
5-10%
20
31.6
632.0
25
7.93
198.25
15
0.16
2.4
25
2.04
51
25
0.66
16.5
U
W
**
10-30%
20
7.37
147.4
25
2.51
62.75
15
0.1
1.5
25
0.83
20.75
25
0.01
0.25
•»
&
>’
>30%
20
3.61
72.2
25
1.00
25
15
0
0
25
0.02
0.5
25
0.05
1.25
Sum
45.75
2,264.0
24.8
610.9
0.6
17.0
6.4
80.5
1.4
18
Average
49.49
24.67
27.42
12.50
12.50
126.6
25.32
61.815
£CC = 61.815
Use £ CC = 62
Model Gully Watershed Area (A) = 62 ha Estimated Peak flow, Qp = 11.05 m /sec
3
Gaachcn Alcm & Associates
| SECTION VIII: CONCLUSION AND 134 RECOMMENDATIONS
Soil Infiltration
Relief or slopeDesign of Large Gully Treatment Works: Tana Bcles Integrated Waler Resources Development Project
Table C. 2 Peak flood (Qp) for Megenagna Model Gully Watershed # 2, Misrak Estie Woreda
Land
use
Cultivate< (LUI
1 Land
1
Barren Land
Grazing Land (LU
3)
Gully (LU 4
1 Garden area (LU
____
Settlement Area
(LU 5)
Total
Averag
eCC
0-3%
5
9.1
45.5
0.3
1.5
0.3
1.5
1
5
0.3
1.5
0.7
3.5
3-8%
8
56.4
451.2
2.2
17.6
2.2
17.6
4.8
38
3
24
5.4
43.2
l 8-15%
10
6.8
68
1.7
16.9
0.1
1
0
0
0
0
0.2
2
15-30%
15
19.4
291
0.4
6
0.7
10.5
1.9
29
0.9
14
1.4
21
Sum
91.7
855.7
4.6
42
3.3
30.6
7.7
72
4.2
39
7.7
69.7
Average
9.332
9.15
9.27
9.3
9.3
9.05
55.43
9.238
0-3%
25
9.1
227.5
50
0.3
15
25
0.3
7.5
30
1
30
25
0.3
7.5
50
0.7
35
3-8%
25
56.4
1410
40
2.2
88
25
25
2.2
55
30
4.8
144
25
3
75
40
5.4
216
8-15%
25
6.8
170
25
1.69
42.3
0.1
2.5
40
0
0
25
0
0
25
0.2
5
15-30%
30
19.3
579
25
0.4
10
30
0.7
21
50
1.9
95
25
0.9
22.5
25
1.4
35
Sum
91.6
2387
140
4.59
155
105
3.3
86
150
7.7
269
100
4.2
105
140
7.7
291
Average
26.05
33.8
26.06
34.9
25
37.8
183.7
30.61
0-3%
20
9.1
182
25
0.3
7.5
40
0.3
12
25
1
25
20
0.3
6
25
0.7
17.5
2.2
55
40
2.2
88
25
4.8
120
20
3
60
25
5.4
8
3
3-8%
20
56.4
1128
25
135
8-15%
20
6.8
136
25
1.69
42.3
25
0.1
2.5
25
0
0
20
0
0
25
0.2
5
15-30%
20
19.3
386
25
0.4
10
25
0.7
17.5
25
1.9
47.5
20
0.9
18
25
1.4
35
Sum
91.6
1832
4.59
US
3.3
120
7.7
193
4.2
84
7.7
193
Average
20
36.36
25
20
[
25
151.4
25.23
65.08
Sum of Runoff Curve CC = 65.08
Use CC =65
Model Gully Watershed Area (A) = 117 ha
Peak flood Q = 20.72 m3/sec
Gctachcw AI cm A: Associates
| SECTION VIII: CONCLUSION AND 135 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Boles Integrated Water Resources Development Project
Table C. 3 Peak flood (Qp) for Megenagna Model Gully Watershed # 3, Misrak Estie Woreda
Land use
Cultivated Land
(LVD
Garden LLU2)
Grazing Land
(LU 3)
Gully
(LU 4,
Settlement Area
(LU 5)
Total
Avg
0-5%
5
0.4
2.1
0.0
0.0
2.4
12.2
1.2
5.9
0.5
2.6
5-10%
10
1.1
11.2
0.1
0.7
3.3
32.9
1.7
16.9
1.5
15.2
10-30%
15
1.7
25.4
0.3
4.4
4.0
60.3
1.5
21.9
7.1
106.4
>30%
22
0.0
0.0
0.0
0.0
0.0
0.2
4.0
Sum
3.2
38.6
0.4
5.1
9.8
105.4
4.3
44.7
9.3
128.1
||
Average
12.0
14.0
10.8
10.3
13.8
60.9
12.2
0-5%
50
0.4
20.5
20.0
0.0
0.0
50.0
2.4
122.0
1.2
59.0
0.5
25.5
5-10%
25
1.1
28.0
20.0
0.1
1.4
40.0
3.3
131.6
1.7
42.3
1.5
38.0
10-30%
20
1.7
33.8
20.0
0.3
5.8
30.0
4.0
120.6
1.5
29.2
7.1
141.8
>30%
10
0.0
0.0
30.0
0.0
0.0
0.0
0.0
0.2
1.8
Sum
3.2
82.3
0.4
7.2
9.8
374.2
4.3
130.5
9.3
207.1
Average
25.6
20.0
38.4
30.1
22.3
136.3
27.3
u
!
J3
*
1
0-5%
20
0.4
8.2
0.0
0.0
15.0
2.4
36.6
25.0
1.2
29.5
25.0
0.5
12.8
5-10%
20
1.1
22.4
20.0
0.1
1.4
15.0
3.3
49.4
25.0
1.7
42.3
25.0
1.5
38.0
10-30%
20
1.7
33.8
20.0
0.3
5.8
15.0
4.0
60.3
25.0
1.5
36.5
25.0
7.1
177.3
>30%
20
0.0
0.0
0.0
15.0
0.0
25.0
0.0
0.0
25.0
0.2
4.5
Sum
3.2
164.6
0.7
7.2
19.5
748.4
8.7
108.3
18.6
232.5
Average
51.1
10.0
38.4
12.5
12.5
124.5
24.9 1
Toati CC
64.4 U
2 CC = 64.35
Use I CC = 64
Gully Sub watershed area = 64 ha Estimated Peak flow, Qp = 12.42 m’/sec
Use Q = 12 m3/sec
Gctachcw Alcm A Associates
| SECTION VII!: CONCLUSION AND 136 RECOMMENDATIONS
Sell Infiltration
I" Relief or slopeDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table C. 4 Peak flood (Qp) for Megenagna Model Gully Watershed # 4, Misrak Estie Woreda
Land use
Cultivated Land
(LUI)
Grazing Land
(LU 3)
Gully (LU
4)
Settlement Area
(LU 5)
Total
Avg |
0-5%
7.17
35.9
0.82
4.10
0.19
0.95
0.98
4.9
■i
5-10%
13.2
131.7
1.82
18.20
0.32
3.2
3.3
33
J
10-30%
5
10
15
17.8
267.6
2.31
34.65
0.65
9.75
9.98
149.7
>30%
22
0.15
3.3
0.01
0.22
0
0
0.43
9.46
Sum
38.3
438.5
4.96
57.2
1.16
13.9
14.69
197.1
Average
11.4
11.5
12.0
13.4
48.4
9.6725
0-5%
50
7.17
358.5
50
0.82
41
50
0.19
9.5
30
0.98
29.4
5-10%
25
13.2
329.3
40
1.82
45.5
40
0.32
12.8
30
3.3
99
10-30%
20
17.8
356.8
30
2.31
46.2
40
0.65
26
30
9.98
299.4
>30%
10
0.15
1.5
30
0.01
0.1
40
0
0
30
0.43
12.9
Sum
38.3
1,046.1
4.96
132.8
1.16
48.3
14.69
440.7
Average
27.3
26.8
41.6
30.0
125.7
25.14
0-5%
20
7.17
143.4
15
0.82
12.3
25
0.19
4.75
25
0.98
24.5
5-10%
20
13.2
263.4
15
1.82
27.3
25
0 32
8
25
3.3
82.5
10-30%
20
17.8
356.8
15
2.31
34.65
25
0.65
1625
25
9.98
249.5
£
m
N*
>
>30%
20
0.15
3.0
15
0.01
0.15
25
0
0
25
0.43
10.75
Sum
38.3
2,092.1
9.9
265.6
2.3
29.0
29.4
367.25
Average
54.58
26.77
12.50
12.50
106.4
21.27
Total CC
56.084
£CC = 56.08
Use £ CC = 56
Gully Sub watershed area = 59.14 ha
Estimated Peak flow, Qp - 8.81 m /sec
3
Use Qp = 9 m /sec
3
Gctachcw Alcm & Associates
| SECTION VIII: CONCLUSION AND 137 RECOMMENDATIONS
Soil Infiltration
Relief or slopeDesign of Large Gully Treatment Works: Tana Boles Integrated Water Resources Development Project
Table C. 5 Peak flood (Qp) for Megenagna Model Gully Watershed # 5, Misrak Estie Woreda
Land
use
Cultivated Land
(LUI)
Grazing Land
(LU 3)
Gully
(LU 4)
Settlement Area
(LU 5)
Total
Avg n
0-5%
5
3.7
18.5
2.7
13.50
0.6
3
1.7
8.5
5-10%
10
11.1
111.0
5.2
52.00
1.2
12
4.3
43
10-30%
15
17.6
264.0
8.8
132.00
1.2
18
9.5
142.5
>30%
22
0.3
6.6
0.1
2.20
0
0
0
0
Sum
32.7
400.1
16.80
199.7
3.00
33.0
15.50
194.0
Average
12.2
11.9
11.0
12.5
47.6
9.5277
0-5%
50
3.7
185.0
50
2.7
135
50
0.6
30
30
1.7
51
5-10%
25
11.1
277.5
40
5.2
130
40
1.2
48
30
4.3
129
10-30%
20
17.6
352.0
30
8.8
176
40
1.2
48
30
9.5
285
>30%
10
0.3
3.0
30
0.1
1
40
0
0
30
0
0
J
Sum
32.7
817.5
16.8
442
3
126
15.5
465
Average
25.0
26.3
42.0
30.0
123.3
24.66 |
0-5%
20
3.7
74.0
15
2.7
40.5
25
0.6
15
25
1.7
42.5
■>
8
It
•1
>
5-10%
20
11.1
222.0
15
5.2
78
25
1.2
30
25
4.3
107.5
10-30%
20
17.6
352.0
15
8.8
132
25
1.2
30
25
9.5
237.5
”1
>30%
20
0.3
6.0
15
0.1
1.5
25
0
0
25
0
0
Sum
32.7
1,635.0
33.6
884.0
6.0
75.0
31.0
387.5
Average
50.00
26.31
12.50
12.50
101.3
20.26 |
Total
CC
54.452 |
£CC = 54.45
Use£CC = 54
Gully sub watershed area = 68 Ha
Estimated Peak flow, Qp -7.6 m /sec
3
Gctacbcw Alem &. Associates
| SECTION VIII: CONCLUSION AND 138 RECOMMENDATIONS
Soil Infiltration
Relief or slopeDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project
Table C.6 Peak flood (Qp) for Enkulal Model Gully Watershed, Dera Woreda
land use
Cultivated Land        (LUI)
Forest Land
Grazing Land (LU
3)
Gully (LU
4)
Settlement Area
(LU 5)
Total
Avg |
0-5%
5
2.29
11.5
0.32
1.6
1.66
8.30
0.02
0.1
0.5
2.5
5-10%
10
7.38
73.8
1.61
16.1
4.44
44.40
0.3
3
0.87
8.7
10-30%
15
28.46
426.9
6.39
95.85
9.34
140.10
0.74
11.1
2.63
39.45
>30%
22
4.58
100.8
1.61
35.42
0.93
20.46
0.0
0
0
0
Sum
42.71
612.9
9.9
149.0
16.4
213.3
1.1
14 2
4.0
50.7
Average
14.4
15.0
13.0
13.4
12.7
68.4
13.69 [
0-5%
10
2.29
22.9
10
0.32
3.2
25
1.66
41.5
25
0.02
0.5
25
0.5
12.5
5-10%
15
7.38
110.7
20
1.61
32.2
30
4.44
133.2
25
0.3
7.5
25
0.87
21.75
10-30%
20
28.46
569.2
20
6.39
127.8
30
9.34
280.2
50
0.74
37
50
2.63
131.5
>30%
30
4.58
137.4
25
1.61
40.25
30
0.93
27.9
50
0.0
0
50
0
0
Sum
42.71
840.2
9.9
203.5
16.4
482.8
1.1
45.0
4.0
165.8
Average
19.7
20.5
29.5
42.5
41.4
153.5
30.71 |
0-5%
20
2.29
45.8
5
0.32
1.6
15
1.66
24.9
u.
CJ
>
8
**
Qfi
■
>
25
0.02
0.5
25
0.5
12.5
5-10%
20
7.38
147.6
10
1.61
16.1
15
4.44
66.6
25
0.3
7.5
25
0.87
21.75
10-30%
20
28.46
569.2
15
6.39
95.85
15
9.34
140.1
25
0.74
18.5
25
2.63
65.75
>30%
20
4.58
91.6
15
1.61
24.15
15
093
13.95
25
0.0
0
25
0
0
Sum
42.71
854.2
9.9
137.7
16.4
245.6
1.1
26.5
4.0
100.0
Average
20.00
13.87
15.00
25.00
25.00
98.9
19.77 1
Total CC
64.17 I
£CC = 64.17
Use JCC=64
Model Gully watershed area = 73 ha
Estimated Peak flow, Qp = 13.27 m /sec
Use Qp = 13 m’/sec
3
Gclachcw Alcm A /tssocialcs
| SECTION VIII: CONCLUSION AND 139 RECOMMENDATIONS
Soil Infiltration
Relief or slopeDesign of Large Gully Treatment Works: Tana Bclcs                In legrated Waler Resources Development Project Table C. 7 Peak flood (Qp) for Tikur Woha Model Gully Watershed, Farta Woreda
Land
use
Cultivated (un:
Land
Forest (LU 2)
Grazing Land (LU 3)
Gully (LU 4
Bare Land (LU 4)
Settlement Area (LU
5)
Tota
Averag  1 eCC
0-5%
5
1.25
6.25
0.88
4.4
4.64
23.2
0.58
2.9
0.68
3.4
0.64
3.2
5-10%
10
2.37
12.5
0.59
5.9
8.38
83.8
0.14
1.4
2.87
28.7
1.69
16.9
10-30%
15
17.71
265.7
2.13
32
24.5
367.1
0.34
5.1
9.57
143.6
4.9
73.5
>30%
20
11.6
232
1.13
22.6
1.92
38.4
0.58
11.6
1.03
20.6
0.72
14.4
Sum
32.93
516.4
4.73
64.9
39.4
512.5
1.06
18.1
14.2
196.3
7.95
108
Average
15.68
13.7
13
17.08
13.87
13.58
86.9
14.49
0-5%
20
1.25
25
10
0.88
8.8
20
4.64
92.8
30
0.58
17.4
30
0.68
20.4
25
0.64
16.0
5-10%
20
2.37
47.4
15
0.59
8.85
20
8.38
168
30
0.14
4.2
30
2.87
86.1
25
1.69
42.3
10-30%
30
17.71
531.3
30
2.13
*63.9
30
24.47
734
30
0.34
10.2
40
9.57
382.8
25
4.90
122.5
>30%
30
11.6
348
50
1.13
56.5
30
1.92
57.6
30
0.58
17.4
50
1.03
51.5
25
0.72
18.0
Sum
32.93
951.7
105
4.73
138.1
100
39.41
1052
120
1.64
49.2
150
14.15
540.8
100
7.95
198.8
Average
28.9
29.19
26.7
30
38.22
25
178
29.67
0-5%
20
1.25
25
5
0.88
4.4
15
4.64
69.6
25
25
0.68
17
25
0.64
16
5-10%
20
2.37
47.4
5
0.59
2.95
15
8.38
126
25
0.14
3.5
25
2.87
71.75
25
1.69
42.25
10-30%
20
17.71
354.2
5
2.13
10.65
15
24.47
367
25
0.34
8.5
25
9.57
239.3
25
4.9
122.5
>30%
20
11.6
232
5
1.13
5.65
15
1.92
28.8
25
1.15
28.75
25
1.03
25.75
25
0.72
18
Sum
32.93
658.6
4.73
23.65
39.41
591
1.63
40.75
14.15
353.8
7.95
198.8
>
Average
20
5
15
25
25
25
115
19.17
I I cc
63.32
Use Z CC = 63
Area = 49 hectare
Q = 9.4 m3/sec
Use Q = 6 m3/sec for finger gully # 2 that has big size gully and with branches Use Q = 3 m3/sec for finger gully # 1
Gctachew Alcm &. Associates
| SECTION VIII: CONCLUSION AND 140 RECOMMENDATIONS
tai cover
Soil Infiltration
SlopeDesign ol Large Gully treatment Works: Tana Bclcs Integrated Waler Resources Development Project Table C.8 Peak flood (Qp) for Biret Afer Model Gully Watershed, Mecha Woreda
Fo
rest (LI
J 2)
Gi
mazing La
(LU 3)
nd
G ully (LI  ’4)
Prot ected    (LU 5)
Tot
Avg 1
Flat (0-3%)
Il S
Farm
(LU 10
and
1)
C1
aa
Settlement
(LU 6
Gently sloping (3-8%)                 8
5.1
40.7
0.4
u.z
2.9
0.0
0.1
0.0
0.6
0.1
0.6
0.3
4.6
0.0
0.1
0.2
1.0
0.8
3.6
—
Moderately sloping (8-
15%)
10
7.3
73.4
0.4
Cl
Q—
4.4
0.5
5.2
0.9
8.8
3.1
30.6
0.7
6.5
0       Sloping (15 30%)
0.5
7.3
0.8
12.6
3.8
V)
15
20
8.7
130.8
0.1
1.7
56.3
1.0
15.0
steep (30-50%)
1.7
33.9
0.0
0.0
0.0
0.0
0.1
2.5
0.3
7.0
0.0
0.1
Very teep slope (> 50%)               25
Sum
Avg
0.0
0.5
0.0
0.0
1.1
27.0
0.0
0.0
0.0
0.0
0.0
0.0
23.9
284.5
1.0
9.18
1.08
25.0
2.5
28.7
7.3
95.0
2.3
26.1
11.91
9.64
23.1
11.6
13.0
11.5
80.7
13.5
Flat (0-3%)
10
1.0
10.2
10
0.0
0.4
20
0.0
0.1
25
0.1
1.3
10
0.0
0.4
25
0.2
4.2
E
0
*<s
m
u
C
c
Gently sloping (3-8%)
10
5.1
50.9
10
0.4
3.6
20
0.1
1.4
25
0.6
14.3
10
0.1
1.2
25
0.5
11.3
Moderately sloping(8-15%)
10
7.3
73.4
10
0.4
4.4
25
0.5
12.9
25
0.9
22.0
10
3.1
30.6
25
0.7
16.3
Sloping (15-30%)
15
8.7
130.8
10
0.1
1.1
25
0.5
12.1
25
0.8
21.0
10
3.8
37.5
25
1.0
25.0
steep (30-50%)
10
1.7
17.0
10
0.0
0.0
25
0.0
0.0
25
0.1
3.2
10
0.3
3.5
25
0.0
0.2
Very teep slope (> 50%)
20
0.0
0.4
10
0.0
0.0
25
1.1
27.0
25
0.0
0.0
10
0.0
0.0
25
0.0
0.0
Sum
23.9
282.7
0.95
9.5
1.08
53.6
2.5
61.7
7.3
73.2
2.28
56.9
Average
11.8
10
49.6
25
10
25
131.5
21.9
Flat (0-3%)
20
1.0
20.5
5
0.0
0.2
15
0.0
0.1
25
0.1
1.3
12
0.0
0.5
25
0.2
4.2
1
Gently sloping (3-8%)
20
5.1
101.8
5
0.4
1.8
15
0.1
1.1
25
0.6
14.3
12
0.1
1.4
25
0.5
11.3
>
"J
Moderately sloping(8-15%)
20
7.3
146.8
5
0.4
2.2
15
0.5
7.8
25
0.9
22.0
12
3.1
36.7
25
0.7
16.3
Sloping (15-30%)
20
8.7
174.5
5
0.1
0.6
15
0.5
7.3
25
0.8
21.0
12
3.8
45.0
25
1.0
25.0
1
43 .
steep (30-50%)
20
1.7
33.9
5
0.0
0.0
15
0.0
0.0
25
0.1
3.2
12
0.3
4.2
25
0.0
0.2
II
>_
Very teep slope (> 50%)
20
0.0
0.4
5
0.0
0.0
15
1.1
16.2
25
0.0
0.0
12
0.0
0.0
25
0.0
0.0
|l
Average
23.9
477.9
30
1.0
4.8
90
2.2
32.4
150
2.5
61.7
72.
7.3
87.9
150
2.3
56.9
Runoff curve, CC            1
20.0
5.0
15.0
25.0
12.0
25.0
102
17. j
Use
Qm’/sec 
52
52.4
UseSm’/sec
Gctachcw AJem & Associates
| SECTION VIII: CONCLUSION AND 141 RECOMMENDATIONS
cm m
66Design of Large Gully Treatment Works: Tana Roles Integrated Water Resources Development Project Table C.9 Peak flood (Qp) for Washaw Model Gully Watershed, Sekela Woreda
Land
use
Cultivated Land
(LUI)
Forest (LU 2)
Grazing Land (LU
3)
Gully (LU 4
Protected area
(LU 4)
Settlement
Area (LU 5)
Total
cce
0-5%
5
1.83
9.15
0.14
0.7
0.00
0
0.2
1
0.06
0.3
0.22
1.1
5-10%
10
12.6
18.3
0.39
3.9
0.68
6.8
1.39
13.9
6.15
61.5
0.94
9.4
10-30%
15
5.38
80.7
0.37
5.55
0.08
1.2
0.69
10.35
0.34
5.1
0.37
5.55
>30%
20
1.61
32.2
0.00
0
000
0
0.12
2.4
034
6.8
0.01
0.2
Sum
21.42
140
0.90
10.15
0.76
8
2.4
26.65
6.89
73.7
1.54
16.3
Average
6.55
11.28
10.53
11.1
10.7
10.6
60.7
10.1
0-5%
20
1.83
36.6
10
0.14
1.4
20
0.0
0.00
30
0.2
6
10
0.06
0.6
25
0.22
5.5
5-10%
25
12.6
315
15
0.39
5.85
20
0.68
13.60
30
1.39
41.7
20
6.15
123
25
0.94
23.5
10-30%
25
5.38
135
25
0.37
9.25
25
0.08
2.00
30
0.69
20.7
20
0.34
6.8
30
0.37
11.1
>30%
30
1.61
48.3
30
0
0
50
0.00
0.00
30
0.12
3.6
25
0.34
8.5
40
0.01
0.4
Sum
21.42
534
0.9
16.5
0.76
15.6
120
2.4
72
75
6.89
139
120
1.54
40.5
j
Average
24.9
18.33
20.53
30
20.2
26.3
140
23 4
,
0-5%
20
1.83
36.6
5
0.14
0.7
15
0
25
0.2
25
0.06
1.5
25
0.22
5.5
“1
g
1°
4-'
too
>0)
5-10%
20
12.6
252
5
0.39
1.95
15
0.68
10.2
25
1.39
34.8
25
6.15
154
25
0.94
23.5
10-30%
20
5.38
108
5
0.37
1.85
15
0.08
1.2
25
0.69
17.3
25
0.34
8.5
25
0.37
9.25
>30%
20
1.61
32.2
5
0
15
0
25
0.12
3
25
0.34
8.5
25
0.01
0.25
Sum
21.42
428
0.9
4.5
0.76
11.4
2.4
55
6.89
172
1.54
38.5
Average
20
5
15
22.9
25
25
113
18.8
52.3
Use runoff curve No £ CC = 52
Area = 34
Q= 4.31 m3/sec
Use 5
m3/sec
Finger H 1 (close to the road with branches for gullies) = 3 m3/sec.
This flow is apportioned into 2m /sec to the big gully Finger #2 = 2 m3/sec
| SECTION VIII: CONCLUSION AND 142 RECOMMENDATIONS
3
Gctachcu Alcm & Associalcs
Soil Infiltration
SlopeDesign of Large Gully Treatment Works: Tana Belcs Integrated Waler Resources Development Project
Table C 10 Peak flood estimation table as presented in Hudson, 1989
Note: The above table is prepared through a watershed study in over 200 sites in USA This table is used in the conversion of the sum of runoff curves developed for each model gully watershed (see Annex C) using GIS based land use data of the watershed; the soil, relief and vegetation cover to produce runoff curve (CC) and the £ cc for each model gully watershed is computed.
Getachew AJem & Associates
SECTION VIII: CONCLUSION AND 143 RECOM M EN DAT1ON SDesign of Large Gully Treatment
Works: Tana Boles Integrated Water Resources Development Projcct^
Annex D: Peak flood Estimation using US SCS method 30
United States Soil Conservation Services Method
The United States Soil Conservation Services (SCS) is one of the several methods used for estimating design peak flood in a watershed. The method is established based on six major parameters; rainfall, hydrologic  soil groups, cover type, land managemen practices, hydrologic  conditions  and topography  (relief) of the watershed. The characteristics of these parameters are detailed as follows.
Rainfall
United States  Soil Conservation Services  has  developed a method for estimating the peak discharge from a small rural watershed. In the method it is well recognized that the catchment characteristics and intense rainfall affects peak floods more seriously than any other factor in the hydrology  of watersheds. The intensity  of rainfall affects  the peak discharge more than it does the volume of runoff.
The recommended watershed for use in this case is small watersheds and for such size of watersheds, storm duration of 24 hours  was  chosen for the synthetic  distributions. Thus, a single storm duration and associated synthetic rainfall distribution can be used to estimate peak discharges for a wide range of watershed areas.
Hydrologic soil groups
The nature of the soil in the small watershed is one of factors affecting surface runoff. In this method soils have been classified into four hydrologic soil groups. The four groups are defined as follows:
Group  A soils  have low runoff potential and high infiltration rates  even when thoroughly  wetted. They  consist chiefly  of sands  and gravels  that are deep, well drained to excessively drained, and have a high rate of water transmission (greater than 0.30 in per hr).
Group B soils  have moderate infiltration rates  when thoroughly  wetted and consist chiefly  of soils  that are moderately  deep to deep, moderately  well drained to well drained, and have moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission (0.15 to 0.30 in per hr)
Group C soils have low infiltration rates when thoroughly wetted and consist chiefly of soils  having a layer that impedes  downward movement of water and soils7 of
"’IXSE")6 'eX,Ure Th6Se S0"5
3
°' — <ransSXn
” Material extracted from SCS Engineering Field Manual, revised edition July 1934
Gciachcw Alcm &. Associates
I SECTION VIII: CONCLUSION AND 144 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development I rojcc£
Group D soils have high runoff potential. They have very low infiltration rates when thoroughly wetted and consist chiefly of clay soils with a high swelling potential, soi s with a permanent high water table, soils with a clay pan or clay layer at or near e surface, and shallow soils over nearly impervious material. These soils have a very low rate of water transmission (0 to 0.05 in per hr).
Cover type
Cover type affects runoff in several ways. The foliage and its litter maintain the soil s infiltration potential by  preventing the impact of the raindrops  from sealing the soil surface. Some of the raindrops are retained on the surface of the foliage, increasing their chance of being evaporated back into the atmosphere. Some of the intercepted moisture takes so long to drain from the plant down to the soil that it is withheld from the initial period of runoff.
Ground cover also allows soil moisture from previous rains to transpire, leaving a greater void in the soil to be filled. Vegetation, including its ground litter, forms numerous barriers along the path of the water flowing over the surface of the land. This increased surface roughness causes water to flow more slowly, lengthening the time of concentration and reducing the peak discharge.
Treatment
Treatment or conservation practices  reduce erosion and thereby  maintain an open structure at the soil surface. This reduces the runoff, but the effect diminishes rapidly with increases in storm magnitude. Contouring and terracing reduce erosion and decrease the amount of runoff by  forming small reservoirs. Closed-end level terraces  act as  storage reservoirs without spillways.
Land areas in which level terraces have been constructed may be excluded from the
drainage area above downstream measures if the terrace system has enough capacity to store the depth of runoff commensurate with the frequency of the runoff event. Gradient terraces increase the distance water must travel and thereby increase the time of concentration.
Hydrologic conditions
In most cases the hydrologic condition of the site affects the volume of runoff more than any other single factor. The hydrologic condition considers the effects of cover type and treatment on infiltration and runoff and is generally estimated from density of plant cover and rescue on the ground surface. Good hydrologic  condition indicates  that the site usually has a lower runoff potential. Crop residue tilled rnto the soil and the res dual root
Shon 9,aSS6S 'ha' b6en ln °r0P r°“°"S
A grassland cover is good if the vegetation covers 75 percent or mrw
a good hJdrolS
„
the plant, whereas trees and shrubs are evaluated on the basis'ct oanop?°'
Gctachcw Alcm & Associates
1 SECTION VIII: CONCLUSION AND ] 45 recommendationsDesign of Large Gully Treatment
Works: Tana Belcs Integrated Water Resources Development Project
For arid and semiarid rangelands, poor conditions exist if ground cover (grass, litter, and brush canopy) is  less  than 30 percent. Fair conditions  exist when the ground cover is between 30 and 70 percent, and good conditions exist when ground cover is greater than 70 percent.
Topography
The slopes  in a watershed have a major effect on the peak  discharge at downstream points. Slopes  have little effect on how much of the rainfall will run off. As  watershed slope increases, velocity increases, time of concentration decreases, and peak discharge increases. An average small watershed is  fan shaped. As  the watershed becomes elongated or more rectangular, the flow length increases  and the peak  discharge decreases
Potholes may trap a small amount of rain, thus reducing the amount of expected runoff. If potholes and marshland areas make up one-third or less of the total watershed and do not intercept the drainage from the remaining two thirds, they will not contribute much to the peak discharge. These areas may be excluded from the drainage area for estimating peak discharge If potholes constitute more than one-third of the total drainage or if they intercept the drainage, the procedures  in NEH-4 should be used to estimate the peak discharge.
Longest Flow length in the model gully
Flow length is one of the design peak flood estimation parameter and it is the longest flow path in the watershed from the watershed divide to the outlet. It is defined as the total path water travels overland and in small channels on the way to the outlet or point of interest The flow length can be determined using a GIS map for the model gully watershed.
If the watershed has Bench Terraces, water flows from the divide to each terrace, and follow along each terrace channel and then along the main drainage stream to the outlet.
Average watershed slope
The average watershed slope (Y) is the slope of the land and not the watercourse II can
«ow. Average watershed stope ts an
t^
be
The average watershed slope can be determined using the Mowing relationship: 1000CI
Sw =
A Where:
Geiachew Alem &. Associates
1 SEC RON VIII: CONCLUSION AND 146 recommendationsDesign of Large Gully Treatment Works: Tana Belts Integrated Waler Resources Des dopment Project
Y = average watershed slope in percent,
C = total contour length in feet,
I = contour interval in feet, and
A = drainage area in square feet.
Some [assumed] characteristics and limitations to this method of design peak flood estimation
The watershed drainage area must be greater than 0 40 ha and less than 810 ha.
The watershed should have only one main stream. If more than one exists, the branches must have nearly equal Tc's,
The watershed must be hydrologicaily similar; i.e., able to be represented by a weighted CN. Land use, soils, and cover are distributed uniformly throughout the watershed. The land use must be primarily rural.
If the computed Tc is less than 0.1 hour, use 0.1 hour. If the computed Tc is greater than 10 hours, peak discharge should be estimated by using the NEH-4 procedures, which are automated in the TR-20 computer program.
The longest flow length is between 61 meters and 7900 meters.
When  the  average  watershed  slope  is  less  than  0.5  percent,  a  different  unit hydrograph shape can be used.
When the weighted CN is less than 40 or more than 98, use another procedure to estimate peak discharge.
When the average watershed slope is greater than 64 percent or less than 0 5 percent, use another procedure to estimate Tc.
Gcuchcw Alcm & Associdics
I SECTION VIII: CONCLUSION AND 14?
RECOMMENDATIONS_______ Design of Large Gully Treatment Works: Tana Belcs Integrated Water Resources Development Project
Estimating Weighted CN
The  Weighted  CN  here  is  the  same  as  the  Runoff  Curve  estimated  for  the  adapted Cook’s method and all CC values in Annex C are used here for US SCS method.
Gctachcw AJcm & Associates
SECTION VIII: CONCLUSION AND 148 RECOMMENDATIONSle               S. Odly Tyealffienl Wwks: T.na Belos    WalCT Dc n .r L.,
Worksheet # 1
l»«h- "*g_
Watershed Name: Megenagna Community Watershed, Finger Gully H 1 Woreda Name: Misrak Estie Region: Amhara
A.       Estimating Time of Concentration 1.       Data :
Rainfall distribution type........................
Drainage area........................................
Runoff Curve number...........................
Watershed slope (average)...................
,A.=  168.5  acres CN .= 62
Flow length.......................................
.....
Y = _2_(%) ...,L= 3511.9 ft
2.
Tc using ,Y, CN and figure 2-27
....= 2.47 hrs
3.
Or using equation 2-5
1000
0.8 r  _ gi
L 1 CN J
ii4or0-5
= 2.47 hrs
B.
1.
2.
3.
Estimating Peak discharge
Frequency..................................
................... Yr
Rainfall, P (24-hours) ...............
.......................... 2 .inches
Initial abstraction , la ................
......................... 1.226 in
4.
5.
6.
7.
Compute la/P ratio.............................................. 0.613..........
Unit peak discharge...................................0.16 cfs/ac/in
Runoff, Q................................................ 0.076....... in (Use P and CN with figure 2-11)
Peak Discharge, q ....................... 2.05.................. cfs.
p
(where p  = q  A. Q ) or 0.058 m /sec
qd
3
Ciclachew Alcm & Associates
I SECTION VIII; CONCLUSION AND 149 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Beles Integrated Water Resot^esDeve^nU^----------------------------------------
Worksheet # 2
Rainfall
distribution
Drainage
type
area
Runoff
Curve
number
Watershed
slope
(average)
Flow length..
.........................
.................
Watershed Name: Megenagna Community Watershed, Finger Gully it 2 Woreda Name: Misrak Estie Region: Amhara
C.       Estimating Time of Concentration
4.       Data:
..=(I,IA,II,III)
,A.= 289.9 acres
CN .= 65
Y = _2__(%)
.... L= 9528.4 ft
....= 4.91 hrs
5.
6.
Tc using
,Y, CN and fi
gure 2-27
Or using
equation 2-5
0.8 r      _ al
1000
1
[ CN -9!
ii4or0-5
=4.91
D. Estimating Peak discharge
8. FrequencyYr
9. Rainfall, P (24-hours)         2 .inches
10.      Initial abstraction , la............... ......................... 1.077 in
Storm
#1
Storm
#2
Storm
11.      Compute la/P ratio...................
0.538.
12.      Unit peak discharge................. ................ 0.10 cfs/ac/in
13.      Runoff, Q................................. ............... 0.14........in (Use P and CN with figure 2-11)
14. Peak Discharge, qp
4.0586cfs.
(where p  = q  A. Q ) or 0.1148 m /sec
qu
3
Gctachcw Alcm & Associates
I SECTION VIII: CONCLUSION AND 150 RECOMMENDATIONSDesign of Large Gullv Treatment Works: Tana Bclcs Integrated Water Resources Development Projert
Worksheet # 3
Watershed Name: Megenagna Community Watershed, Finger Gully # 3 Woreda Name: Misrak Estie Region: Amhara
E.       Estimating Time of Concentration
7.       Data:
8.
9.
Rainfall distribution type.......................
Drainage area.......................................
Runoff Curve number..........................
Watershed slope (average)..................
Flow length........................................... Tc using ,Y, CN and figure 2-27
Or using equation 2-5
. ___ (I,IA,11,111)
A = 168.5 acres
CN = 64
Y = _2_(%)
... L= 4760 ft
....= 2.923 hrs
I 0 8               - 91
ii4or0-5
= 2.92 hrs
F.        Estimating Peak discharge
Storm
#1
Storm
#2
Storm
#3
15.
Frequency.................................................... Yr
16.      Rainfall, P (24-hours)......................................... 2 inches
17.
18.
19.
20.
21.
Initial abstraction , la.......................................... 1.077 in
Compute la/P ratio............................................. 0.538..........
Unit peak discharge.................................. 0.10 cfs/ac/in
Runoff, Q................................................ 0.14........in (Use P and CN with figure 2-11)
Peak Discharge, q ...................... 2.359...cfs.
p
(where p  = q  A. Q ) or 0.0667 m /sec
qu
3
Gclachcw Alctn & Associates
I SECTION VIII: CONCLUSION AND 151 RECOMMENDATIONSteiEn of!          Oully Tns-K*
I, WJ W..«r Rgg^D^P^J^
Worksheet # 4
Watershed Name: Megenagna Community Watershed, Finger Gully # 4 Woreda Name: Misrak Estie Region: Amhara
G. Estimating Time of Concentration
10.       Data:
Rainfall distribution type.........................
Drainage area........................................
Runoff Curve number............................
Watershed slope (average)....................
Flow length............................................. 11.      Tc using ,Y, CN and figure 2-27 12.      Or using equation 2-5
0.8 r1000 _ Qi
..=______ (1,1A,II,III)
.A.= 182.36 acres
CN.= 56
Y = _1_(%)
..._L= 2135.28 ft
... = 2.91 hrs
L [ CN 9J
ii4oy°-5
H. Estimating Peak discharge
= 2.91 hrs
22. Frequency........................................................... Yr
23. Rainfall, P (24-hours)       ......................................... 2 .inches
24.      Initial abstraction , la................ .......................... 1.571 in
25.      Compute la/P ratio....................
0.7855.
26.      Unit peak discharge.............
27.      Runoff, Q..........................        ............... 0.021........ in (Use P and CN with Figure 2-11)
28. Peak Discharge, q ................
p
(where p  = q  A. Q )
qu
........ 0.536...cfs.
or 0.015 m3/sec
Gctachcw Alcm & Associates
SECTION Vllk CONCLUSION AND 1 RECOMMENDATIONS... -1 _ r.„.
Design of Large Gullv Treatment Works Far-a Be_to
Imrcraied Water Resources Development Pro-ect ---------------------------------------- -
Worksheet # 5
Watershed Name Megenagna Community Watershed, Finger Gully * 5 Woreda Name Misrak Estie, Region: Amhara
I. Estimating Time of Concentration
13.     Data:
Rainfall   distribution   type
Drainage
area
Runoff Curve number Watershed slope (average)
Flow length
14  Tc  using  ,Y,  CN  and  figure  2-27 15. Or using equation 2-5
0.8 rlQOO _ g1
=(I.IA.II III)
A.= 141.14 acres
CN = 54
L= 3247.2 ft
....= 4.38 hrs
Y = _1_(%)
[ CN 1
ii4oros
= 4.38 hrs
J.
29.
Estimating Peak discharge
Frequency ............................................
...... Yr
30.
Rainfall, P (24-hours).........................
2 .inches
31.
Initial abstraction , la............................
1.704 in
32.
Compute la/P ratio................................
0.852
33.
Unit peak discharge..........................
34
Runoff. Q........................................
in
(Use P and CN with figure 2-11)
35.
Peak Discharge, q .............
p
0.338 cfs
(where p  = q  A. Q ) or 0.00958 m /sec
qu
3
Gcue hew Alcm 4k AiMKtaio
| SECTION V III: CONCLUSION AND 153
RECOMMENDATIONSof!TK«mCT. T... Beta Inured W.u,
D^^.l
Worksheet # 6
Watershed Name: Enkulal Community Watershed, Enkulal Model Gully Woreda Name: Dera Region: Amhara
K. Estimating Time of Concentration
16 Data:
Rainfall Drainage
distribution
type
area
Runoff
Curve
number
Watershed slope (average)
Flow length............................................ 17.      Tc using ,Y, CN and figure 2-27 18.      Or using equation 2-5
0.8 r1000 _ q-i
.=(I.IA.II.III)
A.= 195.84 acres
CN .= 64
Y = _28_(%)
.... L= 4316.48 ft
....= 0.72 hrs
— c"
ii4oros
J =0.72
L. Estimating Peak discharge
36. FrequencyYr
37. Rainfall, P (24-hours)
38. Initial abstraction ,1a1.125 in
...2.57.inches
39.      Compute la/P ratio.......................................... 0.44
40.      Unit peak discharge0.38 cfs/ac/in
41.      Runoff, Q0.20in
(Use P and CN with figure 2-11)
42 Peak Discharge, qp
(where p  = q  A Q ) or 0.42 m /sec
14.88 cfs.
qu
3
Gc Lac hew Alcm & Associates
I SECTION VIII: CONCLUSION AND 154 RECOMMENDATIONSDesign of Large Gully Treatment Works. Tana
Boles Integrated Water Resources Development Project^
Worksheet # 7
Watershed Name: Awzet Community Watershed, Finger Gully * 1 & 2 together Woreda Name: Farta Region: Amhara
M. Estimating Time of Concentration
19.     Data:
Rainfall distribution type........................
Drainage area........................................
Runoff Curve number...........................
Watershed slope (average)...................
Flow length............................................ 20.      Tc using ,Y, CN and figure 2-27 21.      Or using equation 2-5
0.8 rlOOO _
..=______ (I.IA.II.III)
,A.= 118.92 acres
CN = 64
Y = _37_(%)
...L= 5576 ft
....= 0.77 hrs
Tc =
1  CN J
ii4or0-5
= 0.77 hrs
N. Estimating Peak discharge
43. Frequency........................................................... Yr
44. Rainfall, P (24-hours)       ..................................... 2.37 inches
45.      Initial abstraction , la...........
.......... 1.125 in
46.      Compute la/P ratio.............
.......... 0.47.........
47.      Unit peak discharge.............
..... 0.32 cfs/ac/in
48.      Runoff, Q...................
(Use P and CN with figure 2-11) 49.      Peak Discharge, q ...
p
(where p  = q  A. Q )
qu
5.33...cfs.
or 0.15 m3/sec
Getachew Alcm &. Associates
I SECTION VIII: CONCLUSION AND 155 recommendationsDesign of Large Gully Treatment Works: Tana Bales Integral Water Rcsourc^Pcvelopnwn^^
Worksheet # 8
Watershed Name: Washaw Community Watershed, Finger Gully # 1, 2, 3 and 4 Woreda Name: Mecha Region: Amhara
0. Estimating Time of Concentration
22.     Data:
Rainfall distribution type........................
Drainage area.......................................
Runoff Curve number...........................
Watershed slope (average)...................
Flow length............................................ 23.      Tc using ,Y, CN and figure 2-27 24.      Or using equation 2-5
7 0.8 r1000 _ o]
. =______ (I,IA,II,III)
.A.= 94.3 acres
CN .= 52
Y = _5__(%)
....£.= 4264 ft
....= 2.6 hrs
L L CN J
ii4oy°5
= 2.6 hrs
P.
Estimating Peak discharge
50.
Frequency..................................................... Yr
51.
Rainfall, P (24-hours)................................... 3.75 inches
52.
Initial abstraction , la................................... 1.846 in
53.      Compute la/P ratio...................................... 0.49.........
54.      Unit peak discharge.................................. 0.16 cfs/ac/in
55.      Runoff, Q................................................. 0.315 in (Use P and CN with figure 2-11)
56.      Peak Discharge, q ....................... 2.05...................cfs.
p
(where p  = q  A. Q )
qu
or 0.058 m3/sec
Gctachew Alcm & /Associates
I SECTION VIII; CONCLUSION AND 156 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Boles Integrated Water Resources Developmental
Worksheet # 8
Watershed Name Washaw Community Watershed, Finger Gully 111,2,3 and 4 Woreda Name: Sekela Region: Amhara
Q.       Estimating Time of Concentration
25.     Data :
Rainfall distribution type.......................
Drainage area.......................................
Runoff Curve number...........................
Watershed slope (average)..................
Flow length........................................... 26.     Tc using ,Y, CN and figure 2-27 27.     Or using equation 2-5
.-1000
. =_____ (I.IA.II.III)
,A.= 24.21 acres 
CN.= 52
Y = _24_(%)
....( = 1066 ft
= 0.39 hrs
L CN ii4oy°-5
-9]
= 2.6 hrs
R.       Estimating Peak discharge
57.     Frequency..................................................... Yr
58      Rainfall, P (24-hours)................................... 4.07 inches
59.      Initial abstraction , la...................................1.846in
60.      Compute la/P ratio.......................................0.45......
61.     Unit peak discharge................................... 0.46 cfs/ac/in
62.      Runoff, Q................................................ 0.34 in
(Use P and CN with figure 2-11)
63.      Peak Discharge, q ...................... 3.78.................. cfs.
p
(where p  = q  A. Q )
qu
or 0.107 m3/sec
Gciachew Alcm & Associates
I SECTION VIII: CONCLUSION AND 157 RECOMMENDATIONSDesign of Large Oully Treatment Works: fana
BeJes Integrated Water Resources Development Projo^
Annex E: Chute Design
Maximum Bank Gradient
The recommended maximum desirable side slope for a large roc^ ^d chute is (
V); however, side slopes as steep as 1.5:1 can be also stable if the rock is individually 
placed rather than being bumped.
Placement of chutes
The typically minimum thickness of non-dispersive soil is 200mm, but preferably 300mm. Design of energy dissipation at the base of the chute
The design of energy  dissipaters  is  too complex  to be covered within this  report. The need for specialist hydraulic  advice will depend on the complexity  of the gully  erosion, and the risks associated with ongoing gully erosion
Typically, rock at the base of the chute must:
• 
Extend along the bed of the gully a distance at least 10 times the depth of
the flow passing down the chute; and
• 
Extend up the banks of the gully a distance of at least twice the depth of
the flow (y) passing down the chute.
Hydraulic design of rock-lined chutes31:
Step 1 Determine the design discharge or peak flow (Q ) for the chute
p
Step 2 Determine the slope (S) of the chute from the gully geometry. The face of the
chute should be straight, with no bends or curves unless there exists a bend in the
natural course of the gully,
Step  3  Determine  the  required  geometry  of  the  chute's  crest  using  an  appropriate  weir equation.
Step 4 Determine the design unit flow rate (q) as done for spillway design. This may be estimated by dividing the design discharge (Q) by the crest width determined in step 1
This assumes that the main chute will have the same width as crest of
is strongly recommended).
Step 5 Determine the likely density (specific gravity, Sr - Table 2) and (dso/dw) of the rock to be used on the chute.
the chute (which
c uie '■wn cn
a size distribution
Step 6 Decide the design safety factor (SF) using standard table as a guide.
Catchment & Creeks Pty Ltd^gnof Rockchu e f^ Nyst
1   1    u          abtl
u
GC1^> A.cm * Assoc
at ,on, Apr|| ,Ql0
| SEC noN — 
158
RECOMMENDATIONS_______Design of Large Gully Treatment Works- Tana Belos Integrated Water Resources Development Project
Step 7 using manning's equation determine the uniform flow depth (ymax) and required size of the rock size (dso) for the chute. Manning's equation: Q =A.V = (1/n) A.R .S is used to determine Q
Step 8 The depth of the chute provides  an indication of the minimum height of rock protection. In many cases the rock protection will need to extend to the top of the gully each side of the chute (as per photo 8). The required depth of the chute (h), being the greater of:
(i)  300mm (unless a lower depth is supported by expected flow conditions)
(ii) 0.67(H) plus minimum freeboard of 20 cm; fH' determined from step 4),
(iii)  The uniform flow depth (y) plus a minimum freeboard of 20 cm, or the
equivalent of the flow depth, whichever is smaller.
Step 9 Design the required outlet energy dissipation structure at the base of the chute. The 'local' uniform flow velocity (V) down the chute may be estimated by dividing the design unit flow the rate (q) by flow depth (y), this flow velocity will be slightly greater than the average flow velocity, which equal to the total discharge (Q) divided by the total flow area (A).
Gcuchcw Alem & Assoc iaics
| SECTION Vlll: CONCLUSION AND 159 RECOMMENDATIONS0^ TUre Gully TCTm..i Wwks:
Mo
W~
l>^^2£----------------------------------
Annex  F:  Stability  analysis  and  bearing  capacity  of  soils  for  Check  Dam construction
Introduction
An essential criterion in the design of check dams is that the found^ °'2.
ti
S Pug
against soil rupture and shear failure which is called bearing capacity J.tenon ue. the foundation must have sufficient factor of safety against ultimate load beanng capacily o the soil as a result of the load from the rock check dams or wet masonry check dams
unuer uiib biuuy.
..
..
tho
Foundation design is based on providing a means of transmitting the oa «
structure to the underlying soil without shear failure i.e. a plastic flow and or a lateral expulsion of soil from beneath the foundation or excessive settlements of the soil under the imposed loads. This aspect of the report is therefore, to see if both of these design requirements for stability of the check dam are satisfied.
Geotechnical properties of soils
*
The geotechnical properties  of soils  are (i) grain size analysis; a fundamental measure for the classification of engineering soil, (ii) inter-granular voids, (iii) grain shape and consistency  such as  liquid limit (LL), plastic  limit (PL) and Shrinking limits  (SL). These engineering properties  are determined using laboratory  methods  as  described in Section II: Methodology. They  are soil factors  related to the behaviors  that are problematic  to the construction, functioning and maintenance of the structures. Water content of soils play very important role and affect the engineering properties of the foundation soil.
Two engineering properties  of soils  are especially  important to many  types  of engineering works and these are compressibility and shear strength. Shear strength and compressibility  depend on water content of the particular soil and determine bearing capacity and its stability on slope surface. Compressibility is the decrease in volume of a soil mass  as  a consequence of either natural or artificial means. Compressibility  of the soils  can be roughly  estimated by  Atterberg limits  as  presented in tables  in section III. Consolidation of the soil (the release of air and water from the soil under a static load is Compaction. Compaction is  an artificial densification of soil which is  expressed in the decline in bulk density.
The shear strength is the resistance of soil to sliding of one mass against another. The shear strength of engineering soils  has  an important role in design construction and long-term stabihty  of check  dam structure on and in soil material, stability^ n^tu^ slopes m s°H material is dependent on soil strength. Non-cohesive and cohes ve soHs exhibit different shear strength. The shearing strength index, density  ^osiS water content, compressibility, Permeability, soil compaction acce ding to Kr^nl
" * US,n9 Tab'e °f
VTs g,“ln Xet
Gctachew Alcm & Associates
I SECTION VIII: CONCLUSION AND ] 60 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Belcs Integrated Water R^^soureesDevelopn^^
Table F.l Engineering use chart for soils classified by unified soil classification system
(USCS)
Description of the
soil
Group
symbol
Typical names of soil
groups
Shearing
strength
when
compacted or
as a result of
the load
Compressibi
lity when
compacted
and
saturated
Workability
as a
construction
material
Inorganic clay of high plasticity, fat (swelling)
clay
CH
Poor
High
Poor
Organic clay of mediums to high
plasticity
OH
Poor
High
Poor
Inorganic clay of low to medium plasticity
CL
Fair
Medium
Fair
(Source: Extracted from Krabenobubul and Wagner 19
83)
On the basis  of the engineering properties  of the soils  at the five model gullies  we recognize that the soils in the low lying areas of Megenagna Model gully is vertisol, day soils of high plasticity, and shear strength and compressibility of the soils are poor and high for the respective engineering properties against the load (check  dam). Hence we are required to put a layer of stone and gravel aggregate at the bottom of the foundation before laying the gabion rapped rock check dam over it. A 15 to 20 cm depth rock aggregate is sufficient. In other part of the watershed, elevated areas, the soils are Laptosol and Nitosol shear strength is fair and compressibility is medium to high.
If  need  be  the  weight  of  the  rock  check  dam  can  be  calculated  using  the  followinq empirical equation (Catchment & creeks Pty Ltd).
a__ - 127 x SF X KI x K2 x So0 5 x q° s xy° 2S
50
($r-l)
Where: dso
KI Kz Q Sr
nominal rock size (diameter in meters) of which 50% are smaller = Correction factor for rock shape, 1 for angular (fractured) rock 1 36 for
rounded rock (i.e. smooth, spherical rock),
= Correction factor for rock grading,
= 0.95 for poorly graded rock, ( Cu = d60/dl0 < 1 i nc (Cu > 2.5), otherwise, K2 = 1.0 (1.5<Cu<2 5)
= flow per unit width (m3/sec/m),
' 
' 1-36 f
' 
°' rOd<'thiS Va' can * used for esUma
graded rOck
'" 9 the weight of each
1
Getachew /Mem & Associates
1 SECTION Vlll: CONCLUSION AND 161 RECOM M EN D AT1ON SDesign of Large Gully Treatment Works: Tana Bcles Integrated Water Resources Development Project
So = bed slope = tan 0 (m/m),
SF  =  factor  of  safety  (see  table  below)
Y = depth of flow at a given location
Table F.2 Typical relative density (specific gravity) of rock (Catchment & creeks Pty Ltd)
Rock type
Relative density (Sr)
Sand stone
2.1 to 2.4
Granite
2.5 to 3.1, commonly 2.6
Limestone
2.6
Basalt
2.7 to 3.2
Note: Most rocks in all model gullies are basalt, and the specific gravity ranges between 2.7 and 3.2
The weight of the rock in one meter width gabion check dam can be calculated based on the total volume less the pore space and this value is multiplied by the specific gravity. The total weight obtained acts downward at the center of gravity. An upward force acts opposite to the weight.
In response to the downward force (weight of the rock check dam), different soil types behave differently. A typical feature for deep well drained clay  soil, Nitosol or Laptosl behaves as shown in figure below. Certain soil with high water contains as in vertisol or where there is higher ground water interference, the soil first loses the water and show shrinkage or movement. In the later case the recommended practice for check dam is to remove the surface soil and lay down stone aggregate at the bottom of the foundation. For practical purposes, slight movement will always be their but for check dams this does not have serious damage and risk.
Q ult
1 1 1
I I   1
« I I
I
<- check dam length
..... I1''' ' ’ Original ground surface
0.5 or 0.6 m excavation below the ground surface Modified ground surface
(stone agregate) with an
equivalent surcharge
Figure F.l Direction of failure
Getachcw Alem & Associates
I SECTION VIII: CONCLUSION AND 162 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Boles Imegraied Water Resources Development Projecl
Table F.3 Recommended safety factor (SF) for use in determining rock size
Safety
factor
(SF)
Recommended usage
Example site condition
1.2
•  Low risk structures
■ Failure of structure is most unlikely to cause loss of life or irreversible property damage,
•  Design of permanent rock chutes with all voids filled with soil and pocket planted,
• Rock check dams and chutes located within most rural gullies and low risk urban gullies,
■ Permanent rock check dam and chutes that are likely to experience significant sedimentation and vegetation growth before experiencing high
flows,
1.5
■ High risk structures
• Failure of structure may cause loss of life or irreversible property 
damage,
■ Gullies where failure of the chute is likely to cause ongoing, severe gully erosion and or damage to
assets.
•   Rock chutes in urban gullies located close to homes,
•   Rock chutes and check dam for gully control designed for a storm frequency less than 1 in ten years
Using the above calculation, the total weight of the rock check dam will be estimated. The total area of the check dam structure that came in contact with the ground surface is calculated based on the design sketches.
If the total weight (kg) of the rock check dam structure per square area is greater than the limit for bearing capacity for the specific soil type at the site, there would be instability to the structure. Different measures will be taken to correct this.
(i)        The base area of the rock check dam be increased,
(ii)       Depth of the foundation is increased,
(iii)      Rock foundation will be strengthened
If the bearing limit is greater, then the structure is stable and can be constructed as designed. The table 3 below presents the bearing limit for the different soil types and rock foundation available at site.
Gctachew Alcm & Associates
SECTION VIII; CONCLUSION AND 163 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Belcs Integrated Waler Resources Development Projec£
Table F.4 Limit for bearing capacity of the different soils for different load sizes
SI
No
Types of soil
Angle of
earth
Frustum
(Degrees)
Unit Wt of
soil
(Kg/cum)
Limit
bearing
capacity
(kg/sq.m)
1
Normal dry soil
(a) Without undercut (b) With undercut
30
30
1440
1600
25000
25000
2
Wet soil due to presence of sub soil Water/Surface Water
15
940
1250
3
Black cotton soil
(a) In Dry portion
(b) In Wet portion
0
0
1440
940
12500
12500
4
Sandy soil
(a) With clay content 0-5% (b) With clay content 5-10%
10
20
1440
1440
25000
25000
5
Fissured Rock/Soft rock (with
undercut)
(a) In Dry portion
(b) In Wet portion
20
10
1700
940
62500
62500
6
Hard rock
-
-
125000
7
Normal hard dry soil (Morrum) with undercut
30
1600
40000
Sample Exercise:
(i)    Get the soil type determines as shown in table above
(ii)    Estimate the volume of rock for each check dam less the pore space.
(iii)   Multiply the volume by the density of the rock (S ), and you get the
r
total weight of the check dam.
(iv)   Determine the surface area (A) of the check dam in contact with the ground,
(v)    Divide the total weight by the surface area (A) and get a value kg per sq.m,
(vi)   Compare the kg per sq.m value with the value in table above, and if the value is less than shown, the structure is stable for the soil type and if the value is higher, increase the thickness of the check dam
Getachew Alem & Associates
I SECTION VIII; CONCLUSION AND 164 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Waler Resources Development Project-----------------
Annex G: Calculation for sample peak flood discharge - Direct Measurement
Location: Dera Woreda ,
Gully: Enkulal Model Gully
The following field data are observed in a model gully section approximately 100 meters 
above the mouth of the gully.
Date of observations
Gauge reading in cm
Gauge reading in cm
l32
2 33
Date of observation
1
2
1
6/08/2013
75
16
21/08/2013
-
2
7/08/2013
80
17
22/08/2013
40
3
8/08/2013
100
18
23/08/2013
30
4
9/08/2013
95
19
24/08/2013
60
5
10/08/2013
50
20
25/08/2013
20
6
11/08/2013
110
21
26/08/2013
30
7
12/08/2013
40
22
27/08/2013
r so
8
13/08/2013
50
23
28/08/2013
30
9
14/08/2013
-
24
29/08/2013
-
10
15/08/2013
30
25
30/08/2013
-
11
16/08/2013
92
26
31/08/2013
-
12
17/08/2013
105
13
18/08/2013
13834
72.5
14
19/08/2013
60
15
20/08/2013
-
The direct reading is converted into vertical flow data (d) using the data on channel geometry obtained at the time of the gauge calibration and installation. The slope of the channel (S) is obtained from the GPS readings taken to characterize the gully.
Reading of the discharge as observed on a gauge laid along the slope of the gully bank ’Converted vertical reading in cm of the flow depth in the gully channel
* The highest flood record for the observation period, August
Gctachcw Alcm &. Associates
I SECTION VIII: CONCLUSION AND 165 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources Development Project------------------
Observed peak flood data for the season assuming August is the month: b=50 cm
d=72.5cm
Slope (S) of the gully channel at the point of interest = 0.58
e
e=103.7
z=1.433
Cross sectional area = bd + zd2
= 1.343 + 0.753
=1.13 m2
Using manning equation Q= A (R2/3) (S1/2)
bd+zd2
b+2dy/l+z2
Q = 13.029 m3/sec
= 0.457
The Cook's Method estimate = 13 my/sec
Direct measurement using the highest stage discharge = 13.029 m3/sec
Gclachcw Alem & Associates
SECTION VIII: CONCLUSION AND 166 RECOMMENDATIONSDesign of Large Gully Treatment  Works: Tana Bclcs Integrated Water Resources Development Project
Annex H: Procedures for preparing Bill of Quantities
A. Loose Rock check dam
(i)        Produce a working sketch for each type of Check dam
(ii)       This check dam is recommended for small to medium size gully with stable soil and slope gradient of less than 15°/o.
-f-
1 h= h
0,01
1 m       ------------
0.50 to 0.50 m
—concave surface top
“                               C---- Loose rock \       surface
eifht               1                   \ e dam                                  \
1.25
Loose Rock Chee
k Dam Design                /
Original ground
surface
Figure H.l Design sketch of a loose rock check dam
(iii)      Foundation clearing of about 30 cm depth and a key of about 0.70 m
(iv)     Volume of rock for the foundation work:
Depth x width of the bottom of the gully x length of the dam at the floor
(v)      Volume of the dam above the foundation: you have two triangle volumes at the sides and rectangular volume at the center and determine the individual volumes including the key at each side of the gully,
(vi)     Add all volumes and make adjustment for the concave structure at the top of the dam surface
Getachcw Alem & Associates
I SECTION VIII: CONCLUSION AND 167 RECOMMENDATIONSDesign of Large Gully Treatment Works: Tana Bclcs Integrated Water Resources DevclopmemPr^-------------------------
B. Gabion Rapped Check dam
(i)  Produce a working sketch for each type of Check dam
(ii) This check dam is recommended for small, medium and large size gullies with both stable and unstable soils. It is not recommended for higher slope gradient
(iii)            It is made to have a defined trapezoidal shape as shown in figure
(iv)           Its volume is calculated based on well known geometric
relationships
(v)   Cross sectional area
Trapezoid section, A = d x TW + Zd2
Rectangular spill way, B = (ymax + Free board) x L
Rock check dam C=A-B
Since  the  width  of  the  check  dam  is  one  meter,  the  volume  of  the  check dam structure is estimated by multiplying it by 1 meter.
(vi)           Apron areas
Upstream  apron  is  one  meter  length,  and  use  a  single  stone  lining  of about 25 to 30 cm depth
Downstream apron length is calculated by multiplying the crest height
Gully bank protection is done up to the crest height level with the same depth of stone lining
on the bank
Gclachcw Aicm & Associates
i SECTION VIII: CONCLUSION AND 168
recommendationsDesign of Large Gully Treatment Works: Tana Beles Iniegratcd Water Resources Development Project
Annex I: Bill of Quantities
Gctachcw Alcm & Associates
| SECTION VIII: CONCLUSION AND 169 RECOMMENDATIONSTable 1. Summary of the Bill of Quantities for the Check dam structures with Gabion in Five Woreda Model Gullies
M3 of stone/ Rock
cost In Birr
Cost for the Gabion
Total Cost In
Birr
Woreda
f
Finger #
*43 of soil
excavate
d
Bank
lining
Aprone (up
stream
+downstrea
m)
Foundati
on
Rock 
for
Gabion
Total
Excavati
on
Rock
supply
Construct!
on
Total
1x0.5
1X1
2X1X1
4X1x2
10%
10%
40%
40%
1 Dera
156.97
154.82
153.4
17.35
454.78
780.35
12558
54624.5
62428
129610.1
2842.375
9754.375
156070
312140
2 Farta*
3 M.Estie
#1
156.47
157.97
154.4
16.95
473.8
803.12
12518
56218.4
64249.6
132985.6
2961.25
10039
160624
321248
#2
345.29
333.63
321.8
53.2
994.28
1702.9
27623
119204
136232.8
283059.7
6214.25
21286.375
340582
681164
#3
117.22
98.59
109.56
28.57
348.17
584.89
9377.6
40942.3
46791.2
97111.1
2176.0625
7311.125
116978
233956
#4
72.98
56.23
69.84
19.6
244.42
390.09
5838.4
27306.3
31207.2
64351.9
1527.625
4876.125
78018
156036
#5
197.25
149.31
187.78
55.05
592
984.14
15780
68889.8
78731.2
163401
3700
12301.75
196828
393656
4 Mecha
337.43
348.9
312.82
36.75
975.92
1674.4
26994
117207
133951.2
278152.9
6099.5
20929.875
334878
669756
5 Sekela
264.43!
279.45
244.34
16.1
710.43
1250.3
21155
87522.4
100025.6
208702.8
4440.1875
15629
250064
500128
Total
1,357,375.10
29,961.25
102,127.63
1,634,042.00
3,268,084.00
,,
6 391 589.98
• Farta Model Gully should be left out for other options than the use of Check dam. It is Expensive. Refer recommendation in the report

Summery

Summary: Design of Large Gully Treatment Works

Summary: Design of Large Gully Treatment Works in Tana Beles Project Areas

1. Introduction

The document presents a comprehensive study on gully erosion treatment in five woredas (districts) within the Tana Beles Integrated Water Resources Development Project (TBIWRDP) area in Ethiopia. Gully erosion is identified as a major threat to agricultural lands, infrastructure, and aquatic ecosystems in the region.

            Key Problems:
            Ethiopia loses approximately 1.5 billion metric tons of soil annually to erosion
Gullies contribute 70-90% of total sediment in erosion processes
5% of Abay River Basin (983,850 ha) affected by gully erosion
Severe impacts on Lake Tana from sediment-laden tributaries

        

Project Objectives:

Develop detailed designs for gully treatment in five selected woredas
Characterize and classify gullies through field surveys
Provide knowledge transfer to local staff

2. Methodology

The study employed a comprehensive approach to gully assessment and treatment design:

Key Methodological Components:

Component	Description
Literature Review	Examined local and international experiences in gully control
Field Investigations	Detailed surveys of gully geometry using GPS, tapes, clinometers
GIS Mapping	Production of digital elevation models and land use maps
Hydrological Analysis	Peak flood estimation using three methods (US SCS, Rational, Cook's)
Community Engagement	Focus group discussions with affected communities

Gully Classification:

Gully Class	Depth (m)	Drainage Area (ha)
Small	< 1	< 2
Medium	1-3	2-20
Large	> 3	> 20

3. Study Areas

Five model gully watersheds were selected across two zones in Amhara Region:

Site Characteristics:

Woreda	Zone	Watershed Name	Area (ha)	Key Features
Misrak Estie	South Gondar	Megenagna	7000	Source of Gumera River, severe gully network
Dera	South Gondar	Enkulal	324	Gullies caused by road culvert runoff
Farta	South Gondar	Tikur Woha	503	Steep slopes (up to 100%), intensive cultivation
Mecha	West Gojam	Biret Afer	323	Deep red clay soils (Nitisols)
Sekela	West Gojam	Washaw	9.55	Discontinuous gullies on shallow soils

            Common Issues Across Sites:
            High rainfall (1300-1800 mm annually)
Steep topography (many slopes >30%)
Clay-dominated soils with poor structure when wet
Intensive cultivation and overgrazing
Road construction contributing to gully formation

        

4. Key Findings

Soil Characteristics:

Dominant soils: Vertisols (swelling clays), Nitisols (red clay), Leptosols (shallow)
High clay content (often >50%) leading to poor drainage
Bulk density: 1.3-1.65 g/cm³
Low infiltration rates (0.0017-2.2 cm/hr)

Gully Characteristics:

Site	Gully Length (km)	Max Depth (m)	Soil Loss (m³)	Peak Flood (m³/s)
Megenagna	8.0	7.0	260,466	7-20
Enkulal	0.87	4.4	21,668	13
Tikur Woha	1.64	5.0	94,410	9.4
Biret Afer	1.30	3.0	13,006	5.0
Washaw	0.33	5.0	13,006	5.0

Hydrological Analysis:

Three methods were compared for peak flood estimation:

US SCS Method: Most rigorous but produced lowest estimates
Rational Method: Common but tends to overestimate
Adapted Cook's Method: Most appropriate for Ethiopian conditions

5. Proposed Gully Treatment Design

Key Principles:

System approach considering entire watershed
Community participation in planning and implementation
Combination of structural and biological measures
Treatment of continuous gullies as basic units

Structural Measures:

Check dams as primary intervention, with design specifications:

Materials: Rock, gabion, wood, or concrete
Spacing: 1.2 × (dam height)/(gully gradient)
Design based on broad-crested weir formula: Q/L = C × (2/3)√(2g)H^3/2
Height of flow over crest: 0.4-0.75 m

            Treatment Approaches by Gully Section:
            Headcut: Energy dissipation structures
Channel: Series of check dams
Banks: Reshaping and vegetative stabilization
Mouth: Large check dams for sediment trapping

        

Biological Measures:

Vegetation establishment on stabilized banks and gullies
Species with dense root systems for bank stabilization
Integration with engineering structures for long-term stability

6. Environmental and Socioeconomic Benefits

Environmental:

Reduced sediment load to Lake Tana and tributaries
Improved groundwater recharge
Restoration of degraded lands

Socioeconomic:

Protection of agricultural lands
Income generation through construction works
Improved access across treated gullies
Enhanced water quality

7. Conclusions and Recommendations

Conclusions:

Gully erosion is severe in Tana Beles project areas due to natural and anthropogenic factors
Proper design of check dams requires thorough site investigation
Adapted Cook's Method is most appropriate for peak flood estimation
Integrated approach combining structural and biological measures is essential

Recommendations:

Implement proposed designs with community participation
Prioritize treatment of most active gullies first
Establish monitoring system for structure performance
Combine gully treatment with watershed-scale soil conservation
Build local capacity in gully treatment techniques