THE FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
Arjo-Dedessa Irrigation Project Feasibility Study Report
VOLUME - I
Survey and Investigation
VOLUME - 1(a)
Annexure - 1
Topographic Survey
Annexure - 2
Geomorphological
Investigations
Annexure - 3
Geological & Geotechnical
WATER WORKS OESIGN & SUPERVISION ENTERPRISE
IN ASSOCIATIOH WITH
INTERCONTINENTAL CONSULTANTS AND TECHNOCRATS INDIA PVT LTD
P () Bov 25oI. Addis Ababa Ethiopia
Tcl:(25l)l 6I45IJI/63 lXVii Fax (1251)1 615371 E-mail w w d s.ctfielccom ncl elARJO-DEDESSA IRRIGATION PROJECT
FEASIBILITY STUDY REPORT
CONTENTS OF THE REPORT
SERIAL NO.
VOLUME NO.
PARTICULARS
1
EXECUTIVE SUMMARY
MAIN REPORT
2
Part I - Report
3
Part II - Maps & Drawings
ANNEXURES
VOLUME -1
SURVEY AND INVETIGATION (ANNEXURES - 1 to 3)
4
Volume -1 (a)
Topographic Survey, Geomorphological Studies &
Geological & Geotechnical Investigation
Part I - Report
5
Volume-1 (b)
Part II - Appendices
6
VOLUME -II
NATURAL RESOURCES (ANNEXURES - 4 to 6)
Forestry, Energy & Catchment Development Plan
VOLUME - III
WATER RESOURCES (ANNEXURES - 7 to 11)
7
Volume - III (a)
Meteorological & Hydrological & Hydrogeological Studies
Dam & Appurtenant Works
8
9
10
11
12
13
14
15
16
Volume - III (b)
Part I - Report
Part II - Drawings
Irrigation & Drainage
Volume - III (c)
Part I - Report
Part II - Drawings
Hydraulic Structures
Volume - III (d)
Part I - Report
Part II - Drawings
VOLUME IV
AGRICUTLRUE (ANNEXURES - 12 to 18)
Volume - IV (a)
Soil Survey & Land Evaluation
Volume - IV (b)
Agricultural Planning
Volume - IV (c)
Livestock. Fisheries, Agricultural Mechanization &
Agricultural Marketing
VOLUME-V
ENVIRONMENTAL AND SOCIO - ECONOMIC ASPECTS
(ANNEXURES-19 to 25)
17
18
Volume - v (a)
Environment, Health & Socio-economic Aspects 
Volume - v (b)
Organization & Management, Physical Infrastructure,
Resettlement, Financial & Economic AnalysisANNEXURE- 1
TOPOGRAPHIC SURVEYAr Jo Dedessa Irrigation Project
Topographic Survey
May 2007
Table of Contents
TOPOGRAPHIC SURVEY
1.      INTRODUCTION
2. RECONNAISSANCE
3.       DAM SITE
I
1
4.
RESERVOIR AREA
1
5.
COMMAND AREA.
2
6.
SAMPLE AREA
2
7.
CONTROL POINTS / BENCHMARKS
2
8.
USE OF SOFTWARE
2
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TOPOGRAPHIC SURVEY
1.
Introduction
The
planning,
design  an
dc
onst
ruction
of  water
resource
s  d
evelop
ment
project
s  require
very 
detailed
knowledge
of
the
topogr
aphic  feat
ures  of
the
area.
Acc
ordingly
detailed
topographic surveys have been carried out for the dam & appurtenant works, reservoir area and the command area; and based on the details  of the survey  topographic  maps  on different scales  and contours  at required interval have been prepared to be used for planning and design of various components of the project. These are discussed below:
2.      Reconnaissance
Prior to the start of the detailed surveys a rapid reconnaissance survey was made of the area from the previous Master Plan study document and 1:50,000 EMA base map to define the scope of work  and identify  the irrigable area and dam site. After that a hand GPS Garmin 12xL instrument was used for navigation and fixing the command area and dam site feature to be surveyed by total station at later date.
3.      Dam Site
After  detailed  topographic  survey  for  the  dam  site  the  following  topographic  maps  have been prepared.
i)          topographic  map  of  dam  site  with  one  meter  contour  interval  on  following scales: 1:500, 1:1000 and 1:2000.
ii)         Longitudinal  profile  at  15m  interval  along  dam  axis  and  also  along  lines parallel to dam axis at 25 m interval up to 500 m width on the both upstream and downstream of dam axis.
iii)        Longitudinal  profile  along  the  center  line  of  spillway,  intake  structure  and proposed scheme of diversion.
4.      Reservoir Area
The contour map with 1.0 meter contour interval and scale 1 10,000 for the reservoir area was  prepared by  generating contours  with Global Mapper, since the area was  very inaccessible. The Digital Elevation Model (DEM) used for generation of one meter contour was based on 90 x 90 meter resolution image. National benchmarks were used to calibrate
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the contours. Subsequently  detailed ground topographic  survey  was  undertaken to take cross-sections  across  the reservoir at five representative locations  spread over entire reservoir Cross  sections  were prepared based on the actual topography  survey  and contours developed with DEM. The comparison of the two sets of the cross sections of the reservoirs area shows that they are quite in agreement with each other. This shows that process adopted is quite reliable.
5.     Command Area
The detailed topographic surveys for the proposed command area indicating all features & natural drainage lines have been carried out and the topographic maps on scale 1 10,000 with contour interval of one meter have been prepared. Against a tentative command area of 16,800 ha. as  given in the TOR, the topographic  surveys  have been carried out for 27,000 ha. Altogether, the following seven topographic maps have been prepared for the command area:
Topographic map for Arjo Dedessa Irrigation Sheet No. 1 to 7.
6.      Sample Area
Detailed topographic surveys for sample area measuring about 500 ha have been carried out and topographic maps on scale 1:2000 with contour interval of 0.5 meter have been prepared. These maps will be utilized for detailed planning of the distribution system
7.      Control Points / Benchmarks
A total of 72 control points were established, and concrete pillars were constructed at all
these control points. The topographic survey has been transferred from Ethiopian Mapping Agency (EMP) references and connected to all project topo-maps and references 
Permanent benchmark has also been established at all these points The values of
benchmark along with location of these control points are available in Table 1 1
8.      Use of Software
The topographic surveys have been carried out with the help of Total stations The details of the topographic surveys obtained from the field have been fed directly into the computer for processing and preparation of maps on desired scale and required contour interval The encoding of the surveying data to computer programme and processing to produce topographic maps, profiles, layouts and boundary mark have been done by using LIS CAD, Land CAD. surfer and civil designer software
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Table 1.1 Bench Mark Data
May 2007
EAST
NORTH
|ELEVATION
REMARK
242761
941920.69*
1431295
BM-C
242124
942295.7462
1401.455
BM-1
242087
943147.1824
1323.481
BM-2(INS.)
242108
943074.9411
1324.966
BM-2A(INS.)
243980
943260 2482
1391.683
BM-3
2441II
943364.909
1380.864
BM-3A
242756
941903.1133
1432.625
BM-B
242684
940626.6436
1345.242
BM.G
242806
940663.252
1337.652
BM.1G
243969
942055.7804
1368.515
TP-G11
243984
942072.6493
1367.355
TP-G11A
242464
941619.0438
1392.135
BM-MA-1
243285
942598.6461
1420.167
BM-MA-3
243236
942566.9109
1400.765
BM-MA-4
243228
942539.3341
1389.115
BM.MA-5
243248
942536.5806
1397.182
BM.MA-6
243200
942463.8763
1360.851
BM.MA-7
243207
942476.4079
1366.661
BM.MA-8
243272
942616.6913
1424.712
BM AX-2
243296
942648.9471
1429.786
BMAX-1
242776
941931.5725
1427.346
BM AX3
242746
941890.1834
1431.019
BM AX-5
242688
941809 8812
1415.601
BMAX-7
242732
941870.3174
1426 806
BM AX-7-BM AX-6
239691
942637.7218
1362.275
BM-5A
237253
944231.5579
1328.801
0M-7A
237400
944208.8067
1341 509
BM-7
235691
943730.0781
1364.923
BM-8
235574
943660.43 11
1361.895
BM-8A
234335
947496.9273
1410.409
BM-9 (INSTRUMENT
234232 1
947472.9771
1404.75
BM-9A (INSTRUMENT)
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Table 1.1 (Cont...) Bench Mark Data
EAST
NORTH
ELEVATIOr
q                        REMARK
21394
1    951969.203
8            1321.443
BM-10
21402
1    951993 882
3            1322.904
BM-10A
21403
2     952010.483
8            1335.295
BM-10AA
21528
6     953472.257
2            1301.788
BM-11
21522
3    953511.799
1301.043
BM-11A
226691
943655.677*
1389.991
BM-4GA
226642
943876.9281
1384.721
BM-4G
224194
946721.1277
1431.121
BM-9B
218428
951143.3039
1304 781
BM-1A
218004
951061.7923
1314.351
BM-2A
217986
952161.2236
1310.891
BM-Bz3
215542
948347.2437
1395.541
BM-5G
215535
948373.2142
1395.151
BM-5GA
224929
950733.936
1322.842
BM-MA-13
224507
950957.7891
1354.496
BM-MA-14
221941
954043.6531
1541.758
T.BM-MA-17
221923
954039.8439
1536.355
BM-MA-18
222167
953980.2967
1442.464
BM-MA-15
222145
953996.1451
1453.714
BM-MA-16
218027
951143.7839
1316.445
BM Bz2(MA)Total station TCI 80
218453
951218.0548
1306.236
BM-BZz-1 (MA j
224198
946783.2322
1325.772
BM-MA=I9'
223716
947100.0333
1322.789
BM-MA 21
221969
946730.0293
1368 583
BM-MA-10
221976
946759.3177
1367.659
BM-MA-11
224340
946750.3115
1384 301
BM-MA-12
218432
951172.3198
1307.381
BMBZ
216361
755474.4432
1296.551
BMBZS
215484
756658 2688
1289.021
A-BM6
215658
>56600.6463
1283.421
BM7
218427       9
56025 9567
1418 931
3M MA24
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Tabla 1.1 (Cont...) Banch Marte Data
May 2007
EAST
NORTH
ELEVATION
REMARK
21 8405
956034 5778
1410 951
BM MA25
217724
959120 6036
1312 819
COMMON PT BM 2(Gugsa A Belay)
217688
959003 7803
1312 069
COMMON PT BM 1(Gugsa A Belay)
219469
954372.3694
I3IOO51
BM-16
2I933J
954.161 5599
1291 831
BM-16A
218078
956155 6537
1299371
B M 17
218039
956223 88)2
1302.771
B M 17A
229834
952309.6286
1365 962
BM F3
229767
952311 2645
1364 627
BM F4
235677
948929 9282
1317.871
B M-1G
235686
948850 8103
1317 011
B.M-2G
237871
945813 8663
1316 531
B M14
238005
945922 8191
13 19 471
B M14A
239094
945422.3561
1324 141
BM15
239141
945554 9476
1325 881
BM15A
240907
944859 6997
1333.461
BM-BZ1
240143
944927 8702
1331.561
BM-BZ2
234297
949432.5598
1321.751
B M BZ2
245512
944905.6411
1329.351
B.M BZ3
245256
945206 5074
1335.781
B.M BZ4
240899
948099.8209
1360 931
BM F2
240962
948089 8624
1361.315
BM F1
215261
956558.9158
1293 645
BM-6C
215639
956318 4808
1283 115
BM-7C
215357
955778 3952
1294 415
INST AT BM1 G
217696
959096 0504
1312 915
BM2 G
215640
961429 4492
1274 165
BM 3AC
215439
961490.0176
1275 175
BM-3AB
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GEOMORPHOLOGICAL
STUDIESAr jo Dedessa Irrigation Project
Geomorphological Investigations
May 2007
TABLE OF CONTENTS
TABLE OF CONTENTS
LIST OF FIGURES
1.   INTRODUCTION
1.1 GeneralI
1.2 ObjectivesI
1.3 Methodology2
1 4 Previous Geomorphological Works2
2.   LANDFORMS DESCRIPTION OF THE AREA------------------------------------------------ ------------------------- 3
2.1 General Physiography3
2.2 Landscape evolution3
3.      FLUVIAL GEOMORPHOLOGY OF THE DIDESA RIVER9
3.1 General9
3.2 Stream TypeIO
3.3 Longitudinal profile13
3.4 Entrenchment15
3.5 Channel Reaches16
3.6 Channel disturbance and response potential17
I
1
4.    DAM HYDRAULICS AND RELATED GEOMORPHIC EFFECTS ON DIDESSA RIVER CHANNEL
4 1 General19
4.2    Dam on the Didesa River19
5. FUTURE WORK21
5.1 Stage-Time Relations22
19
19
6. REFRENCES.............................................................................................................................................. 23
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Arjo Dedessa Irrigation Project Geomorphological Investigations
May20g7_
LIST OF FIGURES
Fig 2.1 Type of landforms around the Arjo-Didesa Irrigation Project Area.......................... 5
Fig 2.2 Geological units around Arjo-Didesa Irrigation Project area......................................7
Fig 2.3 Major Lineaments around the Arjo-Didesa Irrigation Project area.............................8
Fig 3.1 Photographs of Didesa River features....................................................................... 11
Fig 3.2 Plan view of Didesa River around the Dam site....................................................... 12
Fig 3.3 Longitudinal, cross-sectional and plan view of major stream types.........................12
Fig 3.4 Longitudial profile along the Didesa River channel.................................................. 13
Fig 3.5 River across sections across the Didesa River...................................................... 14
Fig 3.6 Types of channel entrenchment................................................................................. 15
Fig 3.7 Photograph of pool-riffle channel reach of the Didesa River, downstream of
the Dam axis............................................................................................................... 16
Fig 3.8 The spatial distribution of reach types with a drainage basin
influences.................................................................................................................. 18
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GEOMORPHOLOGIC INVESTIGATIONS 1. INTRODUCTION
1.1      General
May 2007
An understanding of landscape type of an area, river-and stream-channel response to both natural and human factors is necessary for addressing several important issues including the protection of property  and structures; dam-bridge-site selection, design, and maintenance; protection and rehabilitation of riparian and aquatic habitat; channel capacity; ground-water levels; general aesthetics; and recreation. The channel bank erosion that accompanies natural channel migration across a flood plain represents a constant threat to property and structures located in or near the channel. Human disturbances such as dams, channelization, in-channel sand and gravel extraction, and urbanization introduce additional instability  to which rivers  and streams  adjust by  such processes  as  channel-bed degradation (erosion), channel-bed aggradation (deposition of material), and /or channel widening. These adjustments, which represent the channel's attempt to establish a new approximate equilibrium, pose additional threats to property, structures, and habitat located in or near the affected channels.
Channel adjustments are major concern for several reasons. A significant lowering of the channel bed poses  an immediate threat to bridge pier foundations  as  well as  buried pipelines and cables. In addition, significant bed lowering increases bank height and bank instability that may trigger channel widening. Channel aggradation raises the bed elevation, reduces channel capacity, and increases the likelihood of flooding Any channel change that occurs on the mainstream rivers and streams also may migrate upstream on the tributaries where additional property, structures, and habitat may be at risk Finally, any long-term channel adjustment processes also may instigate or worsen local scour problems.
1.2 Objectives
The main aim of this study is to present landscape types in the Arjo-Didesa Irrigation Project area and fluvial geomorphology of the site, and predict future changes that might be expected due to the proposed dam project through a combination of field observation, data analysis and interpretation.
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Knowledge  of  geomorpholgical  condtions  of  an  area  can  be  used  to  plan  and  undertake economically a subsurface investigation and design.
1.3 Methodology
The ability to recognize both landforms and fluvial geomorphology of an area on topographic maps, aerial photographs and site investigation combined with a geologic knowledge of the area, is very important when characterising a site and when locating potential sources for construction materials
Land-sat   imagery   and   aerial   photographs   interpretation   combined   with   brief   on-site inspection were used to determine the geomorphology of Arjo-Didesa Irrigation project area near the dam site. The land-sat imagery available is in 7 bands, is of 1:250, 000 scale taken in 2003. The image cover the project area and its environs, was used in constructing the pleacoenvriment  of  the  project  site  in  general.  The  aerial  photographs  are  those  taken  in 1973  and  are  1:62,  000  scale  Stereoscopic  interpretation  of  the  aerial  photographs  was done to trace lineaments, landforms and plan view of the River channel morphology around the  dam  site  The  site  inspection  included  studying  depositional  landforms,  sediment  types and   channel   geometry.   While   on-site,   photographs   of   the   stream   channel   and   the surrounding   physiography   has   been   taken   Analysis   of   photographs-derived   landforms description and channel information is included in assessment of the plaeoenvirnoment and changes in bank full channel width and shape of the channel upstream and downstream from the dam.
1.4 Previous Geomorphological Works
Geomorphological  investigations  around  the  project  area  have  to  date  barely  scratched  the surface. These may have major implications for a design of an engineering structure on the Didesa River USBR (1964) stated that the main Didesa River around the dam site and in the reservoir flows on volcanic rocks from headwaters to the approximate area of Didesa-Wama junction,  in  the  bend  where  the  River  turns  to  the  northwest  and  then  entrenched  in metamorphic rocks to the rest of the way to the Blue Nile. According to WAPCOS (1990) and BCEOM (1998) studies the NNW flow of  the Didesa River around the proposed dam site is controlled by faults. In the upper reaches the bedrock constituting the River valley is volcanic of Tertiary age
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2. LANDFORMS DESCRIPTION O
F THE AREA
2.1 General Physiography
The Didesa Irrigation Project area and
its environs are divided in to three
physiographic 
regions.  These  are  the  Didesa  plane
(1270-  1400m),  uplands  or  chain  of
ridges  (1400-
2500m), plateau (greater than 2500m) an
d pediments (Fig 2.1 a, b & c).
The Didesa plane is found close to the Didesa River. The chains of ridges are rugged and dissected by the tributaries of Didesa River. The plateau is found on top of the basaltic ridges. The Didesa plane is covered by elluvial soils. They are mainly silty to sandy reddish brown and black cotton soils, are products of the break down of granitoids and basalts. While the Plateau area being made of Tertiary  basalts  the uplands  are made up of Precambrian granitoids and Tertiary volcanic rocks (Fig 2.2). Pediments are gently sloping erosional surfaces, exposing volcanic and basement bedrocks or covered with a veneer of alluvium that can be transported off the pediment surface with the next significant flood event.
Lineaments with general trend of NE-SW, NW-SE and rarely N-S and E-W can be traced from aerial photographs (Fig 2.3) for the area. Locally, the lineaments govern stream and river courses and algimgnt of ridges. They are mainly straight to curvilinear with variable lengths
2.2 Landscape evolution
Knowledge of geological history helps us to understand the present-day landscape of the Didesa valley. Precambrian rocks are exposed in the uplifts surrounding the plateau in the Arjo-Didesa area They  consist of highly  metamorphosed gneissic  country  rocks  intruded by  granites. Approximately  900 million years  ago an east-west compression produced a continent scale North-South trending orogenic belt and the associated Precambrian rocks in western Ethiopia Different Tertiary  to Quaternary  volcanic  rocks  that come through the regional fault lines and fissures characterize the rest of the bedrock geology of the Arjo- Didesa area. Surfacial deposits; alluvium, residual and colluvial soils cover the river valley and the slope areas.
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In Ethiopia and in the Arjo-Didesa area landscape development since early Tertiary has been dominated by episodes of continent-wide uplift. The epiorogenic uplift of Afo-Arabia, that does not involve deformation, which is related to mantle plume whose decompression melting generated an enormous outpouring of large volume of magma, formed the plateau volcanic regions in Ethiopia. The fissures that served as a conduit for the magma are created by upraised and up parched landmass (proto rifting). In response to these uplifts volcanic activity dominated but almost rivers eroded downward, hill slopes retreated, and deep river valleys were formed. Over time, these processes have resulted in the removal of substantial thickness of rocks from across large areas of exposing many of the formerly buried land. Present day streams established their courses at this time and because they were lifted high above base level they began to rigorously down cut. Deep entrenchment of streams and differential erosion of the plateau began at this time. The stream courses were established on flat lying undeformed strata. In valleys such as that of the Didesa, this process of erosion is continuing today, although at a rate imperceptible during a human lifetime.
As rocks have been eroded, deep river valleys have been formed, separated from neighboring valleys by steep, flat-topped escarpments or hills rising up to ^300 m above the adjacent valley floors. The escarpments and hills are mainly formed in volcanic rocks, with trachyte and phonolite often capping these escarpments or forming prominent, boulder- strewn ridges. Soil covers are typically thick but grassy ground covers are extensive.
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Fig 2.1(a): Type of landforms around the Arjo-Didesa Irrigation Project Area
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Fig. 2.1 (a)
Fig 2.1 (c)
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2100C0
220000
230000
2-a2O00
25CUOC
+
970000
900000
940000
030000
220000
230000
240000
250000
Fig 2.2: Geological units around Arjo-Didesa Irrigation Project area
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200
l
Fig 2.3: Major Lineaments around the Arjo-Didesa Irrigation Project area
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3. FLUVIAL GEOMORPHOLOGY OF THE DIDESA RIVER 3.1 General
May 2007
An understanding of the channel geomorphology of a river is important for several issues, including the protection of riparian (streamside) resources; protection of wildlife habitat (for example, the in-stream habitat for the threatened Hippos), and bank stabilization related to property loss. Also, such information may be important for the site selection, design, and construction of dams.
The focus of this study is the channel of the Didesa River, upstream and down stream of the Dam axis. Throughout this  reach an anastomising, silt-bed channel characterises  the Didesa River.
The Didesa channel bed around the dam site is  situated on erosion-resistant bedrock. Elluvium in the Didesa River Valley is typified by silt and clay with a basal layer of sand and gravel. The bank  materials  consist mostly  of cohesive silt and clay  that are relatively resistant to erosion compared to sand banks. Also, the banks  typically  have partial-to- complete mature tree cover that may enhance or reduce bank stability at specific locations (Fig 3.1).
Along its course, the Didesa River is presently eroding perhaps upstream areas where it's young. Pronounced and abrupt changes in the character of the river occur where it flows from one rock type onto another. For instance, when developed on volcanic rocks, valleys are characteristically  wide, and the river meanders  within broad floodplains  marked by numerous  oxbow and abandoned channels. In contrast, on basement rocks, valleys  are narrower, the river follows comparatively a straighter course, and floodplain development is limited (Fig 2.2).
Traditionally, the channel pattern of a river has been classified into straight, meandering and braiding types. However, a more recent classification incorporates anabranching (including anastomosing) as a recognizable pattern. The diversity in the channel patterns is caused due to variability of water discharge, sediment load, bedrock outcrops, ground slopes, human activities and tectonic influence. Tectonic movements change the gradient of a river
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and modify the valley. Alluvial rivers are not significantly affected or constrained by bedrock and flow through sediments  that have been eroded and deposited by  them or their ancestors.
3.2  Stream Type
From  the  Figs  3.1a,  3.2  the  Didesa  River  can  be  classified  into  Stream  type  DA  - Anastomising stream (Fig 3.3) based on the following characteristics:
General:  Anastomising  narrow  and  deep  with  expansive  well  vegetated  flood  plain  and associated wetlands. Very gentile relief with highly variable sinuosity. Stable stream banks.
Landform/soil/features: broad, low gradient valleys  with fine alluvium/lacustrine. Anastomising geological control creating fine deposition with well vegetated bars  that are laterally stable with broad wetland flood plains.
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Fig 3.1: Photographs of Didesa River features a) anastomising b) bank vegetation cover and c) silty bank material
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Fig 3.2 : Plan view of Didesa River around the Dam site
Fig 3.3: Longitudinal, cross-sectional and plan view of major stream types (Modified after Rosgen, 1994)
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3.3
Lo
ngitudina
l profile
Plot
of
the
relative
elevation
of
a  streambed  from
headwaters 
to  mouth  is 
called  its 
‘longitudinal profile’. The longitudinal profile of a stream is generally concave upward which is in accordance with the steady  downstream decrease in slope. The longitudinal profile and river cross  section of the Didesa River channel, which is  inferred from global mapper 5 served as breaking the stream reaches into slope categories that reflect profile morphology (Fig 3.4 and Fig 3.5). Fig 3.3 represents types of dominant slope ranges of major stream types  (Rosegen, 1994). The Didesa river longitudinal profile can be classified into slope range of E, with a dominant slope range of < 2%, and belongs to those of gentle-gradient types. Bed features  generally  are related to channel slope. In such rivers, there is  a little change in the elevation of the riverbed along the river channel. They are also chracterised by lack  of high derbis  transport (supply  limited) and absence of waterfalls; bed forms  are relatively  stable. Longitudinal profile also indicates  a stream reaches  (Fig 3.4) related to slope categories  that reflect profile morphologies. That is  to say  that riverbed features  are related to a channel slope. Alluvial bar development requires a large width-to-depth ratio and small grain easily  mobilized and aggraded by flow. Gradient and bed- feature relationships are integral to the delineation of stream type categories.
Fig 3.4: Longitudinal profile along the Didesa River channel
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From Pos: 242557.31, 941390.62
1380 m
1360 m
1340 m
1320 m
May 2007
To Pos: 244750.00. 942500.00
Upstream
0.5 km
1.0 km
1.5 km
‘870m
2.5 km
From Pos: 242119.67, 942578.49
To Pos: 243620.14, 943141.17
1400 m
1380 m
1360 m
1340 m
Dam Axis - .
1320 m
► 670m
From Pos: 243102.00, 942300.
To Pos: 243605.00, 942809.00
1380 m
1360 m
1340 m
1320 m
Fig  3.5:  River  across  sections  across  the  Didesa  River  a)  up  stream  of  the  Dam  axis  b)  at the dam axis c) down stream of the dam axis. Note Distances in between are indicated
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3.4Entrenchment
May 2007
An important element of delineation is relationship of a river to its valley or/and landform features. This interrelationship determines whether the river is deeply incised or entrenched in the valley  floor or in the deposit feature. Entrenchment is  defined as  the vertical containment of river, the degree to which it is incised in the valley floor. This makes an important distinction of whether the surface adjacent to the channel is a frequent flood plain, a terrace or is outside of a flood prone area. The flood-prone area is defined as the width measured at an elevation, which is determined at twice the maximum bank-full depth. The entrenchment ratio is the ratio of flood prone area to the bank-full surface width of the channel. Comparing the features of River Dedessa as given in Fig 3.4 and Fig 3.5 with those in Fig. 3.6 (Rosgen, 1994), it can be concluded that the Didesa River belongs to moderately  entrenched river with entrenchment ratio of 1.41 - 2.2, typified by  a well developed floodplain.
Fig 3.6: Types of channel entrenchment (Rosgen, 1994)
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3.5 Channel Reaches
May 2007
Channel reaches  exhibit similar bed forms  over stretches  of streams, across  channel widths and its  length. The Didesa River is  characterised by  pool-riffle reaches  (Fig 3.7) in the length under study  It is  unconfined by  valley  wall and consists  of oscillating sequence of bars, pools  and riffles  resulting from oscillating cross-channel flow that causes  flow convergence and scour on alternating banks  of the channel. Concordant downstream flow divergence on the opposite side of the channel results  in local sediment accumulation in discrete bars. Bed form and grain roughness  provide the pnmary  flow resistance in pool riffle reaches. Bed forms  in many  pool-riffle reaches  are relatively  stable morphologic features, even though the material forming the bed is  transported annually  Pool-riffle reaches  are commonly  armored exhibiting a near bank  full threshold for general bed surface mobility
Fig 3.7. Photograph of pool-riffle channel reach of the Didesa River downstream Dam axis
of the
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3.6 Channel disturbance and response potential
Hay 2007
Response to environmental change varies  for different channel. Reach morphologies  are associated with physical morphologies  and the environment that limit the range and magnitude of possible channel responses to changes in hydrualic discharge and sediment supply. The specific  response of a particular channel reach reflects  the intensity  of disturbance or the magnitude of change as well as the conditions of the reach. Table 3.1 illustrates patterns of potential response for Didesa River channel reaches. Lower gradient poole-riffle reaches  become progressively  more responsive to altered discharge and sediment supply  with decreasing ratios  of transport capacity  to sediment supply, smaller grain sizes, and less  channel confinement. Pool-riffle reaches  as  in Gumera River project area allows potentially greater response of bed surface texture, scour depth and generally have significant potential for channel geometry  response to perturbutation in sediment supply  and discharge. Changes  in both channel and valley  storage are also likely responses, as well as channel roughness due to alteration of channel sinuosity.
Interpreted Didesa River pool - riffle reach level channel response potential to moderate changes in Sediment supply and discharges (+ =likely to change; - = unlikely to change) Table 3.1 (Modified from Montgomery and Bufington, 1997)
Grain
Width Depth Roughness Scour depth size
Response +               +              +
+
Sediment Slope Storage
++
Reponses  segments  consist of lower gradient more transport-limited reaches  in which significant morphologic  adjustment occurs  in response to increased sediment supply. The spatial distribution of source, transport and response segments  generally  reflects  the distribution potential of impacts  and recovery  times. Distribution of these segment types define watershed - scale patterns  or sensitivity  to altered discharge and sediment supply (Fig 3.7) Sediments  delivered to transport segments  rapidly  propagates  to downstream response segments, where sediment accumulates. Consequently, locations  in the channel network where transport segments flow into response segments indicate places particularly susceptible to impacts  from accelerated sediment supply  Because response segments  are sensitive to increases in sediment supply, they are excellent sites for monitoring the effects
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of  upstream  actions  and  can  be  considered  locations  of  critical  importance  in  watershed monitoring programs.
Fig 3.8: The spatial distribution of reach types with distribution of potential impacts and responses
drainage basin to disturbance
influences the
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4. DAM HYDRAULICS AND RELATED GEOMORPHIC EFFECTS ON DIDESSA RIVER CHANNEL
4.1 General
May 2007
A River modification by a reservoir will alter significantly the flow characteristics, the channel cross-sections, and the longitudinal profile of a river.
Dam construction changes  both the discharge regime and sediment supply  downstream channels  resulting in channel incision, constriction or widening and changes  in channel substrate. Studies  document channel incision and bed surface coarsening immediately 
downstream of dams  in response to sediment impediment (Williams  and Wolman, 1984). Tributaries  channels  also may  incise in response to mainstream channel incision through upstream knick  point propagation from their confluence Decrease in discharge below a dam may cause narrowing of active channel width. Tributaries sediment inputs downstream of dams  may  cause channel aggradations  and accumulation (Allen et al, 1989) and accumulation of fine sediment because of decreased transport capacity resulting from dam construction. Differences  in channel form and function affect the probability  of specific response to a given perturbation. Channel classification can aid interpretation and assessment of response potential by grouping similar physical environments.
4.2 Dam on the Didesa River
The Didesa River channel is a stable river channel, naturally anastomises across its valley over time while maintaining approximately  the same cross-sectional shape. It has  shown that it has a moderate gradient. Therefore, the expected changes in channel geometry due to damming of the river may be used to infer channel instability.
Reservoirs  can be thought of as  permanent floods  that affect the river system upstream from the dam in a quite different way than the annual floods, which lasted for only a few weeks or perhaps a month. The planned reservoir will cover many of the pools, riffles, and rapids across the reach of the Didesa River.
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Dams trap sediment in the upper reaches of reservoirs where tributaries enter the relatively quiet water of the reservoir and deposit sediment to create deltas and wetlands, which can have effects on the environment that are both positive and negative.
Downstream changes in the stream flow regime attributed to the operation of a dam might be expected due to a decrease in the magnitude of peak flows and an increase in the magnitude of low flows. Also, post-dam suspended-sediment concentrations  could get substantially reduced immediately downstream from the dam. Farther downstream, several tributaries that contribute unregulated flow to the Didesa River will moderate these effects.
Dams result in upstream and downstream changes in a river channel. Upstream from the dams a backwater pool is created, the depth and extent of which depend on the height of the individual dam and the slope of the channel. Within this backwater reach of the river, the decrease in flow velocity may result in some deposition of material on the channel bed. As the  water  flows  over  the  dam,  its  velocity  and  erosive  power  increase.  The  potential consequences   of   these   effects   include   increased   channel   bed   and   bank   erosion immediately downstream from the dam as the river expends the extra energy. Most of the erosion occurs during high flows when the river’s erosive power is the greatest. Bed erosion may  create  a  plunge  pool  at  the  base  of  the  dam,  with  the  formation  of  companion depositional bars just downstream as the river loses its ability to transport the scoured bed load.  The  depositional  bars  may  divert  the  flow  toward  or  away  from  the  banks. Bank erosion  results  in  channel  widening  and  also  may  contribute  to  the  formation  of  the depositional bars. The form and downstream extent of these effects vary from site to site as affected by human-caused and natural conditions that may either promote or inhibit the river’s response to the overflow dams.
Effects of damming will change the character of the Didesa It will cause the water-surface area, the channel area and the sand bars to decrease. The change in channel-surface area is relatively easy to determine from maps and aerial photographs. However, the thickness of sediment on the flood plains is not as easy to determine. This stored sediment also contains large amounts of stored carbon, which is of current concern to many research scientists interested in the global carbon budget (Stallard, 1998).
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5. FUTURE WORK
May 2007
Various forms of channel activity, including widening, bank erosion, meander movement, and degradation, have been identified in the downstream part of the Didesa River basin. Because the channel activity would be intensified during the period following impoundment of  stream  flow  and  sediment  designed  reservoir.  It  would  be  possible  that  part  of  the channel changes is the result of changes in hydrologic regimen by the reservoirs. It is desirable to investigate this possibility, as well as to explore feasible methods of reducing the economic losses and enhancing the environmental character resulting from the channel changes.
A future geomorphologic investigation should be able to provide sediment and channel geometry data and interpretations of those data pertinent to the bank-stabilization project. Therefore, it would include description of the analyses of sediment data and would provide data and interpretations of the channel geometry of the Didesa River.
An objective of the investigation might be directed to determine whether significant alterations in sediment size and supplies to the downstream part of the Didesa River basin might occur in the future. If changes  would occur, an additional objective would be to describe those changes  in quantity  with time. The information may be related to man's effects on land use and construction of the major reservoirs in the Didesa basin and to natural effects of topography, geology, climate, and physiography. To permit reasonable evaluations  of the area and temporal variations  in sediment yield, sediment size, and channel characteristics, the future work should consider the entire Didesa River basin in parts, as well as from adjacent parts of other drainage basins. The study result would be able to report fluvial-sediment transport and channel-morphology  data for the period of extensive stream flow regulation in the basin and for the period immediately  preceding regulation. All of the data and generalizations  to be presented might be intended to represent controlled hydrologic  conditions  or uncontrolled conditions  of a generally agricultural, rather than a pristine watershed.
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5.1 Stage-Time Relations
Changes in the elevation of a stream channel with time provide an additional means of evaluating channel instability. Degradation commonly occurs in channels downstream from reservoirs. The water released from a dam is largely sediment free and thus has a tendency to restore the sediment load to an amount similar to that of unregulated conditions. The occurrence and extent of channel degradation is  a measure of the imbalance that a reservoir has imposed on a stream. Changes of stage (water-level elevation) with time rate largely are related to datum changes of the channel bed (aggradation or degradation). Changes of stage elevation for flow-duration rate result both from changes of bed elevation and from changes in channel conveyance at discharge rates exceeding those equivalent to the active-channel stage. An explanation of these trends would be that aggradations or degradation of the channel bed is not occurring, but that the relatively large release rates from the upstream reservoir are causing erosion of the channel sides below the bank full stage.
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6. REFRENCES
May 2007
Allen, P.M., Hobbs R , Maier, ND., 1989. Downstream impacts of a dam on a bedrock fluvial system. Bulletin of the association of Engineering Geologists 26, 165-189.
BCEOM, 1998. Abbay River basin integrated development master plan project
Hadley,  R.F.  1967.  Pediments  and  pediment  forming  processes.  Journal  of  Geological Education, 15, 83-89.
Montgomery,  D  R,  Buffington,  J.M,  1997.  Channel  Reach  morphology  in  mountain range drainage basins. Geological society of American Bulletin 109, 596-611
Osterkamp, R. E, Crowther H G., 1982. Sediment and channel-geometry investigations  for the Kansas  Riverbank-stabilization study, Kansas, Nebraska, and Colorado. U.S. Geological Survey, Water-Resources  Investigations, Open-File Report 81-128
Rosgen, D.L, 1994. A classification of natural rivers, Catena 22, 169-199
Mengesha,  T,  Tadiwos,  C.,  Workineh,  H.,  1996.  Explanation  to  the  geological  map  of Ethiopia, Scale 1:200000, 2nd edition. Geological survey of Ethiopia
USBR, 1964. Land and water resources of the Blue Nile basin.
WAPCOS, 1990. Preliminary water resources development master plan for Ethiopia Williams,  G.P.,  Wolman,  M.G.,  1984.  Downstream  effects  of  dams  on  alluvial  rivers. United   States   Geological   Survey   professional   paper   1286.  US   Government   Printing office, Washington DC, USA
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23ANNEXURE - 3
GEOLOGICAL AND
GEOTECHNICAL INVESTIGATIONSArjo Dedessa Irrigation Project
Geological & Geotechnical Investigations
TABLE OF CONTENTS
May 2007
TABLE                      OF
LIST OF TABLES
LIST OF FIGURES
LIST
LIST OF APPENDICESIV
CONTENTS
OF
PLATESIV
LIST OF ABREVIATIONSV
1.
INTRODUCTIONI
2.
INVESTIGATIONS....................................................................... ....................................................................... 2
2.1        General2
2.2        Previous investigation in the area2
2.3        Objectives of the present study3
2.4        Methodology3
2.5        Geological mapping3
2.6        Borehole drilling....................................................................................................................................... 3
2 6.1 Location of Borehole J 2.62 Drilling Results ...... ........................ ................................. .......................................... ... ..... 5
2.6             3 Permeability testing
2.6             4 Test pitting and sampling7
2 6.5 Laboratory testing/ /
3.      GEOLOGICAL INVESTIGATION„........................ „I2
3.1 Regional Geology
3 I 1 Metamorphic Rock. 
3 12            Tertiary Volcanic rocks
3 12 I         Jimma Volcanics 
3.1 2.2       Mekoncn Basalt
.... 12
!2
12
12
.12
3 1.2.3
Quaternary plateau basalts 
13 
3.2 Arjo-Dedessa Project area ..
3.2.1      General
13
3.3
Dam Site Geology,
3.3.1     General
Dam Axis Geology 3.3.2
13 14
!4 
3 3 2 1
3 3.2.2
3 3 2 3
Alluvial deposits Colluvial soils 
A Ik aline/ trachy-Basalt
14
14
15
3.4 Geology of the abutments
16
15 
i
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3.4.!     Right abutment. 3.4.2 Left Abutment.
3.5         Valley Bottom....
3.6
3 6.1
Geology of the Saddle Dam site
Colluvial Residual soils
362       Flood Plain Deposits
May 2007
16
16
17 17
18
18
3 6.3
A Ikaline/Trachy-Basalt                                                                                                                                         18
3.7
3.8
Geology of the Spillway Reservoir Area
3.8 I     Alluvial Deposits
3.82 Flood Plain Deposits
V
I
I
I
I
I
I
!
!
I
38 3
3.8.4
38 5
Residual soils Alkaline basalt Aphanitic basalts
3.9 Main Canals
19
26
26
27
27
27
29
28
3.9.1     General29
3.9.2     Left Canal29
3.9 3 Right Canal
3.10   Geological Structures30
3.10.1        General30
3.10.2            Faults 31
3.10 3          Joints
3 10 3.1 General32
3.10 3 2 Right Abutment32
3.10 3.3 Left Abutment 33
3 10.3 4 Saddle dam site
3.10.3.5       Reservoir area34
3.10.3.6       Dikes
3.11      Ground Water Condition37 3.12      Se ismic Hazard Assessment
..30
..... 32
33
3.12.1   Regional Seismotectonics...................................................................................................................................  jg
3.12.2         Seismicity of (he Region 
3.12.3        Seismic Hazard.
3.12.4 Ground Movements/Displacements
3.12.5          Ground A cceleration
3.13      VES Survey Results
4.    CONSTRUCTION MATERIALS
4.1        Introduction
4.2        Clay Materials
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4.3        Shell Material44
4.4        Riprap material44
4.5        Concrete aggregates44
4.6        Sand sources44
May 2007
5. GEOTECHNICAL DESIGN CONSIDERATIONS46
5.1       Construction materials and Design Parameters46
4
5. /. / Clay materials &
5.1.2 Shell Material 48
4
5.1 3 Sandy material  9
5.2        Filter Material49
5.2.1     General4 9
5.2.2     Field Investigation50
5.2.3 Laboratory Testing50
5.2.4     Dam Zoning50
5.2.5     Filler Design51
5.3        Permeability of Main Dam, Saddle Dam abutment areas53
5.3.1 Reservoir area58
5.4        Land sliding59
LIST OF TABLES
Table
2.1 
Borehole
drilling
Details5 
Table 2.2:           Summary of the geologic layers at ihe dam and saddle axis from the drilling6
Table 2.3:           Depth to the water table6
Table 2.4:            Details of clay borrow areas7
Table 2.5:            Estimated Quantities of Borrow and quarry material9
Table
2.6 
Laboratory
tests
Conducted11 
Table 3.1:            Groundwater point data of the project area38
Table 3.2:            VES points at Arjo-Dedessa site41
Table 3.3:            Summary of the VES survey42
Table
5.1                    Summary                       of                       clay                       design                       parameters47 
Table 5.2:            Summary of rock design parameters49
Table
Table
5.3 
5.4 
Location
Selected
of
Clay
Design
source Filter
areas50 
Band52 
Table 5.5:
Summary of falling head permeability test result
Table 5.6. 
Summary of the Lugeon value
Table 5.7:
Summary of permeability test results for both test types
iii
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LIST OF FIGURES
May 2007
Figure 2 l      Borehole Location map...................................................................................................................... 4
Figure 2.2      Borrow areas location map..........................................................................................................................10
Figure 3.1      geological map of the dam and reservoir area..................................................................................... 20
Figure 3.2      Geological map of the dam. saddle and spillway area........................................................................21
Figure 3.3:     Geological cross section of the dam axis along AD2.........................................................................22
Figure 3.4      Geological cross section of the dam site along AD7........................................................................ 23
Figure 3.5      Geological cross section of the saddle dam site................................................................................24
Figure 3 6      Geological cross section of the spillway............................................................................................. 25
Figure 3.7:     stereo plot of the joint direction in the alkaline basalt of right abutment (n=35).................. 32
Figure 3.8:    stereo plot of the joint dip direction in the alkaline basalt of left abutment (n=40)............. 33
Figure 3.9:     stereo plot of the joint direction in the alkaline basalt of saddle dam site (n=30)................. 34
Figure 3.1 0a: Jointing direction of the alkaline basalt at the river course in the reservoir area (n=30) ... 35 
Figure 3.1 Ob: Jointing direction of the aphanitic basalt (left reservoir). The major dip direction is N253°at
AN AVERAGE DIP ANGLE OF 75°(N=40).................................................................................................... 35
Figure 3:10c: Dip direction of Joints of the alkaline basal tall over the area (dam site and reser voir). The
MAJOR DIP DIRECTION IS Nl I6°AT AN AVERAGE DIP ANGLE OF 72°(N=I35)................................. 36
FIGURE 5 I SAND FILTER DESIGN........................................................................................................................................53
LIST OF PLATES
Plate 1          Morphology of dam site looking d/s
Plate 2         Fracture condition of the alkaline basalt as observed in the river course
Plate 3         Jointed aphanitic basalt exposed on the surface on the left side of the river outside the reservoir level
Plate 4 Alkali basalt dike at Ketketo spring close to the confluence of Wama with Dedessa river
LIST OF APPENDICES
Appendix A Borehole logs
Appendix B Photographs of core
Appendix C Field Permeability Test results
Appendix D Test pits log sheets
Appendix E   Laboratory T ests Result
Appendix F Geophysical survey results
Appendix G Piezometer Installation Detail
iv
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Geological & Geotechnical Investigations LIST OF ABREVIATIONS
May 2007
AD1                 Arjo-Dedessa Borehole one at dam site ADCAQ            Arjo-Dedessa Coaese Aggregate Quarry site ADQ1               Arjo-Dedessa Borehole one at rock quarry site ADS1               Arjo-Dedessa Borehole one at saddle dam site ADSP1            Arjo-Dedessa Borehole one at spillway site
AtBTPI
BCEOM
Ate Borrow area Test Pit one
C                      Cohesion
CD                   Confined drained
CDSCO Construction Design Share Company
ChBTPI cm/s2
Chewo Borrow area Test Pit one Centimetre per second square
CU                   Confined untrained
DhBTPI Dhendessa Borrow area Test Pit one
9
K/Ar Kn/m3
Gravitational Acceleration Potassium Argon
Kilo-Newton per meter cube
LL                     Liquid limit
m                      metre
MBTP1 Maribo Borrow area Test Pit one
MDD
N130°
Max Dry Density
130 degrees from north
NE                    North east
NMC                 Natural Moisture content
NW                   North west
ObBTPI            Obose Borrow area Test Pit one Ohm-m             Ohm meter
OMC                 Optimum moisture content
PI
Plasticity index
PL                     Plastic limit
SE                    South east
SSD                  Soaked dry sand SSWIN              SpheriStat for Windows SW                   South west
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ucs
Unconfined compressive strength
USBR
United States Bureau of Reclamation
UTM
Universal Transverse Mercantile
UU
Untrained unconfined
VES
WAPCOS
Vertical Electrical Sounding
WWDSE
Waterworks Design and Supervision Enterprise
XRD
X-Ray Diffraction
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1.    Introduction
May 2007
This  report relates  to the geological and geotechnical investigations  conducted in relation to feasibility  study  of Arjo-Dedessa Irrigation project. It also discusses  the interpretative part on some aspects of the study.
The Arjo-Dedessa Irrigation project development consists of:
•    Main dam
•    Saddle dam adjacent to the main dam
•    Primary Canals running on the left and right banks of Dedessa River towards the command area
•    13665 ha irrigated area
The Report has been organized in the following manner:
Introduction:                  Section  1  provides  the  Introduction.  It  introduces  the project component
Investigation
Section  2  outlines  the  investigation  methodology  and scope of the field investigation works conducted.
Regional Geology         Section   3   discusses   the   Regional   geology   of   the project area.
Construction                  Section 4 presents results of the various construction
Materials
Geotechnical Design Consideration
materials  investigated and studied for use in the dam section and elsewhere during implementation of the project.
Section      5      details      the      geotechnical      design considerations in the design of the dam
The Appendices  contain supporting documents  as  follows: Borehole logs  (Appendix  A); Geophysical Investigation Results  (Appendix  F); Field Permeability  test Results  (Appendix  C); Photographs of cores (Appendix B); Test pits log sheets (Appendix D), Laboratory test results summary  and interpretative charts  (Appendix  E); and Piezometer installation details  (Appendix G).
1
Water Works Design & Supervision Enterprise
In Association with Intercontinental Consultants and Technocrats Pvt Ltd.Arjo Dedessa Irrigation Project
Geological & Geotechnical Investigations
2.   Investigations
2.1   General
May 2007
The aim of the field investigations  was  to address  all issues  necessary  to present the technical and economical feasibility  of the proposed structures  with sufficient level of investigations. At the beginning all relevant information found from previous  studies  of the project area was reviewed and analyzed.
Based on the available information ground conditions  characterization program was  developed giving attention to how the various  construction parts  are affected by  the character of the ground. Consequently, major problems  were defined and the gap of information related to feasibility of the project structures was identified and acted upon.
Rotary  core drillings  was  used for investigation at the dam site, saddle dam sites  and quarry sites. The drilling program was  constantly  reviewed during the field work  and revised in order to get optimum result from the drillings  The final program, including adjustment based on continuous  evaluation of results  obtained during drilling is  shown on Table 2.1. The Rotary  core drilling work was conducted by the Construction Design Share Company (CDSCO).
Vertical electrical sounding work  was  sub contracted to Ethiopian Institute of Geological Survey by  the contractor; CDSCO. The aim of this  geophysical survey  was  to get additional information in between the exploratory  boreholes  along the dam axis, saddle dam axes  and the spillway area. A total of 10 vertical electrical sounding (VES) were conducted along the above mentioned profiles
2.2    Previous investigation in the area
The project site had earlier been studied by  the USBR (1958-64) as  part of the Land and Water Resources  Study  of the Blue Nile Basin, by  WAPCOS (1989 -90) as  part of the Preliminary Water Resources  Development Master Plan of Ethiopia and by  BCEOM (1989) as  part of the Abbay River Basin Integrated Development Master Plan Project.
A  detailed  review  of  the  previous  works  had  earlier  been  reported  in  the  Inception  report.  All the  above  studies  were  general  desk  studies  and  did  not  include  investigations  consisting  of drilling,  mapping  or  laboratory  testing.  The  current  dam  site  investigated  is  a  new  site  proposed by WWDSE So no previous investigation had been conducted at this site.
2
Water Works Design & Supervision Enterprise
In Association with Intercontinental Consultants and Technocrats Pvt Ltd.Arjo Dedessa Irrigation Project
Geological & Geotechnical Investigations
2.3  Objectives of the present study
The objectives of the geotechnical and geological investigations conducted are:
•    To study sub-surface conditions at major structure sites including the dam site
•    Identify suitable construction materials for use in the proposed dam
•    Study the property of the construction materials
•    Assess foundation permeability properties
•    Identify the geology of the project site at feasibility level
•    Prepare preliminary design level to enable to conduct the feasibility of the project
2.4    Methodology
The methodology followed for geological and geotechnical investigations were as follows:
•    Geological mapping of the dam site including the dam axis
•    Borehole drilling at dam site, spillway and quarry sites
May 2007
•    Permeability testing within the boreholes for the dam site and installation of piezometers
•    Geophysical Investigation to augment the core drilling works
•    Test pitting at construction material borrow areas and sampling
•    Laboratory testing of samples collected from boreholes and test pits
2.5       Geological mapping
Geological mapping of reservoir area has  been done making use of aerial photograph, topography  maps  and field work. Outcrops  of rocks  were mapped along with joint orientations, spring areas etc. Details of the mapping conducted are reported under section 3.
2.6       Borehole drilling
2.6.1 Location of Borehole
12 boreholes were drilled with a total depth of 405m. Out of the 12 boreholes. 7 were drilled along the dam axis (Named AD1 to AD7), 1 at spillway (ADSp1), 2 at rock quarry sites (ADQ1 and ADQ2) and two along the saddle dam (ADS1 and ADS2). Except two holes drilled along the dam axis at 60° to the horizontal, the remaining boreholes were vertical. Drilling depth varied between 3m to 92m. The borehole location map is presented in Figure 2.1 and details are available in Table 2 1
3
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Summery

Arjo-Dedessa Irrigation Project Summary

Arjo-Dedessa Irrigation Project Feasibility Study Report

Project Overview

The Arjo-Dedessa Irrigation Project is a major water resources development initiative in Ethiopia, conducted by the Ministry of Water Resources. The feasibility study report was prepared in May 2007 by the Water Works Design & Supervision Enterprise in association with Intercontinental Consultants and Technocrats Pvt. Ltd.

Main Components

Main dam
Saddle dam
Primary canals (left and right banks)
13,665 ha irrigated area

Key Study Areas

1. Topographic Survey

Detailed topographic surveys were conducted for:

Dam site (1:500, 1:1000, 1:2000 scale maps)
Reservoir area (1:10,000 scale)
Command area (27,000 ha surveyed)
Sample area (500 ha at 1:2000 scale)

72 control points were established with benchmarks. Surveying utilized Total Stations and software including LIS CAD, Land CAD, Surfer, and Civil Designer.

2. Geomorphological Investigations

Key findings about the Didesa River:

Classified as an anastomosing stream type (DA)
Moderately entrenched with well-developed floodplain
Characterized by pool-riffle reaches
Expected dam impacts include sediment trapping and downstream channel changes

3. Geological & Geotechnical Investigations

Major findings:

12 boreholes drilled (total 405m)
Geological mapping identified volcanic and metamorphic rock formations
Construction materials investigated including clay, shell material, riprap, and aggregates
Seismic hazard assessment conducted
Permeability tests performed

Key Data Tables

Borehole Drilling Details

Location	Number of Boreholes	Purpose
Dam axis	7	Foundation investigation
Spillway	1	Structure foundation
Rock quarry	2	Material investigation
Saddle dam	2	Foundation investigation

Geologic Layers at Dam Site

Layer	Description	Thickness
Alluvial deposits	Silty to sandy soils	Variable
Colluvial soils	Weathered material	Variable
Alkaline/trachy-basalt	Bedrock formation	Extends to depth

Conclusions

The feasibility study provides comprehensive data for the Arjo-Dedessa Irrigation Project, including:

Detailed topographic mapping of all project areas
Thorough geomorphological understanding of the river system
Geotechnical characterization of foundation conditions
Identification of construction material sources
Assessment of seismic and geological hazards

The study confirms the technical feasibility of the project while identifying areas requiring further investigation during detailed design.