FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
LOWER DIDESSA MEDIUM HYDROPOWER PROJECT RECONNAISSANCE STUDY
- >.’■
> -1
.. v
•  '• r
MAIN REPORT
VOLUME II
(FINAL!
Water Works Design and
Supervision Enterprise
P O.Box 2561
August  2001
Addis Ababa
/§     s—X        £
Addis AbabaVOLUME I        EXECUTIVE SUMMARY
VOLUME III      APPENDICES VOLUME IV      DRAWINGSLmr Didessa Medium Scale Hydropower Project
Table of Contents
INTRODUCTION............................... -
1.1    GENERAL1
1.2    Purpose of the Study-1
Reanoiaisscoia Level Study June 2001
2 DESCRIPTION OF THE PROJECT AREA- -------------------------------------------------------------------------- 3
2.1     Location
2.2     Topography
2.2.1       General
2.2.2 Cross Section Survey.
3. HYDROLOGY
3.1     General
3.2     The Didessa Basin and Availability of Data. 3.2.1       Description of the Didessa Catchment
3.2.2       Meteorological database.................................
3.2.3     Streamflow Data................................................
.............................................................. 5
—.6
.............................................. ^....6
......... _...... „................................. 7
„................................................~9
3.2.3.1 Quality of Streamflow Data and Gap filling...
3 2.4     Data on Sediment Transport.......................
3.2.5 Data on Current Water Use in the Basin...............
3.3     Main Rainfall Characteristics
3.4     Main Runoff Characteristics
3.4.1 Runoff Estimates at Dam Site
3.4.2 Frequency of flow
3.4.3       Floods
.....____________________________ 11
.......................... _......................13
................................................. 14
......... ........ -.16
17
3.4.4      Discussion and Recommendation 
3.5 Evaporation
3.5.1       Open- Water (Lake) Evaporation20
3.5.2     Reservoir Net E v aporat ion L osses22
3.6 Sediment Transport22
3.6 I       Estimation of Sediment Yield of Catchment22
3.6.2       Discussion and Recommendation....23
3.7 CONCLUSION25
....... .............IS
4.       GEOLOGY...
4.1       Introduction26
4.1           I General~...................................................................................................................................... 26
4.1.2       Objective26
4.1.3      Approach and Methodology26
4.1.3.1  Field Investigation26
4.1.3.2  Review of Existing Data26
4.2           Regional Geology27
4.2 1      Geological History2 7
4.2.2  Regional Geology and Structure of the Didessa River Basin28
4.2.3       Physiographic Setting of the Dam Site29
4.3      Geological investigation at Didessa Dam Site29
4.3.1       Geology and Structure of the Dam site. Powerhouse and Reservoir Area..30
4.3.2      Slope Stability of the Dam Site3 / 4.3.3. Construction Materials Availability3/
4.3.3.1    Rock Resource31
4.3.3.2    Sand Resource31
43.3.3       Clay Resource31
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iLoivcr Didessa Medium Scale Hydropower Project
4.4           Regional Seismology
V. 4. /    General
4.4.2     Seismicity of the Didessa Basin 
4.5     Conclusion.....................................
5.       PROJECT LAYOUT
5.1    General34
5. /. /      Identification of AIternatives34
5.1.2     Initial Screening
5.2     Preliminary Design of selected Project: 5.2.1     Design parameters
5.2.2       Dam
Reconmlssivice Level Study June 2001
32
33
34
5.3     Reservoir storage..................................................................................... -
5.4     Spillway
5.5     River Diversion-
5.6     Power and Energy Estimation—.38
5.6.1      Water way39
5.6.2      Surge Chamber Design40
5.7     Electro mechanical41
5.7. / Mechanical41
5.7.2      Electrical-42
5.7.2.1 Generators-................................................................................. -—42
5.7.2.2    Sub-station at sending end-—-44
5.8     INFRASTRUCTURE48
5. <9. /   Selection of access route48
5.8.1.1 Access to Lower Didessa dam site..-...........................................................................................................48
5.8.2      Preliminary cost Estimation48
5.8.2.1 Bases of cost Estimate—48
5.8.2.2    Cost estimate~49
5.8.3 Site Camp 
6. ENVIRONMENTAL & SOCIOECONOMIC ASPECTS
6.1 Introduction
6.1.1 General................................................................................................................................................. 51
6.1.2       Objectives51
6.1.3      Approach and Methodology52
6.1.3.1    Scoping of the Study................................................................................................................................. 52
6.1.3.2         Review of Previous Studies--52
6.1.3.3         Field Investigation and Data Collection—52
6.1.3.4 Data Analysis and Report Preparation-............................................................................. -53
6.2 LEGAL, POLICY AND ADMINISTRATIVE FRAMEWORK-................................................................ 53
6.2.1     Administrative Structure53
6.2.2      Institutional Framework53
6.2.3       Environmental Policy and Guidelines54
6.2.3.1    Environmental Policy-........................................................................................................................ -...... 54
6.2.3.2    EIA Guidelines-..........................................................................................................................................54
6.2.3.3    EIA and International Context................................................................................. ..................................56
6.3    Description of the baseline condition57
6.3.1      BIO-PH YS!CA L baseline condition
6 3.1.1 Land Use/Land Cover
6.3.1.2    Mineral Resources and Mining Activities57
6.3.1.3 Soil Erosion58
6.3.1.4 Terrestrial Ecology58
6.3.1.5    Aquatic Ecology61
6.3.1.6    Public Health62
6.3.1.7 Water Supply and Sanitation
57
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Reconnaissance Level Study June 2001
6.3.1.8    Physical Infrastructure64
6.3.1.9    Historical, Cultural and Religious Sites64
6.3.2     Socio-Economic Baseline Condition64
6.3.2.1    General-64
6.3.2.2    Kainashi Woreda65
6.3.2  3 PAs and Villages in the impact zone in Kamashi Woreda65
6.3.2.4    Bello Jiganfoy Woreda69
6.3.2.5    PAs and Villages in the impact zone in Bello Jiganfoy Woreda69
6.4 Potential Impacts
6.4.1 Potential Positive Impacts•
6.4.1.1    Generation of Electric Power
6.4.1.2    Employment Opportunities
6.4.1.3    Reservoir Fishery
6.4.1.4 Recession Agriculture and Grazing Opportunities-.................................................................................... ' 6.4.1.5    Flood Control
6.4.1.6    Use of Reservoir Area Woody Biomass for Fuel-wood74
6.4.1.7    Potential for Wildlife Reserves75
6.4.1.8    Infrastructure Development and Mitigation Benefits75
6 .4.2 Negative Impacts on Bio-Physical Environment75
6.4.2.1    Impacts on Terrestrial Ecology75
6.4.2.2    Impacts on Aquatic Ecology77
6.4.2.3    Impacts on Water Quality78
6.4.2.4    Risks of Water-related Diseases-............................................................................................................ 79
6.4.2.5    Impacts on Landscape and Aesthetic Value80
6.4.2.6    Slope Stability and Seepage Problems81
6.4.2.7    Watershed Erosion and Sedimentation81
6.4.2.8    Erosion Induced by Project Activities82
6.4.2.9    Air Quality Deterioration82
6.4.2.10 Noise and Vibration83
6.4.2.11 Impacts on Mineral Resources83
6.4.3      Negative Impacts on Socio-Economic EnvironmentS3
6.4.3.1    Displacement of People83
6.4.3.2    Losses of Croplands. Forest and Grazing areas84
6.4.3.3    Impediments to Movement of People and Livestock..85
6.4.3.4    Losses of Religious Institutions and Social Services85
6.4.3.5    Loss of Traditional Mining Sites86
6.4.3.6    Attitudes and Reactions of Local Communities to the Project86
6.5  Proposed Mitigation Measures86
6.5.1  Bio-Physical Mitigation Measures86
6.5.1.1   Terrestrial Ecology86
6.5.1.2    Aquatic Environment87
6.5.1.3    Public Health Issues88
6.5.1.4    Landscape and Aesthetic Value89
6.5.1.5    Slope Stability90
6.5.1.6    Watershed Erosion90
6.5.1.7    Erosion Induced by Project Activities90
6 5.1.8 Water and Drainage........................................................................................................ ........................... 91
6.5.1.9    Air Quality91
6.5.1.10 Noise and Vibration92
6.5.2     Socio-Economic Mitigation Measures93
6.5.2.1    Resettlement.................................................................................................................................... ....... 93
6.5.2.2    Losses of Fixed Assets and Land under Various Uses93
6.5.2.3    Losses of Communication Network93
6.5.2.4    Participation of Communities and Local Authorities94
6.5.3      Preliminary Mitigation Cost94
6.6    Conclusion94
W/iter Wiirfc Design & Supervision EnterpriseLower Dldcmi Medium SwLr HyJnyiwr Project
Reconnaissance Level Study June 2001
7. COST ESTIMATE
8.       ECONOMIC ANALYSIS OF DIDESSA HYDROPOWER
8.1     Basis of Analysis................................................................
8.1.1     Cost of the Project................................................................. 8.1.2       Benefit of the Project
8.2     Economic Viability of the Project....................................
8.3     Sensitivity analysis of the Project
8.4    Conclusion- -
9.      COMPARISON WITH OTHER CANDIDATE PROJECTS
98 
.....................98
~99
99
102
10. CONCLUSION AND RECOMMENDATION-...........................................—------- ----- -------------------106
10.1    General Conclusion~106 10.2    Recommendation-106
REFERENCES110
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ivLmr Dbleai Medium Scale Hydropower Project
List of Tables
RecoiiMi&iitcc Level sbidy June 2001
Table 3.1 List of meteorological stations in and close to Didessa catchment.................................. 7
Table 3.2 Monthly mean values of some meteorological variables........................................................ 9
Table 3.3 List of stream gauging stations in the Didessa catchment.................................................. 10
Table 3.4 List of gauging stations used in the study............................................................................... 11
Table 3.5 Regression Coefficients of Sediment Rating equations.......................................................14
Table 3.6 Mean and dependable (available 80 per cent of time) monthly rainfall............................ 15
Table 3.7 Mean monthly runoff values.........................................................................................................16
Table 3. 8 Runoff Availability at Dam-site (Didessa)...................................................................................18
Table 3.9 Regression Coefficients of Flood Discharge on Area........................................................... 18
Tables, io Flood estimates...............................................................................................................................19
Tables. 11 Mean Monthly Evaporation......................................................................................................... .21
Table 3.12 Net evaporation losses from reservoir surface...................................................................22
Tables. 13 measured Sediment Yield for a Few Rivers............................................................................. 24
Table 5. l Different Dam Type............................................................................................................................34
Table 5.2 Initial Screening................................................................................................. -.................35
Table 5.3 Summary of the Main Technical Features of Lower Didessa Site........................................50
Table 6. I Total population of all villages................................................................................................... 67
Table 6.2 Land use pattern of all villages....................................................................................................67
Table 6.3 Livestock population in all villages..............................................................................................68
Table 6.4 Total population of all villages....................................................................................................70
Table 6. 5 Land use pattern of all villages (ha).......................................................................................... 71
Table 7. I Cost summery...................................................................................................................................... 97
Table 8. l Economic analysis............................................................................................................................ loo
Table 9. I Target Scenario load forecast.................................................................................................. 103
Table 9.2 Comparison with Other Hydropower Projects...................................................................... 104
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vLwcr Didessa Medium Scale Hydropower Project
Reconnaissance Level Study June 2001
List of Figures
Figure 3. I: Didessa river and project site................................................................................................... 5
Figure 3.2: Distribution of Ground Slope in the Didessa Basin.............................................................. 6
Figure 3. 3: Distribution of stream flow gauging stations in and around the Didessa
Catchment...................................................................................................................................... 9
Figure 3.4: annual flow at Didessa (Arjo) and Dabana (Abasina)........................................................ 12
Figure 3.5: Annual flow at didessa (Arjo) and Wama (Nekemte)..........................................................13
Figure 3. 6: Distribution of Mean annual Rainfall.................................................................................. 15
figure 3. 7 monthly and Annual Flow-Duration curves (Didessa)........................................................ 17
Figure 3.8: Scatter plots of mean annual evaporation vs. elevation................................................. 21
Figure 3. 9: Regression coefficients 'a' and 'b' as functions of catchment area............................ 23
Figure 5. I & 5.2 didessa Reservoir area & Capacity Curve...................................................................37
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v>Lower Dfrkat Mciiiiun Scale Hydropower Project
Reconnaissance Level Study August 2001
MCM
Million cubic meter
MFL
Maximum flood level
MFP
Momentary Flood Peak
Mt/yr
Million Tons per Year.
MUSD
Million United States Dollars
MV
Mega Volt
MW
Mega Watt
MWh
Mega Watt hours
MWR
Ministry of Water Resources
NFPA
National Forest Priority Area
NMSA
National Meteorological Services Agency
O&M
Operation and maintenance
ONAF
Oil Normal Circulation & Air Forced Circulation
ONAN
Oil Normal Circulation & Air Normal Circulation
PA
Peasant Association
PH
Power House
PVC
Poly Vini Chloride
RB
Right Bank
RBL
River Bed Level
SCL
Spillway Crest Level
SCRP
Soil Conservation Research Project
2
t/ km /yr
Tons per Square Kilometer per Year
TWL
Tail Water Level
U/S
Upstream
UPS
Uninterruptible Power Supplies
USBR
United States Bureau of Reclamation
use
United States Cents
USD
United States Dollars
V
Volt
WCA
Wildlife Conservation Area
WMO
World Meteorological Organization
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viiiLiwcr Didessa Medium Settle Hydroyowcr Project
1.       INTRODUCTION
1.1     General
Reconnaissance Level Study August 2001
Ethiopia has immense hydropower potential. However, it is estimated that about 1% is utilized, so far. It is also known that its irrigated agricultural development potential is virtually untouched in comparison with the water resources Ethiopia is endowed with. For these reasons the government of Ethiopia has been carrying out basin master plan studies  to prioritize development activities  in the water sector where irrigation and hydropower are given top priority. With respect to hydropower the Ministry  of Water Resources  has  already  undertaken pre feasibility, feasibility studies, design and tender preparation for Tekeze & Gojeb and pre-feasibility studies for Genale, Guder, Baro and Geba projects.
To further make prioritized inventory of projects the Ministry of Water Resources invited Water Works  Design and Supervision Enterprise (WWDSE) to make reconnaissance study of Didessa hydropower project.
WWDSE will therefore, take the responsibility of making reconnaissance study of Lower Didessa hydropower projects.
Previous Study
In 1998 the Abbay River Basin Integrated Development Master Plan Project was commissioned by  the Ministry  and potential sites  for different purpose was identified. The Lower Didessa was  one of the project emerge the other hydropower project. This site was identified in 1964 during the land and water resources  of the Blue nile basin master plan study  by  the USBR and it was known Boo dam (DD2) hydropower project.
Due to the need to further investigate the potential of this  site, the Ministry commissioned a reconnaissance level study in December 29, 2001. This study is  meant to confirm and augment the findings  of the proceeding study. The preceding study  identified a dam site of about 15km upstream of the Anger- Didessa junction and 11km from Kemashi town. The dam is to close the space between elevation 1050m.a.s.l either side of the river. The maximum height is set of 160m.
1.2     Purpose of the Study
The study  conducted by  the previous  consultants  has  preliminary  nature, different from this, the purpose of this reconnaissance level study is to evaluate the potential and to define and formulate the proposed projects. The study also
Writer work Design & Siij/ervlsitni Enterprise
lLower DidesM Medium Seale Hydropower Project
Reconnaissance Level Study August 2001
indicates  whether the potential projects  appear technically, economically  and environmentally  feasible. The report discusses  on whether to proceed with further studies and investigations on the bases of collected data.
And finally  the findings  of the reconnaissance study  will be recommended for further detailed investigations  with all relevant information pertaining to hydropower development.
Water works Design & Supervision Enterprise
2I
I
LEGEND
ERITREA
-16°
SELECTED TOWNS NATIONAL CAPITAL INTERNATIONAL BOUNDARIES
16’-
RIVER
%
PROJECT AREA
SUDAN
-12’
AMHARA
Lower Dlddessa
O ASOSA
ABAB^
SOMALI
SOMALIA
0
SUDAN
z
z
KENYA
UGANDA
KILOMETERS 0 MILES 0Lower Didessa Medium Seofe Hydropower Project
2 DESCRIPTION OF THE PROJECT AREA
2.1     Location
Reconnaissance Level Study AUrfUSt 2001
The Didessa hydropower project is located on Didessa River in Kamashi Zone of the Benishangul-Gumuz  Regional State; specifically  in Kamashi (left bank) and Belo Jiganfoy (right bank) Woredas.
The Didessa river dam site is  reached through the Addis  Ababa-Nekemte-Nejo road turning to the right before Nejo to Kamashi. There is 335km asphalt road from Addis  Ababa to Nekemte and 152km well maintained all weather gravel road from Nekemte to Kamashi Junction. From Kamashi road junction to Kamashi town there is 38km poorly maintained road, which mostly serves only in the dry  season. From Kamashi town the dam site is  reached by  driving about 17km along recently opened unpaved access road that runs through bushes, and walking about 2km up to the dam site. Thus the total distance from Addis to the Dam site via Kamashi is about 544km.
The location maps are:
The geological map at 1:250,000 scale of GIMBI (NC36-12)
The topographical map at 1:50,000 of SAYI (935-D2)
The co-ordinations are:
09°29'00” N Latitude
35°58’30" E Longitude
2.2     Topography
2.2.1  General
The topography  of the area is  rugged & undulating. The Diddessa River originates around Jemma area at maximum elevation of 2620 mask and suffers a drop of 1620m at the dam site area at elevation around 1000 mask  The area covering the potential dam site, the conveyance structures and the power house is  densely  vegetated and detailed topographic  survey  was  not possible without clearing some trees. Given the vegetation and the nature of reconnaissance study cross section survey was therefore adhered to.
Wider Works Design & Supervision Enterpriselower Didcss/t Medium Scale Hydropower project
2.2.2 Cross Section Survey
Recormnissmce Level Sbuly
Ajyust 2001
The existing 1:50,000 scale map of the area has been the guiding base map in identifying the general project area. Once the visiting team accessed the area of interest, a walkover survey was conducted to establish the extent of survey work necessary.
According to the above statement only one dam site was found and three cross section was made by the surveying team, but the topographic map survey was not sufficient to show all the necessary structures, so it has been added another two cross  section from 1:50,000 scale map and try  to show the spillway, the cofferdam and the power house location. The project layout is presented on Drg No. 3.
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4Lower Didessa Medinin Hjdrojwcr Project
3. HYDROLOGY
3.1     General
Recowisswe level Study
This technical report presents the findings of the hydrological investigations for the Lower-Didessa Hydroelectric Project. The Didesa river basin is located in the Blue Nile basin, and Didesa river is one of the major tributaries of the Blue Nile, with a drainage area of about 29 000 sq. km.
\_______ „
ION
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X              Project Site
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36 IS
• 19
37 E
Figure 3.1: Didessa river and project site
(Numbers 1 to 19 indicate locations of meteorological stations)
water Worts Design & supervision Enterprise
5Lower Dhlcssii Medium Settle Hydryowcr project
3.2     The Didessa Basin and Availability of Data 3.2.1  Description of the Didessa Catchment
RnwHb3®f« Level Study
AltfUSt 2001
The Didessa River is one of the major tributaries of the Abbay (Blue Nile) river. Together with its biggest tributary, the Angar, the Didessa basin has an area of about 29 000 sq. km at the confluence with the Blue Nile River. Other important tributaries include the Dabana, Sifa, and Wama. Didessa river originates in the mountainous  region to the west and Northwest of the Jimma town, where the average annual rainfall is about 2000 mm.
The ground slope in the basin varies from 30 per cent to values less than 5 per cent, the distribution of ground slope is given in Figure 3.2. About half of the basin has a ground slope flatter than 5 per cent slope. The mean basin slope is about 7.5 per cent.
Distribution of Ground Slope (Didessa)
Figure 3. 2: Distribution of Ground Slope in the Didessa Basin
The land cover/land use of the area exhibits the following types: forest, dense bush, grassland, cultivated land, or complex, which is  a mixture of cultivated, grass, bush and dense bush. Field visit to the catchment in March 2001 has shown that a good proportion of the catchment is covered by mixture of forest dense bush and grass. The region exhibits  the least soil degradation in the country. This is attributed to the fact that the area was settled relatively recently and the population density is still less. However, recent rural settlement programs observed in the region may  lead to a change of the situation. It has  been observed during a field visit to the area that land clearing in recent time has led to loss of important vegetation cover, which protects the soil from erosion.
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3.2.2 Meteorological database
Reconnaissance Level Sbuiy
AJ^Ilrt 2001
In Ethiopia, hydro-meteorological data are collected primarily  by  the hydrology department and the Ethiopian National Meteorological Services  Authority, both under the ministry  of water resources. The former is  responsible for the installation and operation of stream gauging stations. The ENMSA on the other hand installs  and operates  meteorological stations, which are categorised into four classes. Class 1 and 2 stations are the most well equipped of all, collecting climatic  variables  such as  rainfall, temperature, relative humidity, sunshine duration, etc, whereas  class  4 stations  collect daily  rainfall only. Daily  mean temperature is measured in class 3 stations in addition to daily rainfall.
The total number of meteorological stations of all classes in the basin is well over 30, which makes the gauge density of the network quite good in relation to the minimum required, WMO (1994). Hence, daily  values  of measured climatic variables  have been requested from the National Meteorological Services Authority for some 30 stations in and around the Didessa basin. However, only monthly values have been provided for 9 stations.
Table 3. 1 List of meteorological stations in and close to Didessa
catchment
No
Station Name
Longitud
e
Latitude
Elevation,
m
Class
1
Agaro
36°38 E
7° 51 N
1560
1
2
Alibo
37°04 E
9° 53 N
2000
3
3
Arjo
36°30 E
8° 45 N
2565
1
4
Baco
37°04 E
9° 07 N
1850
1
5
Bedele
36°23 E
8° 27 N
2090
1
6
Anger Gutin
36°24 E
9° 15 N
1500
1
7
Chora
36°18 E
8° 22 N
2000
3
8
Dabena
36°17 E
8° 24 N
1950
4
9
Dembi
36°27 E
8° 05 N
1500
3
10
Didessa (State Farm)
36°06 E
9° 00 N
1200
1
11
Gimbi
35°47 E
9° 10 N
1970
1
12
Haro Aleto
37°08 E
9° 23 N
2395
1
13
Haro
36°31 E
9° 09 N
1900
3
14
Humbe
36°07 E
8° 27 N
1950
4
15
Kone
36*17 E
| 8° 42 N
2000
4
16
Nedjo
35°29 E
| 9° 30 N
1800
1
17
Nekemte
36°36 E
| 9° 03 N
2005
1
18
Shambu
F37*06 E
9° 34 N
2500
1
19
Jimma
36°50 E
7* 40 N
1577
1
Miter worta Design & Supervision Enterprise
7Lower Dtdessa Medium Scale Hydropower Project
RecotmtissMce Level Study
August 2001
Hence, data collected in earlier studies on the basin was sought. For this, about 19 meteorological stations have been selected from those in and close to the catchment, most of which are within the catchment. In all the stations  daily rainfall is measured. The 19 stations have been selected based on the length of gaps  in their record, their nearness  to the project sites  and the information obtained from earlier studies  on the catchment, for instance MoWR (1999), regarding the nature of their data. Table 3.1 presents the details regarding each station, whereas the locations of the stations are shown in Figure 3.1.
Hence, inference regarding spatial variation of meteorological variables, such as mean annual rainfall, have been based on recent studies  carried out on the basin. Data and inferences  made on the hydrology  of the region have been extracted from the reports. In addition, whenever required, the information has been used to augment the data from the 9 station obtained from the NMSA. Reports of previous studies that have been reviewed are:
1.  Abbay River Basin Integrated Development Master Plan Project, Phase 2, Section II-Water'Resources, Parts 1 and 2, (MoWR, 1999).
2.  Resources of the Blue Nile basin, USBR (1964).
3.  Ethiopian National Energy Committee-CESEN, Supplementary Report 3,
(1986).
Among these reports the first one has delivered very useful information on many aspects of the hydrology of the catchment. Moreover this report contains monthly flow series up to the year 1992. The data that have been obtained from these reports have been augmented with the measurements since the publishing of the report.
The meteo-climatic data  available consists of  the  monthly mean  values of the following physical variables, measured at nearby stations:
•    mean monthly wind speed in m/s (measured 2 m above ground level),
•    relative humidity in %,
•    minimum and maximum temperatures in °C,
•    daily sunshine duration in hours
The mean monthly  values  of the above meteorological variables  have been summarised in Table 3.2. These values are meant to apply for the project site and as such do not represent the spatial average over the catchment. They can be used to characterise the project site and also for reservoir operation studies. More statistics  of these meteorological variables  are given in Table1.1, in Appendix 1.3.
VVdti’r works Design & Supervision Enterprbe
SLower Didessa Medium Scale Hydropower Project
Reconnaissance Level Shaly August 2001
Table 3. 2 Monthly mean values of some meteorological variables
(Recorded at Didessa State Farm)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mean monthly rainfall (mm)
2.1
5.3
32.1
58.2
183.3
263.1
335.0
270.8
192.6
105.8
19.8
9.0
Mean temp. (°C)
21.9
23.5
25.4
25.3
24.0
21.8
21.1
21.2
21.8
22.1
21.9
21.6
Rel. Humidity, %
59
52
54
63
71
79
84
84
84
77
71
65
Mean monthly wind speed (m/s)
0.7
0.9
1.2
1.2
1.1
1.0
0.7
0.6
0.6
0.6
0.6
0.6
Daily mean sunshine duration (hours)
8.6
7.9
7.2
7.5
7.0
6.2
5.0
5.0
6.2
8.0
9.7
8.5
Potential Evaporation (mm/day)
3.6
4.0
4.5
4.6
4.1
3.6
3.2
3.2
3.5
3.8
3.6
3.4
3.2.3  Streamflow Data
——------------- ft
i ,-.r ■Jhtls :
Jam Site
%
sjn              X
—J
t♦
<■ -»
X
-
i
X-
IBH
* X < >
\
1
\
>
8" N
21
3
\2SS^                   3
Figure 3. 3: Distribution of stream flow gauging stations in and around the Didessa Catchment
The catchment area of the Didessa river up to the dam site is about 18 200 km2 Dabana, a major tributary to the Didessa, joins at about 75 km upstream from the proposed dam site. Other tributaries include the Sifa and the Wama, which drain Water Worh D«yii & Supervision Enterprise
9
rLower Didessa Medium Scale Hydropower Project
Reconnaissance Level Study
Ajyiirt 2001
but smaller parts of the catchment. A list of gauging station obtained from the hydrology Department of the MoWR shows that there are 13 streamflow gauging stations in the catchment of the dam site. The distribution of these stations in the catchment is shown in Figure 3.3 whereas Table 3.3 presents the catalogue of the stations.
The density of the streamflow stations for the entire Angar-Didessa sub-basin is about 1600 km2 per station, which is  quite satisfactory  according to the WMO (WMO, 1994) guidelines on minimum density for general hydrological purposes.
2
The density for the catchment of the dam site is about 1500 km  per station. However,  the  distribution  of  the  stations  in  the  catchment  is  not  optimum  as downstream  parts  of  the  catchment  remain  ungauged.  As  it  can  be  seen  from Figure 3.3 most of the stations are located following the major access roads. The gauging station nearest to the dam site is Dabana at Abasina, which lies some 75  km  upstream.  Dabana  is  a  tributary  to  Didessa  river,  where  the  confluence lies   a   short   distance   downstream   from   the   gauging   station.   The   most downstream  gauging  station  on  Didessa  lies  near  Arjo,  some  60  km  upstream from  the  confluence  with  the  Dabana  river.  Thus  an  appreciable  part  of  the catchment of the dam site is not gauged, which calls for transfer of hydrological information from the upstream stations.
Table 3. 3 List of stream gauging stations in the Didessa catchment
No
Stn No.
River
Town
Co-ordinate
N
E
Area
Km2
Established
1
114001
DIDESSA
Nr. ARJO
8°41'
36°25'
9981
1959
2
114005
DABANA
Nr. ABASINA
9°02'
36°03'
2881
1961
3
114014
DIDESSA
Nr. DEMBI
8°03'
36°27'
1806
1983
4
114003
SIFA
Nr. NEKEMTE
8°52'
36°47'
951
1960
5
114004
WAMA
Nr. NEKEMTE
8°47'
36°47'
844
1960
6
114013
DABANA
Nr. BEDELE
8°24‘
36°17'
470
1983
7
114006
YEBU
@Yebu
7°48'
36°42‘
47
1979
8
114010
TATO
Nr. GUTIE
9°02‘
36°39’
42.5
1979
9
114011
ME KA
Nr. NEKEMTE
9°03'
36°18'
43.7
1980
10
114012
IDRIS
Nr. SIRE
9°02‘
36°5T
49
1980
11
114009
URGESSA
Nr. GEMBE
7°50*
36 39'
a
19
1979
12
115015
CHERETI
E
Nr. ARMY
CAMP
8°55'
36°22’
62.5
1984
13
114019
TEMSA
Nr. AGARO
7°5T
36°35'
47.5
1989
the area of this station has been reported as 47 sq. km in the station list provided by the Hydrology Department. However, it has been found out in a previous study that the actual area is 470 sq. km.
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10Lower Difani Me fam Scale Hydropower Project
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The station on Meka stream near Nekemte has  been abandoned, thus  the number of stations that could be used for the study becomes 12. As it can be deducted from Figure 3.3, most of the gauging stations have very small drainage areas. Only  few of the total number of stations  are interesting as  far as  the present project is concerned. In fact, it is shown in Figure 3.3 that the stations Didessa near Arjo and Dabana near Abasina drain most of the catchment of the proposed dam site. Hence, based on their aerial distribution, and in consultation with the staff of the Hydrology Department of the MoWR, 5 stations have been selected and their streamflow data obtained. The data provided include monthly volumes  of runoff, monthly  mean flow, maximum and minimum daily  flow for each month. In very few cases, for instance for Didessa near Arjo, momentary peak flows have been provided. However, these values are available only for the first few years after the establishment of the gauging station.
3.2.3.1   Quality of Streamflow Data and Gap filling
A  preliminary  checking  of  the  data  from  these  stations  has  been  conducted. Values that seem to be inconsistent with records from surrounding stations have been  treated  cautiously  or  excluded  altogether  from  analysis.  Such  values, however, were very few in the entire record. A major problem was, nevertheless, longer  gaps  in  the  streamflow  records,  particularly  that  of  Didessa  (near  Arjo). The list of these stations together with the extent of missing records is given in Table 3.4.
Table 3. 4 List of gauging stations used in the study
No
Station
River
Length of
record
(years)
gap
(% months)
1
Nr. Abasina
Dabana
24
5
2
Nr.            Buno Bedelle
Dabana
14
9
3
Nr. Arjo
Didessa
32
23
4
Nr. Nekempte
Sifa
24
10
5
Nr. Nekempte
Wama
24
8
Table 3.4 shows  that the record from the station near Arjo shows  the biggest gap, although this station is located beside a bridge on the route to Bedele. This station has been visited during the field trip in March 2001. The station is in good condition, though some rehabilitation work  is  needed, which is  discussed in Appendix 1.7. To fill in the gaps, correlation and regression analysis has been carried out. Among the five stations selected for the study, the record period at
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Reconnaissance Lew! Study
August loot
the station Dabana near Bunobedelle does not coincide with those of the other stations. Hence, only the other four stations have been included in the regression and correlation analysis.
The correlation between the stations is shown in the form of a cross-correlation matrix given below,
Station
Dabana-AS
Didessa-
Arjo
Sifa
Wama
Dabana-AS
1
Didessa-Arjo
0.6
nr
Sifa
0.703
0.853
1
Wama
0.439
0.898
0.957
hr
The cross-correlation matrix shows a range of values of the cross correlation. It shows that the correlation among the stations Didessa-Arjo, Sifa, and Wama is quite high, whereas that between Dabana-Abasina and the rest is relatively low. It is interesting to see that the correlation between Dabana-Abasina and Didessa- Arjo is, in contrast, quite low. A scatter plot of the flow at Didessa-Arjo and Dabana-Abasina, shown in Figure 3.4, shows that even a correlation of 0.6 would not have been attained had it not been for the outlier point. This point was then removed and the correlation coefficient recalculated, resulting in a value of 0.227! Hence, it can be said that there is a poor correlation between these flow records. On the other hand, the correlation between Didessa-Arjo Wama, Figure 3.5.
Annual flow Correlation: Didessa (Arjo) vs Dabana
(Abasina)
6000
M
V)
■g 5500
Q2
t          5000
c
a c 4500
g 4000
«
*
*
♦
<                3500 _________________________ ________________________________
1000
1500
2000
2500
Annual runoff: Dabana (Abasina), MCM
Figure 3. 4: Annual flow at Didessa (Arjo) and Dabana (Abasina)
W/ikr Work Design & Supervision Enterprise
12Lowr Didessa Medium Seale Hytlroyow Project
Didessa (Arjo) vs. Wama (Nekemte)
RtcoHMissana Level Study AlUJUSt 2001
7000 -
I
6000
5 2000
Q 1000
0 ---------------------------
0 100 200 300 400 500 600
Wama, MCM
Figure 3. 5: Annual flow at Didessa (Arjo) and Wama (Nekemte)
(Solid line indicates regression line)
3.2.4  Data on Sediment Transport
The estimation of annual sediment load was  based on the measured sediment rating curves  for the catchment. There are some sediment measurements  on Didessa and Dabana Rivers. However, the measurement on the Didessa River is done at the upstream gauging station, i.e. near Dembi, which commands some 1800 km2 . In order to assess  the regional condition on sediment transport, sediment rating curves have been used from other nearby stations, namely:
1.  Angar (near Gutin)
2.  Dabana (near Abasina)
3.  Dabana (near Bedelle)
4.  Didessa (near Dembi)
5.  Tato (near Nekempte)
6.  Temessa (near Agaro)
7.  Urgessa (Nr. Gembe)
8.  Wama (near Nekemte)
9.  Yebu(atYebu)
The rating curves have the form:
Where Qs is the sediment discharge, in tons/day, QL is volumetric flow rate, ‘a1 and 'b' are regression coefficients. The regression coefficients for these stations are given in Table 3.5.
Wfllrr worts Dosljii & Suj/ervbloii Euki^rbe
13Ljivcr Didessa Medium Seale Hydropower Project
Reconnaissance Level Study August 2001
Table 3. 5 Regression Coefficients of Sediment Rating Equations
Regression Coefficients
River (Station)
a
b
Angar (Near Gutin)
8.12
1.303
Dabana (Nr. Abasina)
0.87
1.740
Dabana (Nr. Bedelle)
4.97
1.196
Didessa (Nr. Dembi)
4.08
1.330
Tato (Nr. Nekempte)
19.09
1.547
Temessa (Nr. Agaro)
13.71
1.186
Urgessa (Nr. Gembe)
25.89
1.404
Wama (Nr. Nekemte)
8.78
0.947
Yebu (at Yebu)
37.40
1.319
3.2.5 Data on Current Water Use in the Basin
There has been a large-scale State Farm in the Didessa basin upstream of the project site. This  Farm, known as  the Didessa State Farm, involves  rain-fed agriculture, with maize being the main crop produced. The project office in Nekemte has been contacted for information on the current status of the State Farm According to the information obtained, the State Farm is  no more operational. It is being privatised and parts of the Farm have already been sold to private business  firms. As  the schemes  involve rain-fed agriculture no water is drawn from the streams in the basin.
3.3 Main Rainfall Characteristics
The basin falls  within the region whose rainfall characteristics  shows  a monomodal pattern, i.e. the rainfall pattern shows  a single maximum with variable duration of the wet season. The southern part of the catchment, where the river Didessa originates, has its wet period extending from February/March to October/November, whereas  the northern part of the catchment gets  rainfall mainly  from April/May  to October/November (MoWR, 1999). In this  part of Ethiopia, the year is divided into two main seasons, the Kiremt, the wet season and the Bega, i.e. the dry season.
The mean annual rainfall in the catchment ranges  from about 2000 mm in the south and south-east to 1200 mm just at the confluence with the Blue Nile. The basin-wide Mean Annual Rainfall is about 1600 mm.
Writer Wiirh Design & Supervision Enterprise
14Lower Didessa Medium Scale Hydropower project
RecoriMissMce Level Study August 2001
Figure 3. 6: Distribution of Mean Annual Rainfall
(Values are in mm)
Table 3. 6     Mean and dependable (available 80 per cent of time) monthly rainfall
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mean  monthly rainfall (mm)
2.1
5.3
32.1
58.2
183.3
263.1
335.0
270.8
192.6
105.8
19.8
9.0
Dependable
rainfall (mm)
0.0
0.0
4.8
10.5
100.5
184.2
217.4
206.7
130.0
47.1
2.8
0.0
3.4 Main Runoff Characteristics
Following the seasonal distribution of the rainfall in the region, the hydrographs of monthly  flows  in the streams  are monomodal, with peaks  sometime in July  or August. Flows are minimum in the months of January and February.
Water Works Design & supervision Enterprise
15Lower Didessa Medium Seale Hydropower Project
3.4.1 Runoff Estimates at Dam Site
Reconnaissance Level Study AlifflSt 2001
The  total  area  of  the  Didessa  catchment  draining  into  the  dam  site  is  about
2
18200 km , out of which about
12862 km2  has been gauged since 1960 by the
2
2
stations Didessa near Arjo (Area = 9981 km ) and Dabana near Abasina (Area = 2881 km ). There are also other gauging stations in the basin that are located upstream of these two stations.
Before flow was generated for the dam site, preliminary checks were made on the consistency of the reported data and the gaps infilled. As it can be seen from Table 3.4, the flow from Didessa near Arjo exhibits the biggest gap. These gaps were filled by  correlation with the nearby  station and compared with those generated in previous study, as discussed in section 3.2.3.1.
The transfer of river flow data to the proposed dam site was  based on the correlation of annual flows at a number of stations within and around the Didessa sub-basin. The regression equation formulated used the product of mean annual rainfall and catchment areas  as  independent variable. This  has  been selected after a very  good correlation (0.99) was  obtained between the mean annual runoff and the product of mean annual rainfall and catchment area. The stations used in the correlation and regression analysis were Angar near Gutin, Didessa near Arjo, Dabana near Abasina, Sifa near Nekemte, and Wama near Nekemte. A runoff sequence of 33 years long has thus been generated for the dam site and the same has  been used for the computation of relevant flow characteristics, including regulation curve. Mean monthly runoff values for Didessa is shown in Table 3.7.
Table 3. 7 Mean monthly runoff values
o
Qm®an
(m3/sec)
39.83
26.68
22.59
26.96
44.58
140.38
428.19
771.03
692.91
388.39
129.57
65.62
Writer work Design & Supervision Entcryrise
16Lower Didessa Medium Scale Hydropower Project
3.4.2 Frequency of flow
Rccomialssaoiee Level Study August 2001
The flow duration curve has been obtained for annual flows from which flows of various  percentages  of occurrences  have been established. For a continuous time series, the flow duration curve can be taken as  a cumulative frequency curve and for this  reason it is  possible to fit the duration curve by  using an appropriate probability distribution function. In the present case semi-log plots of both annual and monthly duration curves showed that the duration curves can best be described by the log-normal distribution. Hence the same has been fit to the duration curves. The flow-duration curve is presented in Figure 3.7, whereas Table 3.7 summarises the flows at various probability of occurrence.
Annual Flow-Duration Curve, Didessa (dam
Monthly Flow-Duration Curve (Didessa Dam)
percentage of time flow is exceeded or equalled
Figure 3. 7 Monthly and Annual Flow-Duration curves (Didessa)
Wiitor Works Desljn <& Supervision Enterprise
17Lower Dldcssa Medium Scale YAydroyowcr Project
Table 3. 8 Runoff Availability at Dam-site (Didessa)
Reconnaissance Level Study
August 2001
Mean   Annual Flow (MCM)
Area
Km2
Probability of exeedence, %
75
80
90
95
7290.22
18200
6300
6100
5230
4600
3.4.3  Floods
The estimation of design inflow floods has been made based on the following:
-    estimates made in previous studies (Method -1)
relationships developed for the estimation of floods in relevant studies 
(Method-2)
-    regional flood-frequency analysis, Gebeyhu (1989), (Method-3)
Estimates of flood discharges for various return periods have been made in the study conducted by the USBR on the Blue Nile basin. The estimates made in the study have been presented in table 1.4 together with the other estimates.
The Abbay  Basin Integrated Development Master Plan study  has  produced regression equations for the estimation of floods of various return periods. The regression equation developed has the form:
Q = aAb
3
Where Q is in m /s and A is the catchment area in sq. km, whereas 'a' and 'b' are regression coefficients. The coefficients vary based on return period and whether the catchment area is  less  than 10000 sq. km or not. These coefficients  are shown in Table 3.8. No regression equations  have been developed for return periods of 10 years.
Table 3. 9 Regression Coefficients of Flood Discharge on Area
Return period in years
100
1000
10000
a
b
a
b
a
b
2
For tributaries (Area < 10000 km )
47.8
0.339
75.7
0.313
105.0
0.296
For mainstream Abbay  and major
2
tributaries (A >= 10000 km )
216.5
0.326
844.1
0 228
2130.7
0.163
W/itrr Works Design & Supervision Enterprise
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Recoiui/iissrvice level study Allffist 2001
Regional flood frequency analysis has been made to estimate the flood at the proposed dam site. The mean annual flood (index  flood) has  been estimated based on the data from upstream stations within the basin, namely the Dabana at Abasina and the Didessa near Arjo. At both stations, there are records  of Momentary Flood Peaks (MFP) for about 10 to 12 years immediately after the gauging stations  have been established. Graphs  of the ratio of MFP vs. maximum mean daily  discharge have been used to estimate the Flood Peak Factor, as given in Appendix 1.2.
It can be observe from Figure1.1, Appendix 1.2, that the ratio is clearly a function of drainage area. The mean ratio for Didessa (Area = 9981 sq. km) is about 1.1 whereas that for Dabana (drainage area = 2881 sq. km) is about 1.7. Moreover, whereas the ratio for Dabana exhibited dependence on flow rate (r = -0.78), that for Didessa showed very poor correlation (-0.15). Hence, an average value of 1.1 (solid line in Figure1.1a) has  been used for Didessa station to augment the momentary flood peaks based on the maximum mean daily flow. In the case of Dabana, however, a regression equation has  been (solid line in Figure1.1b) established between the ratio of momentary  flood peaks  and maximum mean daily flow and the same has been used. The floods estimated as outlined above are summarised in Table 3.9.
Table 3. 10 Flood estimates
Flood peaks of return period (in years)
Method
10
20
100
1000
10000
1
4200
5300
10500
2
5300
7901
10542
3
1977
2210
2735
3480
4224
Probable maximum flood
3.4.4  Discussion and Recommendation
Table 3.9 exhibits  differences  in flood magnitudes  for different return periods; particularly the floods determined by the regional flood frequency approach are much lower than the other two. The flood estimates by method 1 were made with very short record lengths hence may not be reliable particularly at higher return periods. However, those determined by the second method tally quite well with those determined based the Gumbel distribution. This is because the regression equations  generated were largely  based on the estimates  made by  method 1. Hence, for this  study, it is  recommended to use the floods  estimated by  the regional flood frequency analysis, i.e. Method 3.
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3.5    Evaporation
3.5.1 Open-Water (Lake) Evaporation
Reconnaissance Level Study
2001
This  section focuses  on the estimation of open-water evaporation (lake evaporation) at the site of the proposed storage dam. Daily  data on meteorological variables  could not be obtained from the NMSA. Hence, the estimates made in previous studies shall be used as appropriate.
In the study conducted by CESEN (ENEC-CESEN, 1986), it has been reported that evaporation in the Blue Nile basin is correlated with altitude. Two regression equations have been developed as given below:
For basins that have North-South orientation
EP = 6350.673 - 3.558*EI
and for those that have East-West orientation EP = 4507.399 - 1.939‘EI
(3.5.1a)
(3.5.1b)
Where El is elevation in meters and EP is mean annual potential evaporation in mm.
These regression equations have been checked with the available data on mean annual evaporation. As a result, mean annual evaporation for basins with North- South orientation shows  less  dependence on elevation than that for basin with East-West orientation. The regression coefficients  determined based on the available data are, however, differ from those given above. Scatter plots of mean annual evaporation vs. elevation are shown in Figures  3.8.1a and 3.8.1b. The linear correlation coefficients are -0.52 for North-South oriented catchments, and -0.966 for catchments  with East-West orientation. The regression line for catchments with East-West orientation is
EP = 2217.461 - 0.47408*EI
Writer Werta & supervision Enterprise
20Lower Didessa Medium Seale Hydropower project
Recomnisscmce Level study AUfflst 2001
Evaporation vs elevation
E
c 
o
a re OE
850
elevation, m
E E
c
o
a 2
o
Q. <0
>
(a) Catchments with East-West orientation (solid line shows fitted regression line)
Evaporation vs elevation
1550
[
♦
1350
♦
• ♦. *
»
1150 950 750 -----
1000
♦
i
i
1500            2000
elevation, m
2500
3000
(b) Catchments with North-South orientation
Figure 3. 8: Scatter plots of mean annual evaporation vs. elevation
The Didessa catchment is  oriented in the North-South direction. Hence, evaporation shows  less  dependence on elevation. For this  reason it is recommended to take evaporation estimated at nearest station to the project site, namely Didessa Sate Farm and the same is given in Table 3.10.
Table 3.11 Mean Monthly Evaporation
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Potential Evaporation (mm/day)
. 3.6
4.0
4.5
4.6
4.1
3.6
3.2
3.2
3.5
3.8
3.6
3.4
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21Liwr Diilessa Medfuni Scale Hydropower Project
Reconnaissance Level Study August 2001
3.5.2 Reservoir Net Evaporation Losses
Net water loss due to evaporation from reservoir surface is needed in reservoir operation studies. In this  study  regulation curves  have been developed for the storage scheme at lower Didessa. The regulation curve is developed through a continuous  simulation of the water-balance of the reservoir, for which the lake evaporation estimated above is  used. After the reservoir is  filled the net evaporation loss equals the difference between rainfall and lake evaporation. The lake evaporation tabulated above has  been used together with the rainfall estimates for these sites to compute the net evaporation losses shown in Table 5.11.
Table 3. 12 Net evaporation losses from reservoir surface
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
E (mm)
76.0
78.3
65.6
38.4
-94.3
-187.5
-265.6
-201.4
-119.1
-23.3
55.8
64.8
3.6 Sediment Transport
3.6.1  Estimation of Sediment Yield of Catchment
The estimation of sediment transport in the catchment is first made based on the observed sedimentation rates. The regression coefficients  for the stations  listed in Table 3.5 are plotted and shown in Figure 3.9 against catchment areas of the rivers at the gauging stations.
Coefficient 'a' vs catchment area
Area, km*2
laT
Miter work Design & Supervision EnterpriseLower Didcssa Mcdiioit Scale Hydropower Project
Coefficient'b' vs catchment area
Reconnaissance Level Study August 2001
I■ Iii
ii
ill ' . i|
1 lj
ill
1
1
11
'!
!
1
Ii•
I
<i
1
:
ji   •   i   .♦   . II         ;
l'i
1
♦
H
i
•
1
III
I
'll
i
I*
I IM
I I*
I'
r hi
ii in
III
III
1
III
III
11
III
1.0
10.0
(b)
100.0                   1000.0                  10000.0
Area, sq. km
Figure 3. 9: Regression coefficients 'a' and 'b' as functions of catchment
area
(Solid line in (a) indicates regression line)
From Figure 3.9a, it can be seen that the coefficient 'a' is a function of catchment area, whereas 'b' does not show any dependence on area. Hence, the regression equation for the dam site is established by computing the coefficient 'a' from the regression equation developed and the coefficient 'b' is taken as the average of the values for the stations used in the sediment study.
Finally the regression equation takes the form:
Q, = 0.743^“
The resulting sediment load, after a 20 % allowance has been made for bed load, equals 3.35 M t/yr.
3.6.2   Discussion and Recommendation
2
The total sediment load of the river computed above, which is about 184 t/yr/km , is indeed small compared to reported values of sedimentation in many existing reservoirs, which go well above 1000 t/yr/km2. The situation, however, varies from region to region and depends  largely  on the degree of land degradation prevalent in the basin under question. Reported sediment yields of catchments in Blue Nile and the neighbouring basin of Baro-Akobo are given in Table 3.12.
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23Loiwr Dlilcm Medium Scale Hydropower Project
RccoHnaiswice Level Study August 2001
Table 3. 13 Measured Sediment Yield for a Few Rivers
No.
River
(station)
Station Location
Long.
Lat.
Drainage
area,
km2
Basin/
Sub
basin
Suspended
Sediment
Yield
(t/km /yr)
2
1
Angar (Gutin)
36°35E
9° 30 N
3742
Blue Nile
89
2
Ketto nr.
Chanka
35° 02' E
8 U47'N
1006
Birbir
324
3
Sor nr. Mettu
35 36'E
u
8 U19' N
1622
Baro-
Akobo
124
The values indicated in Table 3.12 are all far on the lower side judged based on the values that are frequently quoted in similar studies. However, it has not been found reasonable to use values  in that order of magnitude without strong justification for it. Hence, information has  been sought on erosion rates  and degree of land degradation in the region.
This region exhibits the least soil degradation in Ethiopia. The vegetation cover is still in relatively good condition. Erosion rates measured in the test catchment (A = 6.72 km ) established by the Soil Conservation Research Project at Dizi in the
2
Bibir  catchment  show  that  the  maximum  value  of  sediment  yield  at  the  small
catchment level was 0.003 t/ha. In another test catchment (A = 2.4 km ) run by 
2
the Soil Conservation Research Project, the average sediment yield of the test catchment was  measured as  944 t/yr/km2, an average value computed for the period 1982-1987.
In light of this  the sediment yields  measured lie between the two extremes observed at small catchment level. It is known that the delivery ratio of sediments decreases  with increase in catchment area. Hence, it is  not expected that the same sedimentation can be applied for bigger catchment even if the same erosion rate may prevail. On the other hand, more spatial variation is expected as  the size of the catchment increases. Hence, it has  been found more appropriate to use higher values  of sediment yield of the catchment than is exhibited by the sediment rating curve in the basin. Pending further refinement thorough measurements in the future, a value of about 500 t/km /yr, i.e. about
1.5   times  the  maximum  measured  in  a  neighbouring  catchment  has  been adopted. The total sediment yield expected is then estimated as 9.1 Mt/yr.
2
Witter work DMijH & Siipen'islon Enterprise
24Lower Didessii Medium Scale Hydropower Project
RecoiunissiVicc Level Study
August 2001
3.7     Conclusion
In this  study  hydrologic  design parameters  have been estimated. Wherever applicable and reasonable, the estimates have been based previous studies in the catchment. The project at hand is  at the reconnaissance level and the methods used to determine the design parameters are acceptable. Regardless of this, the estimation of sediment yield of the catchment, which is  based on information from neighbouring catchment is  very  approximate and hence a thorough study is needed in the future.
W/itcr works Design & supervision Enterprise
25Lower Didessa Medium Scale Hydropower Project
4. GEOLOGY
4.1      Introduction
4.1.1   General
Reconnaissance Level Study August 2001
For the reconnaissance level Hydropower study of the Didessa River, geological and seismological investigations  were carried out at the Didessa river potential dam site and reservoir area.
Geological Map at 1:50,000 scale having a total area of about 320 sq. km. were prepared for the dam site and reservoir area. Regional seismic maps related to the project area were compiled.
4.1.2   Objective
The specific objective of the reconnaissance survey is to identify the geological formations  and structures  of the dam site and reservoir areas; to assess  the availability of Natural Construction Materials and to evaluate the seismicity of the project area.
4.1.3   Approach and Methodology
4.1.3.1 Field Investigation
Geological field investigations were carried out during March 2001. Topographic maps  of 1:250,000 and 1:50,000 scale, aerial photographs  of about 1:50,000 scale and GPS device were used for locating the proposed dam site and geological data recording. Regional geological maps  prepared by  the previous workers (USER, 1964, Mengesha T. etal, 1996 and MWR, 1998) were used for correlation purposes.General geological survey  of the dam site and reservoir areas was made for data collection and geological mapping. Moreover Helicopter survey  was  conducted along the dam axis  and reservoir area during the first week of June 2001.
4.1.3.2 Review of Existing Data
Available geological and seismological maps, aerial photographs  and geological reports related to the project area were reviewed. The geological maps include those published by USBR (1964) study of the Blue Nile River Basin, more recent geological map of Ethiopia, Gimbi and Nekemte sheet by the Ethiopian Institute of Geological survey  (EIGS) and Abbay  River Basin Integrated Development Master Plan Study by the Ministry of Water Resources (1998).
Miter Works Design &. supervision Enterprise
26Lower Diikssa Medium scale Hyilrogower Project
Reconnaissance Level Study August 2001
Aerial photograph about 1:60,000 scales were available for the study of the dam site and reservoir areas.
Seismic  information  comprised  of  the  comprehensive  work  of  Pierre  Gouin (1976, 1979), which is described in chapter 4 is reviewed.
Geological maps were prepared using field investigation data, review of previous work and aerial photograph interpretation.
4.2    Regional Geology
4.2.1 Geological History
The outline of the geological history of the Afro-Arabian as described by Pierre Gouin (1979) is summarized below.
At the end Precambrian Era, some 600 million years  ago, the crystalline basement complex of the present Afro-Arabian swell had been above sea level for a long time and remained for another 370 million years until the end of the Paleozoic  Era. Such a long period of erosion and denudation left the earth’s surface almost completely peneplained.
Crustal motion started in the beginning of the Mesozoic  Era about 225 million years  ago. During the late Triassic  and early  Jurassic  periods, a regional epirogenic sinking of the crust commenced causing a progressive transgression of the ocean from the southeast, that is, from the Indian ocean coast of the present Somalia in the general direction of Lake Tana in the northwest Ethiopia. This  downward Crustal movement, concomitant with a sedimentation process, started a cycle of marine transgression and recession of Mesozoic  sea. Within this  large epicontinental sea, extensive layers  of sediments  were deposited to from hundreds  of meters  of rocks  consisting of sandstone, shale, gypsum, limestone and other varieties of sedimentary rocks.
The crustal movement was  reversed into an upward motion during the late Jurassic  period, which brought the crust’s  surface up to sea level by  marine regression in late Cretaceous period.
The regional uplift resulted in wide spread crustal fracturing during the early Tertiary  period. The crystalline and sedimentary  rock  layers  were fissured mostly, along or in the vicinity of the zone of maximum uplift, thus allowing the outpouring of molten lava to cover the older rock layers.
Major fault displacement along the Rift Valley was initiated during middle Tertiary period. Subsidence of large crustal blocks along steeply inclined fault zones
Writer Worb & Sujiervision Enterprise
27Lcwr Didesw Medium Scale Hydropower project
RecoitMis.wice Level Shuly August 2001
created graben-type depressions  along the Rift Valley  and Lake Tana. Significant volcanic activity was associated with the formation of the graben and young volcanic rocks cover the old Tertiary volcanic rocks in many depression.
Present day  tectonic  activity  occurs  along the Rift Valley  as  evidenced by numerous  earthquakes. More recent volcanism is  associated with tectonic development activity  being concentrated within this structure along the edge of adjoining plateau.
4.2.2 Regional Geology and Structure of the Didessa River Basin
The Didessa river basin is located in the western part of Ethiopia. The Didessa River is one of the main tributaries of the Abbay River.
The geological formation of the Didessa river basin is  represented by  the Precambrian metamorphic rocks of high and low grades and intrusive rocks, the Paleozoic  sedimentary  rocks, Tertiary  volcanic  rocks  and Quaternary  alluvial deposit (USBR, 1964, Kazmin, 1972, Meria etal, 1973, Mengesha etal,1996, MWR, 1998, Tadeese and Tsegaye, 2000, Solomon and Mulugeta, 2000).
The high grades  metamorphic  rocks  are migmatitic  biotite and biotite - hornblende gneiss  and granite gneiss. The low grades  metamorphic  rocks include meta-ultramafic  and meta-sedimentary  units. The meta-volcan ics  are mainly  basic  and the meta-sediments are consisting of quartzite, mica schists, phyllite and graphite schists.
The  Precambrian  intrusive  rocks  are  comprising  of  pre,  syn  and  post  tectonic granite, granodionte and gabro.
The Paleozoic  sedimentary  rocks  are interbeded siltstone clay  stone and subordinate shale. USBR (1964) study did not identify this sedimentary unit in the Didessa basin and the proposed dam site (DD-2) area geology is assumed as Precambrian metamorphic  rock, which is  banded and contorted gneiss. The Abbay river basin master plan study (1998) also did not identify the Paleozoic sedimentary unit at the dam axis. The master plan study reported that the dam axis  is  situated on massive and hard granite like formation included into the basement complex. The sedimentary  rock  named as  Infra Adigrat Clastics (CIAC) is mapped east and west of the dam site. The EIGS study (Tadesse and Tsegaye, 2000) confirmed the presence of this Paleozoic sedimentary rock along the dam axis. The present field investigation proved the works of EIGS.
The Tertiary  volcanic  rocks  include basalt, trachyte and pyroclastic  rocks. The quaternary alluvial deposit is composed of silt to gravel and boulder size of the main rock formation in the Basin.
W/iter wnrh Desyn & Supervision Enterprise
28Lower Didessa Medium scale Hydropower Project
Reconnaissance Level Study
August 2001
According  to  Tadesse  and  Tsegaye
(2000),  Three  deformational  stages  (Di  to
D3) are developed within the Didessa river basin. The early  deformation (Di) resulted in thrust-folding event and related deformation characterized by  West verging thrusts  and associated recumbent, tight to isoclinal folds  with sub horizontal axes  and associated shallow dipping NE-SW trending axial plannar foliation (Si).
The second deformation event (D2) is  a progressive continuation of Di deformation resulted in steepening of Di structures by upright folds and sinistral strike-slip Shear Zones, and parallel to the main NE-SW trending regional foliation.
The third deformation event (D ) characterized by conjugate brittle-ductile strike
3
slop faults/shear Zones  that are superimposed at high angle to Di and D2 deformation structure. The NW-SE trending Didessa Shear Zone is  the major structure of D .
3
4.2.3  Physiographic Setting of the Dam Site
The Didessa river basin geomorphology is characterized by rugged topography; consisting of Tertiary  volcanic  and Precambrian intrusive rocks  at the higher altitude and deeply  incised U-shaped valley  covered by  Precambrian metamorphic and Paleozoic sedimentary rocks.
The proposed dam site is  selected based on the geomorphic  condition, which consider narrow river valley to be blocked and higher elevation of the abutments with respect to the riverbed. The dam site is  located on the Paleozoic sedimentary rock which form steep (sub vertical) slope along the dam axis. The valley  is  V-shaped at the bottom and widened at the top-specially at the right abutment which form relatively gentler slope (Appendix 2.2, Fig. 2.2.) The altitude along the dam axis ranges from 900 masl at the riverbed to 1100 and 1200masl at the top of left and right abutments respectively.
4.3     Geological Investigation at Didessa Dam Site
Field reconnaissance survey was conducted to investigate and identify the rock units, the nature of the topographic  features  and availability  of construction materials around the dam site and reservoir area.
Miter worts Design & Supervision Enterprise
29Lower Didcssa Medium scale Hydropower Project
Reeomiaissauce Level Study August 2001
4.3.1 Geology and Structure of the Dam site, Powerhouse and Reservoir
Area
The proposed dam site and powerhouse area is  geologically  situated on the Paleozoic sedimentary rocks consisting of alternating layers of laminated shale, sandy siltstone and clay stone. The shale layer is thinly laminated (>_1cm thick) with alternating light gray  and yellowish gray  fine layers  similar carbonaceous shale. The mineralogical composition is  mainly  clay  and carbonate. The shale layers split along the bedding plane when hammered.
The silt and clay stone layers are thickly bedded, the major component being silty sandstone and there is a variation to silty mudstone and minor chert at the upper most part of the unit (Luma ridge).
These sedimentary rocks in general appear to have been strongly tectonized by the NW and NNW (N 300° and N 330°) Fault system. The outcrop at the river bed strikes N 45° W and dipping 10°-12° due NE and the dipping angle reaches up to 25° at the Luma ridge. There are also some fractures parallel to the strike of the sediments and dipping sub vertical.
The bedding plane dipping towards the flow direction of the Didesa River and the associated fractures  can induce significant seepage. If this  seepage below the dam exceeds  certain limits  the foundation may  become weak  and the dam is likely  to have a chance to collapse. The seepage will not be limited to the foundation it will also appear on the abutments of the valley.
The reservoir area is geologically located on the Paleozoic sediments and mainly on the relatively  an impervious  high-grade metamorphic  rock. The major rock units outcropped in the reservoir area is migmatitic biotite and biotite hornblende gneiss, minerallogically  composed of 20-35% quartz  35-45% plagioclase, 15- 25% biotite, 10% hornblende.
The linear features  depicted on the aerial photographs  have NW direction and may  be associated with the regional Didessa shear zone and can also cause seepage along the fault zone, which passes across the reservoir.
The quaternary alluvial deposit, which is the weathering products of the reservoir area rocks, consisting of silt to gravel size fractions, is  deposited along the Didesa and Sayi rivers  banks  are a good source of sand for construction purpose.
30Lower Diilcm Medium Scale Hydropower project
4.3.2 Slope Stability of the Dam Site
RAWui&aiec Lad Study Ajyxst 2001
The abutments  of the Didessa River on which the Dam is  supposed to rest have steep slope (sub-vertical angle) that form a narrow (20m) wide V-shaped valley. At the abutments  the upper parts  of the sedimentary  rocks  are weathered and can easily  slide down towards  the reservoir along the bedding plane due to the infiltration and the wave action of the water.
4.3.3. Construction Materials Availability
Natural Construction materials  required for the dam work  and powerhouse are available within 10-20km distances  from the dam site. Approximate locations  of the construction materials are indicated on the geological map.
4.3.3.1  Rock Resource
The Tertiary  volcanic  rock, basalt and the Precambrian metamorphic  rock, migmatitic  biotite and hornblende gneiss  are important rock  resources. The basalt is  fine grained with columnar joints  and can be crushed for gravel requirement. The metamorphic  rocks  are composed of much flaky  minerals  and cannot be used for gravel production; however, they  can be useful for masonry work and rock fill.
4.3.3.2  Sand Resource
The alluvial deposit along the Didessa and Sayi riverbanks  is  free of clay  and composed of mainly  quartz, minor feldspar and micas  in silt to sand size fraction. It can be a sand resource for concrete work and filter material requirement.
4.3.3.3  Clay Resource
Residual soil developed over the surrounding migmatitic  biotite hornblende gneiss  nearby  the dam site and further west of the dam site around Kemashe town is  reddish to brown in color with relics  of original rock  fabrics. In hand specimen test it is plastic and important resource for clay requirement.
water Worts Desyn & Supervision Enterprise
31Lmr Dhlcssa Medium Scale Hydropower Project
4.4 Regional Seismology
4.4.1   General
Reconnaissance Level Study
Ajyilit 2001
Ethiopia has  experienced the effects  of earthquakes  and volcanic  activity throughout its  history  due to its  geographic  location in the vicinity  of two seismically active features (Appendix 2.2, Fig. 2.3.1 & 2.3.2.). These are:
1.     The   major   diverging   tectonic   plate   boundary,   which   divides   Africa   from   the Arabian Peninsula (the Red, sea and Gulf of Aden).
2.     The  East  African  Rift  system,  major  break  in  the  Earth’s  Crust  Pierre  Gouin (1976,1979)   made   a   comprehensive   study   of   the   seismicity   of   Ethiopia   by compiling the earthquake records dating back to 1400 A.D.
This  study  was  essentially  an analysis  of historical data. The data of 1400-1874 events  as  a whole is  neither sufficiently  accurate nor numerically  complete enough to be subjected to statistical analysis.
The modern events, 100 years  records, confirmed by  instrumental recordings from either local or foreign seismic  stations  from 1875-1974 were considered sufficiently  reliable and used as  basis  for the evaluation of the seismicity  of Ethiopia.
Pierre   Gouin   (1976)   evaluated   the   100    years   instrumental   data   to   produce different seismic maps (Appendix 2.2, Fig. 2.3.3. to 2.3.8), such as:
•    Grid location map of seismic events
•    Attenuation curves of different intensities of earth-quakes
•    Ground acceleration map
•    Earth-quake intensity map
•    Seismic map
From the epicenter location map (Appendix  2.2,Fig. 2.3.3. and 2.3.4.) and tectonic  features  map (Appendix  2.2,Fig. 2.3.1 and 2.3.2.), most shocks  originate along the rift structures of Red-Sea, Gulf of Aden and the Ethiopian Rift.
4.4.2   Seismicity of the Didessa Basin
The Didessa river Dam Site and Reservoir area are located within 09°10'-09°35 N latitude and 35° 50' - 36° 10E longitude. This  region is  about 400km. West of the western Ethiopian Rift Valley  escarpment from where an Earthquake epicenter is located (Appendix 2.2,Fig. 2.3.1. - 2.3.3.).
yv<ikr work Drsyn & Supervision Enterprise
32lower Didessa Medium Scale Hydropower Project
Reconnaissance Level Study
Almost 2001
Based on a 100 years  recurrence period, the predicted ground acceleration for the Didessa dam site and reservoir area is  between 2 and 3%g (Appendix 2.2,Fig. 23.6). The predicated earthquake intensity  for the same year is  V to V1 MM (Fig. 3.7) and the seismic  risk  zone belongs  to zone 1 (Appendix  2.2,Fig. 2.3.8) which is a zone of minor damage (Building code for Ethiopia, 1974).
Pierre Couin (1976) predicated the ground acceleration and Earthquake intensities  for water reservoir and Dam sites  as  shown below in Table 4.1. The sites  are located around the Didessa dam site and reservoir area and these values can represent the siesmicity of the project area.
Table 4.1 Intensities and Ground acceleration
Site
Co-ordinate
(Degree)
Estimated Probability
North
East
1/50 yrs.
1/100 yrs.
1/200 yrs.
1/300 yrs.
IA
IA
IA
IA
Anger 1
09.68
36.73
V        1.5
VI        2.6
VI            4.5
VII       6
Dabana
09.48
35.95
V        1.3
VI        2.2
VI            3.8
VII       5
Dabus
09.98
34.90
V        1.0
V         1.7
VI            2.9
VI        4
I = Intensity in Mercal i Modified, MM, (1931) units 
A = Ground Acceleration in percentage of gravity. %g.
For the design purpose, Dam and Power house structure, The Intensity  and Ground acceleration Predicated for Dabena can be applied for Didessa dam and reservoir sites.
4.5 Conclusion
The surface geological condition at the Didessa dam site is  not suitable for foundation due to the bedding attitude of the foundation rock, which is  gently deep towards  the flow direction. This  downstream dipping sedimentary  rock  are thinly  laminated and the lamination planes  are the main path for seepage. Down stream slope of sedimentary  beds, particularly  if the amount of dip is  small, should be looked upon as  an adverse condition at dam site (Mukerjee, 1991) and if such sites  cannot be avoided, due precaution should be planned in advance. The corrective methods  and the cost to control the seepage should be considered in the project cost estimation.
The region, at the project area is  seismically  within minor risk  zone and Earthquake intensity  of V1 and ground acceleration of 2.2% can be considered for 100 years recurring time for the dam design purpose.
Construction materials are available at a reasonable hauling distance. Wider works Design & Supervision EnterpriseLower Didessa Medium Scale Hydropower Project
5. PROJECT LAYOUT
5.1       General
Reconnaissance Level Study
Aiyust 2001
This  chapter describes  the project layout of the various  works, drawings  of, which are included in a separate A3 size album of drawings. The key  drawings  are reproduced in A4 size in this  volume. General design considerations  are stated in section 5.1. The design features  of the Lower Didessa project will be presented in subsequent sections.
5.1.1    Identification of Alternatives
Initially  only  one site was  found based on the 1:50,000 scale map as  well as  site visit. During office study  different dam types  at the same axis  were studied as shown below on Table 5.1.
Table 5.1 Different Dam Type
S/N
Type
Catchment
area
km2
Reservoir
volume
MCM
Mean
Annual
Flow
MCM
Total
head H
m
Dam
volume
Mm3
1
Arch dam
18200
1024
7290.00
80
0.38
2
RCC gravity dam1
18200
2443
7290.00
120
0.93
3
Rock fill
18200
3531
7290.00
140
7.65
4
RCC gravity dam2
18200
1024
7290.00
80
0.31
5.1.2   Initial Screening
The site is  V-shaped particularly  at the lower elevations  suggesting an arch dam. However, it was  omitted because of geological problem on the abutment. For an arch dam to be sound and stable both abutments  have to be geologically  sound. At the Didessa site the abutments are not strong enough.
For gravity  and rock  fill dam types  the initial screening procedure was  mainly focused on calculating the costs  for dam and finally  getting the specific  cost in USD/kw. The criteria for initial screening were to reject that alternative which exhibits  higher specific  cost. As  shown in Table 5.2, the rock  fill dam has  less specific cost hence; it has been selected for further studies.
Water Wjrks Design & Supervision Enterprise
34Lower Didessa Medium Scale Hydropower Project
Table 5. 2 Initial Screening
Reconnaissance Level Study August 2001
S/N
Dam Type
Power
MW
Cost **
MUSD
Cost per/kw
USD/kw
1
RCC Gravity dam1
195
118
605
2
Rockfill
308
86
279
3
RCC Gravity dam2
60
38
633
" Cost of Dam, Spillway, waterway and Powerhouse.
5.2     Preliminary Design of Selected Project
5.2.1    Design parameters
From Table 5.2 a project namely  rock  fill dam is  selected for further study. This item attempts  to carry  out the project. Here the design criteria as  well as parameters  for all elements  going into a hydropower project are presented as below.
•    Dam height (Reservoir optimization)
•    Tunnel optimization
•    Powerhouse design
•    Turbine and accessories selection
•    Generators and accessories selection
•    Switch yard and associated equipment
•    Level of transmission voltage
5.2.2    Dam
The site for the dam has  been selected as  described under section 5.1 The selection of the dam site was  based on the 1:50,000 scale map as  well as  site visit. Site inspection reviled that the observed ground and the map do agree. After having established the dam site on the 1:2000 map, cross-sections  were taken and concurrently  geological study  was  conducted. The location of the test pits was also based on the selected alignment of the dam axis.
Investigation of the topographic  map prepared based on the cross-section survey indicated that a one better site is  available according to 1: 2000 scale map. At this site the volume of the dam would be 7.65Mm3.
Dam Height
Different   reservoir   dam   heights   have   been   considered   to   come   up   with   an optimum dam height The optimization has been done based on the analysis of
Writer Works Design & Siy/ervbimi Enterj/rbe
35Lower Difcw Medium Scale Hydropower Project
Reconnaissance Level Study August 2001
marginal cost and marginal benefits  due to increase in dam height. As  it can be seen in Appendix-3.2, Fig.3.2 in every  height of the dam there is  a benefit, so the optimum economic  dam height with in the limit of topography  has  to be fixed. During optimization process  the following parameter was  considered; 0.06 USD/kwh firm energy  value, 0.00 USD/kwh surplus  energy  value, 50-year project life, 10% discount rate and 0.10 annuity factor.
Dam type
The Didessa dam site is  located within steep sided gorge upto 980 masl and from elevation 980 and above the gorge becomes  flatter at the right side of the river looking downstream which is  shown on Drg.No. 4. According to geological report on chapter 4 the bedding plane dipping towards  the flow direction of the Didessa River, so the seepage problem would occur. Finally  the dam might has  a chance to collapse, so the dam site area need a blanket around 2km2 from the dam axis  to the upstream side of the river. At this  stage clay  blanket with 1 m thick  is  around reservoir area and concert blanket with 0.5 m thick  is  around dam axis area are proposed.
The   selection   of   type   of   material   used   to   construct   the   dam   is   based   on   the following main factors
The dam would be a rock-fill type with a central impervious  material. The downstream and upstream slopes  of the dam are 1:1.5 and 1:1.6 respectively. The foundation of the dam would be at the finished ground level as  indicated on Drg.No. 5. A maximum of 5m overburden would have to be excavated in order to arrive at a sound foundation material.
5.3      Reservoir storage
Storage capacity  versus  elevation curve and area versus  elevation curve of the reservoir is  prepared from topographical map of scale 1:50,000. Based on this curve the storage capacity  is  determined. Hence, the storage capacity  of the reservoir at the normal water level is  3531 MCM. Out of these the live storage capacity  is  3093 MCM, the remaining 329 MCM & 109 MCM are provided for dead & residual storage respectively. This  translates  to useful life of 50 years. Reservoir area & capacity  curve are shown in Fig. 5.1 & Fig. 5.2 respectively. The reservoir area map is shown in Drg.No.2.
The accumulated sediment volume over a period of 50 years  is  329 MCM and 94 % trap efficiency  was  considered from trap efficiency  curve in Appendix  1.8, which is shown in Fig.1.4.
Wiitcr Wnrfc Design & Supervision Enterprise
36Lower Didessa Medium Scale Hydropower Project
Reconnaissance Level Study AJtfUSt 2001
Reservoir Area curve
Figure 5.1
Reservoir Capacity curve
Volume, Mm3
Figure 5.2
Figure 5.1 & 5.2 Didessa Reservoir area & Capacity Curve
\y,iter Works Design & Supervision Enterprise
37Lewr Didessa Medium Seale Hydroymer Project
5.4     Spillway
Reconnaissance Level Study August 2001
The normal reservoir level is  set at 1040 masl while the minimum reservoir level is  set at 952 masl. The maximum reservoir level is  limited to 1040 m beyond which the spillway  starts  functioning. The spillway  is  122m long ogee type and has  a surcharge of 7.33m. The capacity  of the spillway  is  4224m Is corresponding to the 10,000yr flood.
A   chute   is   provided   that   leads   the   high   velocity   spilled   water   to   the   energy dissipating    facility.    In    order    to    avoid    deterioration    of    the    concrete    due    to cavitations    appropriate    aeration    devices    should    be    implemented.    For    spillway parameters refer to Drg.No.6.
The spillway  will have a bridge with 10 span and 12m wide each, it will have also 9 piers  each 1.5m thick. The spillway  will discharge into a 600m long chute, which will fall at a different slope that is  shown in Drg.No.6. The spillway  will terminate in a stilling basin that will discharge in to the river.
5.5      River Diversion
Upstream and downstream cofferdams  of 40m and 30m height respectively would be provided to divert the river during construction. The 1:20 year flood is taken to size the cofferdams  and the tunnel at the right side of the river. The tunnel would be 760m and 640m long. The shape of the tunnel would be one D shaped 9m width by  8.03m height and the other will be circular with 9m diameter. Once the construction is  over, these tunnels  would be plugged with concrete in the upstream side both the tunnel and one on the downstream side.
After construction one of the tunnel will be used for power tunnel and the other tunnel will be for access to the power intake gate chamber which in shown in
Drg.No.3. The power tunnel will be circular but the other tunnel will be D shaped.
5.6      Power and Energy Es timation
For power production the method has  been used empirical formulas  and spread sheet. The first step was  to get hydrological data. According to the hydrology report, the runoff availability at dam site (Didessa) is shown in the table below.
Water Wi’rfa Design & Siyvrviyii’ii Eiiteryrise
3SLiwr Difert Medium Scale Hydropower project
Recouuaiss/otce Level Study August 2001
Mean Annual Flow
(MCM)
Probability of Exceeding, %
75
80
90
95
7290.2
6300
6100
5230
4600
According to the capacity  curve that is  shown in Fig. 5.2 and the regulation curve, which is  shown in Appendix-1.4, Fig. 1.2, 70% of the mean annual flow will be regulated.
The normal tail water level is  taken at 900masl and the gross  head will be 140m. The firm energy  of the project is  calculated base on the energy  equivalent method, which is  defined as  the energy  produced per cubic  meter of water. It is given as:
Energy equivalent = ( p* q * g • h„)/ 3600 kwh/m3
Where, p = density of water (kg/m )
ri = overall efficiency (assumed 0.85)
h  = net head (m), (assumed head loss is 5% of gross head) g = gravitational acceleration (m/s )
3
n
2
The energy  equivalent considering the net head 133m is  then becomes  0.31
kwh/m3,   the   annual   firm   energy   production   for   70%   regulation   becomes   1582
Gwh/yr  and  by  taking  the  plant  factor  0.6,  installed  capacity  becomes  301  MW and the max. turbine discharge is 271m /s.
3
5.6.1  Water way
Power Intake
The power generation civil works are located on the Right bank. The basic 
arrangement is a single power intake feeding a single power tunnel/shaft, and
then will Quadra-furcating upstream of the powerhouse. The intake-inverted level
is found at 946 elevation and also protected with trash rackjt has also stop-log v 
slot to use for maintenance purpose. The intake gate will be provided Bonneted-
sliding gate.
Power Tunnel/Shaft
The power tunnel/shaft diameter D is  selected by  both minimising sum of capital construction costs  and lost energy  due to head losses  during operation. The optimisation curve has  shown in Appendix  3.2, Fig 3.3 & Fig 3.4 and longitudinal profile is shown in Drg No.7 From the curve the power shaft diameter will be
W/i(er Works Design & Supervision Enterprise
39Lower Didessa Medium Scale Hydropower project
Reconnaissance Level Study August 2001
8.5m   internal-diameter   &   51m   long   with   0.25m   thick   concrete   lining   and   power tunnel   will   be   8   5m   internal   diameter,   440m   length   with   0.25m   thick   concrete lining.
Penstock
Four penstock  will be provided after Quadra-furcated for 25m long with 4m diameter that is  shown in Drg.8. Before the penstock  there will be 10m long steel lined tunnel to connect the penstock  with Quadra-furcated steel pipes. The velocity of the penstock is assumed 5m/s.
Power house
The powerhouse is  a surface structure downstream of the dam. It will be arranged vertically  by  Francis  type turbines  (Drg.No.8). There are three operating floors, one floor for the generators, one for the turbines  and a main powerhouse floor on the some level as  top of the generators. The loading bay  and the powerhouse superstructure base would have so be set above the 10,000yr tail water level to avoid flooding.
The  powerhouse  shall  be  64m  in  length  and  24m  wide  and  shall  have  four  turbo generators. Each unit shall have a capacity of 75MW.
5.6.2   Surge Chamber Design
The  water  head  between  the  upper  level  (reservoir)  and  lower  level  (Tail  water)  is far too long to meet the requirement (penstock time constant) Tw < 1 sec.
Penstock time constant is calculated as
T
w
= (Q/(g*h ))*(S(L/A))
n
Where Q = discharge (m /s)
3
g = acceleration due to gravity (m/s )
2
n
2
h  = net head (m)
L = length of conduit (m)
A = area of conduit (m )
In Lower Didessa project the Tw > 1, so the surge chamber has  to be provided. Because of the topography  and to reduce the surge tank  cost, reinforced concert lining air cushion surge chamber is provided and shown in Drg.No.7.
The size of the air cushion is  35m x  30m, the max  pressure rise in the air cushion is  8m water column, the volume of air is  6383m3 and the absolute pressure is  0.2 Mpa.
W/itor World D«yn & Supervision Enterprise
40Lower Di(iessa Medium Scale Hydropower Project
Reconnaissance Level Sbuiy August 2001
Air  compressors  will  be  provided  to  fill  the  chamber  with  air  for  first  start  and during air leakage. The compressor chamber will be nearby the powerhouse.
5.7      Electro Mechanical
5.7.1   Mechanical
Turbine
Francis  turbine, which is  basically  a reaction turbine, is  selected for this  project. The technical criteria for selecting the type of turbine are based on head, discharge and specific  speed. In this  respect the available head, discharge and the calculated specific  speed for this  project makes  Francis  turbine the appropriate selection. In addition to this  attribute, the case that most of the existing hydroelectric  power plants  in Ethiopia employed Francis  turbine qualifies this  type of turbine since it simplifies  spare parts  stocking as  well as  ease of maintenance skill development. Therefore, Francis  turbine with an efficiency  of 90% is  recommended for projects  under study. The type of governor is  digital (PLC). The selection is  in reference to EEPCO current trend of replacing former accelerometric  actuators  by  digital type. Inventory  of each mechanical and hydro mechanical parts  including the turbine inlet valve are mentioned in the cost estimation schedule under broad designations like auxiliaries, accessories... etc.
Characteristics of Turbine(s)
A) Didessa
Type of turbine (reaction)
Francis
Design Head
133m
Arrangement
Vertical
Specific speed
151.83
Number of turbine
4
Nominal speed
300rpm
Design discharge
271m3/s
Runaway speed
575
Installed capacity
301 MW
max.
Powerhouse crane
The power house crane is  a monorail over head traveling with a capacity  of 60t while the gantry  crane is  25t. The selection is  based on the heaviest component to be handle, which the generator part.
Regulating Valve
The   penstok   is   branched   out   into   manifolds   that   feed   water   to   the   individual turbines. Each manifold has its own regulating valve that can be maneuvered
waler Work Design & Supervision Enterprise
41Lew Di/ta Medium swfc Hydrojwwer Project
Reconnaissance Level Study August 2001
independently as the operation of this valve is just at the turbine inlet point or at the entry of the water to the scroll casing in the case of the Francis turbine. The type of the valve should be preferably  a spherical one. The operation is  in controlling the amount of water flow through the turbine runners, as load demand fluctuates.
Draft tube and Tailrace
The draft tube is an elbow type pertinent to the turbine layout drawing.
5.7.2    Electrical
5.7.2.1 Generators
The generator is selected to meat the out put and characteristic of the turbine, in addition to this synchronous generator is selected because of its reactive power advantages. The characteristics of the generator are; salient pole, 50Hz, and will comply with IEC 34 to operate under the ambient conditions prevailing at site. The generators  will be rated for 13.8 kV at 0.9 power factor to suit system operating conditions. The generator air gap diameter will be dimensioned to enable the top cover of the turbine to be removed through the stator bore. The generators  will be mounted in concrete pits  of sufficient dimensions  to provide the required volume of closed circuit air circulation, and will be provided with steel covers.
The rotor weight is expected to be 100 to 150ton.
Assembling the pole and the rotor center to shaft connection on site will reduce the transport weight.
The stator cone shall be built on site. The welded stator frame will be transported in segments  with mechanical flanges  and bolted together on site. This method of construction ensures an integral stator cone and avoids problems associated with core vibrations and differential expansion due to thermal effect.
Fan blades that will be located on the top and bottom outer peripheries of the rotor will circulate the cooling air. Six to eight air-water heat exchangers will be located round the outer periphery  of the stator frame. Because of the large reservoir the cooling water should be clean and no problems are envisaged from the use of raw water.
The bearings  will be immersed in oil contained in oil pots, and will be self lubricating. The thrust bearing has  self aligning bearing pads  and the guide bearings have adjustable pads. The thrust bearing will be equipped with high
Miter Works Design & SHjmWw Enterprise
42Lower Didcssa Medium Scale Hydropower Project
Reconnaissance Level Study August 2001
pressure lubrication for use during starting and stopping of the generator. The bearing oil will be cooled by  water coolers  immersed in the oil pots. Air pressurized glands  will be provided at both ends  of each bearing to prevent oil vapor contaminating the windings.
The generator windings  will be of Class  F  epoxy  resin insulation. The generators will be rated to absorb the full gate turbine output at the turbine's  best efficiency head, without exceeding the temperature limits  for the lower grade of insulation (Class  B). At higher reservoir levels  the generators  will be capable of operating continuously  at up to 115% of their Class  B rating, without exceeding the Class  F temperature limits.
Conventional compressed air operated brakes  will be provided, and the same pistons/cylinders  will be operated by  high pressure oil for jacking. Asbestos  free renewable brake liners  will be provided. The brake units  will be fitted with hoppers to collect dust and a suction unit will evacuate the dust.
Thermostatically  controlled generator heaters  will be provided under the generator and distributed round the periphery. The heaters  will be rated to keep the generator winding temperature sufficiently  high to prevent condensation during shut downs and low load operation.
The static  excitation system for each generator comprises  thyristor rectifier equipment, field suppression equipment and the automatic  voltage regulator, in a suite of cubicles, as  well as  an excitation transformer. The excitation transformer will be naturally  air cooled, dry  type, in a sheet steel enclosure. It will be connected to the terminals  of the generator and will be rated to provide continuous  excitation power as  well as  ceiling excitation power. The excitation system will be designed to operate the generator within the voltage range 13.8 kV±5% at rated output and power factor and to provide excitation under black start conditions to initiate excitation build up.
Medium Voltage Equipment
The generators  will be provided with Neutral Cubicles  and Line Cubicles  located on the generator floor. The Neutral Cubicle encloses  the current transformers for protection, instrumentation and synchronizing, and for the surge arresters.
Generator circuit breakers, rated at 15kV, will be located on the generator floor adjacent to the line cubicles. The generator to transformer connection will be by MV cables.
Waiter works Design & Supervision Enterprise
43Lmr Didessa Mediiun Scale Hydropower Project
Recoiouiissance Level Stiuiy
2001
Generator Cables
Single core, non-armored, XLPE insulated, PVC sheathed, 15 kV rated cables will be provided to connect the generators  to the generator transformers  in the transformer compound.
Generator Transformers
On the unit principle each generator will be connected to a three-phase generator transformer via a generator MV circuit breaker. The three phase 13.8/230KV transformers  will have ONAN/ONAF cooling. The ONAN rating will be matched to the Class  B temperature rise capacity  of the generator. This  will enable the transformer to operate continuously  at its  ONAF  rating when the generator is running at its Class F rating, i.e. at high reservoir levels.
The transformers  will be provided with tapings  to permit voltage adjustments when they  are used to back  feed auxiliary  supplies. An on-load tap changer will be specified in view of extended nature of the transmission system.
Surge   arresters   will   be   provided   near   the   transformer   high   voltage   terminals   to protect the transformers against lightning surges.
The transformer's  transport weight is  not expected to exceed 70t and such a weight is  not expected to present undue difficulty  when transporting units  to site. The transformers  will be located in a transformer compound adjacent to the power house to minimize MV cable costs.
Generator Connections
Connections  between the generator transformers  and the 230KV substation will be by  means  of overhead connections  in horizontal configuration, supported on lattice steel gantries.
5.7.2.2   Sub-station at sending end
Station Service Power
Unit auxiliary  transformers  rated at 13.8/0.4 kV will be provided for each generator. The capacity  of each auxiliary  transformer will be sufficient to supply the auxiliary  power requirements  of generating units  plus  the station service power. The auxiliary  transformers  will be mineral oil insulated ONAN cooled type and will be located in the transformer compound near the generator transformers. Unit auxiliary transformers will be provided with off-load tap changers. They will
Wrtter yViirLr D«yn & Supervision Enterprise
44Lower Didessa Medium scale Hydropower Project
RecoHMlssnncc Level Study August 2001
be connected to the 13.8 kV busbar from the cable terminal to the generator transformer by  HRC fuse switches. The provision of MV generator circuit breakers  between the generator and generator transformer will make it possible for the auxiliary  transformers  to be back  fed from the grid. This  will make the auxiliary supplies very reliable.
A   standby   diesel   generator   set   of   sufficient   capacity   will   be   provided   to   enable black  start  of  a  generating  unit  in  case  the  grid  connection  fails.  The  diesel  set will  start  automatically  if  all  other  sources  of  supply  fail,  or  if  the  station  auxiliary supply   voltage   falls   outside   the   set   range   of   values.   This   generator   will   be connected to the main AC supply bond to supply for all loads connected.
DC Control Power Supply
A  110  V  battery  and  dual  battery  chargers  will  be  provided  to  supply  the  controls, field   flashing,   inverters   supplies,   emergency   lighting,   emergency   closing   and   trip circuits.
Uninterruptible Power Supplies  (UPS) will be provided for the electronic equipment and fire protection systems  through a static  switch which switches  the supply  from AC to DC on failure of AC. The battery  will be rated to provide the DC requirements  of the power station and includes  100% standby  capacity  for the inverter system.
110   V   DC   distribution   boards   containing   switches,   busbars,   links,   fuses,   relays and instrumentation will be provided.
Two 48 V DC batteries  with dual chargers  will be provided for the equipment in this  range of supply. 48 V DC distribution boards  with automatic  change-over switches will be provided for the above.
Power and Multicore Cables
Power   and   multi-core   cables   will   be   XLPE   insulated,   steel   wire   armored,   PVC sheathed, stranded copper conductors rated at 600/1000V.
Lighting
The  lighting  system  shall  provide  adequate  illumination  of  all  points  of  the  power generation area.
Security  lighting  shall  be  installed  around  the  substation  illuminating  the  external area and shall be controlled from the plant building.
Writer work Design & Supervision Enterprise
45Lower Didessa Medium Seale Hydropower Project
Reconnaissance Level Study
August 2001
Lighting shall be designed to produce adequate visual performance, safety  and amenity, and shall be free from excessive glare and flicker from discharge lamps. The system shall include provision for ease of erection, maintenance, cleaning, lamp replacement and future extension.
The battery room shall be supplied with corrosion resistant fittings to IP54.
Lighting    apparatus    shall    be    of    top    quality,    designed    to    ensure    satisfactory operation and service life under all variations of load, frequency and temperature.
Grounding System
The grounding system shall comprise a network  of buried copper earthing conductors  forming a main earth grid with connection to all items  of electrical plant, steel structures and other metalwork inside the substation fence.
The main earth grid shall encompass  as  large an area as  possible within the substation fence and shall be laid at a depth not less  than 500mm. The separating distance between the main earth grid and any  items  connected to it and the fence shall be at least 2 meters.
Each item of high voltage equipment shall be earthed to the main earth grid via a separate branch connection or duplicate connection arranged to form a loop. In which case the conductor size may  be reduced. No sub-branches  shall be permitted with the exception that a three phase group may  be connected to a common branch.
The resulting grounding resistance should in general not exceed 1.0 ohm, and the ground mesh should consist of stranded copper wire with cross  section not less  than 50 sq. mm for mechanical reasons. The mesh should be extended into the tailrace and under the power house foundations. Buried connections  should be brazed, welded or compressed.
Transmission Line
Switchgear
Circuit breakers  will be of the SF6 type suitable for three phase auto-reclosing on line circuit breakers  and complying with IEC 56. Disconnectors  to IEC 129 will be of the single break, horizontal action type, provided with motorized operating mechanisms. Earth switches  will be specified for lines, busbars  and generator circuits. The normal and short time ratings  of switchgear will be specified to be consistent with those adopted on the existing interconnected system unless  plant requirements at the station indicate the need for enhanced performance.
Wilier Worl'5 Design & Supervision Enterprise
46Diver Didessa Medium Scale Hydropower Project
Reconnaissance Level Study August 2001
From the power house of Didessa the power is  transmitted by  a 230KV overhead transmission line to Ghedo substation. The length of the transmission line is estimated to be 128km. The other options  are Ghimbi and Nekemte but their capacity  is  only  132KV. Hence Ghedo will be the first choice. To provide power for construction a transmission line may  be constructed early  during a construction phase otherwise diesel generation can be used.
The     proposed     transmission     line     should     employ     lattice     steel     towers     with conductors arranged in vertical configuration.
Transmission System Analysis
1. Transmission system planning principle
a)        General Principles
In general a new system has been planned in such a way that:
. There is sufficient transmission capacity at each voltage level to
transmit power from the power plants to the distribution network 
. There are no over loading in normal operation.
• Suitable voltage levels are selected for power transmission and that
acceptable voltage profile is maintained in the whole system.
. The generation is dispatched in an optimal way 
. Short circuit levels does not exceed the equipment ratings.
b)        Technical Criteria
The main points of the technical criteria considered are as follows:-
.    The    voltage    at    any    point    in    the    transmission    system    shall    not exceed    the    highest    operational    voltage    allowed    for    the    voltage levels  of  concern  (105%  in  this  case),  the  voltage  shall  not  be  lower than 95% of the normal voltage level.
.   A   20%   overloading   capability   is   allowed   for   transmission   lines   and Transformers during peak hours.
Substation at the receiving end
The Ghedo substation is  230KV scheme. This  substitution is  chosen due to the fact that Finchaa Ghedo line is  230KV. Gilgel Gibe- Ghedo line will be 230KV and Ghedo-Geferssa line is  230KV. Moreover, the Gilgel Gibe - Ghedo line will be energized ahead of the Didessa scheme.
Therefore the line from Didessa can be easily taped to this substitution.
Waler works Design & Supervision Enterprise
47Lower Didessa Medium Seale Hydropower Project
Reconnaissance Level Shuiy
2001
5.8    Infrastructure
Access  road to the project sites  and within it are an important means  inorder to facilitate successful movement of construction machinery, transportation of construction materials, transportation of equipment for power generation and for smooth management of the projects.
Therefore, a new main access  road originating from the nearest available existing road to the proposed site (i.e. Didessa) and its  power house, will be the first necessary and a critical path in order to materialize this project.
5.8.1   Selection of access route
At this  stage of study, reconnaissance site visit and aerial topographical maps (1:250,000 and 1:50,000) of the projects  area were utilized for the approximate selection of access  route. Moreover, the consultation with surveyors  about the sites  and the foot track  shown on the available topo-sheets  render valuable information for locating the access  route on the map (Drg.10). Further to locate the unseen access  route (on Drg.10) to the proposed sites  (dam site and powerhouse) was  made using the project layout plan by  following the gradient of the terrain.
5.8.1.1   Access to Lower Didessa dam site
The Gimbi-Nedjo road branches  to the Kemashi town, which is  all weather road. The route following from Kemashi town to the dam site is  on the footpath, which extends  about 11.5km. Adding up the access  road for dam site to powerhouse totally  14.5km. The condition of access  road to the proposed dam site is possible only  by  small 4WD vehicle, while the access  to powerhouse from dam site is  accessible on foot. The location of this  access  track  is  indicted on the layout plan of lower Didessa dam site (Drg.3), while the access  road from Kemashi town to dam site shown on Drg.10.
5.8.2   Preliminary cost Estimation
5.8.2.1   Bases of cost Estimate
At this  level of study, estimation of costs  for the proposed new access  roads  for this  project was  made based on the international standard road type requirements  and approximated by  current cost applied for the construction of one kilometre of the road section.
water Works Design & Supervision Enterprise
48Lwcr Dldessa Medium Scale Hydropower Project
Reconnaissance Level Study Aqgust 2001
The  unit  rate  of  road  construction  are  derived  from  regional  experience  and  the cost estimate of the road are as follows:
Terrain
Rate (us $/KM)
Very highly, soft rock
Very highly, hard rock 
Undulating
200,000
500,000
100,000
5.8.2.2   Cost estimate
Applying the above unit rate cost for the construction of all road, the approximate
total cost of new access road required for the project has been calculated.
Hydropower site
Total km
cost US$
Didessa (with powerhouse)
14.5
3,200,000
The total length of new access  road required for this  site is  about 14.5 km and the total estimated cost for road construction is  3.2 MUSD. However, building the exact unit rates  and conducting preliminary  alignment survey  during pre feasibility study stage will further refine this estimated cost.
5.8.3 Site Camp
The estimated cost at this  level of study  is  around 4 MUSD. This  cost will cover the cost of all offices  for the client, consultant and contractor, laboratory, water supply, sanitation and drainage and the provision of housing for the clients, consultants and contractors staff.
Writer Work Design & Supervision Enterprise
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Table 5. 3 Summary of the Main Technical Features of Lower Didessa Site
No.
FEATURES
1
Hydroloqy
Catchment Area (km )
Mean Annual Flow (MCM) 10,000 years flood (m /s)
2
3
18200
7290
4224
2
Sediment
Sediment inflow rate (ton/km /yr)
Bulk density (ton/m )
2
3
Total Sediment volume (MCM)
500
1.3
329
3
Reservoir
Max. Flood Level (a.m.s.l)
Max. Operating Level (a.m.s.l) Min. Operating Level (a.m.s.l)
Intake Sill Level (a.m.s.l)
Live Storage (MCM)
1047.33
1040
952
946
3093
4
Dam
Dam type
Crest level (a.m.s.l)
Dam Height at the Max. Section from River Level (m) Crest Length (m)
Total Volume of Dam (MCM)
Rockfill
1050
160
1050
7.65
5
Spillway
Spill way Design Discharge (m /s) Total Length of Spillway (m) Head over Spillway Crest (m) (H )
3
d
4224
122
7.33
6
Diversion
Design Discharges (m /s)
Tunnel Diameter (m)
Total Length (m)
3
2210
9.5
1400
7
Power Tunnel
Internal Diameter (m)
Total Length (m)
8.5
440
8
Pressure Shaft
Internal Diameter (m)
Total Length (m)
8.5
51
9
Air cushion
Area (nr/)
Max. pressure rise (m)
Volume of air (m )
3
35x30
8
6383
10
Power House and Mechanical Equipment
Type of Power House
Dimension of Power House (LxW)
Design Head (m)
Type of Turbine
Number of Units
Turbine Design Discharges (m /s)
Installed Capacity (MW)
3
Surface
64x24
133
Vertical Shaft Francis 
4
271
301
11
Power Generation
Firm Energy (GWh/yr)
Plant Factor
1582
0.60
Wrttrr worb Design & Supervision Enterprise
50Lower Didessa Medium scale Hydropower Project
Reconnaissance Level Study August 2001
6.    ENVIRONMENTAL & SOCIOECONOMIC ASPECTS
6.1     Introduction
6.1.1  General
Large dam projects  usually  cause irreversible environmental changes  over a wide geographic  area and thus  have the potential for significant impacts. The area of influence of a dam project extends from the upper limits of the catchment to far downstream. Therefore, where large dam projects  including hydroelectric dam are designed to enhance economic  development, and bring a better standard of life to people, due consideration should also be given to their adverse environmental effects. The most effective and economic  way  of maintaining the quality  of the environment and enhance sustainable development is  to assess and eliminate or at least minimize the environmental problems  as  early  as possible.
The initial environmental assessment of the Didessa HPP, Reconnaissance Study, was planned and conducted in line of this principle. The findings of this assessment are presented in three main components including description of the existing condition, potential impacts  and recommended mitigation measures. Description of the project is given in chapter 5.
6.1.2 Objectives
The main objective of the environmental and socio-economic  studies  is  to describe the environmental and socio-economic  baseline conditions  of the study area, assess  the potential effects  of the proposed project, and recommend possible mitigating measures for the significant adverse impacts.
Specific objectives include:
•      Preliminary   description   of   the   bio-physical   and   socio-economic   baseline conditions;
•    Preliminary assessment of the impacts of the proposed HPP on terrestrial and aquatic ecology, and quantification of the impacts wherever possible;
•     Preliminary  assessment  and  quantification  of  the  loss  of  agricultural  lands, grazing lands, forestlands, etc., if any;
•     Preliminary assessment of the number of families or persons to be displaced, and their fixed assets to be affected, if any;
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51Li’ivcr Diitaw Medium Scale Hydropower Project
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•    Preliminary assessment of the current public health status of the project area and  project  impacts  on  this,  particularly  the  impacts  on  the  prevalence  of water related diseases;
•    Preliminary assessment of the social and physical infrastructure of the project area and extent of project impacts on these and the beneficiaries;
•    Evaluating the attitude of community members towards the envisaged project;
•     Identifying  other  major  environmental  issues  that  might  be  caused  by  the project;
•     Identifying  and  proposing  mitigation  and  management  measures  that  would help to avoid or reduce the negative impacts.
6.1.3 Approach and Methodology
The approach and methodology used to carry out the environmental assessment include:
6.1.3.1  Scoping of the Study
The first step in the environmental and socio-economic  impact assessment process was scoping of the study using checklists prepared for initial assessment of hydropower projects. In addition, topographical maps  (1:50,000 & 1:250,000 scales) were used to locate the proposed dam sites, diversion weir, waterways and powerhouse, and resources within the impact zone. The scope of this study is  preliminary  description of the existing environment and identification of potential impacts  of the proposed projects  at reconnaissance level study. The major issues to be dealt with are described in the objective section.
6.1.3.2  Review of Previous Studies
Relevant literature on previous studies and available documents including those given in the list of references were reviewed to abstract relevant information.
6.1.3.3  Field Investigation and Data Collection
Secondary  and primary  data were obtained using various  data collection methods. Field trip to the sub-basin and the project site was  conducted in March 2001 to study  the existing biophysical and socio-economic  conditions  of the study  area, identify  the existing environmental constraints  and assess  the potential environmental effects of the proposed hydroelectric project.
Biophysical and socio-economic  data were collected from local Government Offices  (Zonal, Woreda and Development Center Offices). In addition, site specific data were collected through focus group discussions with key informants
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(PA leaders, elders, and other community  members, and DAs), transact walk, social and resource mapping and direct observation. Based on the developed checklist for group discussion, different groups  were formed for all villages. In addition, the team conducted transact walk  in the proposed reservoir area through different directions to get a first hand observation on the site. The team received useful information on the envisaged project through informal discussions  with farmers  and observation. Key  informants  (like DAs, Woreda Administrators, respected community elders and Woreda Agriculture staff) were interviewed in an effort to probe more deeply into the attitudes and feelings of community members.
For ecological study vegetation survey of the dam site and reservoir area was made through a transect walk  to identify  the vegetation types, species composition and collect specimens of plant species not identified on the site.
6.1.3.4 Data Analysis and Report Preparation
At the final stage the collected data and information as well as relevant data generated by the engineering, hydrological and geological studies were analyzed and this initial environmental impact analysis report was prepared.
6.2     legal, Policy and Administrative Framework
6.2.1  Administrative Structure
The Federal Democratic  Republic  of Ethiopia (FDRE) comprises  fourteen member States  or Regions  with their own legislative, executive and judicial powers. The FDRE has  a parliamentary  system of government, with two councils; the Council of Peoples' Representatives and the Federal Council.
Each of the fourteen States has powers under the State Council for planning, determining and implementing social and economic  programs  within its  own region, and each is responsible for the development and protection of its natural resource base. For administrative purposes, the States are divided into Zones, which are in turn sub-divided into Woredas. Each Woreda is again sub-divided into Kebeles, which provide local level organization and administration.
6.2.2  Institutional Framework
The National Environmental Protection Authority  (EPA) was  established under Proclamation No. 9/1995 as an autonomous government body entrusted with the protection and conservation of natural resources in Ethiopia. The general role of the EPA is  to provide for the protection and conservation of the broad environment, through environmental protection policy  and legislation, directives 
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for evaluating the impact of social and economic  development projects, programmes or policies on the environment and standards for the protection of soil, water, air and biological systems. The specific  powers  and duties  of the EPA are defined in Proclamation No. 9/1995.
6.2.3   Environmental Policy and Guidelines
6.2.3.1 Environmental Policy
In the Constitution of the Federal Democratic Republic of Ethiopia, Article 92 sets out the broad environmental objectives of the Government as the need to ensure that:
•   All Ethiopians live in a clean and healthy environment;
•    The design and implementation of program and development projects do not damage or destroy the environment;
•     People  participate  fully  in  the  planning  and  implementation  of  policies  and projects that affect them directly; and
•      The  government  and  citizens  have  fulfilled  their  duties  to  protect  the environment.
The Environmental Policy  of the Federal Democratic  Republic  of Ethiopia recognises  the importance of incorporating environmental factors  into development activities  from the outset, so that planners may take into account ecological protection as an essential component of economic, social and cultural development.
6.2.3.2 EIA Guidelines
The EPA has  introduced procedural guidelines  for EIA to aid development planning and natural resource management in Ethiopia. The long-term objectives of the EIA system as set out by the EPA are:
•    Conservation and sustainable use of natural resources;
•       Integration   of   environmental   considerations   in   development   planning processes;
•    Protection and enhancement of the quality of all life forms; and
•       Attainment    of    environmentally    and    socially    sound    and    sustainable development.
Competent Agency:- This  may  be the EPA or other regional agencies mandated to screen, review, decide and follow up the existing and proposed development projects in the EIA process.
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Jurisdiction   of   Competent   Agencies:-The   Competent   Agency   has   jurisdiction over the following projects:
•    Projects directly undertaken by federal agencies.
•    Projects supported in whole or in part through federal contracts, grants, subsidies, loans or other forms of funding, except where there is no federal agency control over subsequent use of such funds.
•      Projects   involving   a   federal   lease,   permit,   license,   certificate   or   other entitlements for use; and in accordance with other criterion to be determined by the relevant laws; evaluation of environmental impact assessment shall be carried out by the authority.
In cases other than the above, evaluation of the EIA process shall be carried out by the Competent Agencies in each Regional State.
Project  Schedules:-After  an  Initial  Environmental  Examination  (IEE)  projects are assigned to one of two categories.
•    Schedule 1 - Projects which may have significant environmental impacts and therefore require detailed field review and a full Environmental Impact Study.
•     Schedule  2  -  These  are  projects  whose  type,  scale  or  other  relevant characteristics   have   potential   to   cause   some   significant   environmental impacts but are not likely to warrant an Environmental Impact Study.
Hydropower projects  are not specifically  mentioned in the list of scheduled projects; but the activities  to be carried out for a hydropower project to be commissioned would result in a designation as Schedule 1 (except for a very small-scale scheme).
The EIA Process in Ethiopia:- The EPA guidelines on EIA call for a systematic approach to project evaluation. The sequential steps are:
1.  Screening - carried out by the project proponent.
2.    Scoping  -  to  determine  the  extent,  magnitude  and  likely  significance  of adverse impacts and possible mitigation measures.
3  Environmental  Impact  Study  -  with  full  public  participation  to  determine  in more  detail  and  quantify  the  extent,  magnitude  and  likely  significance  of adverse impacts; cost adverse impacts where appropriate. Provide detailed mitigation  measures  with  costs;  recommend  a  monitoring  program;  discuss alternative interventions.
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4.    Review  of  Environmental  Impact  Statement-  to  be  undertaken  by  the Competent Agency and other interested parties.
5.    Decision-Making   Process   -   the   Competent   Agency   provides   a   clear statement   of   rejection   or   conditions   attached   with   the   granting   of   an environmental clearance certificate.
6.    Grievance  -  the  proponent  may  make  grievance  with  the  Head  of  the Competent Agency within 14 days of receipt of decision.
7.     Environmental   Audit   and   Surveillance   -   if   deemed   necessary   the Competent  Agency  may  conduct  an  audit  or  surveillance  of  the  project  to assess  the  effectiveness  of  environmental  management  measures  decreed for the development.
6.2.3.3 El A and International Context
The specific guidelines for EIA in Ethiopia have been developed from World Bank guidelines. In this  study, therefore, the principles  applied by  the World Bank have been selected as  representing a consensus  concerning the international standards and criteria upon which a project should be judged. A project meeting the criteria for World Bank environmental clearance or acceptability will normally also meet the demands of all other internationally recognized donor or funding agencies  and the National Environmental Protection Authority  of the Federal Democratic Republic of Ethiopia.
As  a hydropower project (HPP) involving the construction of a high dam and associated inundation of land to form the reservoir, the proposed Birbir-A and Birbir-R Hydropower Projects  qualifies  as  a World Bank  Category  A Project (World Bank, 1991). In the Environmental Assessment Sourcebook  Update (World Bank, 1993) a Category A project is defined as having one or more of the following attributes that make the potential impacts "significant": direct pollution discharges that are large enough to cause degradation of air, water or soil; large- scale disturbance of the site and/or surroundings; extraction, consumption or conversion of substantial amounts  of forest and other natural resources; measurable modification of hydrologic cycle; hazardous materials in more than incidental quantities; and involuntary displacement of people and other significant social disturbances. The Category  A designation may  be equated to the Schedule 1 designation in Ethiopia.
By virtue of requiring the compulsory relocation of people and appropriating their associated natural resource base (in terms of agricultural land), destroying forest and modifying the river regime significantly, the project meets  the specifics  of both World Bank Category A criteria and Schedule 1 under Ethiopian guidelines.
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6.3   Description of the baseline condition
6.3.1  BIO-PHYSICAL baseline condition
Descriptions of the hydrology, climate and geology of the study area are provided
in chapter 3 and chapter 4. Therefore, relevant information can be referred from
those chapters.
6.3.1.1  Land Use/Land Cover
The main land use and land cover within the sub-basin include cultivation and
wood land vegetation. Cultivation is mainly carried out in the upland areas that
have favorable agricultural soils  and climatic  conditions. Most of the upland areas are moderately cultivated, and some localities are intensively cultivated. A variety  of crops  are grown; maize being the predominant. Others  include sorghum, teff, finger millet, wheat, barley, chickpeas, beans and oil seeds. Coffee production is also a significant land use particularly in the eastern and southern part of the sub-basin (in Jimma, llubabor and West Wollega Zones). Large proportion of the slopping areas between the upland and the valley is cultivated, and expansion of farmlands and settlements into this landscape is significant with increasing population and land degradation in the uplands.
Currently  most part of the escarpment slopes  and lowland areas  are mainly covered by  open or dense woodland vegetation and grasses. The woodlands are normally always associated with a grass understorey or grasslands between areas of low woody vegetation. In some parts of the valley the woodland and grassland areas have been converted to farmlands for large scale state farming (now being transferred to private business  farms) and small-scale peasant farming (mostly  by  new settlers  coming from highland areas), and settlement areas.
Other important components of the land use and land cover of the sub-basin are forestland, bamboo and grassland. There are some disturbed remnant forests in the upper part of the basin in areas generally occupied for human settlement and agricultural use. Extensive stands  of the lowland bamboo, Oxytenanthera abyssinica, occur in the project area particularly on the left side of Didessa river, including the Kamashi area.
6.3.1.2  Mineral Resources and Mining Activities
The major minerals  found in the Abbay  Basin, where Didessa is  the major tributary, include gold, platinum, copper, nickel, iron and molybdenum. The principal industrial rocks  and minerals  are marble, limestone, siliceous sandstone, clay and lignite. Within the Didessa sub-basin particularly in the
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catchment draining to the proposed dam site, an iron ore deposit is  currently being prospected at an area called Bikilal located near Gimbi (20km Northeast of Gimbi or 465km West of Addis Ababa) with regard to proving the size of the deposit. Most of the mineral deposits are found in the western part of the basin.
Gold is  the main mineral that has  been mined mainly  from alluvial deposits located near Assosa. Very limited traditional gold mining from alluvial deposits along tributaries of Didessa such as Jirma, Sayi, etc. is practiced as observed during the field visit to the area.
Marble is  the main industrial rock  and is  well represented at several sites throughout the lower basin, and limestone is  worked in the Muger valley  for cement. Other minerals and construction materials are widely worked but on an insignificant scale.
6.3.1.3  Soil Erosion
The rate of soil erosion in the sub-basin especially  in the uplands  is  high. Intensive cultivation without basic soil conservation practices is the main cause of soil erosion. With rapid population growth and land degradation in the highlands there is a great conversion of areas of vegetation cover especially in the valley slopes  (sloping lands  between the uplands  and low lands) for expansion of agricultural lands.
6.3.1.4 Terrestrial Ecology
Flora and Vegetation
The natural vegetation of the Didessa valley  including the project area is dominantly  broad-leaved deciduous  woodland, which is  usually  termed as Combretum-Terminalia Woodland. This  vegetation is  relatively  dense and its floristic  composition is  particularly  rich in trees, bushes  and shrubs. The vegetation is mostly characterized by small trees and bushes with fairly large deciduous leaves, and tall grasses and the lowland bamboo.
The floristic  composition of the woodland vegetation of the project area is dominated by Combretum spp., Terminalia spp. and a species with local name
Miwa’ (Gumuz)/'Miroro’ (Oromo) in the layer of small trees, mostly associated with Hyparrhenia spp. (tall grasses) and Oxytenanthera abyssinica (the lowland bamboo) thickets. This vegetation is also characterized by other small trees and bushes including Stereospermum kunthianum, Piliostigma thonningii, Entada sp, Lannea schimperi, Grewia bicolor, Protea sp, Lonchocarpus  laxiflorus, Anogeissus  leiocarpa, Maytenus sp, etc. Extensive stands  of O. abyssinica (bamboo) occur in the area, maintained by burning which has mostly produced a
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moisaic  of thick  bamboo stands  interspersed with grasslands  and fire resistant trees such as P. thonningii, T. schimpehana, Combretum spp., S. kunthianum, Maythenus sp, etc.
The woodland (woody vegetation) of the project area (dam and reservoir area) and surroundings is associated with a grass layer understorey. The woodlands of the valley are maintained as a fire deflected succession by repeated burning.
There is  no significant riverine vegetation (forest) along Didessa river and its tributaries  However, there are some larger tree species  on the riverbanks including Manilkara butugi, Syzygium guineense, Cordia africana, Croton macrostachys, Tamarindus indica, Aningeria sp., Ficus sp., Celtis sp., Berchemia discolor and species with local names-'Dijaha' (Gum.)/‘Qasso' (Orom.), 'Tonkola’ (Gum)/ 'Chamo' (Orom.), ‘Quni’ (Orom.) and ‘Qolati’ (Orom.). Detail list of the main plant species identified in the project area is given in Appendix 4.5.
The commercially  important tree species  Boswellia papyrifera, which produces resin (incense) is not available in the project area (dam site and reservoir area) but it is present in the downstream reach of the valley.
Detail and site-specific studies on floristic composition of the project area have not been carried out. As a result it is not easy to determine whether there are any  threatened plant species  in the impact area. Information available from previous  studies  is  general and not specific  to the project area. According to Ensermu K., et al (1992) about twenty threatened endemic plants species have been identified from the Combretum-Terminalia Woodland vegetation type (see Appendix 6 for the detail list). Of these plant species nine are endangered, seven are rare and three are vulnerable while the remaining one is  endangered or extinct. Therefore, the presence of these and any other threatened plant species in the project area particularly in the direct impact zone must be subject to further study.
Extensive  burning  of  the  woodland  vegetation  is  a  well-known  practice  in  the Didessa  valley  including  the  project  area.  The  vegetation  in  the  Broadleaved Deciduous Woodland zone has developed under the influence of fire, and it is thus not affected heavily by controlled burning (Ethiopia-UNCED National Report, 1992). Most woody species are relatively fire tolerant, having evolved under the influence of regular burning (Pratt and Gwynne, 1977 cited in Woodroof, 1995). Burning of the vegetation takes place every year during the dry season mostly when the grasses are sufficiently dry. During the reconnaissance field visit it was observed that burning was widely practiced in the woodlands mainly too improve the  grazing  for  livestock,  to  help  control  tsetse  fly  and  to  reduce  cover  that harbors predators and other unwanted game animals. It was also evident that
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burning was carried out to clear land
of woody vegetation and tall grasses for
cultivation; land clearance by burning
is usually supplemented by cutting of the
woody vegetation using hand tools.
In  some  parts  of  the  valley  particularly  along  the  main  road  to  Gimbi,  around Kamashi (left side of Didessa) and in Belo Jiganfoy Woreda significant areas of woodland  vegetation  are  being  cleared  for  shifting  cultivation,  establishment  of new   settlements   and   for   mechanized   farming   by   private   business   firms (investors). As realized during the visits to these areas there is no land use plan for  these  areas  and  unoccupied  (unused)  areas  are  just  provided  by  the local/regional authorities for the investors who request for land. In addition, the local people widely practice shifting cultivation where they clear areas of dense natural vegetation and cultivate only for a few years (mostly two or three years) to minimize labor requirement to cultivate the land and the problem of weeds.
Unless  this  unsuitable or unplanned land use system is  changed timely, undoubtedly it will result in rapid degradation of the natural resource (mainly the woody  vegetation and land), which is  the major resource base for the local people. The destruction of the vegetation means the loss of the natural resource base on which human livelihood depends, and the habitats  where the wild animals live, resulting in the reduction in their number and the extinction of some of them. It also means the loss of the plant cover that maintains the soil and hence the dry season water supply to the rivers.
On right bank of Didessa River, (just upstream of the proposed reservoir) large plain areas  had been deforested or cleared for mechanized state farms. Recently these state owned farms were abandoned but currently being privatized (transferred to private investors) according to the privatization policy  of the Government. Some have already  started farming particularly  the areas  called Belo and Bereda Farms.
Fauna
Most part of the proposed project area (dam site and reservoir area) and its surroundings  are characterized by  relatively  undisturbed woodland habitats except the temporarily disturbance caused by the widespread burning during the dry season. Although the area is not designated as Wildlife Conservation Area, it provides ideal habitats for a variety of wild animals.
The main larger animals  (according to information obtained from local offices, interviews with local people and personal observations during visit in the project area) present in the area include Anubis  baboon (Papio anubis), CoIobus monkey  (CoIobus  guereza), Grivet monkey  (Cercopithecus  aethiops), Bush pig (Potamochoerus larvatus), Warthog (Phacochoerus africanus), Bushbuck
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(Tragelaphus scriptus), Bush duiker (Sylvicapra grimmia), Gazelle (Gazella sp.), Lion (Pathera leo), Leopard (Pathera pardus), Buffalo (Syncerus caffer), Jackal (Canis sp ), Hippopotamus  (Hippopotamus  amphibius), Hyena, Porcupine, and Cheetah. None can be considered rare, while some such as the Anubis baboon, Grivet monkey, warthog and Bush-pig damage crops and others such as the lion and leopard can be a danger to livestock.
The project area is  also a habitat for a diversity  of avifauna but their species composition has  not been studied. With regard to other animal groups  a wide variety of species, particularly amphibians and reptiles (e.g. Crocodile, Tortoise, Python and a number of snake species) are present in the area.
6.3.7.5 Aquatic Ecology
Flora
Scientific study on aquatic flora of the Didessa River has not been carried out. As  observed during the reconnaissance field visit no significant aquatic  plant communities are believed present in the Didessa River and in the riverside areas.
Fauna
There is  little information on aquatic  fauna within the Didessa River, and its tributaries. With regard to fishery no systematic fish identification has been done within the sub-basin, and the fish species composition in the Didessa river is still uncertain. As information yielded by study carried out by the Joint Ethio-Russian Biological Expedition (JERBE) (cited in BCEOM, 1998) indicates fish species in the genera Garra and Varicorhinus, and Mormyrus  hasselquistii are known to occur in the Didessa River.
During the public  consultation the local people and development agents  have confirmed the presence of a number of fish species  in Didessa river that are edible and fished by the local people. There is no commercial fish production in the area, but subsistence fishing is  important to the local people. As  they indicated about 13 fish species  are caught (by  the local people) from Didessa River, that are mainly  used for household consumption or sold to local consumers  at the nearby  villages  and towns  like Kamashi. The local names (Gumuzigna) for the fish species  include Badana, Piza, Tseya, Jaja, Sileya, Bagogue, Bagaba, Hepala, Hepatua and Hekakima. They mostly use traditional fishing equipment such as  fishhooks  and fish traps. Other significant larger aquatic animals include hippopotamus and crocodile.
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6.3.1.6 Public Health
Current Health Status
Reamilswice Level Study AMtflst 2001
Morbidity  statistics  recorded by  Kamashi Health Center and Clinics  in the Kamashi Zone were collected from the Zone Health Department, and Kamashi Woreda Economic  Development and Social Services  Office, supplemented by information obtained from the local people, to show the health status  of the population in the zone in general and around the project in particular. Appendix 4.7A and 4.7B give morbidity statistics recorded by the health center and clinics in the zone and woreda for the top leading causes of outpatient visits. As these morbidity data show, the majority of the health problems are communicable and directly  or indirectly  related to the low socio-economic  status  of the population and environmental factors. Malaria is the top most leading cause of morbidity, accounting for about 30% of the total outpatient visits. Intestinal parasites, diarrheal diseases, respiratory tract infections and skin diseases are among the major causes of outpatient visits. Other main health problems include gastritis, rheumatism and duodenitis. The main health problems reported by the clinics in Kamashi Woreda are malaria, intestinal parasites, duodenitis, diarrheal disease, skin infections and respiratory tract infections.
Water-related Vector-borne Diseases
Malaria
Malaria is the top most leading cause of morbidity and mortality in the project area. The Didessa HPP area is highly malarious as it is located in lower altitude, below 1500 masl, which has favorable ecology for the disease vectors. It occurs through out the year but its  main transmission season from September to December, which is  after the main rainy  season. And the second important transmission is  March to May, which is  associated with short rainy season or 'Belg'.
Of the four Plasmodium species  known to cause human malaria, the two epidemiologically  important species, Plasmodium falciparum and P. vivax are prevalent in the area. As indicated by the zone health department as the whole P. vivax infected cases are most frequent, and P. falciparum cases are dominant during the main transmission season. P. falciparum is the cause of severe and complicated malaria, which causes high fatality rate. P. vivax is more common in the dry  season. This  species  is  not a major cause of mortality  but is  an important cause of morbidity due to the phenomenon of relapse. The dominant vector responsible for transmitting the disease is  Anopheles  arabiensis, which belongs to the Anopheles Gambiae complex.
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The responsibility for controlling malaria in the Kamashi Zone lies on the Malaria Control Zone Office at Kamashi, which is  organized under the Zonal Health Department. Chloroquine, Premaquine and Fansidar are the main prophylaxis used to treat malaria-infected patients. Mostly  Fansidar is  ordered where Chloroquine resistance is  suspected. Control of the malaria vector is  achieved by  selective spraying of resting sites. In addition, some environmental management activities  including drainage and removal of habitat to prevent vector breeding practiced. However, the extent of chemical spraying and environmental management is very small compared to what is required due to shortage of resources  (trained manpower, equipment, etc.) and accessibility problems.
Onchocerciasis
Onchocerciasis is endemic in southern Ethiopia including the project area. This disease is caused by a filarial worm called Onchocerca volvulus. In Ethiopia two vectors, Simulium damnosum and S. woodi ethiopiense have been identified as being responsible for the transmission of Onchocerciasis. These vectors prefer fast flowing and shaded rivers and streams in forests and tree-fringed savannahs in southwestern and western Ethiopia. These conditions are found in the project area, along Didessa River and its tributaries.
Health Infrastructure
The currently  operating health infrastructure of the Kamashi Zone includes  1 health center (located at Kamashi town), 4 clinics (located at Belo/Sogie, Agelo, Sirba & Yaso), and 5 health posts (4 located in Belo & 1 in Yaso). The health center, Belo clinic  and the four h. posts  in Belo Woreda are providing health services for the population around the proposed project.
6.3.1.7 Water Supply and Sanitation
Water Supply
In the area around the HPP the development for potable water supply is at very low levels. At present even the nearby zone and woreda towns (Kamashi and Sogie/Belo Jiganfoy) are have no safe water supply system. For Kamashi town it is under construction in which the source is groundwater. Therefore, both the urban and rural population depends  on unsafe sources  mainly  unprotected springs, streams  and rivers. Only  a few rural villages  have developed water supply systems that are hand pumps and protected springs. The lack of safe drinking water is reflected in the prevalence of water-borne diseases, which are among the common health problems.
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Sanitation
Development for sanitary  facilities  is  extremely  at low levels. The urban population mostly  depends  on pit latrines  for excreta disposal. In rural areas sanitary  facilities  are almost non-existent. Therefore, the rural population uses the open field for waste disposal. Environmental sanitation in Kamashi is  also poor due to lack of dry waste disposal system and presence of large livestock population in the town.
6.3.1.8  Physical Infrastructure
Road infrastructure development is at very low levels. The major road link close to the project is  the Addis  Ababa-Nekemte-Nejo Road, which is  mostly  well maintained There is also about 38 km long poorly maintained gravel road that makes junction with the main road at about 152km and connects Kamashi, the nearest town to the dam site. From Kamashi there is a recently opened unpaved access road or trail (about 17km long) leading to the dam site. The total distance from Addis to the dam site via Kamashi is about 544km.
The other important village close to the dam site is Sogie, capital of Belo Jiganfoy Woreda, and located on right bank  of Didessa. This  village is  linked by  a dry weather road (about 60 km long) that starts at Arjo town, which is located on the Nekemte-Gimbi Road at about 43 km from Nekemte.
6.3.1.9  Historical, Cultural and Religious Sites
There are no known historical or cultural sites within or in close proximity of the project areas. In addition, there are no religious  institutions  within the direct impact areas  (reservoir area, dam and powerhouse site, and access  road) but there are two churches in the vicinity of the reservoir in the upper part (right side).
6.3.2  Socio-Economic Baseline Condition
6.3.2.1 General
Administratively, the Didessa HPP is  located in Kamashi Zone covering two Woredas, namely: Kamashi and Bello Jiganfoy. The nearest town to the dam site is  Kamashi - zonal and Woreda capital. Accessibility  is  the critical problem for this HPP, especially the right side of the dam site is completely inaccessible and even the Kamashi side is  difficult. The distance of the dam site from Kamashi town is about 19km. The HPP affects 9 Peasant Associations (5 PAs in Kamashi and 4 in Bello Jiganfoy) and 12 villages (6 in Kamashi and 6 in Bello Jiganfoy) in both Woredas.
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6.3.2.2  Kamas hi Woreda
Kamashi Woreda is located in the center of Kamashi Administrative Zone, and Kamashi town is the capital for the Zone and Woreda. The total population of the Woreda as  per the 1999 Central Statistics  data was  9,621 with a 45-50% of active labor force (15-64 years of age) and 5.9 persons/km2 population density. The total area of the Woreda is  about 1,622.5 km2 with rugged and hilly topography dissected by river valleys.
Of the total PAs in the Woreda, five PAs - Hena Dugda Gibie, Daguba Bedessa Jirma, Soyoma Gawla, Mukarba Hunda and Jallo Laika are located in the impact zone of the HPP.
6.3.2.3  PAs and Villages in the impact zone in Kamashi Woreda
Peasant Associations
There are five Peasant Associations (PAs) in the area to be influenced/affected by  the project, namely  Hena Dugda Gibie, Daguba Bedessa Jirma, Soyoma Gawla, Mukarba Hunda and Jallo Laika. These PAs  were inhabited by  4,740 people (798 household heads and 3,942 family members) during the time of the assessment. Most of the area in all PAs is not under intensive cultivation mainly due to less population pressure and availability of large area compared to the size of the population. About 60-70% of the PAs  total area is  covered by woodland (trees bushes, shrubs and grasses) and bamboo thickets. As per the information received from the DAs  and local offices, the PAs  are sparsely populated with 6 persons/household.
According to the information gathered from the DA Offices, the average land holding (cropland only) in all PAs  is  estimated to be 1.5 -2 ha/household. Sorghum, maize, millet, sesame, haricot bean and cotton are the major crops grown in these PAs. The yield of sorghum, maize and millet is 13,10 and 8 qt//ha respectively. The yield of haricot bean is 8 qt/ha, sesame 6 qt/ha and cotton 6 qt/ha. All farmers  use local seeds, practice shifting cultivation and crop production is not intensive. The price of agricultural produces is very low. For example, the price of sorghum and maize is Birr 40 and Birr 45/qt respectively. The price of millet is Birr 50/qt, sesame Birr 200/qt, haricot bean Birr 150/qt. In the PAs, the total number of livestock population is estimated to be 4,198 with an average holding of 5 livestock/household.
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Villages in the impact zone
In All PAs, six  villages, namely: Hurungu, Sayi Jirma, Godere, Daweta Meti, Sarnie and Shimala villages  are located in the impact zone of the proposed reservoir area - mainly along Sayi River. Since Sayi river is a major tributary of Didessa, the reservoir area will extend following Sayi River. The distance of the villages from Sayi River is as follows: Sayi Jirma about 0.5 km, Sarnie 1.5 km, Dawota Meti 4 km and Godere 2km. The distance of Shimala village from Didessa is  5 km. Most of these villages  were established in the 1960s  - the oldest being Shimala (1964 E.C.), and the recently  established one Hurungu (1980 E.C.). In all villages, there were 137 houses (all thatched roofs) during the assessment period.
The local people are moving up voluntarily from Sayi River to areas of relatively higher elevation. For example, the reconnaissance team witnessed that villagers from Sayi Jirma were moving up from the riverbank, and were establishing new settlement site. There is also a resettlement program designed by the Regional Government to resettle these villagers  and other PA members  in suitable settlement areas  where social services  can be better provided to the people. Although the plan is  not yet implemented, the Zonal officials  informed the reconnaissance team that the study  was  completed and implementation will follow soon. Therefore, in both cases, these are good opportunities to the project because the cost and risk  of relocating people will be assumed by  the local population and the regional government. What is  expected from the project holder is to follow-up the implementation of the resettlement plan and strengthen its  working relations  with the local authorities  before the actual project implementation started and other mitigating measures considered.
All villages  were inhabited by  about 124 households  and 427 family  members during the survey period. The settlement pattern of all villagers is scattered, with an average family  size of 4 persons/household and male: female sex  ratio of 1:0.8 in all villages. The villagers are followers of Christian religion (Protestant and Orthodox), and a few are pagans. There were no religious  institutions  in these villages; they were all located in Kamashi town.
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Table 6. 1 Total population of all villages
Reconnaissance Level Study
August 2001
PA
Village
Household Heads
Family
/ members
Male
Female
Total
Male
Female
Total
Hena         Dugda Gibie
Hurungu
10
4
14
27
29
56
Sayi Jirma
21
2
23
25
38
63
Daguba Bedessa
Godere
25
3
28
28
33
61
Soyama Gawla
Dowata Meti
10
3
13
24
31
55
Mukqrba Hunda
Sarnie
24
5
29
36
45
81
Jallo Laika
Shimala
16
1
17
53
58
111
Total
106
18
124
193
234
427
Source: DA Offices
The total area available in the impact zone is about 413 ha - this is under forest and grazing area. Of this area, 295 ha is under bamboo forest and the remaining 118 ha is  a grazing land. In these villages, sorghum, maize, millet, sesame, haricot bean and cotton are commonly  grown crops. But there is  no cultivated area in the proposed reservoir area along Sayi river - all croplands are above the villages, croplands therefore will not be in the impact zone. The villagers practice shifting cultivation because the number of population is less, the area is vast and hoe culture is dominant.
Table 6. 2 Land use pattern of all villages
Village
Grazing (ha)
Forest (ha)
Total
Hurungu
10
25
35
Sayi Jirma
20
35
55
Godere
20
50
70
Dowata Meti
30
90
120
Sarnie
15
30
45
Shimala
23
65
88
Total
118
295
Source: DA Oflfices
Livestock is part of the farming system in both villages. Accordingly, there were about 805 livestock population in all villages, and the average livestock holding is 6.5 livestock/household.
413
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Table 6. 3 Livestock population in all villages
Type
Size/number
Average/household
Cattle
47
0.4
Sheep
36
0.3
Goats
125
1.0
Equines
19
0.2
Poultry
578
4.7
Total                      805
6.5
Source: DA Offices
Current Use of the Proposed Dam and Reservoir Area
The reservoir area along Didessa and Sayi rivers is not currently under intensive
cultivation and/or densely  populated. The local population derives  limited benefits  from this  area. However, the 124 households  in all villages  and other PA members use the proposed reservoir area as a:
Grazing area Grazing is  practiced on communal area along Didessa and Sayi Rivers. The total area under grazing in all villages  is  estimated to 118 ha, on which livestock depends on for grazing.
Forest area: The bamboo forest area is  also along Didessa and Sayi Rivers. From the forest area, villagers  and other PA members  collect grasses  for different purposes  (such as  for thatching roof), collect forest products  (such as poles  and bamboo - mainly  for house construction) and collect firewood. The estimated total forest area under bamboo, bushes  and shrubs  in all villages  is 295 ha
Traditional mining site: All households  (except in Shimala village) practice traditional gold mining to generate additional income along Sayi and Jirma Rivers. The estimated income from the traditional gold mining is  Birr 20- 30/week/person. The traditional mining is in the impact zone.
Fishing site: Local people practice traditional fishing activities  mainly  for consumption. All households  practice fishing. As  per the key  informants, there are about 13 fish species in Didessa, Sayi and Jirma rivers.
Crossing points: In the dam and reservoir area, there are no bridges to cross Didessa, Sayi and other tributaries. However, there are wooden boats to cross 
Didessa River above and below the dam site. Using these wooden boats, people from Kamashi and Bello Jiganfoy communicate each other. People cross also
Didessa and Sayi Rivers on foot when the volume decreases. The crossing points (on foot) are at Bela Kobbo, Melka Serie and Melka Yambo for Sayi River,
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2
6.3.2.5  PAs and Villages in the impact zone in Bello Jiganfoy Woreda
Peasant Associations
There are four Peasant Associations (PAs) located in the impact zone of the proposed hydropower project, namely Bello Didessa, Bello Didiga, Sayi Dalecha and Dimtu. These PAs were inhabited by 4,426 people during the time of the assessment. Most of the proposed reservoir area in all PAs is not under
cultivation mainly due to less population pressure and unsuitable topography.
As per the information received from the DAs, the average land holding in all PAs is estimated to be 6 ha/household. Sorghum, maize, millet, teff, sesame, haricot bean and noug are some of the major crops  grown in all PAs. The yield of sorghum, maize and millet is 20,15 and 8 qt//ha respectively. The yield of teff, haricot bean and sesame is  8 qt/ha and noug 4 qt/ha. All farmers  use local seeds, practice shifting cultivation and crop production is not intensive. The price of agricultural produces  is  very  low. For example, the price of sorghum and maize is Birr 20/qt. The price of millet is Birr 50/qt, sesame Birr 220/qt, haricot
and for Didessa upstream of the dam site but at a very close distance from the dam site. When the volume of Didessa and Sayi Rivers increase as the result of the project, crossing on foot and using the wooden boats may not be possible so that the movement of people and livestock will be affected.
Settlement sites: In all villages, there were about 137 houses  (all thatched roofs) during the assessment period. The current villages may not be suitable for settlement because the reservoir area will increase the rate of malaria incidence markedly. For example, houses  in Sayi Jirma village (located within a 0.5 km from Sayi river) are very close to Sayi River although the villagers were moving out to a new settlement site (at 1.5-2 km from the river) by abandoning the old village during the assessment period. There were no religious  or any  other institutions in these villages.
Water sources: The villagers  use Sayi, Jirma and other water sources  for domestic  and livestock  purposes. Over 551 people and about 227 livestock access drinking water from these sources.
6.3.2.4  Bello Jiganfoy Woreda
Bello Jiganfoy  Woreda is  the second Woreda which will be partly  affected by HPP. The total population of the Woreda as  per the 1999 Statistical Abstract was 13,006 with a 45-50% of active labor force (15-64 years of age) and 8.1 persons/km2 population density. The total area of the Woreda is about 1,611.88 km with rugged and hilly topography dissected by river valleys.
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bean  Birr  150/qt.  In  the  PAs,  the  total  number  of  livestock  population  is estimated to be 4,404 (about 6 livestock/household), poultry being the major one.
Villages in the impact zone in the Proposed Project
In the above-mentioned PAs, six villages, namely: Kersa (in Bello Didessa PA), Senkole and Gawo (in Bello Didiga PA), Dalecha & Arkokasso (in Sayi Dalecha PA) and Kutamuri (in Dimtu PA) villages are located in the impact zone along Didessa, Didiga and Jirma Rivers. The reservoir area is  expected to extend following Didiga and Jirma Rivers. The distance of the villages from these Rivers is as follows: The distance of Kersa & Gawo villages from Didessa is about 5 km, Senkole, less  than 1 km & Arkokasso, 5 km from Didiga. Dalecha village is located at less than 1 km from both Didessa and Didiga rivers, Kutamuri 1 km and 2.5 km from Jirma and Didessa Rivers respectively.
All these villages were established in the 1980s - the oldest being Senkole (1980 E.C.), and the recently established ones Dalecha & Kutamuri (1987 E.C.). In all villages, there were 533 houses (522 thatched roofs & 11 Corrugated) during the assessment period. About 55% of all houses  were in Arkokasso followed by 28% in Dalecha Village ((this village is at less than 1 km distance from Didessa and Didiga Rivers - both villages are in Sayi Dalecha PA). The remaining houses (17%) were in Kutamuri, Gawo, Kersa and Senkole villages.
About 324 households and 1,902 family members are located also in the impact zone. The settlement pattern of all villagers is scattered, with an average family size of 6 persons/household. The villagers are followers of Christian (Protestant & Orthodox) and Islamic religions, and a few are pagans. Ethnically, the majority of the villagers belong to Gumuz with some Oromos and Amharas. There were two Churches  with their cemeteries, and two market places  in Arkokasso and Kutamuri villages. There were also one elementary school and one developed water source at Kutamuri village.
Table 6. 4 Total population of all villages
PA
Village
Household Heads
Family members
Male
Female
Total
Male
Female
Total
Bello Didiga
Senkole
12
-
12
31
41
72
Gawo
33
7
40
90
110
200
Bello Didessa
Kersa
34
3
37
118
104
222
Dimtu
Kutamuri
43
2
45
126
122
248
Sayi Dalecha
Arkokasso
79
11
90
231
309
540
Dalecha
91
9
100
316
304
620
292
Total
Source: DA Offices
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324
912
990
1,902
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The total area available in the impact zone is about 1,944 ha - over 80% of this area is  under forest and grazing area, and 12% is  occupied by mountains/gorges. Only  8% of this  area is  under cultivation mainly  along the tributaries  of Didessa - Didiga and Jirma Rivers. In most of these villages, sorghum, maize, millet, teff, sesame, haricot bean and cotton are some of the major crops  grown. The villagers  practice shifting cultivation because the number of population is less, the area is vast and hoe culture is dominant.
Table 6. 5 Land use pattern of all villages (ha)
Village
Cultivated
Grazing
Forest
Mountain/Gorge
Total
Senkole
15
15
32
10
72
Gawo
20
80
100
40
240
Kersa
-
22
180
20
222
Kutamuri
25
50
175
20
270
Arkokasso
40
195
215
90
540
Dalecha
50
200
280
70
600
Total
150
562
982
250
1,944
Source: DA Offices
Livestock is part of the farming system in all villages. Accordingly, there were about 1,296 livestock population in all villages, with an average livestock holding of 4 livestock/household. The livestock  partly  depend for grazing on the proposed reservoir area.
Current Use of the Proposed Dam Reservoir Area
The proposed reservoir area along Didessa and its  tributaries  is  not under intensive cultivation. The local population derives limited benefits from this area. The 324 households in all villages and other PA members use the proposed reservoir area therefore as a:
Cropland. Cultivation on the proposed area, especially along the Didessa River is non-existent. However, limited area (150 ha) is put under cultivation along the tributaries of Didessa (Didiga and Jirma) River.
Grazing area: Grazing is practiced on communal area along Didiga and Jirma Rivers. The total area under grazing in all villages is 562 ha, which the livestock population depend on for grazing.
Forest area The forest area is also along Didessa and its tributaries. From the forest area, villagers  and other PA members  collect grasses  for different purposes (such as for thatching roof), collect forest products (such as poles for
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71Lower Didessa Medium Scale Hydrojowcr Project
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house construction) and collect firewood. The estimated total area under forest, bushes and shrubs in all villages is 982 ha.
Fishing: The local people practice traditional fishing activities  mainly  for consumption. All households  practice fishing during the dry  season using traditional fishing techniques.
Crossing points: There are no bridges  to cross  Didessa and its  tributaries. However, there are wooden boats to cross Didessa River above and below the dam site. Using these wooden boats, people from Kamashi and Bello Jiganfoy communicate each other. The local population cross also Didessa River on foot during the dry  season to go to different places. During the rainy  season, it is impossible to cross Didessa on foot - the only communication system is using the traditional boats  at a limited scale. When the volume of Didessa and its tributaries increases as the result of the project, crossing either on foot or using the traditional boats may not be possible so that the movement of people and livestock will be affected.
Settlement sites: In all villages, there were about 533 houses  (522 thatched roofs and 11 corrugated) during the assessment period. Since malaria is a major disease in the area, the current villages  will not be suitable for settlement because the reservoir area will increase the rate of malaria incidence markedly. Some houses particularly in Dalecha village (located within 1 km distance from Didessa and Didiga Rivers) are very close to both rivers. There were also two Churches  with their cemeteries, and two market places  in Arkokasso and Kutamuri villages, one elementary  school and one developed water source at Kutamuri village.
Water  sources:  The  villagers  use  Didiga,  Jirma  and  other  water  sources  for domestic and livestock purposes.
Traditional bee keeping: There were about 850 traditional beehives  in all villages  during the assessment period. Most of the households  practice bee keeping, and according to the information received from the DAs, Birr 34,000 is generated from this activity annually.
6,4    Potential Impacts
6.4.1 Potential Positive Impacts
Community  members  and particularly  the local authorities  are optimistic  about the expected benefits  of the envisaged project, thus  there is  high expectation from the local population and government officials. The following are amona the potential benefits expected from the envisaged hydropower project
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6.4.1.1  Generation of Electric Power
Hydroelectric projects have the potential to create a range of beneficial impacts.
The main benefit of a hydroelectric project is the generation of electric power,
which  can  support  economic  and  social  service  development  and  improve  the
quality of life in the service area. With the presently proposed dam, the Didessa
HPP would produce a firm energy of 1582 GWh/yr, which is equivalent to 94.92
million US dollar per annum.
At present the nearby towns such as Kamashi (Zonal & Woreda capital), and
Sogie (Woreda capital of Bell Jiganfoy Woreda) do not have electric light. The
local authorities expressed their interest and hope to get this service when the
project is realized. Due to this expectation, the cooperation of the government
offices and other community members during the reconnaissance study was very 
high. Therefore, the project is  expected to provide this  service to both towns after its completion and enhance the social benefits attributed to electricity (such as  good educational services, cottage industries  in the long run, attraction for private investors, etc.) in the project area.
6.4.1.2  Employment Opportunities
The hydroelectric project will bring a substantial possibility of job opportunities for the local people and migrant workers during the construction period. About 45- 50% of the local population is estimated to be active labor force (15-59 years of age), which is an asset for the project because it requires large labor force for construction works  and other labor-intensive activities. In addition, there is  a possibility of hiring some community members as permanent employees of the project during the operation phase.
6.4.1.3  Reservoir Fishery
The creation of large reservoirs  may  provide a significant potential for new fisheries  development. There is  a culture of fishing despite the fact that it is backward and limited to household consumption. The local people practice traditional fishing activities on Didessa and Sayi Rivers mainly for consumption using traditional fishing instruments. However, there is  no information on potential fish yields for reservoirs in the basin. For Lake Tana the annual catch and yield are 15,000 tones and 43 kg/ha/yr respectively (BCEOM, 1998 (Vol.8)). This result cannot be extrapolated to reservoirs in the Didessa basin, however, without further study.
The reservoir fisheries may provide commercial fishing industry, which would be a valuable source of protein and income for the local community. However, the fishing industry need to be carefully managed for the benefit of the local
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community  especially  the displaced population, and in-migration controlled to avoid over fishing, which may  lead to the collapse of the reservoir induced fisheries. The suitability  of the reservoirs  for stocking with commercial fish species and the potential fish yields of the reservoirs need to be studied in the subsequent study phases.
6.4.1.4  Recession Agriculture and Grazing Opportunities
The proposed dam reservoir will have about 48.4 Km2 of draw-down area between the maximum water level (at 1040 masl) and the minimum water (operating) level (at 952masl). However, this  area is  not expected to provide substantial potential for recession agriculture and livestock grazing during the dry season because the reservoir is  mostly  located in steep sided valley. In fact it has  some potential in its  upper part particularly  on right bank  that has  gently sloping land. This part of the reservoir area and some pocket areas along some of the tributaries  of Didessa could be used for controlled farming and grazing through a permit system. This condition can allow a certain number of people to grow short duration crops on these areas as the reservoir level falls. However, the adverse effects of these activities (such as potential for siltation, and addition of nutrients to the reservoir from livestock droppings/organic matter) need further assessment.
6.4.1.5  Flood Control
The total volume of the reservoir is 3531 Mm , and the mean annual discharge of
the Didessa river at the dam site is 7290 Mm3 or 234.4 m3/s. Average reservoir retention time, computed by  dividing volume by  flow, is  5.81 months  (the reservoir would be replenished only 2 times per year), giving a high potential for flood control. The low level of development and sparse population of the region makes  flood control benefit seem unnecessary  consideration at this  time. However, it could be an advantage for the people who use river crossings downstream as  the dam storage would control the peak  flows  that makes impediment to movement of people across the river.
6.4.1.6  Use of Reservoir Area Woody Biomass for Fuel-wood
Among the mitigations  required is  the clearance of biodegradable material from the reservoir area. The woody  biomass  of the reservoir area has  not been surveyed. However, the results of the Woody Biomass Survey by Tecsuit (1995) which shows the potential dry woody biomass of 10 to 30 tons/ha, can be applied to estimate the biomass of the proposed reservoir area. For the 64km2 reservoir area, this  translates  into a potential fuel-wood supply  of between 64 000 and 192,000 tons  or between 38,000 and 113,000 person years  of rural supply based on consumption rates given by Tecsuit. At 5 Birr per 20kg bundle of fuel-
3
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wood, its gross resource value is between 16.0 and 48.0 Million Birr (exclusive of the cost of collection and preparation).
6.4.1.7  Potential for Wildlife Reserves
The reservoir could provide a major refuge for waterfowls  whose habitat is threatened elsewhere, and for migratory  birds. The creation of a buffer zone around the reservoir (that may be proposed to prevent degradation of the lake environs) may attract a variety of birds, which have been losing habitat due to degradation of the natural environment. In addition, wildlife reserves  can be established within the buffer zone, which can be mitigation for the loss of habitat due to reservoir area inundation.
Furthermore, the regulated flow regime may be beneficial to some aquatic wildlife species, as ecological stress naturally arising during critically low flow conditions will be reduced. Aquatic  macrophytes  and larger animals  such as  the hippopotamus  and crocodile will benefit from sustaining or increasing areas  of wetland habitat in the dry season.
6.4.1.8  Infrastructure Development and Mitigation Benefits
The local communities  will likely  benefit from the social and physical infrastructure development to be implemented with realization of the proposed project. The local people may have greater access to health care, education and water supply facilities, and improved road infrastructure. They could also benefit from the mitigation and compensation measures to be implemented to minimize the adverse impacts of the proposed project. Families that are directly impacted including those to be resettled will be provided with necessary  mitigation and compensation schemes in such a way that they are better off than before.
6.4.2 Negative Impacts on Bio-Physical Environment
6.4.2.1  Impacts on Terrestrial Ecology
6.4.2.1.1 Vegetation Damage and Loss of Habitats
Construction of the proposed dam, and flooding by the new reservoir will bring a significant damage/loss  to the natural vegetation and terrestrial habitat Degradation of the terrestrial habitat will also be associated with the construction of access road, quarrying activities, and the establishment of construction camps and spontaneous settlements by immigrant people (who would be coming to the area during the construction phase) resulting in increased pressure on natural resources.
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Construction of the proposed dam will cause a loss of terrestrial habitat with a total area of about 6,400ha at maximum regulated level (MRL). The most significant impact will be on the woodland vegetation found in the inundation area. This vegetation is mostly dense and its woody biomass is relatively higher.
6.4.2.1.2 Impacts on Wildlife
As  discussed in section 6.3.1.4 the woodlands  of the project area (dam site, reservoir area, and access road) are important habitats for a variety of wildlife species. Certainly  with implementation of the proposed project the reservoir, dam and other infrastructure would replace significant areas  of these habitats and disrupt habitat use patterns  of the wild animals  including obstruction of migratory pathways.
Implementation of the project together with expansion of farmlands will certainly cause significant degradation of the wildlife habitats. In fact most of the wild animals can move to the remaining parts of the habitats (adjacent to the reservoir and at downstream of the dam). However, they may get closer to areas of human activities  and likely  be exposed to contact with the local inhabitants, and will therefore be more vulnerable to poaching activities. As the animals approach to areas  of human habitation or activities  there could also be conflicts  between human uses and wildlife conservation as some of the animals can damage crops cultivated by the local people, and some are danger for livestock.
6.4.2.1.3 Loss of Rare or Endemic Species
Detail and specific studies on flora and fauna of the study area particularly the zones  of disturbance by  the hydropower project have not been carried out. Therefore, the extent to which there are any  particularly  rare or vulnerable species  in the impact zones  is  not certain. Available information from previous studies  is  very  general and is  based on vegetation types. About twenty threatened endemic  plant species  are known to be found in the broad-leaved deciduous woodland vegetation type (Ensermu, K., et al, 1992), which includes the vegetation of the proposed dam and reservoir area (see Appendix 4.6). It is assumed that losses will most likely be of numbers in individual species, not in species themselves. Thus, the loss is probably not one of biodiversity. However, this must be subject to confirmation during the subsequent studies.
The vegetation and flora found in the impact areas  is  widely  distributed in northwestern and western parts  of Ethiopia (in the Abbay  and Baro-Akobo Basins) and in southwest of Ethiopia, along the Ethio-Sudan boundary. Most of these areas have had some shifting cultivation but have otherwise not been used agriculturally for a long time. Recently, however, there has been an intensification of agricultural activity together with increasing population due to schemes to
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settle people from famine stricken areas (Ethiopia: UNCED National Report on Environment and Development, 1992).
Concerning the impacts on the diversity of fauna, there is unlikely to be any loss of species  in the disturbed areas. It is  likely  that both the larger and smaller mammal species will move to adjacent areas of similar or identical habitat. There may be a decrease in relative abundance, but not loss of bio-diversity. Further study on fauna of the project areas especially insects, reptiles, amphibians and avifauna is required to establish baseline data and to give more detailed analysis on the impacts of the projects.
S.4.2.2 Impacts on Aquatic Ecology
Impacts on Flora and Fauna
The flooding of the reservoir area will destroy a length of about 50 Km of the Didessa river, 20 Km of the Sayi river and 20 Km of other major tributaries, whilst the dam will provide a barrier to movement of aquatic  fauna. No significant aquatic  plant communities  are present in the riverside areas  where the construction works are to be located. However, the activities at the dam site will disturb and probably  displace the populations  of large aquatic  animals particularly hippopotamus and crocodile, and fish in this locality.
During the period of reservoir impoundment the river flows below the dam will be reduced to 30% of the mean annual flow (as 70% of it will be regulated). This will reduce the habitat for aquatic fauna, which will induce stress, particularly for fish, hippopotamus  and crocodiles. The significant impact will be along the 17Km length of the Didessa river between the dam and confluence of Angar river. The inflows from the streams that feed Didessa at downstream of the dam will reduce the extent of the impact.
The compensation flows  are significantly  lower than those experienced under normal operation, and their persistence during the period of reservoir filling will undoubtedly  induce stress  on the mammals  and fish communities. Linder operation, however, it is likely that the larger aquatic mammals will benefit from the regulated downstream flow regime, since periods of low flow stress will be reduced, but the diversity and abundance of aquatic fauna, which are adapted to seasonal flow variations will be reduced.
The scheme may  have a major impact on existing fish communities within the Didessa river. Riverine fish will be replaced by  stagnant water species  in the reservoir, and species composition and biomass in the river will be affected by the regulation of the flow regime particularly downstream to the confluence with the Angar river, a major tributary to Didessa at about 17 Km downstream of the
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Didessa dam site. However, the proposed reservoir will provide an aquatic environment, which could significantly  increase the present fisheries  potential. The presence of any seasonal migration of fish within the affected reach of the river system is subject to future assessment.
Riverine fish communities are adapted to the flow regime, and alterations to this can lead to a change in the community structure in the river. Appropriate flows are also essential for the breeding and migration of many species of fish, and unless  a seasonal flood regime is  maintained these fish species  may  fail to reproduce successfully. Abrupt changes in flow characteristics can also have an adverse effect upon spawning success. Very rapid changes in water level can leave nests and spawning grounds exposed at critical periods. Larvae or eggs may be stranded in isolated pools and streams or washed away.
G.4.2.3 Impacts on Water Quality
Physical Changes of Water Quality
The physical nature of water is associated with color or turbidity, taste, odor etc. such physical change of water is caused by the interaction of the river water with the geologic  materials and organic materials decomposed from the vegetation cover of the reservoir and water shed area. As a result color and turbidity is observed in the water due to sediment and organic colloidal materials from the clay  and silt of the minerals  of geologic  origin and decomposable vegetation respectively.
In the case of lower Didessa the reservoir and dam sites are proposed to be established on geological rock formation composed of gneiss and granitic rocks or the reservoir area is sited partially on the old Precambrian basement complex and partially may be the dominant part of the reservoir and the dam site is, well defined by a local restriction formed by the ridge of a massive hard granite like formation included in to the basement gneiss of the PULC series.
The sandstone which forms the partial reservoir area under lain by Precambrian basement complex have relatively higher permeability though heterogeneity in permeability of the geological formation is expected.
Therefore, the interaction of the river water with the superficial material or soil of the reservoir area which is supported by the heterogeneous rock complexes like gneiss, magnetite, and stone or granitic rock may cause acidity in the reservoir water.
The  existence  of  the  reservoir  water  on  sand  stone  can’t  avoid  such  acidity problem since there is vertical and horizontal continuity between Precambrian
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78Lower Didcssa Medium scale Hydropower Project
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basement complex  and Adigrat sand stone. Furthermore, there is  a higher chance of flow of run-off from higher areas around the reservoir which can alter the pH of the reservoir water. Such acidic water will have corrosion effect on the turbines and other metallic installations. Also such condition has a series effect to the extent of death on the fauna of the reservoir and downstream.
There is a report made that iron smelting for agricultural instruments and utensils from the iron containing ores found in the geological formation of the Wollega region was exercised by the inhabitants. Iron is soluble under reduced condition and low pH therefore such information helps the environmentalist to be careful in future fate of reservoir water not to be acidic.
Hence a careful geological survey has to be made before one is reaching on decision of the possible reservoir area and dam sites or a possible solution to resolve such problem in case once such sites are selected and implemented has to be suggested before hand.
Chemical Changes of Water Quality
The  colloids  and  decomposed  vegetable  material  have  the  following  Bio chemical effects.
Oxygen Depletion:- Before the reservoir filling any  woody  biomass  in the inundation area has to be cleared in order to reduce organic matter, which affects water quality due to its decomposition. Otherwise oxygen depletion due to the consumption of oxygen in oxidizing those bio-degradable organic  material that
may result in low pH, and high CO2
and H S;
effects  to  turbines.  Further  oxidation  may  result  in  anaerobic  condition  of  the
2       this in turn may bring corrosion
aquatic environment and formation of CH , NH , H S, CO etc. At
442               2
reservoir water may lose its use for any purpose.
this stage the
Residence time of reservoir water:- The reservoir water is  replenished every 7.94 months. This period of time is very long for the reservoir formed in such geologic and vegetative region. Therefore a good mixing of the reservoir water is a must. Detailed analysis on impacts of water quality is given in Appendix 4.9.
6.4.2.4 Risks of Water-related Diseases
During construction period there will be job opportunities  attracting labor force from outside the area. This  new influx  will change the existing population structure in number, age, and health history. The main effects will be increased exposure of workers  and their families  to locally  endemic  diseases  such as malaria and onchocerciasis  through contact with insect vectors. This  situation might be exacerbated by in-migrant laborers and technical personnel who may
water works Design & Supervision Enterprise
79Lower Dldcsw Medium Seale Hydropower Project
RecoKwiiswtee Level Study
AltfUSt 2001
bring  non-endemic  diseases  to  the  area.  There  will  be  increased
risks  of  the
transmission  of  endemic  diseases  and  sexually  transmitted  diseas
es  including
AIDS, due to increased movement and interaction between the local community and the construction workforce.
Among water related vector-borne diseases, malaria and onchocerciasis are the main concern during the operation of the hydropower project. The risk of malaria will change from being seasonal to becoming perennial. The increase in incidence of the disease will result from improved breeding habitats offered by the reservoir and its shoreline vegetation. The dam will create an impoundment with shallow banks  at some localities. As  reservoir water draw-down shallow water points or pools can be formed, which certainly become favorable breeding sites for vectors that transmit the disease to the communities living around the reservoir. The communities  residing within 2kms  distance from mosquito sources/breeding sites  would be most affected. Away  from 2kms  the rate of transmission will reduce as  the human/vector contact is  reduced since only  a fraction of the mosquito population will reach the villages.
For onchocerciasis  the impoundment will have a positive effect due to the replacement of the fast flowing and vegetation-fringed river regime by a standing water body. However, downstream of the dam there would be favorable habitat for breeding of the vector due to almost constant and fast flowing river regime during most of the time.
Schistosomiasis  is  not prevalent in the project area at present. It does  not present the same risk for hydropower reservoirs as it does for irrigation, which brings man into intimate contact with water. There is no information about the presence of snail intermediate hosts, Biomphalaria or Bulinus spp. Were these intermediate hosts to penetrate the reservoirs, then the risk  of infection would increase. Infection would affect people who use the reservoir for fishery, recreation or as a source of domestic water. The greatest risk would be on the lee shore of the reservoir, especially if vegetation were allowed to grow there. Control measure will include excluding the human population from the vicinity of the reservoir, together with training in hygiene and health.
6.4.2.S Impacts on Landscape and Aesthetic Value
Major impacts  on landscape are associated with reservoir inundation, construction of the dam, excavations  for dam foundation and powerhouse, operation of borrow areas and quarries, and disposal of materials generated from the excavation activities. These activities  especially  the creation of a reservoir will bring a significant alteration in the river regime and natural landscape. The effect of this on visual amenity is often subjective. There are likely to be those who feel an artificial lake is a positive visual asset. Equally there could be those who hold that such alterations to the river landscape is unacceptable and
W/iter works Do'ijn & Supervision Enterprise
SOLower Didessa Medium scale Hydropower Project
Reconnaissance Level Study
AJ^USt 2001
considerably lowers the intrinsic value of the scenic feature of the area. One of the main impacts on the scenic feature of the area will be associated with the replacement of the dense woodland and bamboo thickets  along the Didessa River by the dam reservoir.
6.4.2.6  Slope Stability and Seepage Problems
Operation of the reservoir within the steep sided valley can pose sliding/slumping of side slopes. The abutments  of the Didessa river at the dam site are steep (sub-vertical angle), which form a narrow V-shaped valley geologically consisting of sedimentary rocks (with alternating layers of laminated shale, sandy siltstone and clay-stone). At the abutments the upper parts of the sedimentary rocks are weathered and can easily  slide down towards  the reservoir along the bedding plane due to the infiltration (resulting in saturation of the material) and the wave action of the reservoir water.
The reservoir area is  located on an impervious  high-grade metamorphic  rock, and the sedimentary  rocks  of the dam and powerhouse area appear to have been strongly tectonized by NW and NNW Fault System. There are also some fractures  parallel to the strike of the sediments  and dipping sub-vertical. The bedding plane dipping towards  the flow direction of the Didessa river and the associated fractures  can induce significant seepage. The linear features  can also cause seepage along the fault zone, which passes  across  the reservoir. Filling and draw-down of the reservoir within a steep sided valley, which is geologically susceptible, may induce slope instability problem in some part of the project area. However, the degree of this problem should be analyzed in further studies when more detail information is available.
6.4.2.7  Watershed Erosion and Sedimentation
With increasing pressure on natural resources  resulting in vegetation clearance and intensive cultivation, as  well as overgrazing, together with high intensity of rainfall in the catchment, the rate of soil erosion is expected to accelerate in the near future. Extension of cultivation into those areas where vegetation clearance was absent, especially the escarpment areas between the uplands and lowlands (mostly with sloping lands) could result in serious erosion. Even in the lowland part of the sub-basin particularly  in the vicinity  of the proposed reservoir (right bank) vast areas of woodland vegetation are being converted into farmlands and settlement areas  by  private business  firms  and small-holder peasants  including new settlers. This  extensive vegetation clearance could result in significant erosion. Close to the reservoir (on right bank), there are a number of large-scale farms, formerly under the Didessa State Farms, currently being privatized' some have started farming. Erosion from these farms is likely to be considerable
Water works Design & Supervision Enterprise
81Lower Dtdessa Medium Scale Hydropower project
Reconnaissance Level Study August 2001
A sediment measurement at the recording station on the Abbay River just east of the Sudanese border shows a sediment load of 19.46 t/ha/yr, which translates into 195t/ha/yr of erosion for the basin as  a whole applying the EHRS 10% estimated delivery rate to the rivers (BCEOM, 1998). The total sediment yield at the proposed dam site is estimated at about 9.1 Mt/yr. This high sediment yield may  result in the reduction of reservoir capacity, which jeopardizes  energy generation, and reduces  the economic  life of the structure. Therefore, it is necessary  to control the existing degrading factors  and implement soil conservation measures  particularly  in the significantly  degraded parts  of the catchment.
6.4.2.8 Erosion Induced by Project Activities
Construction activities will increase the risk of soil erosion and physical impacts on soil structure. Earthwork activities for roads, civil works, construction camps, etc. will remove and destruct the natural vegetation and topsoil particularly  on steeper slopes  increasing the potential for erosion. Implementation of the proposed HPP will require construction of about 20km new road from Kamashi to dam site (assuming the route through Kamashi would be used for the project) and about 40km road improvement (from Kamashi junction to Kamashi town), powerhouse and diversion tunnel. In addition, implementation of the projects will involve quarrying activities that would expose the soil to erosion.
Roads  are important contributor to soil erosion, primarily  because they concentrate and distribute runoff as channel flow rather than a uniform overland or subsurface flow. Roads also dissect fields and interrupt established drainage; at crossings  they  risk  disrupting peak  flows, accentuating backwater, and accelerating bank  erosion. At present the landscape of the project area is well protected by  vegetation cover. But, it will rapidly  degrade once the vegetation cover is removed. The degree of erosion is severe along the road to Kamashi (from Kamashi junction to Kamashi), which mostly  runs  through steep terrain characterized by highly erodible soils.
The erosion from the disturbed/exposed area could be significant and result in increased sediment loading of the river. In fact, the scale of these activities  is small in the context of the erosion potential of the total catchment, and there are no sensitive receptors in the downstream river, which are not already adapted to high sediment levels.
6.4.2.9 Air Quality Deterioration
Air quality  deterioration will be due to the generation of suspended particles during construction of access roads, dam, diversion tunnel and powerhouse, and excavations at borrow sites and quarries. In addition the transportation of
Miter Worlts Doyii & Supervision Enterprise
82Loircr Dftoi Medium Seu/e Hyz/ropmr Project
Reconnaissance Level Study August 2001
construction materials and machinery by trucks will affect the air quality due to their frequent movements  on unpaved roads  that generate polluting particles. These impacts will be localized and may cause significant health effects to site workers  and local people living around the project activities  mainly  due to inhalation of dust and exhaust fumes that may sensitize the respiratory tracts and lungs, unless adequate mitigation measures are taken.
For plants and animals, air pollution problems can cause physical and chemical stresses that may affect the organisms' physiology. In practice it is usually dust generation that is  the major problem, particularly  for plants  in the proximity  of roads.
6.4.2.10 Noise and Vibration
Excessive noise levels can occur from the operation of construction equipment, aggregate generating equipment, the concrete batch plant and blasting. During construction operations  both mobile and stationary  equipment such as bulldozers, graders, cranes, concrete mixers and air compressors can generate excessive noise, which is a nuisance to local communities and site workers. In addition, rock removal will be required for construction of diversion tunnel and powerhouse. Furthermore, rock  removal can be required for the production of construction aggregates. Blasting can cause both noise and vibration impacts as well as damage from fly rock.
6.4.2.11 Impacts on Mineral Resources
No significant mineral occurrences have been identified from the impact areas (dam & powerhouse area, reservoir area and access  road). Therefore, implementation of the proposed hydropower project will not bring major impacts on mineral resources.
6.4.3 Negative Impacts on Socio-Economic Environment
6.4.3.1 Displacement of People
In this assessment, it is considered that all villagers settled within a 5 km distance from Didessa and its tributaries will be affected. Accordingly, about 2,777 (448 household heads and 2329 family members) will be directly or indirectly affected. Of the total households covered in this assessment, 237 household heads settled in six villages (Sayi Jirma, Sarnie, Godere, Serikole, Kutamuri and Dalecha) with their family members (1,145) will be displaced because they are located within the reservoir and its close vicinity. The other villages (listed in the baseline section) will be affected indirectly mainly due to increased health hazards.
W/iirr vwrb Doi/jii & Supervision Enteiprise
83Liwcr Didessa Medium Scale Hydropower Project
RecoHiuitssaHce Level Study August 2001
In all villages, there were also 670 houses (659 thatched and 11 corrugated) and 2 Churches  (corrugated) with their cemeteries, one elementary  school, one developed water system and market places close to the reservoir area mainly along the tributaries of Didessa. About 353 (342 thatched and 11 corrugated ) houses, 2 churches, the school and developed water source are located in the directly  affected villages. As  per the information received from the area, one thatched house with an average size costs Birr 1,000 and one corrugated house costs Birr 5,000 on the average. The total value of all houses, which belong to those household heads  who will be displaced is  estimated ([342 houses*Birr 1,000]+[11 houses*Birr 5,000]]) to be Birr 397,000.
However, the displacement effect of this HPP would be lessen for the following major reasons: 1) The reservoir area is not densely populated and intensively cultivated area. 2) There is  a trend of moving out of settlers  to higher areas voluntarily. For example in Kamashi Woreda, the reconnaissance team witnessed that villagers from Sayi Jirma were moving out from the river bank, and were establishing new settlement site. 3) There is a resettlement program planned by the Regional Government to resettle people from remote areas and scattered settlements  to suitable areas  where social services  can be better provided and economic development enhanced. Although the implementation of the resettlement program is  not yet started, the Zonal officials  informed the reconnaissance team that the study was completed recently and implementation will follow soon. This is a good opportunity to the project because health hazards will be reduced and the costs and risks of relocating people will be assumed by the regional government. From socio-economic point of view, all these and other factors make the HPP more feasible and justifiable. What is expected from the project holder is to follow-up the implementation of the resettlement program and strengthen its  working relations  with the local authorities  before actual project implementation started. Therefore, the overall suggestion on this  HPP is  that there will not be much disturbance of human; and the negative impacts of the HPP will be manageable if project implemented in collaboration with the regional government and communities. The estimated number of people, fixed properties and area of land under various uses to be affected (directly and indirectly) by the proposed project is summarized in Appendix 4.8.
6.4.3.2 Losses of Croplands, Forest and Grazing areas
The proposed reservoir area along Didessa River and its  tributaries  is  mainly under forest, bushes and shrubs with limited land under cultivation. And, hence the envisaged project will not affect a significant area under different land use systems that may affect the livelihood of the villagers.
In all the 12 villages of the 9 PAs and two Woredas covered in this assessment, about 680 ha of grazing land and 1,277 ha of forest area used by the local
84Lw DideM Medium Sc«Ic Hydropower Project
RccoiwiiSMnce Level Study
Ajyust 2001
communities  will be inundated by  the reservoir. Cropland within the reservoir area is limited (150 ha) as most croplands are situated above the settlement sites in most of the villages.
The limited benefits such as grasses, poles, firewood, etc. derived from the 1,277 ha of forest area in all villages might be affected. In addition, traditional bee keeping is one activity performed in the forest area. Traditional bee keeping is an accepted activity, and hence most household heads in Bello Jiganfoy Woreda practice it. In all villages, there were about 850 beehives during the time of the assessment. According to the information received from the project area, Birr 34, 000 is generated from the traditional bee keeping activity, and this income will be affected. As  per the information collected from the project area, 672 ha of grazing (and over 2,101 livestock population depend for grazing on this area) area will be also affected. The benefit generated from the 150 ha of cropland will be also lost. Sorghum being the dominant crop in the area (sorghum occupies over 90% of the cropping pattern in most villages and it is assumed that the 150 ha is occupied by this crop) with an average yield of 15 qts/ha with a price of Birr 40/qt at the local market. The estimated benefit generated from the 150 ha (150 ha*15 qt/ha*Birr 40/qt), which is equivalent to Birr 90,000 will be lost.
6.4.3.3  Impediments to Movement of People and Livestock
The villagers in and around the proposed reservoir area cross Didessa and its tributaries using locally made wooden boats and on foot when the volume of the water decreases  during dry  season. All these communication systems  help connecting people from Kamashi with Bello Jiganfoy and vice versa. About 448 households, their family  members  and other people from different areas  use these communication systems to cross Didessa River and its tributaries. The negative effect of this  project on existing communication systems  will be damaging; it will disturb existing communication systems, disrupts  social, economic  and administrative relations  among the local population, and life becomes complicated and difficult to continue as it is now.
6.4.3.4  Losses of Religious Institutions and Social Services
There were two Churches  with their cemeteries, one elementary  school, two market places, one developed water source (located along Jirma and Didiga Rivers) and might be partly  affected by  the reservoir area. Apart from the spiritual value attached to the religious  institutions  and the social services rendered to the communities by these social institutions, their monetary value is high. According to the information collected from the area, one church costs Birr 9,500, and the estimated value of the churches is about (2 churches’Birr 9,500) Birr 19,000. Whereas  the estimated value of the school ranges  from' Birr 600,000-700,000 and the developed water source from Birr 40,000-50,000.
VV.itrr Works Desyn & Supervision Enterprise
85Lmr Didcssa Medinin scale Hydropower Project
6.4.3.5 Loss of Traditional Mining Sites
Reconnaissance Level Study Al^JUSt 2001
There is  limited mining practice along Sayi river. Some villagers  from the Kamashi side are involved in a traditional gold mining activity. The estimated income generated from the traditional mining is  Birr 20-30 /week/person. The reservoir area will partly inundate the traditional mining sites; thus it will affect this activity and the income derived from it.
6.4.3.6  Attitudes and Reactions of Local Communities to the Project
There is  no a feeling of insecurity  among all villagers  because the benefits generated from the proposed reservoir area are limited. In addition to this, most villagers are moving out from the reservoir area and its surrounding voluntarily so that there is no much concern except the health aspect associated with malaria. The proposed reservoir area serves  partly  as  forest and grazing area but not intensively  used. The limited benefits  generated from the forest area, grazing land and the effect on a few settlement sites can be mitigated by compensation and well-planned resettlement program. Apart from these, there will not be serious problem due to the project. The project will not affect the interest of the local communities. The only concern that was reflected by the local communities was  the disturbance of existing communication systems. In addition to the communications  disturbance, there are people also who generate income from the locally  made wooden boats  as  a sideline income. As  an option, most villagers suggested that as long as the project compensates for all benefits lost due to the project and minimize other negative impacts  (such as  disturbing existing communication systems, health issues, etc.) the local population would be happy and accept the proposed project.
6.5 Proposed Mitigation Measures
6.5.1 Bio-Physical Mitigation Measures
6.5.1.1  Terrestrial Ecology
Ecology  includes  the analysis  of biotic  communities  and their habitats  The ecological component of the initial impact assessment looks  at the potential impact of the project on terrestrial habitats  through project activities  includinq those previously  outlined. Measures  to minimize loss/degradation of terrestrial
habitats include:
Design Phase:
. Ensure the zones of disturbance avoid as far as possible ecoloaicallv 
sensitive habitats and/or wildlife migratory routes.
ecologically
Water works Design & Supervision Enterprise
86Lc’ivcr Dfdessa Medium scale Hydropower Project
RecoHMiSMitce Level Study
August 2001
Construction Phase:
•     Minimize  the  off-take  of  woody  biomass  for  buildings  and  fuel  by  the workforce    and    resettled    local    people;    prohibit    the    people    from unsustainably exploiting the local natural resource base.
•     Implement  replanting  program  in  order  to  rehabilitate  habitats  and/or  to provide alternative sources of wood for buildings and fuel.
•     Construction  site/compounds  should  undergo  restoration  after  project completion.
•    Woody biomass within reservoir area should be harvested and utilized for buildings of the resettled families, and fuel wood.
Operation Phase:
•     Ensure  the  conservation  of  the  forest  and  woodland  resources  in  the adjacent, upstream and downstream areas by establishing a buffer zone around  the  reservoir  where  planting  of  indigenous  tree  species  may  be implemented and human activities be restricted in the area.
•    Ensure that vegetation replanting programs are completed.
•    Minimize areas disturbed for operational purposes and employ formulated best practice guidelines.
•    Monitor for the spread of invasive alien species.
6.5.1.2 Aquatic Environment
Protecting river ecosystems  involves  maintaining the wide range of habitats associated with the river channel, the subterranean hyporheic zone and the river corridor or flood plain. To preserve and protect the aquatic ecosystems likely to be affected by the project and to maximize efficiency and longevity of the project the following measures are recommended:
Design Phase;
•    Ensure the zones of disturbance are minimized and avoid as far as possible ecologically sensitive habitats.
•   Attempt to minimize residence time for water in the reservoir to discourage aquatic weed growth.
•   Consider an integrated catchment management to minimize sedimentation in the reservoir.
•   Allow for a compensation flow release downstream during reservoir filling.
•    Provide for the setting up of a fish hatchery in the reservoir.
Miler Works Design & supervision Enterprise
87Lower DiSessa Medium scale Hydrojwver Project
Reconitatssaiice Level Study
AMJHSt 2001
Construction Phase:
•    Careful environmental management to avoid polluting watercourses.
•    Clear biomass in the inundation zone to avoid eutrophication problems,
and water quality deterioration.
•    Implement erosion protection and stabilization measures, e.g. replant
disturbed areas as soon as possible to prevent erosion.
•    Implement hydraulic removal of sediments during operation.
•   Allow minimum flow release during reservoir filling.
Operation Phase:
•    Careful environmental management to avoid polluting watercourses.
•    Management of the reservoir to minimize residence time for water in
the reservoir.
•      Make provision for staged release of water from the reservoir to
maintain natural water temperatures downstream.
•    Releases at any time should be regulated to avoid sudden changes in
water level downstream both to minimize the risk to people visiting or
crossing the river and to give animals and other aquatic fauna time to
adjust to the change in levels and flow velocities.
6.5.1.3 Public Health Issues
Of the potential adverse social and health impacts, some of the most serious are
the transmission of sexually transmitted diseases and water related vector-borne
diseases.  Mitigation  measures  for  controlling  these  diseases  and  other  health
problems include:
•    Relocate the population living within the reservoir area and its vicinity at
least those within 2kms radius;
•    Control wetland vegetation, possibly by recession grazing;
•    Mount an awareness campaign about hygiene and sexually transmitted
diseases  and their prevention methods  in advance of the construction work; Workers  and residents  alike should be sensitized to the risks, particularly of HIV/AIDS.
•    Provision of adequate health services, potable water supply and sanitary facility  to  the  project  workers  and  local  population;  Employment  of measures to reduce transmission of communicable diseases between the workforce and the local population;
•    Medical screening and treatment of workers coming from outside;
Waler works Desyn & Supervision Enterprise
88Lmr Didcssu Medium scale Hydropower project
Recounaissiuice Level Study August 2001
•    Establishment of a health center in the project area and adequate health services provided to the workers and locals;
•    Construction works do not create pools of water where insect vectors of disease especially mosquitoes may breed. Such pools should be drained as quickly as possible if near settlements.
•    Support local health offices and institutions to enhance their coordinating roles  and  upgrade  their  capacity  in  preventing  and  controlling  health problems.
6.5.1.4 Landscape and Aesthetic Value
The loss  of visual amenity  due to the project can be reduced with sensitive design, careful construction techniques  and effective rehabilitation measures particularly  in areas  other than the reservoir area. Some of the mitigation
measures that will reduce the adverse effects include:
Design Phase:
•     Appropriate  engineering  designs  and  materials  should  be  selected  to minimize the disturbance to the visual integrity and physical continuity of the landscape.
•    Vertical and horizontal alignment of new roads should follow the natural relief as closely as possible.
Construction Phase:
•   Sensitive siting of construction compounds and material sites.
•    Minimize clearance of vegetation for project infrastructure.
•     Utilize rock spoil as far as possible in the works and consider dumping excess into the reservoir area.
•     Restore  the  borrow  and  quarry  areas  as  closely  as  possible  to  natural contour after completion of the construction work taking consideration also the drainage point of views.
Operation Phase:
•     Replant  and  maintain  vegetation  to  restore  the  natural  appearance  of construction areas.
•     Maintain a re-vegetation program and discourage the unsustainable off take of woody biomass.
•    Maintain water quality in the reservoir to avoid eutrophication.
Wrtti’r work & Supervision Enterprise
89Loivfr Didessa Medium Scale Hydropower Project
6.5.1.5  Slope Stability
RecomaissMice Level Study Adjust 2001
Best  engineering  practice  should  identify  this  problem  at  the  time  of  site evaluation.
6.5.1.6  Watershed Erosion
Reducing land degradation and the sediment load reaching the river depends on controlling soil erosion on cultivated/grazing land and other disturbed areas, and stabilizing the catchment area. It is important to control erosion to maintain the live storage capacity  of the reservoir and to protect the generating machinery. Implementation of effective soil conservation and watershed management practices or measures are required in order to reduce the amount of sediment transported into the reservoir.
6.5.1.7  Erosion Induced by Project Activities
The design of the project should integrate measures that would help to reduce
erosion, including:
Design Phase:
•      Ensure  that  disturbance  and  vegetation  clearance  are  minimized  in erosion prone areas.
•     Make allowances for seasonal climatic variations particularly rainfall and adjust the program of work accordingly.
•     For  any.  road  construction  side  drains,  longitudinal  drains,  culverts  and appropriate angles of cut- and -fill should be incorporated to combat soil erosion and accommodate run-off.
Construction Phase:
•     Keep land clearing and disturbance to absolute minimum particularly in erosion prone areas such as steep and unstable slopes.
Construction Phase:
•    Erodible surfaces should be cut only during dry weather and re-planted as soon as possible.
•    Best practices should be made not to disturb subsoil and normal surface drainage system.
•     Layer  borrow  pits  with  soil  and  plant  trees  within  unless  required  to provide material for maintenance.
.  Execute  erosion  protection  measures  without  extra  delays  including  re filling borrow and quarry sites.
Wiitrr work Doyu & Supervision EHteiprise
90Lower Didcssn Medium Seale Hydropower Project
Reconnaissance Level Study
Aygust 2001
Operation Phase:
•     Maintain a re-planting program and control the
unsustainable off-take of
woody biomass.
•     Maintain  erosion  protection  measures  on  access  roads  and  discourage off-road driving.
6.5.1.8  Water and Drainage
Alteration to the natural drainage pattern of the area can create unhealthy pools of standing water, water-logging, increased sediment loading and water quality 
deterioration. Amelioration measures can include:
Design Phase:
•     Align  any  new  roads  and  buildings  to  avoid  disturbance  of  the  natural drainage pattern as far as possible.
•    Identify where contamination would be serious, lithological characteristics of the sub-surface rock should be determined. If not impermeable, it would certainly  transport  effluent  to  the  aquifers,  thus  where  it  is  porous  and permeable, no discharge should be permitted.
Construction Phase:
•      Provide   adequate   settling   facilities   for   particulate   matter   held   in suspension.
•     Storage  facilities  should  be  located  far  away  from  natural water  bodies and in a bounded enclosure with an impermeable liners in order to prevent accidental spillage of pollutants to surface and ground water.
Operation Phase:
•   Avoid conflicting with water demands for domestic and livestock consumption.
•      Ensure   that   disturbed   watercourses   are   re-engineered   to   prevent excessive erosion.
6.5.1.9  Air Quality
The proposed project is not expected to cause significant air pollution, except in
localized instances. Nevertheless best practices should be employed as follows:
Design Phase:
. Consider the use of vegetation in the design to filter particulate matter (subject to bio-physical constraints)
Writer Work Design & Supervision Enterprise
91lower Didessii Medium scale Hydropower Project
RecoivuiissMce Level Study
August 2001
Construction Phase:
•    Traffic speeds should be restricted to minimize dust and approved dust
suppressants including periodic application of water should be applied as 
required.
•    Dust collectors or water spray systems may be required as appropriate to
prevent high dust emissions from bath plant operations.
•     Diesel  engines  of  construction  equipment  should  be  subject  to  regular
checking and cleaning of the injectors to minimize emissions.
Operation Phase:
•      Access   roads   should   be   watered   periodically   to   minimize   dust   if
applicable.
6.5.1.10 Noise and Vibration
Construction equipment or machinery  will create noise significantly  above the
ambient levels. Negative impacts can be mitigated by:
Design Phase:
•     Make  provision  for  the  imposition  of  speed  restrictions  in  areas  where there are communities in close proximity to roads.
Construction Phase:
•     Consider limiting working hours in more environmentally sensitive areas such as settlements, recognized religious places etc.
•    Ensure construction vehicles are well maintained and noise suppression systems are operable.
•     Minimize  the  use  of  explosives  and  promote  a  systematic  blasting schedule.
Operation Phase:
•    Ensure construction vehicles are well maintained and noise suppression systems are operable.
Ensure that ancillary machinery is contained and silenced as far as possible.
W/ilt’r works Design & Supervision EnterpriseLower Didessa Medium scale Hydropower Project
Reconnaissance Level Study
August 2001
6.5.2  Socio-Economic Mitigation Measures
6.5.2.1  Resettlement
The envisaged project will displace 237 household heads  because of their proximity to the proposed reservoir area. As stated in this report, the regional government is planning to implement a re-settlement program in both Woredas including the potentially  affected villages. Close collaboration with local authorities is essential to facilitate the relocation of the affected households and implement the HPP smoothly  with minimum negative effects. However, the project holder should be able to carry out an intensive study on the situation of the population to be resettled in order to ensure that the settler population will be provided with resources and facilities they have now.
6.5.2.2  Losses of Fixed Assets and Land under Various Uses
The proposed reservoir area is  not either densely  populated or intensively cultivated. However, as stated above, some fixed assets and land under crop cultivation, grazing, settlement and forest will be lost due implementation of the HPP. Mitigation for the lost houses, religious and social service institutions, and benefits from crop, grazing and forestlands as well as incomes generated from bee keeping, may include:
•    Financial compensation for residential houses, religious and social service institutions, croplands, and incomes derived from the natural resources of the reservoir area based on a detailed assessment that will be carried out in the future;
•    Infrastructure development like improved water sources, health facilities, rural roads, etc.;
•    Enhancement of reservoir fishery development and extension services.
6.5.2.3  Losses of Communication Network
In the project area, there are communication systems  established by  the communities  themselves. Unfortunately, these systems  will be affected by  the HPP, and this  could be one source of conflict between the project and the communities. Due to the losses  of the river crossings  and foot paths, social, economic  and administrative relations  will be hampered. As  a mitigating measure, the project should provide alternative crossing points so that relations will continue without any problems.
Wfller Work* Design & Supervision Enterprise
93Ltwr Dldessn Medium Scale Hvdroyower Project
6.5.2.4 Participation of Communities and Local Authorities
Recomiatssance level Study Adjust 2001
The involvement of local people in the project will minimize the negative effect of the project because locally  tuned solutions  could be part of the proposed mitigation measures. By working closely with the communities, it is possible to build trust and resolve conflicts that may arise among community members and the project/project staff.
6.5.3 Preliminary Mitigation Cost
The preliminary mitigation cost estimates were based on data collected in this assessment and unit costs were adopted from recent hydropower project studies including the Gojeb & Tekeze Medium Hydropower Projects, Feasibility Study, and Geba & Baro Hydropower Project, pre-feasibility  study, and EEPCo. In addition, the environmental expert's judgment was applied to estimate some of the environmental costs. The total estimated environmental and socio-economic cost for Didessa HPP is 9,904,345 US Dollars. Details of the cost estimates are given in Appendix 5.
6.6 Conclusion
The construction and operation of the proposed hydroelectric power project will bring about many impacts (both positive and negative) to the natural and socio economic environment of the project area. The main advantage of the project would be generation of electric power, which can significantly enhance economic and social service development at local, regional and national level. Other advantages of the project will include provision of job opportunities for the local communities and in-migrant population, enhancement of fisheries development, and improvement of social and physical infrastructure.
Most of the significant negative impacts will result from the impoundment of water and alteration of water flow downstream. In particular flooding of the reservoir and alteration of the river regime would bring significant impacts on terrestrial environment (losses of natural vegetation and wildlife habitats, and changes in landscape), aquatic environment (deterioration of water quality, alteration of the river regime, and dam barrier to fish migration), and socio-economic environment (losses of the existing communication systems/river crossings and footpaths, and health hazards from increased prevalence of water related diseases).
Many of the negative environmental and socio-economic impacts are capable of being reduced to an acceptable level through implementation of mitigation and compensation measures. The direct environmental impacts associated with the construction of the dam can be reduced to acceptable level through a
Miter Wi>rh Design & supervision Enterprise
94Lower Didcssa Medium Scale Hydropower Project
Reconnaissance Level study August 2001
combination of “best practice" techniques  during construction, sensitive micro siting of project components and holistic environmental planning. The significant adverse impacts on aquatic ecology, which are associated with changes to the river regime, in the operational phase can be held to acceptable level given that minimum release measures are implemented.
In terms of population displacement, there is a trend that the local population is moving out from the proposed reservoir area and its vicinity to uplands or to nearby  major settlement areas, and the resettlement program planned by  the regional government if implemented makes  the HPP more feasible and justifiable. Therefore, the overall suggestion on this HPP is that there will not be much disturbance of human population, and the negative impacts of the HPP will be manageable if implemented in collaboration with the regional government and communities.
These adverse effects on socio-economic environment can be mitigated through a coherent and well-planned project planning and implementation process. Some of the suggested mitigating measures  include compensation payments, adequate collaboration with local authorities, initiation of infrastructure and agricultural development packages  worked out through participation with the communities and government offices interactively.
Water works Design & Supervision Enterprise
95Lmr Dtdessa Medium Scale Hydropower Project
Reconnaissance Level Study
August 2001
7.    COST ESTIMATE
The cost estimate for various civil works components of the project is based on
unit costs compiled for the year 2001. The main assumptions made during the
preparation of the estimate include those unit prices  and lump sums  that are deemed to include all contractors  direct and indirect costs, including lobar, equipment, material, freight, insurance, overhead and profits.
Estimation of costs  for the proposed new access  roads  for the projects  were made based on the International standard road type requirements  and approximated by current cost applied for the construction of one kilometer of the road section. The road standard and the estimated cost will be verified according to the national experience, at the pre-feasibility and feasibility stage.
The price estimation for mechanical equipment and accessories is based on the type and ratings (capacity) correlated to actual costs of currently on-going and recently  commissioned similar projects. An adjustment is  made to account for price escalation since the procurement of the equipment taken as  a reference datum. The prices are lump sum for a set of items in each stations since there is no quotation for an individual item.
The   reconnaissance   cost   estimates   for   electrical   equipment   is   based   cost information obtained from studies recently carried out in Ethiopia. Appendix- 5,
The total project cost comprises  of the sum of project capital cost and an allowance for contingencies. The contingencies represent an allowance required to cover the anticipated discrepancies  between the estimated cost and actual expenditure, over head cost by the client and engineering supervision costs. It is expressed as  a percentage of the project base cost. The cost summaries  for Lower Didessa are given in Table 7.1. Table 5.1 & Table 5.2 present’s unit rates and detail cost break down for civil work respectively.
Milrr Works Design & Supervision Enterprise
96Lower Didessa Medium Scale Hydroyower project
Table 7.1 Cost summery
Reconnaissance level study AUfflst 2001
SNo.
Description
Local (MUSD)
Foreign
(MUSD)
Total
Cost
(MUSD)
1
Main civil work
1.1
Blanket (upstream)
10.482
24.458
34.94
1.2
Diversion work
1.368
3.192
4.56
1.3
Embankment dam
24.321
56.749
81.07
1.4
Spillway
6.36
14.84
21.2
1.5
Waterway
0.545
1.272
1.817
1.6
Surge chamber
0.363
0.847
1.21
1.7
Powerhouse*
0.010
0.023
0.034
1.8
Hydro-mechanical works
2.559
14.501
17.06
1.9
Infrastructures
2.175
5.075
7.25
Total item No. 1
48.183
120.957
169.141
2
Electro-mechanical equipment
2.1
Mechanical works
1.964
37.321
39.285
2.2
Electrical works
1.774
33.706
35.48
Total item No. 2
3.738
71.027
74.765
3
Environment & socio-economic
5.744
1.436
7.18
Total item No. 3
5.744
1.436
7.18
Sub-total
57.665
193.420
251.086
4
Physical contingency(25% of 1)+(15%of 2+3)
13.468
41.109
54.577
Total cost
71.134
234.529
305.662
5
Management & supervision 8% of total
5.691
18.762
24.453
6
Miscellaneous  owner's  cost  2% of total
1.423
4.691
6.113
Grand total
78.247
257.982
336.229
(• Source, Tekeze medium HPP pre-feasibillty, HOWARD HUMPHREYS et al ,1996)
Wafer Work Dwi/pi <& Suyervision Enterprise
97Lower Dldessa Medium scale Hydropower Project
Rccom/ussancc Level Study
August 2001
8.      ECONOMIC ANALYSIS
The preliminary economic analysis of the Didessa hydropower project is carried out considering the contribution of its  economic  value from the hydropower project. The project is analyzed by comparing the benefit of the project with its different cost structures.
To show the best use of the resources, the investment is thoroughly examined by discounting all benefits and costs using the appropriate discount rate.
When doing the analysis the following basic assumptions and parameters are used.
Project life
50 years
Discount rate
10%
Construction period
5 years
SCF
0.9
Production to start on year
60
8.1     Basis of Analysis
8.1.1  Cost of the Project
Investment Cost
The investment cost of the Didessa hydropower includes costs of the power, environmental, and socioeconomic  mitigation. The investment costs  are converted in to economic values using the standard conversion factor of 0.9. The total investment cost is  327.72 MUSD for the power, environment and socioeconomic. The interest during construction (IDC) is  not included in the economic analysis due to that the discounting is started from the first year of construction period.
The cost on environment and socioeconomic  is  the secondary  cost that is incurred out side the project as  a consequence of implementation of the hydropower project. The cost is included in computation of the project cost as the mitigation of the environmental and socioeconomic effect.
Operation, Maintenance and Replacement Costs
The operation, maintenance and replacement costs  in the operation phase is considered in each year of the project life. This cost is estimated to be 2% (on average) of the total investment cost for the power, environment and socioeconomic mitigation measures.
Water Wi>rt> Dwirjn & Supervision Enterprise
98Lower Didessa Medium Scale Hydropower Project
Reconnaissance Level Study
August 2001
8.1.2 Benefit of the Project
The benefit of the Didessa hydropower project is derived from the power. Energy 
produced  for  sale  is  converted  in  to economic value by  the conversion factor,
which is 1.77. The total estimated energy to be generated is 1582 Gwh/yr. The
financial value estimated by  the hydropower engineer is  94.92. MUSD. This financial value is changed to economic value using the power conversion factor and it is 168 MUSD.
Secondary Benefits
There are some benefits that are created outside the project, which could not be obtained without the implementation of this project. These are benefits such as the creation of employment resulting from the hydropower to be implemented. This will create improvement of wealth for the new workers and contributes to the reduction of unemployment rate.
8.2    Economic Viability of the Project
By discounting the net incremental benefits and total costs, the economic viability analysis  of Didessa hydropower is  evaluated. The discounting measures  used are benefit cost ratio (B/C) obtained by dividing the sum of discounted net benefit to total sum of discounted cost, net present value (NPV) also found by deducting the total sum of the discounted cost from the total sum of discounted benefit and internal rate of return (IRR) is calculated using two discount factors by trial and error that will generate zero value of net present value from the discounted benefit and cost. The formula used to calculate IRR is:
R1+(R2-R1) NPV1/NPV1 + NPV2.
In which,       R1
R2 NPV1 NPV2
= the first (lower) discount rate
= the second (higher) discount rate
= net present value at lower discount rate
= net present value at higher discount rate in absolute value
The analysis result shows that benefit cost ratio of 3.3 at 10% discount rate, a net present value of 656.55 MUSD and an EIRR of 27%. The detail is on appendix 6.
8.3     Sensitivity Analysis of the Project
Project designing starts  by  establishing assumption and conditions  based on inputs, outputs, costs, prices and revenues. Since the assumptions will not hold true with certainty, the sensitivity analysis is important to be sure about some of the assumptions. Therefore this sensitivity analysis is done for Didessa
W/iti’r Works D«yn & Supervision Enterprise
99Lower Diikssa Medium scale Hyilrywcr project
RecoiowtoHce Level Study
AJIJJUSt 2001
hydropower project assuming that what will be the economic  viability  of the project if the price of out put drops by 20%. This sensitivity analysis showed that the project is still viable with a higher internal rate of return than the cut-off rate. The sensitivity internal rate of the project with a decrease of price by 20% is 23 % and it's B/C and ENPV is 2.6 and 482.671 MUSD, respectively.
Table 8. 1 Economic Analysis
COST(MUSD)
Benefit
Year
Investment Cost
Net cash flow
Power          Env & Sco            Sub total
O&MR             Total Cost
BP
1           32.04                        0.73              32.77
2             64.1                        1.45              65.55
3         112.16                         2.5            114.66
4           80.11                       1.81               81.92
5           32 04                       0.73              32.77
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
32.77
65.55
114.66
81.92
6 54                39.31
6.54                   6.54
6.54                  6.54
6.54                   6.54
6 54                   6.54
6.54                  6.54
6.54                  6 54
6.54                  6.54
6.54                   6.54
6.54                  6.54
6.54                   6.54
6.54                  6.54
6.54                  6.54
6.54                  6.54
6.54                  6.54
6.54                   6.54
6.54                   6.54
6.54                   6.54
6.54                   6 54
6.54                   6.54
6.54                  6.54
6.54                   6.54
6.54                  6.54
6.54                  6.54
6.54                  6.54
6.54                  6.54
6.54                  6 54
6.54                   6.54
6.54                   6.54
6.54                   6.54
6.54                   6.54
6.54                   6.54
-5860780
-38973646
-68245551
-49389877
-21456781
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161 46
168                 161 46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
168                 161 46
168                 161.46
168                 161.46
168                 161.46
168                 161.46
Writer Works Design & Si^ervisiofi Eiite^rise
100Li’ivcr Dldcssn Medium Scale Hydropower Project
ReeowMb&uux Level Shaly
August 2001
8.4     Conclusion
Because of the high benefit derived from the power, the cost benefit ratio, the net present value and the internal rate of return is also very high compared to the cut off rate (10%). According to the rule of economic  viability  evaluation, it is worthiness, since the EIRR is greater than the cut off rate.
Besides these tangible benefits, there are intangible benefits, which cannot be evaluated concretely that do have a time value. The contribution of the project to these intangible benefits such as better income distribution, regional integration and a better life for the population will increase the acceptance of the project to be materialized.
W/itrr work Design & Supervision Enterprise
101Lower Diiicssa Mi’diKiit Scale HjJriymr Project
9. COMPARISON WITH OTHER CANDIDATE PROJECTS
Reconnaissance Level Study August 2001
In order to compare a project with other projects the market to which each project sells its generated energy should be well known. In the present case the market can not be any thing other than the Interconnected System of EEPCO and any expansion thereof.
The TOR calls  for a market assessment for the potential generation from the power plant. Due to the nature of the type of the plant (large hydro) a regional consideration does not warrant proper evaluation of the plant. The magnitude of energy that can be generated at this plant by far exceeds the load that can be served in a regional context. This study has therefore concentrated on forecast related to the national grid and the potential expansion thereof.
The most recent load forecast is  prepared on a national basis  under the Ethiopian Power System Expansion Master Plan Project report (EPSEMP, prepared for EEPCO by ACRES March 2001 Draft). Two scenarios have been looked at, the 'moderate scenario' and the 'target scenario’. For the purpose of this study the target scenario has been adopted (the reason being, it is standard forecast and also with this forecast it is possible to consider even the extreme case). Table 9.1 depicts the result of the target forecast
Wilier Worb Desyn & Supervision Enterprise
102Lower Didcssa Medium Scale Hydropower project
Table 9.1 Target Scenario load forecast
Reconnaissance Level Study August 2001
Year
Net Generation
Required MWh.
Peak Demand
MW
2001
1,726,008
342
2002
1,849,177
367
2003
1,998,225
397
2004
2,144,079
425
2005
2,292,409
455
2006
2,459,675
488
2007
2,636,585
523
2008
2,828,431
561
2009
3,034,298
602
2010
3,265,463
648
2011
3,506,190
696
2012
3,766,063
747
2013
4,047,434
803
2014
4,346,601
862
2015
4,667,485
926
2016
5,010,851
994
2017
5,379,271
1,067
2018
5,755,439
1,146
2019
6,501,740
1,230
2020
6,663,268
1,322
2021
7,159,999
1,420
2022
7,698,838
1,527
2023
8,279,734
1,643
2024
8,905,098
1,767
2025
9,581,444
1,900
(Source EPSEMP, EEPCO 200l).
The Ethiopian Power System Expansion Master Plan project report indicates that after Tekeze HE Plant is  commissioned there remain six  potential hydropower schemes that are most promising. These are Aleltu East Stage I Aleltu West Chemoga-Yeda ( Stages  1 and 2) and Halele-Werabessa ( stages  1 and 2)' During the current study comparison of the above referenced plants with those of the Birbir and Didessa has  been carried out. The result of the comparison is presented here below Table 9.2.
Writer Work Design <& Supervision Enterprise
103Linvrr Didessa Medium Scale Hydropower Project
Table 9. 2 Comparison with Other Hydropower Projects
Reconnaissance Level Study Autist 2001
Description
Geba
Birbir A
Birbir R
Didessa
Aleltu
WEST
Chemoga- Yeda stage 1
Genale
2&3
Halele-
Werabessa
Stge 1
Total Net Capacity (MW)
154
97.4
467
301
265
118
338
96
Fixed Cost
Capital ($x 10 6)
A
Transmission ($ x 10 6)
A
Total ($ x 10 6) [b+c]
A
Total Proj. Cost adju. for
IDC
350
170
482
336
394
245
651.5
Unit Cost ($/kw) (without
IDC)
Included
Included
included
included
included
included
included
216.6
included
IDC Factor
359
170
482
336
394
245
651.5
216.6
C.R.F
2331
1745
1032
1116
1487
2076
1928
2256
Interim Replacement +lns.
0.1009
0.1009
0.1009
0.1009
0.1009
0.1009
0 1009
0.1009
Unit Cost ($/kw) (with IDC)
0.50%
0.50%
0.50%
0.50%
0.50%
0.50%
0.50%
0.50%
Fixed Ann. Capital
(S/kw*yr)
246.8
184.8
109.3
118.2
157.4
219.8
204
238.8
Fixed 0 & M (S/kw*yr)
6.0
6
6
6
6
6
6
6
Total Fixed Cost (S/Kw*yr)
252.8
190.8
115.3
124.2
163 4
225.8
210
244.8
Total Outage Rate
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
Outage Adjustment
1.02
1.02
1.02
1.02
1.02
1.02
1.02
1.02
Adj. Fixed Cost ($/kw*yr)
257.9
194.7
117.6
126.7
166.7
230.4
214.3
249.8
Variable Cost
0
0
0
0
0
0
0
0
Firm Energy Costs
Firm Energy (GWh).......
857
512
2459
1582
981
534
1752
377
Firm Energy Factor.........
0.64
0.6
0.6
0 60
0.42
0.52
0.59
0.45
Cts in USD/kWh (based on Total Fixed Cost)
4.5423
3.629
2.1889
2.3625
4.41
4.99
4.05
6.23
Cts in USD/kWh (based on Adjusted Fixed Cost)
4.6350
3.7031
2.2336
2.4107
4.5
5.09
4.13
6.36
From the results on the table one can conclude that the three projects, Birbir A, Birbir R and Didessa exhibit far more economic  merit than the previous  four projects. As can be seen from the table the range of unit energy costs is between 2.36 USC/KWh in the case of Didessa and 2.19 USC/KWh in the case of Birbir R. Actually with the combined effect of Birbir A and Birbir R the picture would even be better.
It is understandable that the cost estimate is perhaps the single most important variable in the validity  of the assertion made above. However, even if one assumes that the cost estimate is 1.5 times the current estimate for the Lower Didessa project the result would still be better than for the five projects
vvakr Warlt5 Dui/pi & supervision Enterprise
104Lower Didessa Medium scale Hydropower Project
Reconnaissance Level Study August 2001
The target forecast prepared under the EPSEMP project suggests that capacity addition would not be required from the time Tekeze HE plant materializes up to the year 2013. This conclusion of the EPSEMP is reached without considering regional interconnections  (such as  Ethiopia-Sudan and Ethiopia-Djibouti) which have currently gained impetus. Moreover forecasts of long-term period are less reliable for the latter part of the forecast than short-term forecasts.
Even with EPSEMP's  assumption, the 12 years  of seemingly  a period of relaxation does not exist in that the finance seeking, tendering, hiring and going through each phase from pre-feasibility  to design and tender document preparation alone would take anywhere between six to eight years depending on ease of financing. If one includes the construction period, about 5 years for the size of the projects in question, the total lead time before commissioning any one promising power plant is in the order of 12 to 13 years.
The EPSEMP study estimates that for the least-cost long term expansion plan the economic Long Run Marginal Cost (LRMC) of energy is in the order of 5.5 USC/KWh. By comparison the unit cost of energy in the case of the Didessa, Birbir A and Birbir R projects is well below this value.
Wrtter works Desigt & Supervision Enterprise
105Lower Ditert Medium scale Hydropower Project
10. CONCLUSION AND RECOMMENDATION
10.1 General Conclusion
RccoitwfssHHce level study Adjust 2001
In this study it was tried to assess the hydropower potential site on the Didessa River in reconnaissance level.
Detail cost estimate was  worked out this  project, by  categorizing various components of the project as civil, mechanical and electrical. The total project cost is estimated as 336 MUSD where as the annual energy production of the project is 1582 GWh. The result of economical analysis shows that the unit cost of energy for Didessa is 0.0236 USD/KWh. Comparisons of this project with other previously studied potential projects are also performed. The result indicates that, Didessa project is more attractive than the previous potential projects even the cost estimate for the project under study is increased by 50%.
Based on the information obtained from the reconnaissance study described in this report, it can be concluded that the increasing power and energy requirement in the national grid can be partially met by constructing a power plant on Didessa River.
As it is discussed in chapter 9, additional power plant is not required until year 2013, however it takes more than 12 years to put in to operation the project still under planning stage. Therefore, the overall conclusion from the study is that on technical, environmental and economic ground this project is promising. Hence this project is recommended for further study in pre-feasibility level after detail geological investigation has been done properly.
10.2 Recommendation
Hydrology
A few gauging stations have been visited in this study. Full report on the visits is given in Appendix 1.3. The recommendations given below are based on the field visits to these gauging stations.
a) Rehabilitation of gauging stations
1) Didessa near Arjo: with a catchment area of about 10000 sq. km, this station is the most important of all stations within the Didessa basin. The station is located just upstream of the Bridge on the road from Nekemte to Bedelle. Major recommendations of earlier visits in 1995 pertain to the rehabilitation of the staff gauges, and the maintenance of the river bank so that the bridge and the structure supporting the overhead cable way remain stable. The
Wrtler Wurfc Doyii <& Siy/crvbifH Enterprise
106Lower Didessa Medium Scale Hydropower project
Reconnaissance Level Study August 2001
foundation of the right support to the OCW under threat. A small water course just by  the side of the support carrying seasonal runoff has  eroded the overburden soil and exposed one of the footings. It appears  that no rehabilitation work  has  been done since the previous  visits  in 1995. This should be rehabilitated as soon as possible.
2)  Dabana  near Abasina: this station has been  established  in 1960. It has a catchment area of about 2900 sq. km. The Overhead Cable Way including its supports is in good condition. However, the third element of the sectional staff gauge is in bad condition. The staff gauge lies below the surrounding ground level and has rusted throughout its height. The observer has dug a trench so that  water  can  easily  reach  the  staff  gauge  when  it  is  above  the  highest reading  of  the  2nd   element.  However,  this  is  not  satisfactory  and  the  staff gauge should be relocated.
b) New gauging station
The gauging station near Abasina and Arjo have measure the flow from about 70 per cent of the catchment area of the dam site. Hence, a good proportion of the total catchment area is not gauged and it recommended to plan a new station closer to the dam site. The recommendation made in earlier study (MoWR, 1999) to install a gauging station a short distance downstream of the confluence of Dabana with Didessa has not been found useful for this purpose, as this gauging station could increase the area upstream to about 80 per cent of the total area up to the dam site.
c) Sediment sampling
Although the station Didessa near Arjo is the most important of all in the basin, no sediment-rating curve is  available for this  station. Moreover, the record of stream flow in the river shows  the longest gaps  in all gauging stations. This shows that recording is carried out intermittently. This should be improved and the gauging station be given important consideration, both from streamflow as well as sediment gauging points of view.
Geology
Lineaments interpreted as faults, should be verified, by detail geological mapping and geophysical investigation.
Subsurface  geotechnical  and  geophysical  investigations  should  be  carried  out along the Dam axis for sound foundation.
water Work Doyn & SiijKrvbw Extr^riie
107Lower DidtssA Medium scale Hydropower Project
Reconnaissance Level Study
AUflllSt 2001
Quantitative and qualitative investigation should be carried out on the existing construction material. The investigation include, pitting Trenching, Sampling and Engineering tests.
Engineering
i)
Ahead of any further study, detail geological survey has to be done around
the dam site and reservoir area.
ii)
According to this level of study the project looks attractive economically so
it is recommended for further study on pre-feasibility level.
Environment & Socio-economy
Major recommendations include:
i)
A detailed survey of the flora and fauna of the anticipated impact zones to
explore the presence or absence of any rare or endemic species.
ii)
A limnological study to determine likely impacts of the reservoir and dam
release  patterns  on  the  aquatic  ecology  of  the  new  reservoir  and  the
downstream river reach. The potential for reservoir fishery development
should also be studied by a fishery specialist.
iii)       A survey to identify the type of disease vectors and the potential breeding sites  especially  in  the  draw-down  area  of  the  reservoir  and  develop appropriate  mitigating  measures  that  will  be  incorporated  in  the  design and implemented during construction and operation phases.
iv)       A detailed socio-economic assessment should be carried out to determine the  actual  number  of  displaced  people.  Since  relocating  people  is  the most  difficult  task,  the  project  holder  in  collaboration  with  concerned regional government offices should conduct a thorough  assessment on how, where, who and when to relocate the people, and prepare a detailed resettlement plan for the communities to be relocated.
v)       A reasonable compensation payment (based on a detailed house to house inventory) should be paid for all benefits lost due to the project. Based on a detailed assessment again, the project holder should provide adequate social services such as potable water supply, medical services, etc. for those who may remain near to the proposed reservoir area.
vi)        The  existing  communication  systems  are  highly  useful  for  the  local population.  Disturbing  these  systems  means  making  life  more  difficult, which may lead to developing a negative attitude towards the envisaged
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project. The project should consider therefore options for crossing points while designing and implementing the HPP.
vii)        Community  trust  is  the  most  important  precondition  for  a  successful implementation  of  the  HPP.  The  involvement  of  local  communities  and concerned local authorities - from the initial period of the project planning - is therefore fundamental to create this trust. Although the local population feels   less   secure,   it   is   strongly   recommended   that   the   concerned stakeholders  should  participate  interactively  in  the  process  of  project planning and implementation from the very beginning.
viii)       The  incidence  of  water-related  vector-borne  disease  will  considerably increase   with  implementation  of   the   HPP.  This  requires  coordinated health  interventions  between  different  actors.  The  adverse  effect  of  the project  has  to  be  controlled  through  coordinated  health  interventions.  In addition, health education should be provided to community members so that their awareness on preventive and curative measure will increase.
ix)        Feedback  mechanisms  should  be  systematically  incorporated  into  the feasibility  and  design  phases  to  ensure  that  engineering  and  technical solutions are developed in response to and in phase with environmental assessment.
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REFERENCES
Hydrology
-    Brune, G. M. (1953). Trap Efficiency of Reservoirs. Trans Am. Geophys. Union 34 (3).
-    Ethiopian National Energy Committee, ENEC-CESEN (1986): Supplementary Report 3: Hydrometerorologica! Database.
-    Gebeyehu, Admassu (1989): Regional Flood Frequency Analysis, Bulletin No. TRITA -VBI-148, The Royal Institute of Technology, Stockholm, Sweden.
-    Haan, C. T. (1977): Statistical Methods in Hydrology. The Iowa Sate University Press, Ames, Iowa 50010.
-    Shaw E. M. (1994): Hydrology in Practice. Chapman and Hall, London
-    USBR (1964): Land and Water Resources of the Blue Nile Basin, Ethiopia.
-    Appendix Ill-Hydrology.
-    WMO (1994): Guide to Hydrological Practices. Geneva, Switzerland.
Geology
-    Building Code for Ethiopia (1974) revised draft, part E, in pierre Gouin (1976), seismic zoning in Ethiopia, Bulletin of the Geophysical observatory of Aau, No. 17, 46 pp.Addis Ababa, Ethiopia.
-    Kazmin V., 1972, Geological map of Ethiopia (compile)
1:2,000,000 scale EIGS, Addis Ababa
-    Mengesha T. etal, 1996, Geological map of Ethiopia (Compile)
1:2,000,000 scale, EIGS, Addis Ababa
-    Mengesha T., and Seife M. Berhe (1987), Geological map of the Gore Sheet, 1:250,000 scale, EIGS, Addis Ababa
-    Meria, etal, 1973, Geological map of Ethiopia and Somalia,
1:2,000,000 scale EIGS, Addis Ababa
-    Pierre Gouin, 1976, Seismic Zoning in Ethiopia, Bulletin of the Geophysical observatory of Addis Ababa University, Ethiopia, No. 17, 46 pp Addis Ababa.
-    Pierre Gouin 1979, Earthquake History of Ethiopia and the Horn of Africa, IDRC, Ottawa, Canada, 258 pp, Addis Ababa.
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110Lower Dife/i Medium Scale Hydropower Project
Engineering
Reconnaissance Level Study August 2001
-     Dandekar, M.M and Sherma,  K.N,  1979,  Water  Power  Engineering, Vikas Publishing House Pvt. Ltd.
-     Ethiopian  Electric  Power  Corporation,  Ethiopian  Power  System  Expansion Master Plan Project report, ACRES March 2001.
-    Ministry of water resources, 1998, Abbay River Basin Integrated Development Master Plan Project, Phase 2, "Vol.VI, Water resources Development Part 2- Large dams & Hydropower Schemes".
-     Ministry  of  Water  resources,  Tekeze  Medium  Hydropower  Project,  Pre feasibility study, Main report Volume 2, Nov. 1996.
-       Norwegian   Institute   of   Technology,   Jarole   Ravn   1992,   Hydropower Development  Volume  No.  5,  "Planning  and  Implementation  of  Hydropower projects".
-     Norwegian  Institute  of  Technology,  Ola  Gunner  (1998),  Lecture  Notes  on Technical and Economic analysis in Hydropower planning.
-    Rozgar Baban, 1995, Design of Diversion weirs " Small scale Irrigation in Hot climates".
-    Santosh Kumar Garg, 1999, Irrigation Engineering and Hydraulic Structures, Khanna Publishers, Delhi.
-    USBR (1964), land and water resources of the Blue Nile Basin
-     USBR,  1979,  Design  of  Small  Dams,  United  States  Government  Printing Office, Washington.
-    Ven Te Chow (1986), Open Channel Hydraulics, Mc.Graw-Hill International editions, Civil Engineering service.
-    Water Resources Development Authority, Medium Scale Hydropower Plants Project, Tekeze Hydroelectric Reconnaissance Study, Main report, Aug
1995.
Environment
-      Anonymous   (1992).   Ethiopia:   National   Report   on   Environment   and Development. A Report prepared for the United Nations on Environment and Development, Rio de Janeiro, Brazil. Addis Ababa, Ethiopia.
-    Azene Bekele-Tesemma, et al (1993). Useful Trees and Shrubs for Ethiopia. Identification,  Propagation  and  Management  for  Agricultural  and  Pastoral Communities.
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111Lower Didessa Medium Sm/c Hydropower project
Reconnaissance Level Study August 2001
-     BCEOM,  BRGM  &  ISL  (1998).  Abbay  River  Basin  Integrated  Development Master Plan Project. Phase 2. Data Collection-Site Investigation, Survey and Analysis. Section ll-Sectoral Studies. Volume XIII. ENVIRONMENT.
-    Central Statistical Authority (2000). STATISTICAL ABSTRACT - 1999.
-     Ensermu K., et al (1992). Some Threatened Endemic Plants of Ethiopia. In Botany 2000: East and Central Africa. NAPRECA Monograph 2: 35-52.
-      Howard  Humphreys,  et  al  (1997).  Gojeb  Medium  Hydropower  Project. Feasibility  Study.  Volume  5.  Annex  E-Environmental  Impact  Assessment (Draft).
-     Mesfin Tadesse (1992). A Survey of the Evergreen Forests of Ethiopia. In: The Status of Some Plant Resources in parts of Tropical Africa. Botany 2000: East and Central Africa. NAPRECA Monograph No. Series 2.
-    NORPLAN A.S., Et Al (1998). Baro Medium Scale Hydropower Project, Pre Feasibility Study. Volume 5. INITIAL ENVIRONMENTAL ASSESSMENT.
-    NORPLAN A.S., Et Al (1998). Geba Medium Scale Hydropower Project, Pre Feasibility Study. Volume 5. INITIAL ENVIRONMENTAL ASSESSMENT.
-     Planning  and  Economic  Development  Bureau  (2000).  Physical  and  Socio economic Profile of 180 Districts of Oromiya Region. March, 2000, Finefinne.
-    USBR (1964): Land and Water Resources of the Blue Nile Basin, Ethiopia.
-    Water Works Design & Supervision Enterprise (2000). Jemma and Dawa Hydropower Projects - Reconnaissance Level Study. Annex VI - JEMMA PRELIMINARY ENVIRONMENT IMPACT ASSESSMENT (Final Report)
-    Water Works Design & Supervision Enterprise (2000). Jemma and Dawa Hydropower Projects - Reconnaissance Level Study. Annex VI - DAWA PRELIMINARY ENVIRONMENT IMPACT ASSESSMENT (Final Report).
Economic Analysis
-    Audace I. Knshahu, 1996, Planning and Implementing Sustainable Projects in Developing Countries: Theory, Practice and Economics.
-     FAO,  1986.  Guide for Training in the Formulation  of  Agricultural and rural Investment Projects.
-    J. Prize Gittinger, 1982. Economics Analysis of Agricultural Projects, second edition.
-     Water  Works  Design  and  Supervision  Enterprise,  2000,  Dawa  Hydropower Project Reconnaissance level Study.
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Summery

Lower Didessa Hydropower Project Summary

Lower Didessa Medium Scale Hydropower Project - Reconnaissance Study

Project Overview

The Lower Didessa Medium Scale Hydropower Project is located on the Didessa River in Kamashi Zone of the Benishangul-Gumuz Regional State, Ethiopia. The project aims to harness Ethiopia's significant hydropower potential, with only about 1% currently utilized.

        Key Project Details
        Location: 09°29'00" N Latitude, 35°58'30" E Longitude
Catchment Area: 18,200 km²
Mean Annual Flow: 7,290 MCM
Proposed Dam Type: Rockfill dam (selected after screening alternatives)
Installed Capacity: 301 MW
Firm Energy: 1,582 GWh/year

    

Technical Components

Hydrology

The Didessa River is a major tributary of the Blue Nile with significant water resources:

Parameter	Value
10,000-year flood	4,224 m³/s
Mean annual evaporation	1,450 mm
Estimated sediment yield	9.1 Mt/yr (500 t/km²/yr)

Geology

The dam site is located on Paleozoic sedimentary rocks with potential seepage issues due to bedding planes dipping towards the flow direction. The area has minor seismic risk with predicted ground acceleration of 2.2%g for 100-year recurrence.

Project Layout

Dam: 160m height, 1,050m crest length, 7.65 MCM volume
Reservoir: 3,531 MCM total storage (3,093 MCM live storage)
Spillway: 122m long ogee type with 4,224 m³/s capacity
Powerhouse: Surface structure with 4 vertical shaft Francis turbines
Transmission: 230kV line to Ghedo substation (128km)

Environmental and Socioeconomic Aspects

Positive Impacts

Electric power generation supporting economic development
Employment opportunities during construction and operation
Potential for reservoir fishery development
Flood control benefits
Infrastructure development (roads, utilities)

Negative Impacts and Mitigation

Impact Area	Potential Impact	Proposed Mitigation
Terrestrial Ecology	Loss of 6,400ha woodland habitat	Vegetation clearance before flooding, wildlife management
Aquatic Ecology	Disruption of river ecosystem	Maintenance of minimum flows, fish passage studies
Human Displacement	2,777 people potentially affected	Resettlement program in collaboration with regional government
Public Health	Increased malaria risk	Vector control programs, health education

Economic Analysis

The project was found economically viable with sensitivity analysis showing robustness to cost and benefit variations. The estimated annual benefit from firm energy alone is approximately $94.92 million.

Conclusion and Recommendations

The reconnaissance study confirms the technical and economic feasibility of the Lower Didessa Hydropower Project. Key recommendations include:

Proceed with more detailed feasibility studies
Conduct comprehensive environmental impact assessment
Address geological concerns regarding seepage
Develop detailed resettlement action plan
Implement watershed management to control sedimentation