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FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
LOWER DIDESSA MEDIUM HYDROPOWER PROJECT
RECONNAISSANCE STUDY
EXECUTIVE SUMMARY
Wafer Works Design and
Supervision Enterprise P.O.Box 2561
Addis Ababa
August 2001
Addis AbabaVOLUME II
VOLUME III
VOLUME IV
MAIN REPORT APPENDICES DRAWINGSDidessa Medium Seale Hydropower Project
Reconnaissance Level Study
August 2001
Table of contents
1.    INTRODUCTION................................................................................................................... 1
2.     HYDROLOGY......................................................................................................................... 2
3.     GEOLOGY............................................................................................................................... 5
3.1     Regional Geology and Physiography of the Didessa River Basin........................ 5
3.2       Geology and Structure of the Dam site, Powerhouse and Reservoir Area....6
3.3       Slope Stability of the Dam Site...................................................................................8
3.4       Construction Materials Availability..................................................................... 8
3.5       Seismicity of the Didessa Basin.................................................................................. 9
3.6       Conclusion & Recommendation................................................................................. 9
4.     PROJECT LAYOUT............................................................................................................ 11
5.     INITIAL ENVIRONMENTAL IMPACT ASSESSMENT FOR DIDESSA HPP............ 13
5.1       Introduction.................................................................................................................13
5.2       Description Of The Baseline Condition.................................................................. 13
5.3       Potential Impacts........................................................................................................ 15
5.3. I Positive Impacts.............................................................................................................. 15
5.3.2       Negative Bio-Physical Impacts................................................................................ 15
5.3.3      Negative Socio-Economic Impacts.............................................................................17
5.4       Conclusion And Recommendation............................................................................18
waiter works dwijii & supervision EnterpriseDidessa Medium Scale Hydropower Project
Reconnaissance Level Study August 2001
List of Tables
Table 1.1*: Runoff Availability (Didessa)............................................................................ 3
Table 1.2 Flood estimates for dam site (Didessa).............................................................. 3
Table 1.3 *: Net evaporation losses from reservoir surface................................................ 4
Table 4.1 Technical Features of Lower Didessa.............................................................. 12Didessa Medium Scale Hydropower Project
1.     INTRODUCTION
Reconnaissance Level Study
Adjust 2001
Ethiopia is bordered by Eritrea to the North, Sudan to the west, Djibouti & Somalia and Kenya to the South. The country  is  very  rich in its  water resources  potential even though the resource has  not been exploited adequately  so far regarding hydropower potential only 1% has been used.
Lower Didessa is one such a project, which is taken up for reconnaissance study. The Didessa River originates around Jemma area at maximum elevation of 2620m.a.s.I and suffers a drop of 1620m at the dam site area at elevation around 1000m.a.s.I. and it is located on Didessa river in Kamashi zone of the Benishangul-Gumuz regional state; specifically  in Kemashi (left bank) and Belo-Jigonfoy  (right bank) woredas. Geographical location of the site is  at 09°29'11-15N' latitude and 35°58’55”-59"E longitude that is shown in Figure 1.
Objective of this Study
The objective of this study has been to provide in general terms a review of earlier studies and the identification and further study of potential hydropower sites.
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1LEGEND
ERITREA
-16*
SELECTED TOWNS
NATIONAL CAPITAL INTERNATIONAL BOUNDARIES
%
RIVER
PROJECT AREA
-14’
SUDAN
-1?*
AM ARA
Lower DkJdesso
,ASOSA
\ H&RAR
(JuZlVAr
SOMALI
SOMALIA
SUDAN
_ I KENYA J] UGANDA^ S
LOCATION MAP
KILOHC'ERS 0 HILES 0DiSessa Medium Scale Hydropower Project
2.    HYDROLOGY
Reconnaissance Level Study
AUtflSt 2001
The storage site of Lower Didessa has  a drainage area of about 18200 km2. The altitude in the catchment upstream of the site ranges from about 2600 m around the drainage divide and at selected spots within the basin to about 1000 m.
The climate within the basin is largely a function of altitude and falls mainly within the category of 'Kolla', where the average annual temperature ranges from 20°C to 28 C and 'Weina-Dega', where the average annual temperature ranges from 16°C to 20°C. The area of the basin to the left and right of the main river Didessa shows the 'Kolla' climate, whereas  most of the basin falls  under 'Weina-Dega' climatic  classification. Very small areas within the basin can be classified as 'Dega'. These are areas lying at altitudes above 2400 m.
The temporal rainfall pattern in the catchment shows  a single peak  around July and August with no distinction between the smaller and the main rain season, which is prevalent in some parts of Ethiopia. The rain period extends from May to October. The mean monthly rainfall shows its maximum in July, with June and August following. The mean annual rainfall is about 1600 mm in the catchment.
There are quite a few stream flow gauging stations  in the basin but the lower-most
2
gauging on Didessa river is that near Arjo and commands an area of about 9980 km . There is one gauging station on Dabena river near Abasina (A = 2880), which is the major tributary to Didessa upstream of the project site. The two stations thus gauge a
total area of about
12860 km , which
2                    is about 70 per cent of the catchment area up to
the dam site. Flow at the dam site has been estimated based on the measurements of stream flow at these two gauging stations.
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Following the temporal pattern of rainfall distribution, the stream flow also shows the same variation in time. Mean monthly flows in Didessa River measured at Arjo show that more than 90 per cent of the flow occurs from June to November. Besides, the stream flow shows  big variability. The minimum and maximum mean monthly  runoff values exhibited are about 20 MCM (for March) and abovelOOO MCM (for August).
Monthly flows at the project have been generated based on the catchment area ratios and spatial rainfall distribution. In addition, flow generated for the project sites  in previous  studies  have been reviewed, adjusted, and extended. Based on the generated flows, the mean annual runoff together with runoff at various probability of occurrences have been estimated as given in the Table *.1.1
Table 1.1 *: Runoff Availability (Didessa)
Mean Annual
Flow (MCM)
Probability of excedence, %
75
80
90
95
7290.22
6300
6100
5230
4600
Observed momentary flood peaks in Didessa near Arjo exhibit values that range from
3
3
about 1200 m /s to 600 m /s. These values have been used to determine design floods at various  levels  of probability  using the Regional Flood Frequency  approach. The floods at different return periods are given in Table *.1.2.
In order to get the momentary flood peaks the flood peak factor of 1.3-1.5 has been used.
Table 1.2 *.: Flood estimates for dam site (Didessa)
10
Flood
3
oeaks of return period (in years), m /s
20
100
1000
1977
2210
10000
2735
3480
4224
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Net water loss  due to evaporation from reservoir surface is  needed in reservoir operation studies. After the reservoir is  filled the net evaporation loss  equals  the difference between rainfall and lake evaporation. Based on the potential evaporation obtained for the project sites, lake evaporation was  estimated by  introducing a pan coefficient of about 0.7. The latter is then used together with mean monthly rainfall at project sites to estimate net evaporation losses computed, Table *.1.3.
Table 1.3 *: 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
The estimation of annual sediment load was based on the measured sediment rating curves for the catchment. These rating curves have been analysed and the parameters were determined for the project site using regression analysis.
Based on the above the average sediment yield of the catchment at the dam site was 
2
obtained as 184 t/yr/km . Reported values of
specific sediment yield in the Blue Nile
basin range from values less than 100 t/yr/km2 to values above 600 t/yr/km. This region of the country  is  still less  degraded and the vegetation cover is  in good condition. Hence, one cannot expect high erosion rate. However, the situation may not continue as before as more and more settlements are established that lead to deforestation and thereby enhancing erosion. In the estimation of sediment transport rates for the project sites  recourse has  also been made to data obtained at nearby  research stations. Accordingly a value of 500 t/km /yr has been adopted to estimate the sediment delivery rate at the project sites. This brings the total sedimentation rate at about 9.1Mt/yr for the storage site at Didessa. It should be emphasised at this point that future refinement of sedimentation estimates  should be done based on actual measurement in the same catchment.
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3.       GEOLOGY
3.1     Regional Geology and Physiography of the Didessa River Basin
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-volcanics  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 Didesa basin and the proposed dam site (DD-2) area geology is assumed as Precambrian metamorphic rock, which is banded and contorted gneiss. The Abay 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
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developed within the Didesa river basin, river basin, which resulted thurst-folding; shear zones and brittle-ductile strike-slip faults/shear zones
The Didesa 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 (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.
3.2  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 (^Icm thick) with alternating light gray and yellowish gray fine layers similar carbonaceous shale. The mineralogical
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.
According to Tadesse and Tsegaye (2000), Three deformational stages (D, to D ) are
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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 Didessa 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 the Paleozoic  sediments  and mainly  on relatively impervious high-grade metamorphic rock. The major rock units outcropped in the reservoir area is  migmatitic  biotite and biotite hornblende gneiss, mineralogicaly 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.
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Quaternary alluvial deposit consisting of silt to gravel size fraction of the reservoir area rock products are deposited along the banks of the Didessa and Sayi river which are a good source of sand for construction purpose.
3.3    Slope Stability of the Dam Site
The abutments of the Didessa River on which the Dam is supposed to rest have steep slope (sub-vertical angle), which 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.
3.4     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.
The Tertiary volcanic rock, basalt and the Precambrian metamorphic rock, migmatitic biotite and hornblende gneiss are important rock resource. 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 the production of gravel, however, they can be useful for masonry work and rock fill. The alluvial deposit along the Didessa and Sayi river banks is free of clay and composed of mainly quartz, minor feldspar micas in silt to sand size fraction. It can be a sand resource for concrete work  and filter material requirement. Clay  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 the original rockDidessa Medium Scale Hydropower Project
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fabrics.  In  hand  specimen  test  it  is  plastic  and  important  resource  for  clay requirement.
3.5     Seismicity of the Didessa Basin
The Didessa river Dam Site and Reservoir area are located within 09°10'-09°35 and 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.
Based on a 100 years recurrence period, Pierre Couin (1976) has predicted the ground acceleration for the Didessa dam site and reservoir area to be between 2 and 3%g). The predicated earthquake intensity for the same year is V to V1 MM and the seismic risk zone belongs to zone 1, which is a zone of minor damage.
3.6    Conclusion & Recommendation
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 included 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.
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Construction materials are available at a reasonable hauling distance.
It is recommended that 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.
Quantitative and qualitative investigation should be earned out on the existing construction material. The investigation includes, pitting Trenching, Sampling and Engineering tests.
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4.       PROJECT LAYOUT
4.1      This section has been prepared in the base of hydrology, topography survey and geology report. Depending on those reports the initial screening was prepared and rockfill dam was selected.
4.2      Description of Recommended Scheme
The general layout of the recommended scheme has been shown in Figure 2. This scheme envisages construction of a rockfill dam on Didessa River at down stream with utilizing head of about 133m and generating 1582 Gwh/yr.
The project envisages construction of 160m high dam on Didessa river at downstream. The water is diverted through 491m long tunnel to a surface power house locating at the toe of dam. A 122m long ogee spillway has been provided to pass the design floods at 1040 masl of overflow crest. The maximum water level for the design flood
3
Q,0000 year 4224m /sec. The diversion tunnel
is proposed to discharge Q20  year flood
of 2210 m3/s.
The intake has been design with level at 946 masl to pass the design discharge of 271m3/s, through 9m internal diameter of tunnel with total length of 491m and shall merge into four penstocks with 4m diameter each.
The power house shall have 4 unit of Francis turbine of 75MW each, and formally the water will pass through the draft tube then back to the Didessa river.
The project is likely to be commissioned by the end of five year. The project will cost around 336 MUSD.
It is recommend that pre-feasibility study of lower Didessa project be prepared after detail geological survey has been done properly.
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Table 4.1 Technical Features of Lower Didessa
No.
Features
Didessa
1
Catchment area
18200km2
2
Mean annual flow
7290Mm3
3
Type of dam
Height
Crest length
Crest level
Bed level
Rockfill
160m
1050m
1050masl
890
4
Reservoir area
HRL
TWL
Gross head
Firm flow
Firm Energy
64 km2
1040masl
900masl
140m
5103 Mm3
1582Gwh/yr
5
Spillway type
Crest level
Crest length
Design discharge
Ogee & ungated 1040masl
122m
4224m3/s
6
Water way
Power tunnel length (Internal diameter) Pressure shaft (Internal diameter) Penstock length (Internal diameter)
440m (8.5m) 51m (8.5m)
25m(4m)
7
Power house
Turbine type
Installation capacity 
Turbine design discharge
Surface type Francis (with 4 unit) 301 MW
271m’/s
8
Economic analysis
Construction time
Capital cost
Interest rate
B/C ratio
EIRR
Unit energy cost
5 year
336 MUSD
10%
3.3
27%
0.0236 USD/kwh
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5.       INITIAL ENVIRONMENTAL IMPACT ASSESSMENT FOR DIDESSA HPP
5J Introduction
Large dam projects  usually  cause irreversible environmental changes  over a wide geographic area and thus have the potential for significant impacts. Therefore, where large dam projects including hydroelectric dams are designed to enhance economic development 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 minimize the environmental problems as early as possible.
The main objective of the environmental and socio-economic studies is to describe the existing environmental and socio-economic  condition of the study  area, assess  the potential effects of the proposed project, and recommend possible mitigating measures for the significant adverse impacts. The methodology used to carry out the assessment include scoping of the study using checklists, review of previous studies, secondary and primary  data collection through field investigation, discussion with local government authorities  and communities. Focus  group discussions  with key  informants  and communities, transact walk, social and resource mapping and direct observation were main tools used to collect site-specific data, and to obtain public attitude to the project.
5.2    Description Of The Baseline Condition
The Didessa sub-basin in which the proposed hydropower project is  situated, is characterized by land use/land cover system dominated by 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 to intensively  cultivated and a variety  of annual crops  are grown; maize being the
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predominant Coffee (a perennial crop) comprises  a significant proportion of the cropping system in the eastern and southern part of the sub-basin. The escarpment slopes  and lowland areas  are mainly  covered by  woodland vegetation, which is normally associated with a tall grass understorey.
The natural vegetation cover 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  mostly  characterized by  small trees  and bushes with fairly large deciduous leaves, usually associated with tall grasses and the lowland bamboo. Its floristic composition is dominated by Combretum and Terminalia spp. in the layer of trees, Hyparrhenia spp. (tall grasses) and Oxytenanthera abyssinica (the lowland bamboo). Extensive stands of bamboo thickets occur in the project area particularly on the left side of Didessa river. Extensive burning of this vegetation, which takes place every year during the dry season, is a well-known practice in the area. Large proportion of the woodland areas  are being cleared for shifting cultivation, establishment of new settlements and for mechanized farming by private business firms particularly on the left side of Didessa.
The woodland areas of the proposed project area (dam site and reservoir area) and its surroundings are mostly little disturbed (except the burning practice). Therefore, it is a favorable habitat for a variety of wild animals including larger animals such as Anubis baboon, CoIobus  monkey, Grivet monkey, bush-pig, warthog, bushbuck, bush-duiker, gazelle, lion, leopard, buffalo, jackal, hyena, porcupine and cheetah. Aquatic  larger animals particularly hippopotamus and crocodile, and a variety of fish species are also known to occur in the Didessa river.
Administratively, the proposed project is located in Kamashi Zone of the Benishangul- Gumuz Regional State and in two Woredas, namely Kamashi and Bello Jiganfoy. The nearest town to the dam site is Kamashi, capital of Kamashi Woreda and Kamashi
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Zone. There are 9 Peasant Associations (5 in Kamashi and 4 in Bello Jiganfoy) and 12 villages (6 in Kamashi and 6 in Bello Jeganfoy) around the proposed dam site and reservoir area.
The health status  of the population around the project is  among the lowest in the country, which is primarily related to the low socio-economic status and environmental factors. Malaria is the top most leading cause of morbidity, accounting for about 30% of the total outpatient visits at the health units. Intestinal parasites, diarrheal diseases, respiratory tract infections and skin diseases are among the major causes of outpatient visits. In addition to malaria, onchocerciasis is endemic and an important water related vector-borne disease of concern in the area.
5.3    Potential    Impacts
5.3.1 Positive Impacts
The main benefit of the proposed project is the generation of hydroelectric energy. Other potential positive impacts include employment opportunity for the local people and in-migrant population, reservoir fishery  development, infrastructure development (water supply, health facilities, schools & roads), and flood control.
5.3.2 Negative Bio-Physical Impacts
Impacts  on Terrestrial Ecology:- The inundation by  the new reservoir 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 reservoir area. This vegetation is mostly dense and its woody biomass is relatively higher. In addition, there will be a significant disturbance in the dam, powerhouse, access  road, quarry  and construction camp areas  during the construction phase. Impacts on wildlife would be significant due to the loss of this important habitat. The
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wild animals  would be displaced and their habitat use patterns  disrupted including obstruction of migratory pathways. The animals can move to adjacent habitats but they might be exposed to contact with the local inhabitants, and will therefore be more vulnerable to poaching activities or they may migrate to other places.
Impacts on Aquatic Ecology:- During the construction period the activities at the dam site will disturb and probably  displace the populations  of larger aquatic  animals (hippopotamus and crocodile) and fish in this locality. The flooding of the reservoir area will destroy  a length of about 50km of the Didessa river, and 40km of the main tributaries, whilst the dam will provide a barrier to movement of aquatic fauna. During the reservoir-filling period the river flows below the dam particularly along the 17km length of the Didessa river between the dam and confluence of Angar river will be reduced to 28% of the mean annual flow. This will reduce the habitat for aquatic fauna, which will induce stress  particularly  for larger animals  and fish. Under 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 might be reduced.
Risks  of Water-related Diseases:- During the construction period there will be increased risks  of the transmission of endemic  diseases  and sexually  transmitted diseases. There will be increased exposure of workers  and their families  to locally endemic diseases, and introduction of non-endemic diseases to the area by in-migrant laborers  and technical personnel. During operation the risk  of malaria and onchocerciasis will change from being seasonal to becoming perennial. The increase in incidence of the diseases will result from improved breeding habitats offered by the reservoir and its  shoreline vegetation for malaria vectors, and downstream flow conditions for onchocerciasis vectors.
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Impacts  on Landscape:- Flooding of the reservoir area, 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 would affect the landscape and its aesthetic value. In particular creation of a reservoir will bring a significant alteration in the river regime and natural landscape. The scenic feature of the area will be changed due to the replacement of the terrestrial habitat along about 90 Km length of the Didessa river and its major tributaries by artificial lake (reservoir).
Erosion:- Construction activities will increase the risk of soil erosion. Earthworks for road construction, civil works, construction camps, and quarrying activities will remove and destruct the natural vegetation and topsoil particularly on steeper slopes increasing the potential for erosion.
5.3.3 Negative Socio-Economic Impacts
The number of people and fixed properties, and area of cultivated lands within the direct impact zone (reservoir area, dam site, and access road) is limited as the area is not densely settled or intensively cultivated. However, there is significant number of people living in the close proximity of the reservoir area. Totally about 2,777 people (448 household heads  and 2329 family  members  in about 12 villages, 9 Peasant Associations) were identified to be directly or indirectly affected by the proposed project. Of these, about 237 household heads settled in 6 villages with their family members (about 1,145) will be displaced because they are located within the reservoir and its vicinity. The remaining population will be affected indirectly mainly due to increased health hazards.
In all villages, there are 670 houses, 2 Churches, one elementary school, and one developed water system mainly along the tributaries of Didessa, at localities close to the reservoir area. Of these about 353 houses, 2 churches, the school and developed
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water supply are located in the directly affected villages. The area of land being used by the people in the potentially affected villages include about 150 ha of cultivated land, 700 ha of grazing land and 1,300 ha of forestland/woodland. As there is already a trend of moving out from these villages to uplands or to areas close to major settlement areas such as the Woreda capitals, the extent of displacement by the proposed hydropower project may  not be significant. 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.
The major socio-economic  issue related to the development of the proposed hydropower project is the disturbance of existing communication systems. There are critically important river crossing points within the reservoir area and downstream of the dam site that link the Bello Jiganfoy and Yaso Woredas with Kamashi, zonal capital. The people in these woredas frequently use these routes for economic, social and administrative interactions. Wooden made boats  are mostly  used to cross  the river (Didessa), and on foot crossing during low flows.
As to the present condition there is no shortage of land to relocate the affected population. The adverse effects can be mitigated through proper resettlement plan and provision of adequate financial compensation for the lost properties and benefits, and development of infrastructure (water supply, health facilities, roads and alternative crossings) and reservoir fishery, and enhancement of extension services.
5.4 Conclusion And Recommendation
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 including losses of natural vegetation
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18Didessa  Medium scale Hydropower Project
Reconnaissance Level Study
2001
and wildlife habitats, aesthetic/landscape intrusion, deterioration of water quality, alteration of the river regime, dam barrier to fish migration, losses  of tradition communication networks/river crossings, and increased risks of water related diseases.
Many  of the negative environmental 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 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.
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Summery

Lower Didessa Hydropower Project Summary

Lower Didessa Medium Hydropower Project - Executive Summary

Project Location: Didessa River, Kamashi Zone of Benishangul-Gumuz Regional State, Ethiopia

Coordinates: 09°29'11-15"N latitude, 35°58'55"-59"E longitude

Study Date: August 2001

Prepared by: Water Works Design and Supervision Enterprise, Addis Ababa

1. Project Overview

The Lower Didessa Medium Hydropower Project is a proposed hydroelectric development on the Didessa River in western Ethiopia. The project aims to harness Ethiopia's significant untapped hydropower potential (currently only 1% utilized).

2. Hydrology

Catchment area: 18,200 km²
Mean annual flow: 7,290 MCM
Climate: 'Kolla' (20-28°C) and 'Weina-Dega' (16-20°C)
Rainfall: Single peak season (May-October), mean annual 1600mm
Flood estimates: 10-year flood = 1,977 m³/s; 10,000-year flood = 4,224 m³/s
Sedimentation: Estimated at 9.1 million tons/year

3. Geology

Dam site located on Paleozoic sedimentary rocks (shale, siltstone, claystone)
Concerns about bedding plane orientation causing potential seepage issues
Construction materials available within 10-20km
Seismic risk zone: Minor (Zone 1), predicted ground acceleration 2-3%g
Recommendations include detailed geological mapping and geophysical investigations

4. Project Layout

Feature	Specification
Dam Type	Rockfill, 160m high
Reservoir Area	64 km²
Spillway	Ogee type, 122m long
Power Generation	4 Francis turbines (75MW each), total 301MW
Annual Energy	1,582 GWh/year
Construction Time	5 years
Estimated Cost	336 MUSD

5. Environmental Impact Assessment

Positive Impacts:

Clean energy generation
Employment opportunities
Reservoir fishery development
Infrastructure improvement
Flood control

Negative Impacts:

Terrestrial Ecology: Loss of 6,400ha woodland habitat, wildlife displacement
Aquatic Ecology: Destruction of 50km river habitat, barrier to fish migration
Health: Increased risk of malaria and onchocerciasis
Social: Displacement of 1,145 people in 237 households
Infrastructure: Loss of 353 houses, 2 churches, 1 school, water system

Mitigation Measures:

Resettlement plan with compensation
Alternative river crossings
Disease control programs
Erosion control during construction

Conclusion: The project offers significant energy benefits but requires careful implementation of mitigation measures to address environmental and social impacts. A pre-feasibility study with detailed geological survey is recommended before proceeding.