Environmental and Social Impact Assessment - Gibe III

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Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Seismology While the Gibe III dam is located in Ethiopia, in the vicinity (about 70 km) of the eastern branch of the east African rift system the entire area interested by the project, according to the LEVEL 1 DESIGN Geological Report, doesn’t show any evidences of present existing seismic activity. Hydrogeology Available data concerning the reservoir were collected from boreholes drilled at the dam site, from documents of the Gibe II project, and from remote sensing. The stratigraphy and structural geology of the reservoir banks were defined on the basis of the interpretation of ASTER scenes and other remote sensing data. Local permeability values were acquired from boreholes drilled at the dam site. According to the geological report no relevant filtration couloirs from the reservoir have been observed along the river basin. Permeability would be of secondary type (i.e. related to fracturing of the rock mass) while primary permeability (i.e. related to porosity) would be limited to the pyroclastic units interbedded in basalt flows Hydrology The Omo River Basin is drained by two major rivers from the highlands, the Gibe River flowing southwards and Gibe River flowing eastwards. Downstream of their confluence only minor tributaries join, as the river continues southwards and enters the deep gorge where the Gibe III dam site has been identified. At the dam site, the catchment area is some 34,150 km 2 that represents 45% of the total catchment area at Lake Turkana and contributes 80.5 % of the basin flow. The Omo River has an average slope of 3.1 m/km. The long term mean flow at Gibe III site is estimated to be 435 m 3 /s or 13.5 Billion m 3 per annum. Seasonal variations are extreme, with monthly mean flow ranging from around 60 m 3 /s in March to more than 1,500 m 3 /s in August. Water quality In order to describe water chemical characteristics upstream and downstream the dam site, a water sampling campaign has been done to define the abundance of defined chemical parameters in three stations (downstream Gibe II Power House, at Bele Bridge and near the Gibe III dam site). Considering very low salinity levels on the usual mineral water scale, the Omo River water can be classified as earthy-brackish, alkaline, and with bicarbonate. 0.4.1.2 Biological Environment Land Use and Land Cover Land cover assessment was carried out for the Gibe III reservoir area and it was based on satellite images interpretation, field observations, field data collection and analysis. The classification resulted into the following four classes: The riparian vegetation in the reservoir area was observed along the river sides and it occupied 1,839 ha of land (8.8 %), and deciduous woodland covers about 17,158 ha (82.2%) and it is characterized by approximately 2% tree cover and 98% grass at the time when the field survey was carried out. The exposed surface and silt/gravel land covers 4.7 % of the reservoir area and it mainly occurs along the lower parts of the river, on steep sides of rivers and degraded hillsides and rock outcrops. River/ water body covers 4.3 % of the reservoir area. Farming practices and settlement are concentrated in areas (outside the valley on the highland) which are not affected by the future reservoir. CESI SpA - Mid-Day International Consulting Engineers Page 9
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Natural Vegetation and Forest Resources The vegetation on the hill slopes of the valley is characterized by deciduous phenology of the woodland species which shed their leaves during the dry season and regain them during the wet season as an adaptive mechanism for the prolonged dry season. The plant species of the Omo Valley have over time developed adaptive mechanisms and traits that allow them either to survive fire, germinating after the heat shock or to regenerate after a fire episode. In general, the vegetation which has evolved (around the reservoir area) as response to the frequent fires is poor in species composition. There is a narrow band of riparian vegetation of almost similar species composition as the woodland on the hill slopes. Due to ample moisture, trees found at the edge of the riverbank are not affected by fire as the rest of trees in the upper parts of the study area. The altitudinal ranges, temperature, humidity and the floristic and physiognomy composition of the vegetation in the Omo river basin and Gibe Basin provide ideal conditions for Tsetse fly infestation. Wildlife Resources Based on the assessment, the number of wildlife species in the project area is low and does not rate well with areas in downstream of Gibe III dam (the Mago and Omo National Parks) and harbours only limited number of wildlife. However, the local residents and professionals from the offices of Agriculture interviewed during the field studies reported the presence of wildlife within the project area. The wildlife that have been recorded for the reservoir area are very common in many parts of the country and none of these species are endangered or threatened. The vegetations provide good habitat to support diverse wildlife species. However, the wildlife habitats have always been under threat due to wildfires that are practiced during the dry season and it is greatly affecting the overall ecology and resource base. The woodlands, in particular are under high threat from fire. Fishery Resources The Omo-Gibe river basin is known to contain high diversity of fish species with over 70 species listed. The fishery in the study area includes: the riverine fishery along the mainstream Omo River, flood plain fishery (and Dipa Hayk), and the lacustrine fishery at Lake Turkana (Bubua and Toltale). Studies on the biology of the different fish species have shown that they have different seasonal and spatial distribution for optimal growth – feeding, reproduction and survival. The large variety of species found in the river system is distributed in all sorts of habitat like in the deep open river channel or in pools (Hetrobranchus longifilis), in the floodplains, in rocky habitats like Labeo cylindricus, demersal areas (Hyperpisus bebe), etc. Similarly their feeding habit varies enormously among the species covering all available niches. Some species undergo seasonal migration from the lake feeding grounds to spawn in the River Omo – like Distichodus niloticus, Labeo horie. Others prepare their own spawning sites (nests) close to the vegetated shore areas in the lake and complete the life cycle (Oreochromis niloticus). There are also fish species that spawn in the lake and also some of the population in the river systems (Hydrocynus forskalii). There are few species which are endemic to the Lake Turkana and lower delta of the Omo River. However, none of the reports referred in the studies indicate the occurrence of endangered or threatened fish species in the Omo-Turkana system. The endemic fish species breed in different locations in or close to the lake - like CESI SpA - Mid-Day International Consulting Engineers Page 10
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Environmental and Social Impact Assessment - Gibe III

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