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 values of rainfall, recorded at the same stations considered for the evaluation of air temperature. In the table is reported also the estimation of the monthly rainfall at the selected dam site. Table 5.4: Mean monthly rainfall (mm) Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Year Agaro 28.5 39.5 102.8 114.2 172.5 253.0 262.2 242.5 196.2 128.9 58.3 37.9 1636.5 Gojeb 42.4 121.8 85.4 128.3 189.9 226.1 203.2 212.0 171.6 93.2 65.8 37.1 1577.1 Sekorru 30.0 38.1 71.0 105.9 149.2 222.6 216.2 324.2 166.0 79.5 17.7 25.8 1446.2 Walyita Sodo 37.5 39.7 82.2 150.1 162.5 127.8 177.2 173.9 112.3 95.4 41.9 29.0 1229.5 Dam Site 27 40 84 112 152 211 248 243 164 81 33 24 1426 It appears that 71 per cent of the total amount of annual rainfall occurs between May and September, 14 per cent from February to May, and 12 per cent between October to January. The natural evapotranspiration (ETP) is estimated for each considered station using the FAO Penman- Monteith. The expected ETP in Site 5 watershed (without reservoir) is computed by mean of the Thiessen polygons. The mean ETP are reported in the following Errore. L'origine riferimento non è stata trovata.. Table 5.5: Mean monthly ETP (mm) Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Year Agaro 141 140 166 168 169 148 141 143 143 155 143 136 1791 Gojeb 150 146 173 172 172 149 146 148 146 162 152 144 1859 Sekorru 135 134 159 159 158 136 128 129 131 147 137 131 1685 Walyita Sodo 136 132 152 151 147 125 120 123 127 141 132 129 1614 Dam Site 136 135 157 160 159 139 132 134 135 148 137 130 1702 A comparison between the losses due to evapotranspiration (in the area without reservoir) and rainfall is reported in Table 5.6: the ETP should exceed rainfall rates during the rainy season (becoming runoff), while in the dry season all the rainfall should be lost due to evapotranspiration. However these are indicative only values since the ETP has been preliminarly estimated (i.e. not measured) and the dynamics of the control of the transformation between rainfall, infiltration and evapotranspiration are not known on a monthly basis. Table 5.6: Comparison between Mean monthly rainfall and ETP at dam site (mm) Dam site Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. Year EPT 136 135 157 160 159 139 132 134 135 148 137 130 1702 Rainfall 27 40 84 112 152 211 248 243 164 81 33 24 1426 CESI SpA - Mid-Day International Consulting Engineers Page 77
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 5.1.2 Geology 5.1.2.1 Regional Geology 1 The regional geology of southern Ethiopia consists mostly of metamorphic rocks of green schist, amphibolite and granulite facies that represent the southern margins of the Arabian-Nubian shield. The Precambrian basement rocks are the result of a complex tectonic and metamorphic history during the Proterozoic times. During the Palaeozoic, the Precambrian rocks were eroded to a vast peneplan, apparently a continuation of the Kenya -Sudan plains to the South and West. In early Mesozoic times, a major marine incursion from the east deposited the Triassic and Jurassic Adigrat sandstones and limestones that cover much of the eastern part of Ethiopia and Somalia. The central and western parts of Ethiopia remained a continental land area throughout the Mesozoic, with a thin unit of presumed continental sediments being the only record of deposition. Marine sedimentation was brought to an end by widespread regional uplift starting in the Eocene period, associated with the initial development of the Red Sea and the East African rift systems. This uplift was accompanied by extensive volcanic activity. At first, this took the form of huge outpourings of basalt lava (flood basalts) from long fissures that opened along the line of the rifts. These flood basalts erupted for almost 20 million years and accumulated to a total thickness of several thousand metres. They now form the foundation of the Ethiopian Plateau. However, the details of the geological history between about 30 and 15 millions years ago has some uncertainties, owing to the scarcity of absolute ages for the felsic volcanic rocks, and it is likely that basaltic and more acid lavas erupted contemporaneously from different volcanic centres (Davidson 1983) with a progressive evolution from dominantly basaltic lavas to mainly trachytic and rhyolitic lavas. It is inferred that volcanic activity declined about 20 million years ago, leading to the localised formations of a series of shallow lakes and swamps which developed on top of the flood basalts. This resulted in the deposited series of shallow-water lacustrine sediments containing carbonaceous shales, lignite and coal deposits. Around 13 million years ago, the development of the main Ethiopian Rift Valley commenced, reaching something similar to its present form by the early Pliocene, about 5 million years ago (Davidson, 1983). This inaugurated a period of felsic volcanism, including rhyolites and trachytes, where the available dates indicate a duration between 10 and 3 million years ago; these felsic volcanics extended over much of the central plateau of Ethiopia. 1 Main data on which the present geological study is referred are: · 300 GEN R BR 001 Seismic Hazard Assessment · 300 GEO R SP 001 Geological Report · 300 GEO R SP 003 Hydrogeological Report · 300 GEO R SP 002 Reservoir seepage risk analysis In particular a large campaign of site investigations has been carried out and is presently in progress. The investigations already completed include: · Boreholes (also with lugeon tests) > 13000 m, 120 No · Geophysical Investigations · Geostructural Surveys > 5000 joints · Laboratory Tests · Inspection Adits > 350 m, No. 7 CESI SpA - Mid-Day International Consulting Engineers Page 78
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