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 5.4.4 Hydrology Downstream of the dam the Omo river flows “entrenched” for about 200 km and through an “incision”, generally more than 10 m deep, for about 150 km. Along the reach heading W several small though perennial effluents (Sana, Deme, Zage, Irgene) join the river draining the highlands from E/S. Average rainfall in these sub-basins ranges from about 1500 mm/year (south) to 1000 mm/year (east). Along the reach heading SW the Zinga and Mansa tributaries (smaller) and the Denchiya and Toshi/Sherma rivers (larger) join the Omo after draining the upper highlands (mainly covered with a vegetation of wooden shrubland). The average rainfall ranges from about 1600 mm (north) to 900 mm (south). About 80 km downstream the longitudinal slope of the riverbed flattens and gorge opens as the river crosses a hilly and semi-arid landscape. The river changes in characters with a meandering river channel ending at the river delta area. In this stretch the river is clearly in an evolutionary process, probably related also to the “shrinking” of the Turkana lake, where abandoned meanders and upcoming ones are clearly visible. Along this stretch the Aku and particularly the Maiki, tapering streams with perennial tributaries, drain the highlands along the Omo basin boundaries. Average rainfall in this area ranges from about 1500 mm/year (north-west) to 500 mm/year (south). The Omo River then flows south across the arid plains and the Delta for some 100 kms. The total catchment of the Omo at the Lake Turkana is about 73500 km 2 . The average rainfall within the whole Omo River watershed varies from 1900 mm/year (north-west) to less than 300 mm/year (south). The mean annual flows of the Omo River at the Lake Turkana have been evaluated 20 by means of an estimate of the mean flows in the residual basin Gibe III-Turkana. Since no robust flow measurements series are available downstream Abelti/Gojeb the analysis has been carried out by elaborating rainfall data of the residual basin. The table below shows the rainfall rates, the runoff coefficient and the mean flows for the main subcatchments of the Omo basin: the mean annual rainfall for the residual basin downstream Gibe III site (denoted as “Residual 2”) is 870 mm/year. This value is far below the rainfall rate of the upper part of the basin, characterised by a wetter climate. 20 300 ENV RAG003A Additional Study on Downstream Impact – April 2008 CESI SpA - Mid-Day International Consulting Engineers Page 161
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Table 5.44: Rainfall rates, runoff coefficients and flows for the main sub-catchments of the Omo River. A H Φ Q (km2) (mm/year) (m3/s) Gibe nr. Abelti 15853 1450 0.26 190.0 Wabi nr. Wolkite 1869 1430 0.39 33.0 Gojeb dam site 5186 1710 0.32 91.3 Residual 1 11251 1253 0.27 120.7 Dam site 5 34159 1424 0.28 435.0 Residual 2 43847 871 0.19 230.0 Lake Turkana 78006 1113 0.21 663.9 The estimation of the runoff coefficient for the Residual 2 catchment was carried out through the extrapolation of the values of ETP (Potential EvapoTranspiration) of the upstream catchments. Table 5.45: Runoff coefficients for the main sub-catchments of the Omo River. ETP Φ (mm/year) Gibe nr. Abelti 1124 0.266 Wabi nr. Wolkite 1108 0.360 Gojeb dam site 1101 0.333 Residual 1 1215 0.266 Residual 2 1369 0.19 For the Residual 2 catchment a reasonably conservative value of φ = 0.19 was adopted, well below the values obtained for the upstream part of the basin. This assumption leads to a mean flow at Lake Turkana of about 650 m 3 /s. The monthly variability of natural flows has been assumed to be the same as that reported in the Master Plan, shifted to the mean value of 650 m 3 /s. The result put in evidence that the monthly variability at Lake Turkana is smoother than the one at Gibe III. This is basically due to the variation of the rainfall pattern of the Gibe III basin and of the southern residual basin, which, however, produces proportionally less flows. At Gibe III the summer peak is higher than at Lake Turkana (the coefficient is equal to 3.5, against the 2.5 at Lake Turkana). On the contrary the dry season flow is generally higher at Lake Turkana. While the mean annual flow is higher at Lake Turkana, the summer peak is proportionally less evident and the August flow is not much higher than that at Gibe III. On the contrary high flow rates persist in September also due to the “ponding” effect of such a wide catchment. The table below, illustrating the monthly variability of the flow rates at Lake Turkana, indicates that highest flows are to be found within the months of August / September. CESI SpA - Mid-Day International Consulting Engineers Page 162
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