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 It is worth underlining the greatest mean runoff occurs in August, reaching the considerable value of 1509 m 3 /s. Table 5.13: Monthly variability of runoff at Dam Site 5 in m 3 /s m 3 /s JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Mean Abelti 41.4 33.1 23.5 28.5 52.0 132.1 364.9 616.5 509.9 294.6 112.7 60.4 190.3 OM19 17.7 14.3 18.2 24.6 54.1 116.8 184.5 226.3 212.9 137.7 58.9 28.0 91.7 Wabi 3.9 4.7 4.1 6.9 7.8 18.3 77.8 132.2 65.0 26.8 8.3 5.3 30.4 Residual 16.2 19.3 16.9 28.5 32.2 76.0 322.2 547.8 269.1 110.8 34.5 21.9 125.7 Total 79.2 71.4 62.6 88.5 146.0 343.3 949.4 1522.9 1056.9 569.8 214.5 115.6 438.1 Table 5.14: Monthly variability of runoff at Dam Site 5 in Mm 3 Mm 3 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Abelti 108.1 78.2 62.9 73.8 139.3 342.5 977.5 1651.4 1321.6 789.1 292.1 161.8 OM19 48.6 36.4 48.6 63.8 144.9 310.0 494.1 606.1 551.9 368.7 152.7 74.9 Wabi 10.5 11.3 10.9 17.8 20.8 47.5 208.3 354.2 168.4 71.6 21.6 14.2 Residual 43.5 46.6 45.3 73.9 86.1 197.0 863.0 1467.3 697.5 296.8 89.5 58.7 Total 210.7 172.5 167.8 229.4 391.1 897.0 2542.9 4078.9 2739.4 1526.3 555.9 309.6 Extreme flood runoff Table 5.15: Total annual and mean monthly runoff at Dam Site 5 in Mm 3 Mm 3 Total Mean Abelti 5998.2 499.8 OM19 2900.7 241.7 Wabi 957.2 79.8 Residual 3965.3 330.4 Total 13821.3 1151.8 The analysis of the extreme flood flows are based on: • Standard statistics of the precipitation 4 5 6 7 8 ; • A largely adopted procedure for estimating the PMP 9 10 11 ; • U.S. Weather Bureau method for transposition of point rainfall data to areal precipitation 12 2 ; • U.S. SCS rainfall-runoff model 4,5,6,8 ; • A former flood analysis for the gauged catchments included in the Omo River Basin closing at the dam site 2 ; 4 USBR, “Design of Small Dams”, United States Department of the Interior – Bureau of Reclamation, 2006 5 Maidment D. R., “Handbook of Hydrology”, Mc Graw Hill, 1993 6 Chow V. T., “Applied Hydrology”, Mc Graw Hill, 1998 7 Weiss L.L., “Ratio of True to Fixed-Interval Maximum Rainfall”, Proceeding of the American Society of Civil Engineers, Journal of the Hydraulics Division, Vol.90, n.HY1, 1964 8 Moisello U., “Idrologia Tecnica”, La Goliardica Pavese, 1998 9 Hershfield D. M., “Estimating the Probable Maximum Precipitation”, Proceeding of the American Society of Civil Engineers, Journal of the Hydraulics Division, Vol.87, n.HY5, 1961 10 Hershfield D. M., “Method for Estimating Probable Maximum Rainfall”, Journal of the American Waterworks Association, Vol.57, 1965 11 WMO “Manual for estimation of Probable Maximum Precipitation”, Operational Hydrology Report N.1, World Meteorological Organization n.332, Geneva, Second Edition, 1986 12 Allen R.J., De Gaetano A.T., “Areal Reduction Factors for Two Eastern United States Regions with High Rain- Gauge Density”, Proceeding of the American Society of Civil Engineers, Journal of the Hydrologic Engineering, ASCE, July/August 2005 CESI SpA - Mid-Day International Consulting Engineers Page 107
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 2 13 3 • Hydrological studies for other regions comprised in the catchment of interest • Gibe III Hydrological Study of the Basic Design 3 . The transformation of rainfall into runoff has been carried out developing a hydrological model of the whole watershed by means of HEC HMS 3.1.0 software in order to better control the process of formation of the flood over the contributing catchments taking into account the routing effect due to the operating upstream Gibe I Dam, the routing effect along the river, and the hydrographs combination in correspondence of the junctions. Geomorphologic parameters of the single catchment have been derived from a digital terrain model using GIS tool. Rainfall analysis Daily precipitation records have been used to generate 3-, 5-, 7-, 10- and 15-day records. The longest duration of 15 days has been preliminarily chosen as the critical storm duration basing on experience of catchments of comparable extension (about 35,000 km 2 ) and basing on some empirical expression 2 such as dcrit = (A/100 + 0.5) 0.5 , which provides 18 days for the given area. Rainfall-runoff analysis for longer durations adopting extrapolated data has shown, indeed, that peak flood increase after 15 days is practically negligible. Frequency analysis of extreme precipitation A brief analysis of the mean of the annual max records shows that this value varies in a narrow range 61.5 mm – 49.2 mm if Gedo with 28.7 mm is excluded. This is likely due to the comparable altitude of the stations. Gedo station is located just outside of the Gibe III catchment and its point precipitation is strongly affected by the highland which bounds the basin. Moreover Gedo is not only out of the boundary but also very close to Ejaji station, therefore as specified later on, its areal weight in the hydrological elaborations is very modest. Consequently all the stations have been considered in the frequency analysis. A “regional” approach, based on the assumption of the hydrological homogeneity of the sites in the Gibe III catchment area, has been applied. This approach is considered since no large number of records is available while a long dataset is mandatory for a high return period analysis in a homogeneous region. The method assumes that the probability distribution of rainfall at different sites within a region is the same except for a scale parameter, also known as “index storm”, which reflects the rainfall characteristics of the particular site and is usually assumed to be the mean rainfall at the station. The following table lists the main results 14 of the statistical elaborations for 100-, 1,000- and 10,000-year frequencies. 13 EEPCO, “Gilgel Gibe Hydroelectrical Project, Feasibility Study Main Report”, ELC Electroconsult - ENEL, Feb 1998 14 300 HYD R SP 001 EXTREME FLOODS REPORT Jun 07 lc, studio pietrangeli, rome CESI SpA - Mid-Day International Consulting Engineers Page 108
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