Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
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• 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.
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EEPCO, “Gilgel Gibe Hydroelectrical Project, Feasibility Study Main Report”, ELC Electroconsult - ENEL, Feb
1998
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300 HYD R SP 001 EXTREME FLOODS REPORT Jun 07 lc, studio pietrangeli, rome
CESI SpA - Mid-Day International Consulting Engineers Page 108