Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
Conclusions
Main results of the hydrological elaborations are summarized here below:
Gibe III Dam Site
Peak
Discharge
Time of Peak TP Total Runoff Volume
[m 3 /s] [hr] [Mm 3 ]
10000yr 10606 240 12193
PMF 18661 240 19741
The whole flood events lasts about 30 days with a lag time of about 4 days.
Comparison of the results with other studies confirms the consistency of the estimates.
Table 5.27 illustrates the comparison with the results of preceding extreme flood studies for basins within the
Gibe III catchment, achieved adopting the same procedure illustrated in this report.
10000yr
PMF
Dam/River Site +
Reference
Table 5.27: Comparison with Other Hydrologic Studies
Catchment
Area [km 2 ]
Peak
Discharge
[m 3 /s]
Specific Peak
[m 3 /s/ km 2 ]
Gojeb Shebe [12] 3614 1770 0.490 3.80
Gojeb OM19 [12] 5136 2250 0.438 3.83
Halele [15] 6126 2420 0.395 3.79
Gibe III 34159 10606 0.310 4.30
Gojeb Shebe [12] 3614 2050 0.567 3.95
GojebOM19[12] 5136 2590 0.504 3.97
Halele [15] 6126 5760 0.940 4.68
Gibe III 34159 18661 0.546 5.01
Francou-
Rodier’s
Coefficient [-]
The highest values of the Francou-Rodier’s coefficient 17 at Gibe III dam site prove the precautionary
approach of the analysis.
It’s easy to notice in the table above that specific peak values for the 10000 yr analysis well follow a
decreasing trend as the catchment extension increases.
On the contrary, Gojeb PMF data seem to reject this tendency. The PMF peak for OM19 assessed in the
present study equals 3486 m3/s being about one third higher of the value in Table 5.27.
This discrepancy in the PMF calculation does not surprise if three factors are considered:
1. the adoption in the Gojeb Study of a constant ARF of 0.54 inevitably involves an underestimate of
the areal PMP for longer storm duration such as 15 days (critical duration for Gibe III Basin);
2. a larger record including more intense precipitation was made available for the latest studies (Halele
and Gibe III);
17
Francou J., Rodier J. A., “Essai de classification des crues maximales observees dans le monde”, Cah. ORSTOM, ser.
Hydrol., vol. IV, n. 3, 1967
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