Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
Table 5.21: Gibe III Basin – 360 hrs (15 days) Design Hyetographs
Time P/Pcumulated P10,000yrs PMP Time P/Pcumulated P10,000yrs PMP
[hrs] [-] [mm] [mm] [hrs] [-] [mm] [mm]
12 0.014 5.076 8.284 192 0.035 12.561 20.500
24 0.017 5.972 9.746 204 0.031 11.304 18.450
36 0.023 8.352 13.632 216 0.030 10.766 17.572
48 0.026 9.280 15.147 228 0.030 10.766 17.572
60 0.032 11.462 18.708 240 0.030 10.766 17.572
72 0.038 13.485 22.010 252 0.029 10.362 16.913
84 0.046 16.679 27.223 264 0.028 9.869 16.107
96 0.049 17.557 28.655 276 0.026 9.420 15.375
108 0.056 20.154 32.894 288 0.025 8.972 14.643
120 0.059 20.994 34.265 300 0.024 8.478 13.838
132 0.060 21.532 35.143 312 0.023 8.075 13.179
144 0.063 22.430 36.608 324 0.021 7.536 12.300
156 0.050 17.800 29.052 336 0.020 7.177 11.714
168 0.040 14.355 23.429 348 0.020 7.177 11.714
180 0.036 13.063 21.320 360 0.020 7.177 11.714
S 1.000 359 585
HEC HMS program internally runs the SCS loss model and operates the abstraction calculation cumulating
the design hyetograph, applying
( P()
t −0.
2S
)
2
P =
P()
t + 0.
8S
(4.1)
And finally replicating the hyetograph pattern in order to form the excess hyetograph.
CN values for Abelti, Wabe and Gojeb at Shebe catchments have been directly derived from the OGMP
Flood Study 2 which calibrated them on each of these three gauged catchments. The CNs from the OGMP
were already computed considering the sever antecedent moisture condition.
CN value of Abelti and Shebe have been respectively attributed to Gibe I and Gojeb at OM19 while a
weighted average according to the basin extension has been computed in order to come up with the CN value
to be assigned to the Residual catchment.
Table 5.22: Gibe III HMS model – CN values
Area CN S Ia=0.2S
Catchment [km 2 ] [-] [mm] [mm]
Abelti 11516 74.1* 88.78 17.76
Gibe I 4230 74.1 88.78 17.76
Wabe 1866 82.1* 55.38 11.08
OM19 5136 74.3* 87.86 17.57
Residual 11411 74.8 85.57 17.11
* - From the OGMP 2
The synthetic unit hydrograph used by the SCS was developed averaging a large number of individual
dimensionless unit hydrographs. The final product was made dimensionless by considering the ratios of q/qp
(flow/peak flow) on the ordinate axis and t/Tp (time/time to peak) on the abscissa, where the units of q and
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