Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
qp are m 3 /s/mm. Its shape is characterized by a time-to-peak located at approximately 20% of its time base
and an inflection point at 1.7 times the time-to-peak.
Concentration time has been here estimated on the basis of the travel time of the main channel (having
conservatively neglected the rather short travel time of the unconcentrated flow). Mean velocity reference
values of the stream flow in natural channels are provided in the most common international references 6 .
The following table provides the lag times estimated for the 5 basins and used in the software elaborations.
Table 5.23: Gibe III HMS model – Tlag values
Catchment
Area
[km
L Slope avg Mean V tC tlag
2 ] [km] [%] [m/s] [hr] [hr] [min]
Abeti w/o Gibe I 11516 210 3.6 0.73 79.74 47.85 2871
Gibe I 4230 147 2.8 0.57 71.77 43.06 2584
Wabe 1866 91 5.7 1.16 22.09 13.25 795
Gojeb at OM19 5136 188 4.0 0.81 65.04 39.02 2341
Residual 11411 224 2.1 0.43 146.66 88.00 5280
These values are fairly consistent with the estimates contained in the Omo Gibe Master Plan 2 .
A time interval of 12 hrs has been adopted for defining the design unit hydrographs. This time step looks
adequately stable for the purpose of the simulation with HMS.
Flood routing effect due to Gibe I Dam has been processed in the conservative assumption of complete
filling of the reservoir. In this condition Gibe I spillway operates according to the rating curve of the spillway
itself. For these exceptional precipitation all the 4 gates are assumed open. Although many methodologies for
estimating the baseflow of the hydrograph are available, the choice of this parameter is here merely
arbitrarily, since no prediction on the occurrence of such extreme events can be done. According to the usual
operation of hydroelectric plant, the circumstance of having Gibe I Reservoir at the maximum operating
level could happen at the end of the wet season but many factors may affect the seasonal regulation and
refute this logic.
However, in the present analysis a fairly conservative value of baseflow equalling the average monthly of the
wetter period (June – September) has been assigned to each catchment. Monthly flows have been taken from
the hydrological report of the Basic Design 16 . Gibe I catchment and Abelti without Gibe I have been
computed through an area proportion from the Abelti series.
Results
Two scenarios have been investigated in the rainfall – runoff analysis:
1. a 15 day storm of 10,000 yr Return Period;
2. a 15 day storm of PMP (Probable Maximum Precipitation).
This chapter only limits to present the relevant hydrologic parameters of any hydrologic element of the
whole system (Table 5.24 and Table 5.25) and shows the two flood hydrographs at the Gibe III dam site
(Table 5.26 and Figure 5.10).
16 200 HYD R SP 001 A – Hydrological Report Vol. 1
CESI SpA - Mid-Day International Consulting Engineers Page 115