Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
OM19 were suitably infilled for the years following 1996 according to the Shebe runoff, following the
deterministic correlation existing between these two sites The transposition of the Shebe flow series to the
dam site OM19, adopted in the mentioned report, was based only on catchment areas. The difference in
rainfall ratio (equal to 0.98) was considered negligible. So the conversion factor between Shebe (3356 km 2 )
and OM19 Dam site (5136 km 2 ) was the ratio between areas.
Tab. 10 and Tab. 11 of the [200 HYD R SP 001 A - Hydrological report, Vol 1] report the runoff series for
the OM19 Dam site location in m 3 /s and Mm3 respectively. The mean runoff is 91.66 m 3 /s. The yearly
runoff is almost 2900 Mm 3 . The monthly variability is obviously the same of the Shebe station, due to the
deterministic transposition. The highest runoff is reached in August with a mean monthly flow of 226.3 m 3 /s.
d) Wolkite station on the Wabi River
Also the Wolkite hydrometric station on the river Wabi is of great interest because its catchment lies
downstream the Abelti’s one covering an area of 1866 km 2 equal to 5% of the total project area.
The basin is located in an area with a rainfall regime similar to that of Abelti. The south-eastern part of the
global basin, as discussed in a previous paragraph, is in general characterised by a drier climate. Nonetheless
the Wabi basin, according to the isohyetal map of Figure 5.4, is interested in the upper part by high relieves
which should sensibly increase its rainfall rates.
The hydrological series is quite long, having an average recording period of 35 years even if not continuous.
The average runoff resulting from these data is 30.9 m 3 /s. In order to extend the observation period to a full
40 years record, the integration and infilling of the runoff series was carried out by mean of the
contemporary observation of the Abelti station, which lies nearby showing a good correlation, and has a long
recording time.
Tables 14 and 15 of the [200 HYD R SP 001 A - Hydrological report, Vol 1] reports the integrated data: the
average runoff is 30.35 m 3 /s, substantially confirming the value coming from the non-integrated data set, the
mean annual runoff is 957 Mm 3 .
Estimation of mean runoff
As no hydrometric station is located nearby the dam sites under investigation, the estimation of the mean
runoff is carried out with “indirect” methods which are based on the analysis both of runoff and rainfall time
series.
A good number of hydrometric stations are present in the upper part of the watershed covering an extension
of almost 67% of the basin, while no instrumentation is present in the downstream part. On the contrary
rainfall gauges are present all over the catchment area, also in the remaining downstream part even if in a
small number. This circumstance makes impossible to estimate directly runoff through the elaboration of
runoff series, but implies an indirect estimation by mean of runoff in the upper part of the basin or rainfall
rates in all or part of the basin. Due to the uncertainties introduced by this indirect estimation on such a wide
area the determination of runoff was carried out with four different procedures:
1) elaboration of available runoff time series,
2) elaboration of rainfall and estimation of the runoff coefficient,
3) direct observation of runoff on 67% of the basin and elaboration of rainfall on the residual 33%,
4) estimation of losses by mean of evapotranspiration.
CESI SpA - Mid-Day International Consulting Engineers Page 105