Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
Unfortunately no hydrometric station is located in the residual part of the basin, downstream the Gibe River
at Abelti, Wabi River at Wolkite and the Gojeb River (at Shebe or at the dam site). This residual part
represents almost 33% of the total area.
Integration of runoff time series
The integration of data was carried out for the hydrometric stations of Abelti on the Gibe River, Wolkite of
the Wabi River and Gojeb dam site OM19 on the Gojeb River. These stations, as discussed in the previous
paragraph, are of particular importance in the calculation of the mean runoff of the total project area. For
these three sites 40 years long time series were obtained after infilling.
a) Abelti station on the Gibe River
Ideally, the infilling of data should be based on the correlation with other reliable flow records nearby the
considered station. Unfortunately no station is present on the same river reach, with a sufficiently long
record. Abelti is in fact the station with the longest time series with more than 30 years long records (1963-
2004) even if not continuous. At this point the problem was to individuate a suitable station, in terms of
reliability of measurements and length of recording period.
After a first unsuccessful attempt with the flow series analysed during the OGMP study of 1995, the Abelt
runoff time series was in filled and extended using the runoff data for the Shebe station on the Gojeb river
reported in the mentioned Gojeb Medium Hydropower Project, Pre-Feasibility Study. In that study this series
was integrated and extended from the OGMP Shebe record suitably adjusted, obtaining a 40 years long series
(1955-1994). The series was finally completed with the more recent observation (1995-2003) available.
To obtain a full 40 years series the extension was truncated to the last complete year of observation at Shebe
(2001). To account for the two missing data of Jan and Feb 1996, which is not clear if they are missing or if
they are nil data (the first hypothesis is more probable), the real data of the first months of 2004 were
included.
In conclusion Tab. 4 and Tab. 5 of [200 HYD R SP 001 A - Hydrological report, Vol 1] report the integrated
runoff series for the Abelti station in m 3 /s and Mm 3 respectively. In light yellow are reported the infilled
data. The mean runoff is 190.35 m 3 /s (obviously the mean runoff was calculated as a weighted average
according to the number of day in each month). The yearly runoff is almost 6000 Mm 3 . The monthly runoff
follows the typical variability of rainfall with a dry and a wet season. The highest runoff is reached in August
with a mean monthly flow of 616.5 m 3 /s.
The mean value of 190 m 3 /s substantially agrees with the value computed in the OGMP, that, as already
noticed, tends to overestimate runoff. The addition in our elaboration of slightly wetter annual record has
balanced the result to 190 m 3 /s. The same cannot be said for the other downstream sections or subcatchments.
In fact the OGMP itself states that “it is believed that the flows upstream Abelti are reasonably
well simulated. However, because of the limited number of gauging stations in the southern part of the basin,
flow there are likely to be less accurate, especially the distribution of flow into sub-catchments.” In Figure8
the annual runoff for the Abelti station is reported. Moreover a five years moving average was added to the
plot. It can be noted a marked trend to relatively larger runoff beginning in 1989. This is probably due, as
already noted by OGMP, to a progressive deforestation which has take place in favour of agriculture in the
highlands of the upper part of the basin.
CESI SpA - Mid-Day International Consulting Engineers Page 103