Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
number of hydrometric stations are present in the upper part of the watershed, no instrumentation is located
in the downstream part. On the contrary rainfall gauges are present all over the catchment area, even if with
lower frequency in the downstream part. This circumstance makes impossible to estimate directly runoff
through the elaboration of runoff series, but implies an indirect estimation by mean of 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. mixed use of runoff (on 67% of the basin) and rainfall (on the residual 33%),
4. estimation of losses by mean of evapotranspiration.
Finally the monthly variability is evaluated and the historical runoff series at Dam Site 5 determined.
All the elaborations were carried out considering the rainfall and runoff data provided mainly by gauges of
the Ethiopian National Meteorological Service Agency and Ethiopian Water Resources Authority. In
addition external data were used coming only from officially approved documents related to other project in
the same area (listed in the following). All these documents provided very useful information both in terms
of data, and in giving elements for comparison with the results obtained in the elaboration. Special attention
was devoted to the “Omo-Gibe River Basin Integrated Development Master Plan Study”, Vol. VI - Water
Resources Surveys and Inventories, of Dec 1996, which contains a lot of hydrological information for the
entire Gibe – Omo River watershed, thus supplying those information lacking in the downstream part of the
basin under investigation.
The extreme flood flows have been calculated by applying a consistent rainfall-runoff model.
5.1.7.1 Physical and meteorological features of the area
The physical and hydro-meteorological features of the area under investigation are extensively exposed in
the “Omo-Gibe River Basin Integrated Development Master Plan Study”, Vol. VI, Water Resources Surveys
and Inventories, Dec 1996, carried out by Richard Woodroofe & Associates for the Ministry of Water
Resources of the Federal Democratic Republic of Ethiopia.
The following information are mainly extracted from the mentioned document.
Description of the watershed
The Gibe River rises just north of latitude 9°N and longitude 37°E of the Ethiopian Plateau, geologically an
area comprised of Tertiary volcanics. The Gibe headwaters are at an elevation of about 2200 m asl. Although
there are some important tributaries from different directions, the general direction of flow of the Gibe River
is southwards, towards the Omo River/Lake Turkana Trough, a fault feature filled with alluvial and
lacustrine sediments of recent origin associated with the Great Rift Valley. The northern part of the
catchment has a number of tributaries from the NE, the largest ones being the Walga and the Wabi River.
These drain largely cultivated land, much of it with rather impeded drainage. This is an area where erosional
processes are important. The Tunjo and Gilgel Gibe Rivers are important tributaries, also draining mainly
cultivated lands from the SW. These have a higher proportion of more permeable soils than the Walga and
Wabi catchments.
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