Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
• Identification of the disposal areas considering the environment. Minimization of the impact related
to the flora
• Definition of proper final slopes according to the nature of the materials to be disposed
• Identification and definition of the drainage works to protect the area from the running rainwater
• Identification and definition of any retaining structure (gabion walls, concrete retaining walls) at the
toe of the slopes to avoid landslides.
During the working phase the whole monitoring system (i.e. piezometers, inclinometers, extensometer and so
on) prepared for the ordinary dam control, will be maintained operative and in perfect efficiency. Along the
reservoir banks, is better to plan periodical controls to verify the possible triggering of banks movements
(eventually of wide dimensions) due to the filling and partial emptying of the reservoir.
7.2.6 Hydrology and Water quality
7.2.6.1 Hydrology
Gibe III is planned to have a reservoir which is very large and the project will regulate the downstream Omo
flow at that location markedly increasing the low flows and decreasing the high flows in the river reaches
further downstream.
During the construction phase, the diversion tunnels allow the full passage of the incoming flows.
Downstream the distance between the dam and the outlet is very short (less than 800 m) and although during
operation the river bed in this stretch will remain dry the construction of the project and the change in the
hydrologic regime of the river will not affect fishery and other valuable aquatic flora and fauna.
However, when the plant is not operating the environmental flow shall be released downstream by means of
one of the several outlet works available. During the first reservoir filling, the two outlet devices will
discharge the release required for environmental uses and for impounding control operations. During
reservoir filling, it is also recommended to release the environmental flow.
Construction stage of RCC dam will be extremely safe and doesn’t present any potential risk in case of
exceptional flood.
7.2.6.2 Water Quality
Water storage in reservoirs induces physical, chemical and biological changes in the stored water all of
which affect water quality. The chemical composition of water within a reservoir can be significantly
different to that of the inflows. The size of the dam, its location in the river system, its geographical location
with respect to altitude and latitude, the storage detention time of the water and the source(s) of the water all
influence the way that storage detention modifies water quality.
Major biologically induced changes occur within thermally stratified reservoirs. In the surface layer,
phytoplankton often proliferate and release oxygen thereby maintaining concentrations at near saturation
levels for most of the year.
Upon dam closure, the lentic system establishes itself rapidly as the reservoir fills. Thereafter, a microbial
population explosion occurs, releasing nutrients from the newly submerged organic matter. This stimulates
an equally rapid development of the phytoplankton, which harnesses solar energy. The enrichment of
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