Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
5.1.5.2 Hydrogeology at Dam Site
The hydrogeological studies at the dam site area are illustrated in the report:
• 300 GEO R SP 003 a Hydrogeological study whose findings are recalled here below.
Hydrogeological Features of the Formations
The main rock formations mainly present a “secondary” permeability controlled by the density of fracturing;
consistent deposits of recent alluvium can fill the river bed with various thicknesses, and ancient alluvial
deposits occupy limited areas on the upper sides of the plateau or they are present as intra-formation.
Analyzing the hydrogeological behaviour and the permeability of the formations, the lithologies at dam site
have been divided in five groups. The classification is not a recognized standard, but mainly point out the
general hydrogeological features of the formations in the area. Some formations with a wide range of
permeability can record a moderate or low infiltration on the basis of the proneness to fracturing, be or not to
be site of suspended aquifer on the basis of the aptitude at decomposition, or being at the same time roof and
body of a confined aquifer.
In light of the above the permeability degree is not “absolute” but mostly related at the behaviour of the
formations in this area. Clay, laterite and silt, pyroclastic sand and ashes and their mixes have been grouped
together with not any type or a very low permeability. According with a characteristic of a low permeability
a second group has been selected. This class put together all the main formations present at dam site such as
trachyte in dark colour, basalt, slightly vesicular basalt, brecciated basalt, tuff and silty sand. A third class
has a low-moderate permeability and is shared between I (porosity) and II (fractures) type. Backfilling and
soil, sand and colluvium are joined in the first type; conglomerate, slightly weathered “zonated with
concentric bands” trachyte and vacuolar basalt are collected together as second type. The fourth group
collects grey ignimbrite, ancient alluvium, breccia, and unweathered - weathered light grey trachyte, all of
them classified as moderately permeable. High permeability characterizes the fifth group where recent
alluvium and highly weathered and/or fragmented trachyte are present. the following figure lists the five
groups of permeability
CESI SpA - Mid-Day International Consulting Engineers Page 88