Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
colour. They generally come from the alteration of the basaltic basement and are found on the hills and
ridges. They are deep sandy-clay-loam soils, quite draining, but they have a tendency to erosion.
The second ones are faintly stratified, dark reddish brown soils (named Eutric Cambisols according to the
FAO classification). Found in the valleys, these sandy-clay soils are composed of colluviums taken on the
near slopes.
The third group of soils that can be found in the area are dark brownish black soils (that belong to the Pellic
Vertisols according to the FAO classification). These clay predominantly soils are generated by the alteration
of tuffs and fine ashes deposits.
The main characteristics of this different soil types are summarized in the following Table 5.7.
Table 5.7: Description, according to FAO classification, of the soils identified in the reservoir area
Soil type (FAO classification) Description of soil characteristics
Soils having an argillic B horizon with a clay distribution where the percentage
of clay does not decrease from its maximum amount by as much as 20 percent
Eutric Nitosols
within 150 cm of the surface; lacking plinthite within 125 cm of the surface;
lacking vertic and ferric properties
Ortic Luvisols Other soils having an argillic B horizon
Soils having a strong brown to red B horizon (rubbed soil has a hue of 7.5YR
Chromic Luvisols
and a chroma of more than 4, or a hue redder than 7.5YR)
Soils having an argillic B horizon; with a base saturation which is less than 50
Orthic Acrisols
percent (by NH4OAc) in at least some part of the B horizon within 125 cm of
the surface
Soils having a cambic B horizon or an umbric A horizon which is more than 25
Eutric Cambisols
cm thick
Soils which, after the upper 20 cm are mixed, have 30 percent or more clay in
all horizons to at least 50 cm from the surface; at some period in most years
have cracks at least 1 cm wide at a depth of 50 cm, unless irrigated, and have
one or more of the following characteristics: gilgai microrelief, intersecting
Pellic Vertisols
slickensides or wedge-shaped or parallelepiped structural aggregates at some
depth between 25 and 100 cm from the surface. Pellic Vertisols have moist
chromas of less than 1.5 dominant in the soil matrix throughout the upper 30
cm.
It is estimated that Vertisols comprise about 24% of all cropped highland soils (Jutzi and Haque, 1985).
Vertisols are potentially among the most productive soils of sub-Saharan Africa, but they are agriculturally
underutilised within the traditional farming practices due to excess soil moisture from waterlogging during
the heavy rains.
5.1.7 Hydrology
EPT 27 40 84 112 152 211 248 243 164 81 33 24 1426
The hydrological elaborations which are reported hereafter are aimed at the study of the mean annual runoff
and its monthly variability, and the extreme flood runoff, for the basins of the Gibe-Omo River with closing
section located at Dam Site 5 as defined in the general plan [200 GEN D SP 007 A]:
The first important aspect is the determination of the mean runoff which is a key issue for the evaluation of
the productivity of the power plant.
As no hydrometric station is located nearby the dam site, 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. While a good
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