Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
The downstream cofferdam, raised at el. 691 m a.s.l., will protect the dam foundations area from the
backwater flows.
3.7 Dam
3.7.1 General
The dam is a classical gravity structure to be constructed using the technology of RCC (rolled compacted
concrete).
The dam comprises a central overflow block, including the spillway structure, and two lateral emerging
blocks.
The design of the transversal sections is based on the classical basic triangle with vertex at el 893 m a.s.l. and
a total opening of 0.75 (u/s slope = 0.10, d/s slope = 0.65).
The large spillway, designed for Q= 10.600 m 3 /sec (10.000 yrs return period flood) includes:
• an overflow crest controlled by # 9 radial gates (12x19 m each)
• the chute on the d/s slope, divided into three independent canals, with H = 103 m (largely lower than
the H= 140 m max. suggested by the ER)
• a deflector bucket (ski jump) energy dissipating device (lip el = 790 m a.s.l., lip angle = 20°)
Two middle outlets are foreseen through conduits embedded in the lateral blocks with the intake at el. 755. m
a.s.l. and capable of discharging up to 800 m 3 /sec each.
The grouting screen will be carried out from the dam u/s toe, on a vertical plane, reaching a maximum length
of about 80 m below the dam foundations.
The screen will be carried out in subsequent phases (from the primary injections with about 8 m distance up
to the quaternary, where necessary, with an interval of 1 or 2 m) applying the Grouting Intensity Number
(GIN) methodology.
A large drainage system is envisaged to control the seepages both in the dam foundations and within the dam
body, comprising:
• the u/s screen in the foundations (D=102 mm, @1.5m, H = 20m) behind the grouting curtain, to
control seepages from the reservoir
• the d/s screen in the foundations (D=102 mm, @1.5m, H = 20m) to control the backwater seepages
from the Power House
• the transversal screens in the foundations, controlling seepages from the abutments
• the drainage curtain in the dam body (D=250 mm @3m) behind the enriched RCC layer, will control
the seepages between the lifts within the dam body
Five longitudinal inspection galleries are located at various elevations, together with the transversal ones.
These galleries will collect and remove the seepages controlling the uplift pressures and allow the installation
of the internal monitoring instrumentation.
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