Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
The length of the two power tunnels equal approximately 660 m for the right tunnel and 870 m for the left
one with a longitudinal slope of approximately 0.6% and 0.75%, respectively.
The flow section of 95 m 2 implies a maximum flow velocity in the power tunnel of about 5 m/s.
The tunnel lining will be executed by a 50 cm thick cast in place concrete with reinforcement bars over the
temporary shortcrete layer.
Two surge shafts, one each line, are required for the Gibe III Power Plant, in order to mitigate the water
hammer effects in the gallery due to rapid change in the flow caused by turbines maneuvers.
Internal diameter of the shafts is fixed at 20 m and an orifice diameter of 5.8 m has been selected. The two
structure have been designed to be almost entirely underground having a total height of 35 m and 25 m (for
the left and the right tunnel, respectively) above the ground level.
Two penstocks with a diameter of 7.7 meters have been foreseen. The diameter has been selected to maintain
flow speed not much larger than 10 meters per second. At their lower end five plus five wye branches with a
diameter of 3.5 m, intercepted by butterfly valves, feed the generating units. Wye branches have been
designed for smooth hydraulic flow to avoid excessive head losses, vibration and cavitations.
3.11 Power House
The power house is located on the Omo river left bank, about 550m downstream of the dam axis.
The Power House building is disposed parallel to a steep (almost vertical) rock front, approximately along
the N-S direction, slightly rotated in respect to the river alignment, in order to reduce the impact angle
between the turbine tailraces and the river stream.
This location has been decided taking into account the geomorphology of the river banks, trying to reduce as
much as possible the rock excavations.
The river right bank will be excavated in order to found the Power House structure on sound rock and at the
same time to assure the submergence of the turbines draft tubes.
The power house will be a reinforced concrete structure with a total length of about 250m and a width of
about 57m.
The main body of the building is about 37m wide; behind the building a 5m wide draft tube and a 21m wide
upper transformer deck complete the structure.
Inside the power house are foreseen n.10 Francis turbines each one with an installed power of 187 MW.
The n.2 penstocks excavated inside the rock exit directly at the turbine elevation, behind the Power House,
where n. 2 manifold with 5 branches each is connected to the n.10 turbines.
The main elevations of the structure are the following:
• Upper floor (transformer deck) (el. 702.0 m a.s.l.)
• Machines hall floor (el. 678.7 m a.s.l.)
• Generator floor (el. 674.6 m a.s.l.)
• Turbine floor (el. 670.3 m a.s.l.)
• Spiral case floor (el. 665.5-662.0 m a.s.l.)
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