Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
It can be noted that, in normal operating conditions, the device will be necessary and effective only for the
very first trunk of the reach downstream of the plant.
This since, while going downstream, the lamination effects along the valley will however guarantee a
continues flow during the whole day (n.d.r. in case no energy production will be envisaged during the night
for some periods).
3.9.3 Temporary environmental discharge
The scope of the temporary ecological discharge is twofold:
• to release the required ecological discharge during the first impounding
• to contribute to the controlling of the reservoir impounding during the tunnel closure
The main components of this outlet device are:
• two pipes
D=1.6 m (approx.)
installed through diversion tunnels plugs
embedded in plugging concrete (TL or TR
according to construction program)
• four butterfly valves D = 1.6 m
max design pressure = 2.1 Mpa
This device will allow a rated discharge of about 35 m 3 /sec for each of the two pipes (for the lowest water
levels about el 700 m a.s.l.) progressively increasing up to reaching about 70 m3/sec with the reservoir
impounding.
The required ecological releases during first impounding will vary depending on the downstream flows (i.e.
wet or dry year in the residual basin) on the impounding starting and of the inflows sequence of the specific
year.
However the maximum discharges allowed by this outlet device (varying from about 35 to 70 m 3 /sec with
water levels for each pipe) can be compared to the runoffs at Gibe III during the dry season varying from a
minimum of 25 m 3 /sec in March 1973 to the averages of 61 to 141 m 3 /sec January – May.
This comparison shows that the designed device guarantees a full flexibility, together with the higher middle
outlet works, to the selection of the optimum ecological release during first impounding.
3.10 Power Waterways
The power system, including mainly two power tunnels, two surge tanks, two vertical underground
penstocks and above ground Power House, is located in the left abutment.
The inverts of the tunnels have been set at el. 780m a.s.l., that is low enough below the minimum operating
level (800 m a.s.l.) in order to assure at the same time, a good submergence and avoid vortices, and to reduce
the internal pressure in the tunnels as much as possible.
The trashracks will be installed to protect the EM equipment from possible debris entering the tunnels.
Downstream of the intake structures, a transition section is insuring the transition to the 11.0 m diameter
circular tunnels.
The tunnel bottom is located at el 780 m a.s.l. at the intake and at el. 775 m a.s.l. at the base of the shaft.
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