Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
• a first part with length about 25 m and section 6.5 x 4m
• two sliding gates (one working fully closed or opened and the second one for emergency) having a
max statical load of about 150 m and a max. operating load of about 140 m;
• a second part about 80 m long
The main results of the preliminary hydraulic computation considering max and min operating level of the
outlet are listed hereafter.
Table 3.3: Middle Level Outlet – Hydraulic calculations main result
Eu/s Q V1 V2 V1 2 /2g V2 2 /2g SDHlocal DHfrict Ed/s
Res.lev. Q/A1 Q/A2 Hc1 Hc2
m a.s.l. m 3 /s m/s m/s m m m m m a.s.l.
780 315 12.12 19.70 7.49 19.77 4.49 1.74 773.77
892 725 27.92 45.37 39.73 104.92 23.84 9.23 858.92
The detailed hydraulic studies will be completed by means of the physical hydraulic modeling under
construction.
The Mid-Level Outlet rating curve (for one of the two outlet devices) can be defined having expressed the
terms of equation as a product of a constant for Q 2 obtaining:
H w
=
-4 2
4 + 2.61858E Q
being Hw the headwater on the outlet sill (reservoir elevation – sill elevation).
The jet trajectory of the flow discharged by the middle outlet leads to the min distance of impact of about 25
m from the d/s toe of the dam (Q=315 m 3 /s, Res. elev. =780 m a.s.l.) and the max distance of 135 m from the
Power House u/s wall (Q=725 m 3 /s, Res. elev. =892 m a.s.l.)
3.9.2 Permanent environmental discharge
The environmental releases will be possible by means of two separate devices:
• a conduit within the PH deriving from the u/s manifold and returning flows to the river bed
(permanent device, operating from above el. 800 m a.s.l.)
• conduits discharging the flows into a diversion tunnel (temporary device operating during the first
impounding only up to max. el. 800 m a.s.l.)
The scope of the permanent ecological discharge is to release a minimum vital flow downstream of the plant
when it might be fully inoperative (maintenance, possibly during night operations, etc.).
The main components of this outlet device are:
• a penstock
D = 0.8 m
starting from branch of manifold of unit No. 10 and
ending to valves at south corner of the PH
• two butterfly valves D = 0.8 m
max design pressure = 2.8 Mpa
This device will allow a rated discharge of about 25 m 3 /sec in order to guarantee, with the lowest normal
reservoir operating levels, a released flow higher than 15 m 3 /sec.
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