Environmental and Social Impact Assessment - Gibe III

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Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 Stoplogs are foreseen for the temporary closures of the tunnels allowing the bottom outlet works achievement. The tunnels, running below the right abutment of the dam, have the geometrical characteristics indicated in the following table: TR TC TL Table 3.1: Geometrical characteristics of the diversion tunnels Tunnel Cross-section Horse shoe D1/D2=14/13 m Horse shoe D1/D2=14/13 m Circular D=7 m Inlet – outlet invert level [m a.s.l.] Section S [m 2 ] 680 - 674 150.4 1’209 680 - 674 150.4 1’029 687 - 681 38.5 968 Length [m] The flows will be returned to the Omo river downstream of the Power House site with each tunnel separately discharging into the river bed through a relevant canal. 3.6.3 Cofferdams The diversion works include the construction of two cofferdams (in two stages) on the Omo river bed: • main pre-cofferdam (upstream) 696 m a.s.l. crest elevation stage 1 • main cofferdam 720 m a.s.l. stage 2 • secondary pre-cofferdam (downstream) 680 m a.s.l. stage 1 • secondary cofferdam 691 m a.s.l. stage 2 The two pre-cofferdams (stage 1) will be constructed at the beginning of the dry season. These are zoned earth-rock embankment with basalts and trachytes used for the main body. The impervious layer is constructed using the silty sands (SM) from quarry 8 integrated with 15 % of high plasticity clay (CH). The main cofferdams (stage 2) will be constructed after the river closure. The cutoff will be built in order to reduce the seepages in the alluvium. Therefore the embankment construction will be carried out using the materials from the basalts, trachyte and alluvium quarries. The impervious layer of the main cofferdam will be carried out fixing a geo-composite membrane to the cutoff and therefore raising this membrane with geotextile protections at “zig-zag” on the embankment compacted layers. The crest and the downstream face will be constructed using selected basalt more resistant to any minor overtopping. This main cofferdam will reach elevation 720 m a.s.l required for diverting floods with a return period of 30 years using the three diversion tunnels. CESI SpA - Mid-Day International Consulting Engineers Page 53
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. CESI SpA - Mid-Day International Consulting Engineers Page 54
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