C153514 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I I I liner sections. The existing 3D-inch diameter steel pipe at the base of the control tower and the oval steel pipe downstream of the lower tunnel are generally in good condition and will not be relined; 5. Construction of new intake structure. including installation of a new reinforced concrete structure, air vent pipe system, hydraulic cylinder, hydraulic lines, gate, manual control system and trash rack. The working area excavated at the new intake structure will be backfilled with sandstone riprap materials; 6. Completion of miscellaneous repairs to the existing outlet conduit and tower including welding repair of the leaking joint in the oval steel liner pipe, and mortaring to close off the existing inlet port for hydraulic line access on the side of the tower; and 7. Breaching of the coffer dam. The coffer dam shown on Fi gure 11 has been desi gned to store approximately 3.5 acre-ft of water maximum on a temporary basis. This storage volume should be sufficient to handle expected spring flows and seepage from sediment consol idation with the immediate catchment area of Gurley Reservoir assuming that all upstream flows are di verted away from the Gurl ey system and assumi ng that the coffer dam storage is released on a daily basis. The coffer dam is not designed to store storm events of any significant magnitude due to the high costs associated with the relatively large coffer dam embankment which would be required. Expected runoff from the 10-yr, 24-hr storm event for this area is estimated to be approximately 200 to 220 acre-ft which would require an embankment top elevation of approximately 8,225 ft. Therefore, if the storage capacity of the coffer dam is expected to be exceeded, the construction contractor must be prepared to evacuate the outlet conduit in preparation for possible overtopping of the coffer dam followed by -22-