Page - 1542 - in Book of Full Papers - Symposium Hydro Engineering

requirements. The most economical location to pass around or under the dam and meet the foregoing requirements should be selected. An alternate plan used for some deep reservoirs has a high-level intake at the head of an inclined tunnel connecting with a tunnel through the abutment near river level. After serving its purpose of diversion during construction, the low-level tunnel is plugged at the upstream side of the intersection and the inclined tunnel and downstream portion of the low-level tunnel with a connecting curved transition from the permanent outlet. This plan may be economical and satisfactory under special conditions where the reservoir is very deep, the minimum water surface is high, and rock foundation is adequate. Because of the difficult foundation and structural problems involved in this type of construction, thorough foundation investigations and comparative studies should be made and this plan should not be selected without full analysis of its practicability and economy. The advantage of the high-level intakes for a very deep reservoir is the lower intake structure and lower head on intake gates, resulting in a less expensive intake structure. Disadvantages of this type of outlet are: cost of the inclined tunnel, difficulty of obtaining an adequate foundation for the intake structure, and the difficulty of excavation for and the complexity of the transition structure at the junction of the inclined tunnel with the low-level tunnel. Particular care should be taken to obtain high quality concrete, smooth gradual curves, and good alignment at the junction. If the intake structure is located directly over the low-level tunnel, there should be an ample thickness of good, sound rock between the intake base and the tunnel to transmit the foundation loads to the main rock mass. Sometimes conditions may be favourable for locating the intake far enough to one side of the low-level tunnel to simplify the problem of supporting the intake structure. For some high dams, outlet works separate from diversion facilities and located much higher than the river channel may be suitable. In such cases, overall economy may result from use of separate high-level outlets and use of the downstream portion of the diversion tunnel as part of the spillway outlet. 1542