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14. Hydrologic Requirements. - A tentative design flood inflow hydrograph was furnished by the Hydrology division. Because of the topography and subsurface conditions, a 500-foot-wide overflow spillway (fig. 8) seemed to have adequate capacity and be the most economical. Flood routing studies showed that the spillway would discharge a maximum of 20,000 cubic feet per second and the water surface would rise 6 feet, giving a maximum water surface elevation of 1058. 0. The crest of the dam was set at elevation 1062. 5, providing 4-1/2 feet of freeboard. A revised hydrograph was later furnished reflecting drainage area boundary changes and reconsideration of other pertinent data; and upon receipt of this hydrograph the spillway was redesigned for a maximum discharge capacity of 7, 500 cubic feet per second. The spillway width of 500 feet was retained and the maximum water surface elevation was calculated as 1055.0. After consideration of the factors influencing wave height and wave rideup, including maximum wind velocity, its direction and duration, offshore reservoir depths, fetch, slope of dam and shore, and the nature of the surface protection, the freeboard was increased to 6 feet. The crest of the dam was finally set at elevation 1061. 0 and the spillway crest at elevation 1052.0.
15. General. - One possible location for the spillway was in the saddle at the right abutment An examination of the topography revealed that the proposed spillway would dump water into a pothole with no suitable outlet. To provide an outlet, a large amount of excavation would be necessary for the required 4, 000-foot channel and this location was abandoned. Another possible location, in dense basalt, was in the vicinity of axis station 145+80. This site was chosen. The downstream toes of the embankments would be protected by confining the flow to a channel, and it was thought that the spillway excavation materials would provide the rock required in the dam construction. The centerline length of this spillway is about 925 feet.
The spillway discharge was computed as for a broad-crested weir with C ■ 2.65. The depth of the water throughout the channel was calculated. The calculated maximum depths at channel grade indicated the flow would probably be in rock throughout the length of the channel. The crest of the spillway was constructed of concrete.
C. Outlet Works
16. General. - The contributory watershed area for Potholes reservoir is
4, 000 square miles. There are many small lakes and depressions which tend to retard the runoff. During the summer, streamflows come mostly from waste water and the return flow from irrigated lands upstream. No hydrograph for a 5-year or 10-year flood for diversion studies was provided. It was assumed that a 7-1/2-foot-diameter tunnel would have adequate capacity for stream diversion during construction of the dam. Accordingly, an outlet tunnel of that size was provided, to serve also as a diversion tunnel during construction for which purpose it proved to have adequate capacity.
Two outlets were provided downstream from the gate chamber (fig. 9), one consisting of the 7-1/2-foot-diameter tunnel controlled by a 4- by 4-foot high-pressure gate, and the other consisting of a 22-inch-diameter pipe beneath the tunnel, controlled by an 18-inch pivot valve. The former is used periodically for sluicing the silt from the trashrack structure and the latter is used for supplying existing water rights downstream. The impounded water is distributed for irrigation purposes through canal systems with headworks at other locations. Both outlets, while their discharge is independently controlled, utilize the same trashrack structure, section of tunnel from trashracks to gate chamber, and stilling basin.
Because of the excellent grade of rock existing at the dam site, a tunnel-type outlet works seemed most desirable (fig. 9). Owing to the concave conformation of the left abutment, it was necessary to aline the tunnel with a sharp bend upstream from the gate chamber in order to keep the tunnel excavation in solid rock. The outlet works structures were designed to be constructed from concrete and to act as continuous frames in resisting loads.
17. Design Details. - The outlet works consist of a trashrack structure, tunnel, gate chamber7 valve house, outlet pipe, and stilling basin. (See section 18 for discussion of the outlet pipe.)