periods of the switches can be manually adjusted or set. The operation of lighting units and lighting outlets in public areas of the elevator towers and at elevator landings in the elevator towers and dam, and the floodlighting units on the spillway structures is controlled directly by the circuit breakers serving the circuits to these lighting units and outlets.

The dam and related structures are provided with a grounding system which generally consists of main runs

of bare copper cable extending exposed along galleries and otherwise embedded in concrete of structures. The main runs of ground cable extend from and are connected to the powerplant ground mat. Electrical equipment enclosures and cabinets, metal conduits, structure handrailings and metalwork, machinery bases, and crane rails are connected to main ground cable runs with branch ground cable taps and extensions to provide a basically common and interconnected grounding system.

40. GENERAL. One spillway is provided on each abutment. Each spillway consists of an approach channel, intake structure, spillway tunnel, and deflector bucket. Spillway discharges are controlled by two 40 by 52.5-foot radial gates in each intake structure. General plans and profiles of the spillways are shown on figures 100 and 101.

41. HYDRAULICS. To determine required spillway capacity, two inflow floods were considered:

(1) A maximum probable snowflood which had a peak inflow of 380,000 cubic feet per second and a 122-day volume of 29,060,000 acre-feet.

(2) A maximum probable rainflood which had a peak inflow of 417,000 cubic feet per second and a 6-day volume of 2,063,000 acre-feet.

Studies indicated the snowflood to be the critical flood and it was therefore adopted as the inflow design flood.

The design flood was routed through the reservoir, assuming that the reservoir was at elevation 3700, top of conservation storage, at the beginning of the flood. The maximum reservoir water surface obtained from the routing was elevation 3711 and the maximum outflow was 300,000 cubic feet per second. The spillways were sized to have a combined capacity of 276,000 cubic feet per second with the reservoir at elevation 3711. The remaining 24,000 cubic feet per second would be released through the river outlets and turbines.

The locations of the spillways were set to provide a satisfactory alinement with the river. This resulted in the intake structures being located well back of the canyon rim. Intake channels were provided for flow from the reservoir at the canyon rim to the intake structures.

The intake structures were designed to control releases and provide an entrance to the tunnels. The concrete crest is at elevation 3648.00. Two 40- by 52.5-foot radial gates are installed at the crest of each intake structure to control releases. Discharge curves for the radial gates are shown on figure 102. The crest shape is designed to follow the under nappe for a gate opening of 10 feet. The coefficient of discharge for the uncontrolled crest with maximum reservoir water surface is 3.46. The crest and gates are set at a converging angle of 84° from the centerline of the

tunnel to start the transition and to decrease the tunnel portal width.

The spillway tunnels for the greater part of their length are 41 feet in diameter. The transition section downstream from the intake structure changes from a flat-arch-roof section 89 feet wide by 52 feet high to a circular section 48 feet 3 inches in diameter. From this point there is a further transition of the circular section to the 41-foot-diameter tunnel. The tunnels were designed to flow partially full, and at all sections the depth of the water will be 0.7 times the height of tunnel, or less.

At the downstream portals a concrete deflector bucket was designed to lift the jet of water a safe distance into the center of the river channel and also to deflect the jet away from the canyon wall.

42. MODEL STUDIES. The original layout for the spillway tunnels was based on data obtained from other tunnel spillways built by the Bureau, routing of the flood through the tunnels, and adapting the lower ends of the tunnels to the diversion scheme.

Extensive hydraulic model studies of both spillway tunnels were made on a 1:63.48 scale model.1 These studies aided in determining the final dimensions of the approach channel, the transition section of the tunnel, and the location and final shape of the deflector buckets. Model studies indicated that the maximum velocity in the spillway tunnel will be 162 feet per second. Discharge curves shown on figure 102 were determined by the model studies.

43. CONCLUSIONS FROM MODEL STUDIES. In addition to the maximum spillway tunnel velocity stated above, other conclusions from the model studies

are:

(1) The alinement of the tunnels was satisfactory for diversion flows and spillway flows.

(2) Preliminary tests on a 1 to 88 scale model indicated that the most satisfactory invert angle for the flip buckets was 35°. Subsequent tests on the 1 to 63.48 spillway model confirmed this.

(3) A low curved concrete wall placed adjacent to each canyon wall protected the canyon walls from further undermining and erosion damage by diversion flows.