Spillway Modifications for Chief Joseph Dam Columbia River, Washington. Hydraulic Model Investigation.

Spillway modifications were required in plans to increase power production by raising the level of the forebay pool and adding generating units. Pool was to be raised 10 ft initially and later an additional 14 ft. Effect of heads on the crest 1.33 to 1.70 times the design head was studied initially in a 142.47-scale model of Lower Granite spillway. Pressures on the spillway were satisfactory with gated flows but indicated cavitation with free flow. Surging occurred in the bays during gated flow. Preliminary studies indicated surging could be controlled by modifications. A modified Chief Joseph spillway with the existing crest, 36-ft-wide bays, and higher gates was studied with heads as high as 1.65 times the design head in a 143.35-scale, 4-bay sectional model. In a 172-scale comprehensive model of the project, flow conditions with the modified spillway without a flow deflector were satisfactory. The spillway also functioned satisfactorily with a flow deflector when the spill was regulated to avoid surging in the stilling basin. If surging was permitted, high long-period waves along the powerhouse and on the south bank at Foster Creek. Existing riprap on the north bank of the tailrace was adequate with the high head flows.