Model tests of hydraulic performance of Pit 6 Dam stilling basin

Pit 6 Dam, located on the Pit River in northern California, was constructed by Pacific Gas and Electric Company in San Francisco in 1965 for hydroelectric power generation. The gross head of the hydroelectric installation is 48.5 m for a rated generating capacity of 89 MW Flow over the ogee spillway is regulated by two radial gates, and it discharges into a 33.5-m-wide and 19.5-m-long stilling basin. Since completion of the dam, there have been chronic problems with the floor blocks, some of which were dislodged more than once. To elucidate the primary cause of floor block failures and develop a new type of block, a 1:28-scale undistorted hydraulic model was constructed, and direct measurements of the longitudinal and lateral components of the forces acting on the model floor blocks were taken using a load cell. Extremely large fluctuating loads acting on the existing blocks in the lateral direction were found to be the primary cause of the block failures. Triangular hollow blocks, which substantially reduce the lateral loads, were then developed using the model.