Mossyrock Arch Spillway

Design of the Mossyrock spillway on the Cowlitz River for the Department of Public Utilities of the City of Tacoma required careful selection of the spillway arrangement to be used in order to dissipate 11,000,000 hp of energy from a flow of 276,000 cfs falling 340 ft within a limited area of a deep, narrow gorge. A combination of warped buckets and plunge pool was selected to dissipate the energy. Hydraulic model tests were conducted to verify and modify the design, where necessary, in order to provide a final design for the spillway system. The spillway went into operation in November, 1968; maximum gate opening to date was 5 ft to 6 ft. The flow conditions for the spillway openings were good and were as predicted in the model; no air-entraining vortices or gate vibrations were noticed.