Avista is implementing spillway modifications to reduce Total Dissolved Gas (TDG) supersaturation downstream of Cabinet Gorge Dam. The key feature of the modifications is the addition of roughness elements, similar to supercavitating baffle blocks, to break up the spillway jet, thereby reducing the depth of plunge and TDG supersaturation. The work is progressing in a step-wise manner. A single bay was modified in 2012 and field tested in 2013. Following the initial field tests, a CFD model was

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... eveloped to aid in design refinements for the prototype and to improve the design prior to modification of subsequent spillway bays. The prototype demonstrated that spillway modifications are an effective method to reduce TDG downstream of a spillway discharging freely into a deep plunge pool. The CFD model has allowed the design to be simplified while both maintaining the plunge depth improvements of the initial prototype and reducing the effect of the modifications on spillway capacity. This paper presents the prototype design and the design that will be implemented for the next two bays, summarizes the results from the prototype field tests, and describes the CFD model and results.