Garrison Dam - Iterative CFD Modeling to Evaluate Spillway Overlay Measures with 70 ft of Uplift Resistance

Differential uplift forces on the Garrison Dam concrete spillway lower chute and stilling basin slabs of up to 70 feet are expected during infrequent flood events, including the 1,150,000 cubic feet per second probable maximum flood event (PMF), and exceed existing uplift resistance. Uplift resistance configurations were iteratively developed and included “smooth” and stepped concrete overlays up to 16 feet thick (i.e., mass gravity resisting force) and associated stilling basin modifications. These configuration geometries were unique with atypical entrance and exit transitions, which resulted in uncertain hydraulic performance and adequacy. As such, computational fluid dynamics (CFD) modeling was undertaken to provide an iterative “proof of concept” understanding of the hydraulic adequacy of various modification configurations. The results of CFD simulations provided preliminary confirmation of the potential hydraulic adequacy of the overlay measure to mitigate a potential failure mode associated with the excessive uplift forces. Simulation result adequacy ranged from marginal for a smooth chute overlay without stilling basin modifications to near complete for a stepped chute overlay with stilling basin modifications.