CS10D - Concurrent Session 10D: Garrison Dam - Iterative CFD Modeling to Evaluate Spillway Overlay Measures with 70 ft of Uplift Resistance

The U.S. Army Corps of Engineers recently completed an Issue Evaluation Study (IES) and subsequent Dam Safety Modification Study (DSMS) for Garrison Dam. The IES identified several risk driving potential failure modes (PFMs) for the 800 ft wide, 3,000 ft long, and 1,150,000 cfs capacity reinforced concrete spillway. One of the highest risk driving PFMs was associated with uplift forces of approximately 70 ft on the stilling basin and downstream half of the lower spillway chute slabs due to the differential water surface upstream and downstream of the hydraulic jump. This uplift was not capable of being adequately resisted by the existing configuration; therefore, numerous lower chute and stilling basin modification measures (i.e., alternatives) were iteratively developed as part of the DSMS to mitigate this uplift.

One such measure included supplementing existing uplift resistance with additional weight in the form of a substantial concrete overlay (i.e., gravity resisting force). The magnitude of uplift and associated required overlay weight for resistance varied longitudinally, therefore, the overlay varied in thickness and its extent was limited to only a portion of the lower chute and stilling basin. The maximum required overlay thickness to resist full uplift was estimated to be about 16 feet and was located at the upstream end of the stilling basin. As a result of the overlay configuration, a grade break was present in the lower chute and the overlay terminated in a vertical curve to reduce stilling basin length encroachment. The hydraulic adequacy and performance of this unique configuration was uncertain. As such, computational fluid dynamics (CFD) modeling was undertaken to provide a “proof of concept” understanding of the hydraulic adequacy of various modification configurations.

Configurations evaluated using CFD modeling included “smooth” and stepped chute overlays in combination with and without stilling basin modifications. Of particular interest was the hydraulic performance of the stepped chute overlay configuration with its atypical entrance and exit transitions. Stilling basin modifications were generally focused on baffle block size and location as well as end sill size. The results of CFD simulations provided preliminary confirmation of the potential hydraulic adequacy of the overlay measure. 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.