CS12C - Concurrent Session 12C: Garrison Dam Spillway Modification - Spillway Overlay Alternatives Analysis using CFD Modeling (Hydraulic White Paper Series)

Abstract Description: The U.S. Army Corps of Engineers is currently engaged in the Preconstruction Engineering and Design phase for the Garrison Dam Spillway Modification Project. The implemented project modifications ($1B+) will mitigate several risk-driving potential failure modes for the 3,000 ft long reinforced concrete spillway required to convey 1,150,000 cfs within a width that contracts from 1,300 to 800 ft.

The previous study phase identified an alternative to mitigate uplift forces of up to approximately 70 feet in the lower chute and stilling basin, which included supplementing existing uplift resistance with a substantial concrete overlay (i.e., weight). This unique configuration featured an overlay with a stepped profile with varying step heights up to 3 ft tall on the lower portion of the spillway chute and into the stilling basin. The chute overlay thickness varied from 2.5 to 16 ft with a convex vertical curve terminating in the stilling basin. This configuration also included a 4-ft-thick overlay in the stilling basin and new baffle block and end sill arrangements. The adequacy of this configuration was previously confirmed using computation fluid dynamics (CFD) modeling.

The scope of the current design effort was to further evaluate and refine the aforementioned lower chute and stilling basin modification configuration by achieving uplift resistance through a balance of concrete and water weight. This was initiated by evaluating an array of additional modification configurations using CFD modeling to yield a range of potential hydraulic performance and uplift resistance data. A total of fourteen additional modification configurations were developed, which featured three lower chute overlay thicknesses, two lower chute overlay types (i.e., smooth and stepped), two step face types (i.e., vertical and beveled), two stilling basin overlay thicknesses, three baffle block and end sill heights, two baffle block stations, several baffle block widths and spacings (including variable), and two end sill slopes.

The CFD simulations provided a valuable range of data for use in design advancements. The modeling indicated that a stepped lower chute overlay profile with a thickness of 8 ft coupled with 20-ft-tall baffle blocks and end sill provided the greatest balance between hydraulic performance, uplift resistance, and economy. Some additional findings included improved hydraulic performance for stepped versus smooth overlays as well as increased energy dissipation for beveled versus vertical step faces and for baffle blockage percentages greater than 50 percent.