Is that a hydraulic jump basin or a roller bucket? Physical modeling of the Gross Dam Spillway Stilling Basin

Physical modeling was performed, at a 1:24 (model:prototype) length scale, to support the design, and optimization, of the proposed spillway and stilling basin on the raised Gross Dam. The spillway design included physical characteristics that were atypical and generally beyond the criteria presented in industry design guidelines. This included a 456-foot-tall, stepped spillway chute on an arch dam with a downstream slope of 0.5 horizontal to 1 vertical (0.5H:1V) with converging chute walls. Site conditions dictate the need to hold the location of the existing spillway toe in the proposed raise and existing infrastructure requires the stilling basin contain and control the hydraulic jump. The initial stilling basin design was based on a Reclamation Type III hydraulic jump stilling basin incorporating super-cavitating blocks. Physical modeling showed uncharacteristic performance of the preliminary Type III basin and demonstrated the need to move the baffle blocks significantly farther upstream to contain the hydraulic jump inside the basin and thus properly dissipate the energy. Further testing also showed that the overall length of the stilling basin could be significantly reduced while maintaining required hydraulic performance. This presentation will provide a summary of the initial design development, the physical hydraulic model results, the unique hydraulic characteristics of the basin with the steep approach spillway chute, and the corresponding design modifications implemented based on the physical model results.
The Gross Reservoir Expansion (GRE) Project is located on South Boulder Creek near Boulder, Colorado. The objective of the GRE Project is to increase the storage capacity of Gross Reservoir from approximately 42,000 acre-feet to about 119,000 acre-feet (AF). This expansion will be accomplished by raising the existing 340-foot-high Gross Dam by 131 feet (FT) to an ultimate height of 471 FT, resulting in the highest stepped spillway in the United States.