A common concern with new and existing dams is ensuring adequate spillway capacity for large flood flows. This is often mitigated by providing longer spillway crests, increasing parapet wall height, or improving the efficiency of the crest. Scott Dam, located on the Eel River approximately 100 miles northwest of Sacramento, provides an example of a structure where the spillway chute played a more significant role than the crest in determining the overall spillway discharge capacity. The dam impounds Lake Pillsbury and is owned and operated by Pacific Gas & Electric (PG&E). Flow over the ogee spillway is controlled by a series of 26 rectangular slide gates and five radial gates. Unique to this structure is a spillway chute that converges from approximately 400 ft wide at the dam crest to approximately 185 ft wide at the toe of the dam with training walls bounding the chute along its right and left sides. The converging chute width results in strong flow run-up on the training walls which generates standing waves / rollers on both sides of the chute. At higher discharges these waves can produce significant overtopping of the training walls, which has become the largest constraint in determining the spillway discharge capacity. Energy dissipation was also of concern due to flow splashing over the training walls, the jet from the flip bucket impacting the downstream banks, and the potential for scour immediately downstream of the dam. The capacity of the training walls were found to be particularly sensitive to the operation of the slide gates situated along the spillway crest. This paper presents the results of a physical model study conducted on Scott Dam and describes the balancing of spillway crest capacity with training wall capacity in determining the maximum safe discharge for the project.
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