The Folsom Dam Auxiliary Spillway Physical Model Study as a Design Tool

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Folsom Dam is on the American River about 20 miles northeast of Sacramento, California. The dam was designed and built by the Corps of Engineers (COE) and transferred to Reclamation for operation and maintenance in 1956. The dam is a concrete gravity structure 340 ft high and impounds a reservoir of a little more than one million acre-ft. The existing dam features two tiers of four outlets each, controlled by 5- by 9-ft slide gates having a discharge capacity of approximately 700,000cfs. Updated PMF calculations require a total flood discharge in excess of 1,000,000cfs. To safely pass the PMF, an auxiliary spillway is proposed and as a design tool to ensure safe and efficient operation of the structure, a physical model study was utilized. Physical modeling was performed on the upstream portion of the proposed auxiliary spillway near the left abutment of Folsom Dam. The auxiliary spillway control structure will contain six submerged tainter gates (also known as top-seal radial gates) that can discharge about 300,000 cfs at a maximum pool elevation of 481 ft. Ayres Associates and Utah State University, with guidance from the USACE have utilized a physical model of the auxiliary spillway and the approach channel in the reservoir as a design tool.