Scotts Flat Spillway Replacement Project
Scotts Flat Dam and Spillway is located on Deer Creek in Nevada County, approximately 5 miles east of Nevada City, California. The dam is a zoned earth and rockfill embankment, originally constructed in 1949 and subsequently raised in 1964 by 35 feet. Scotts Flat Dam is owned, operated, and maintained by the Nevada Irrigation District (NID) as a multi-use facility to address domestic and agricultural water needs as well as recreation and power generation. As such, the dam is part of NID’s FERC-licensed Project No. 5930 and is classified as High Hazzard due to the potential for loss of life in the event of failure. The spillway is located approximately 400 feet south of the left abutment of the embankment dam.
In the wake of the Oroville Spillway incident, DSOD and FERC required that a focused assessment be performed. Extensive field exploration was performed at the Scotts Flat Spillway to characterize the subsurface conditions. Major deficiencies were found at the Scotts Flat Spillway, including insufficient chute flow capacity and extensive erosion potential at the lower plunge pool, and improvements are necessary.
After an alternative study was conducted in 2019, and a physical hydraulic model was built to simulate the flows at the spillway under the probable maximum flood (PMF), the District determined that of the three alternatives developed, Alternative 3: The replacement of the spillway chute with a rectangular chute geometry, i.e., new floor slabs with new vertical side walls, and a new sub-drain system is the superior alternative.
The project is currently in the design process, and a thorough and complete California Environmental Quality Act (CEQA) and National Environmental Policy Act (NEPA) evaluations for Alternative 3 are underway. We will post the Draft documents once completed and the comment period begins.
