Silver Lake is in Amador County, approximately 19 miles south of Lake Tahoe, California. Silver Lake Reservoir is formed by the dam at its northwest end and is located adjacent to State Route (SR) 88/Carson Pass Highway. The boundary of Amador and El Dorado counties crosses SR 88 and runs along the existing dam. Silver Lake Reservoir is part of the District’s Project 184, which consists of a series of dams, canals, flumes, siphons, penstock, and powerhouse to deliver water from the South Fork American River for drinking water, agriculture, and power generation. Silver Lake Dam (Dam) was originally constructed in 1876. Silver Lake has an area of approximately 534 acres and a storage capacity of 8,912 acre-feet above the invert of the low level outlet works. The existing Dam is a 32-foot-high and 280-foot-long timber-crib and masonry embankment dam, buttressed by a downstream rock-filled concrete crib wall. The Dam has a 2-foot-high masonry parapet wall along the upstream edge of the crest and a reinforced concrete spillway structure with steel radial gates on the west side of the embankment. There is an existing steel outlet tower on the upstream side of the Dam. The Dam crest is currently used for access by day use recreationists and maintenance personnel. A reinforced concrete spillway was added to the Dam in 1940. The existing outlet works conduit consists of a 26-inch-diameter welded steel pipe with an upstream invert at elevation (El.) 7,242.5 feet. The Dam does not meet current dam safety standards, has exceeded its service life, and requires replacement. The Dam’s long-term reliability and integrity are compromised by voids developing due to rotting interior logs encapsulated as fill as part of the original rock and soil-filled timber crib structure constructed in 1876. In addition, the upstream gunite face of the Dam is at the end of its useful life and lacks long-term reliability. Defects in the gunite face facilitate leakage through the Dam that could potentially create more voids within the Dam caused by soil particle migration (i.e., piping). Additionally, the dam’s spillway does not have adequate capacity to pass the Probable Maximum Flood as required under FERC regulations and is susceptible to loading damage that could occur during an earthquake. The Project would replace the existing dam with a new combined gravity dam, overflow spillway structure, and associated appurtenances. A 780-foot-long and 34-foot-high conventional concrete gravity dam would be constructed at the location of the existing Dam. The new Dam would meet regulatory dam safety requirements for overall stability and freeboard. The Project would include raising Kit Carson Road approximately four feet to accommodate the probable maximum flood. Project components include; • Removal of the existing Dam, spillway, and appurtenances • Construction of the replacement Dam, spillway, and appurtenances o Combined replacement gravity dam and spillway structure, including labyrinth weir, stilling basin, wing dikes on the left and right abutments, and low level outlets o Spillway access and bridge o Cutoff wall o Grading and bank stabilization • Kit Carson Road improvements • Relocation and establishment of utilities
