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The El Dorado Forebay Dam and Reservoir (Forebay) was constructed in 1923 as a key part of the El Dorado Hydroelectic Project and stores water diverted from the South Fork of the American River. The diverted water is received from four upper watershed reservoirs, Caples and Silver Lakes located along Highway 88, and Echo Lake and Lake Aloha along Highway 50. Water is delivered into Forebay, after being diverted from the El Dorado Diversion Dam in Kyburz, through the El Dorado Canal. The water is conveyed approximately 22 miles through a series of man-made structures including earthen and concrete-lined canals, wooden and concrete flumes, bedrock and concrete-lined tunnels, and steel-pipe siphons. Forebay water is utilized both for consumptive and power generation purposes. A portion of the water delivered to Forebay is diverted to the Main Ditch and conveyed to the District’s water treatment plant, where the water is treated and distributed in the drinking water system. The other portion of water delivered to Forebay is diverted into a penstock and conveyed to the El Dorado Powerhouse. The diverted water flows through the El Dorado Powerhouse, where it is used to generate hydroelectric power for delivery to a Pacific Gas and Electric Company transmission system, and the outflow is discharged back into the South Fork American River.
Project Background - Remediation
The Forebay Dam Modification Project (Project) will remediate Forebay and its associated facilities. The primary remediation activities will be to construct an earthen stability buttress that will both strengthen Forebay dam and increase its water storage capacity. While the District is currently working on design, environmental review, permitting, and other administrative aspects of the project, the active construction period is anticipated to begin in the summer of 2017 and be completed in 2019.
Reasons Why Project is Required
State and federal regulators—California Department of Water Resources – Division of Safety of Dams (DSOD) and Federal Energy Regulatory Commission (FERC)—required EID to review the stability of the Dam in 2003. A series of technical investigations and studies of the Dam's stability required extensive time to complete. Analyses from the studies completed over the past decade indicate that stability improvements are required to the dam to better protect public safety by protecting downstream residents, life, and property. EID has since been working with DSOD and FERC to determine the engineering design and extent of the modifications needed to achieve these stability requirements.
Since the Forebay was constructed in 1922, the reservoir has lost approximately one third of its storage capacity from siltation. While the Project is primarily designed to stabilize the dam, it will also recover this lost storage capacity which will benefit District customers by improving the reliability of emergency drinking water storage from less than one day to 6 days. Finally, it will protect District ratepayers from the cost of required repairs through optimizing hydroelectric revenue to offset capital costs.
The Project is comprised of the following major elements: Constructing an earthen stability buttress below the Dam and raising the Dam 10 feet; repairing the unstable reservoir inlet; and armoring the upstream face of the Dam.
In summary, the project is required to satisfy specific regulatory mandates issued by both state and federal dam safety agencies, while improving the reliability of the drinking water system and minimizing impacts to District ratepayers through increased hydroelectric revenue.
Community Benefits From Project
Dam stability remediation projects, by their very nature, are significant financial and construction undertakings. While the District acknowledges that the lengthy construction period will impose some inconvenience to neighbors surrounding the construction site, it is also anticipated that the project will provide revenue generation brought about by outside workers utilizing the food, lodging, and other services from businesses in the area.
Frequently Asked Questions (FAQs) Updated May 23, 2016
Draft Environmental Impact Report (DEIR) October 2013
Final Environmental Impact Report (EIR) March 2014
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