San Francisco, CA – The San Francisco Public Utilities Commission (SFPUC) has begun critical improvement and repair work on facilities at Moccasin Dam and Reservoir, a Tuolumne County site that suffered damages during heavy rains last March.
Crews at the SFPUC-owned facility will repair erosion of Moccasin Dam and appurtenant facilities, including its auxiliary spillway and create barriers to direct flows downstream of the dam. In addition, work will be carried out to repair the diversion dam upstream of the reservoir, fix culverts and water distribution lines, install debris and log booms, and provide access and automation to reservoir outlet works. Furthermore, improvements will be made to raise the internal core wall within the main dam to prevent internal seepage should a similar storm event occur in the future.
On March 22, heavy rainfall sent a huge surge of water and debris into Moccasin Reservoir, overwhelming its diversion system and nearly overtopping the dam. While the dam functioned properly under this stress, an evacuation order was issued for all people potentially in harm’s way, including individuals at the Moccasin Fish Hatchery and Moccasin Point Marina and Campground. SFPUC crews drained the reservoir before it overtopped, and at no point was there any threat to the Bay Area’s water supply.
“We are taking the necessary steps to improve and repair this crucial piece of infrastructure to ensure that we are prepared as soon as possible for the next major weather event,” said SFPUC General Manager Harlan L. Kelly, Jr. “The safety of our staff and facilities is our number one priority, and through this project, we will restore the functionality of Moccasin Dam. At the same time this extreme weather event makes it clear that we have to fully evaluate this facility and consider long-term improvements that will allow it to serve us for the next 100 years in the face of climate change uncertainty.”
The SFPUC has kept Moccasin Reservoir empty since the March event, as directed by the California Department of Water Resources Division of Safety of Dams (DSOD) while interim repair and improvement projects are being implemented. Inspections by the DSOD and the SFPUC informed the basis of the repair project, which will enable the facility to resume operations this fall while preparing the site for the upcoming rainy season. The work is scheduled to be concluded by early 2019. A separate project to implement longer-term improvements will be considered after full evaluation of the facility to prepare for uncertainty brought on by climate change developments.
"During the response to this incident, SFPUC and DSOD worked closely together to take immediate actions to mitigate a developing dam safety emergency situation,” stated Sharon Tapia, DSOD Division Chief. “DSOD continues to work with the SFPUC as the design and construction of the repairs progress, keeping public safety at the forefront of any decision.”
The $21.5 million interim improvement project will be funded through the Hetchy Hetchy Water and Power 10-Year Capital Improvement Program, a $949 million long-term plan to upgrade the City’s water and hydroelectric power systems. As part of regular agency protocols, the dam will continue to receive weekly visual checkups and monitoring of instrumentation from SFPUC crews and biannual inspections from dam safety engineers.
Moccasin Reservoir is one of the SFPUC’s smallest reservoirs and is used for regulating the operation of the Hetchy Hetchy Water and Power system. Moccasin Dam has been operating since 1929.