Repairs on the Moccasin Dam and Reservoir in Tuolumne County, operated by the San Francisco Public Utilities Commission, have begun five months after heavy rains caused severe damage.
The dam and reservoir, which is part of the Hetch Hetchy Project and helps provide water and power to San Francisco Bay, suffered severe erosion during the March storms.
The repairs will address the erosion to the dam and its buildings, including its auxiliary spillway, while creating barriers to direct flows downstream of the dam. Additionally, the diversion dam upstream of the reservoir will be repaired, as will culverts and water distribution lines.
Debris and log booms will also be installed, as well as access automation to reservoir outlet works. Improvements will also 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.
According to Dan Wade, the SFPUC’s Director of the Water System Improvement Program, while the repairs being made currently are permanent, SFPUC officials are looking into long-term projects at the facility, in anticipation of even heavier storms in the future.
“We anticipate a longer term project for larger floods in the future, with climate change uncertainty,” he said.
Heavy rainfall descended upon the area on March 22, sending a surge of water and debris into Moccasin Reservoir, overwhelming the reservoir’s diversion system and nearly overtopping the dam.
Wade said that within a 24-hour period, the area received up to five inches of rain, causing significant erosion, flooding and clogging.
Despite the damage, the dam still managed to work during the storm. Residents in the area were evacuated as a precaution, including the Moccasin Fish Hatchery and Moccasin Point Marina and Campground.
SFPUC crews drained the water from the reservoir before it overtopped, ensuring that water supply to the Bay Area was never in danger.
The reservoir has remained empty since March as the repairs are being made, as directed by the California Department of Water Resources Division of Safety of Dams.
SFPUC General Manager Harlan Kelly, Jr. said in a statement:
“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. … 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.”
The repairs, worth $21.5 million, are scheduled to finish sometime in early 2019. They’re being funded by the Hetch 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.
Moccasin Reservoir, operating since 1929, is one of the SFPUC’s smallest reservoirs and helps regulate the operation of the Hetch Hetchy Water and Power system.