San Francisco, CA— Following the completion of interim repairs carried out by the San Francisco Public Utilities Commission (SFPUC) to address damage from the March 22, 2018 storm event, Moccasin Reservoir has been returned to service, supporting water reliability for the agency’s 2.7 million customers.
“We prioritize the safe operations of our system, and after a series of rigorous and necessary improvements projects, we are ready to bring the Moccasin Reservoir back into service,” said SFPUC General Manager Harlan L. Kelly, Jr. “Since last year’s extreme weather event at the dam, we pursued a top-to-bottom safety review of the facilities, and we have made interim repairs and improvements to enhance the safety of this important facility while we continue to develop plans for long term improvements.”
On March 22, 2018, a heavy rainfall storm event resulted in a surge of water and debris down several creeks Creek into Moccasin Reservoir. The surge overwhelmed the Moccasin Creek Diversion Dam upstream of the reservoir, and threatened to overtop Moccasin Dam. At no time was there a threat to the Bay Area’s water supply, and SFPUC crews were able to lower the water level in the reservoir before it overtopped the dam.
Following the incident, SFPUC assessed damage at the site and determined that debris from the storm had damaged and blocked a crucial outlet and culverts at the facility, and the surge caused severe erosion at the toe and abutments of the Moccasin Creek Diversion Dam and along portions of the Moccasin Dam Auxiliary Spillway. The Moccasin Dam itself also suffered minor erosion, and damage to various infrastructure components—including potable water distribution lines, metal gates, fences and culverts.
SFPUC crews have completed a series of interim repairs which were divided into two phases. Phase I focused on repair of the erosion at the toe of Moccasin Dam and improving access to an outlet tower and gate valve that releases water from the reservoir. Work on Phase I is complete, with the exception of a new gate valve that will soon be installed with remote operation capability.
Phase II of the interim repairs extended the core of Moccasin Dam to the full height of the dam, addressed damage to the Moccasin Creek Diversion Dam and Moccasin Auxiliary Spillway, and repaired several culverts. In the coming months, crews will be constructing an additional debris barrier above the Moccasin Creek Diversion Dam to prevent debris from blocking the outlet in the future and a new berm to similarly protect the fish hatchery located downstream of the dam.
The work has been sufficient for the California Division of Dam Safety (DSOD) to allow the SFPUC to bring Moccasin Reservoir back into service, with full capacity restored on June 6.
As part of the SFPUC’s ongoing dam safety program, the SFPUC will continue to evaluate Moccasin Dam and its ancillary structures to identify any further improvements that might be needed. Any identified long-term improvements would be undertaken over the next 5 to 10 years.
Moccasin Dam is a rockfill dam with an interior concrete core wall that was completed in 1930. Although it is one the SFPUC’s smallest facilities, the Moccasin Dam and Reservoir are critical to the SFPUC’s Regional Water and Power System. Moccasin Reservoir impounds Hetch Hetchy water released from the Moccasin Powerhouse before the water enters pipelines that deliver it to the Bay Area.