SFPUC updates commission on Moccasin dam emergency after intense March 22 storm
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Summary
Assistant General Manager Steve Ritchie told commissioners heavy rains on March 22 produced extraordinary seepage at Moccasin Dam, an evacuation order for downstream residences, extensive hatchery losses and ongoing inspections; staff said repairs and safety assessments are underway and Hetch Hetchy deliveries are expected to be restored the following week.
Steve Ritchie, the SFPUC’s Assistant General Manager for Water, told the commission on March 27 that a nearly five-inch rain event on March 22 — including about 3.1 inches in a four-hour span at Priest Reservoir — triggered widespread runoff into the Moccasin/Moxon watershed and produced ‘‘extraordinary leakage’’ through the face of Moccasin Dam.
Ritchie said the agency activated its emergency action plan, notified the Division of Safety of Dams and Tuolumne County, and issued an evacuation order for residences downstream and some Moccasin cottages. ‘‘That was the point at which we said this is truly an emergency and invoked the evacuation order,’’ he said, describing seepage and muddy discharges from the dam face.
Staff described visible damage across the watershed: hillside gashes and large debris flows into Moxon Creek, a clogged diversion structure, an emergency spillway flowing heavily, and an aerial photo sequence showing sediment-laden inflows to the reservoir. The Moccasin Creek fish hatchery was heavily affected; Ritchie said roughly 90 percent of fish were ‘‘either washed out or killed,’’ and the remaining stock had been trucked to Don Pedro Reservoir for temporary holding.
Ritchie said crews lowered the reservoir level to stabilize conditions, replaced damaged pressure-relief stacks on a critical supply pipeline, and began cleanup and repairs to potable water lines for downstream residents; in some cases the SFPUC was delivering trucked water until permanent repairs are complete. A consultant team completed field inspections and delivered immediate-action recommendations; staff said further geotechnical investigations — including borings — will determine whether physical modifications to the dam or spillways are required.
On operations, Ritchie said the SFPUC had planned to restore Hetch Hetchy deliveries on April 5 but that the work would require more time: ‘‘We had planned on restoring the water supply today, but there’s too much work to do. It’s gonna be next week before we can actually restore the Hetch Hetchy supply,’’ he said, noting the system could be served in the interim by other local supplies.
Commissioners pressed staff on the magnitude of seepage and the length of evacuations. Ritchie said that while ‘‘all dams leak a little bit,’’ the observed muddy seepage was extraordinary and ‘‘a sign that there is a real problem’’ requiring additional investigation. He added that the Division of Safety of Dams was on site the afternoon of March 22 and ‘‘concurred that our staff took all the necessary actions to deal with it.’’
The presentation closed with a list of next steps: continued inspections of dams, spillways, tunnels and pipelines; repair planning; hatchery cleanup and restoration assistance; potable-line repairs for affected residences; and follow-up reporting to the commission after consultants complete their investigations.
The commission received the update and entered a period of questioning; no formal vote was required on the briefing.
