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SFPUC details Moccasin flood emergency; interim repairs planned before October
Summary
SFPUC staff reported that the March 22 Moccasin flash flood overtopped diversion structures and led to a March 23 emergency declaration; crews restored bypass flows, Moccasin Reservoir remains drained pending Division of Safety of Dams oversight, and interim repairs are targeted for completion by Oct. 1, 2018.
Dan Wade, director of water capital projects, gave a detailed update on the Moccasin emergency stemming from March 22 flash flooding. Heavy debris—sediment, trees and logs—clogged a diversion conduit, caused overflow that reached near the crest of the dam, and produced seepage on the downstream face. Staff declared an emergency on March 23, evacuated downstream areas during the acute response, and worked with contractors around the clock to clear debris and reestablish bypass flows by March 24.
Wade said investigators do not believe the seepage represents internal erosion sufficient to cause a dam-safety failure; instead, the seepage likely resulted from reservoir water surging over porous crest materials and flowing around the buried concrete core wall. The California Division of Safety of Dams (DSOD) has required a full condition assessment and indicated it may take jurisdiction of the diversion dam as part of oversight. Moccasin Reservoir has been drained and potable flows currently bypass the reservoir via the Moccasin powerhouse and a bypass pipe to Foothill Tunnel and San Joaquin pipelines.
Operations staff identified three operational concerns while the reservoir remains out of service: reduced operational flexibility if a power unit is disrupted, potential pressure/transmission issues in San Joaquin pipelines, and less ability to manage turbidity. Wade said interim repairs and improvements are requested to be completed by Oct. 1 so DSOD and SFPUC would be comfortable returning the reservoir to service while longer-term fixes and environmental review proceed. SFPUC has issued small construction contracts for interim work, will issue more, and is proceeding with environmental documentation and 35% design for longer-term projects.
Commissioners asked for clarification about what 'interim' means for the Mountain Tunnel and lining repairs; Wade said the interim work addresses critical voids and those repairs will be incorporated into the permanent fix for the affected tunnel sections. The Moccasin penstock rehabilitation contract approved earlier in the year was canceled in light of the emergency; staff plan to rebid that work once the facility is stabilized.
Next steps include completing DSOD-requested assessments, continuing debris removal and interim repairs, advancing design and environmental work for longer-term projects, and identifying funding needs for both interim and long-term work.
