Repairs are needed to a dam in the Sierra foothills that came close to overflowing last week as well as to the water system and roads serving the surrounding communities, San Francisco Public Utilities Commission officials said Tuesday.
Five inches of rainfall in Tuolumne County on March 22 caused a surge of water and debris into the Moccasin Reservoir. After officials saw water leaking from the downstream face of the Moccasin Dam, evacuations were ordered for the Moccasin Fish Hatchery and Marina.
“It was just a massive amount of water in a short amount of time,” Steve Ritchie, assistant general manager for water at the San Francisco Public Utilities Commission, said. “In the case of Thursday’s event, the water got within 1.2 feet of the top of the dam and overtopping a dam is the last thing you want to have.”
Within 36 hours of the incident, the reservoir was completely emptied into a larger dam, the Don Pedro reserve, so that The City’s utilities department and a third party inspector could look for any necessary repairs for the reservoir.
Residents were able to move back into their homes on Saturday.
The Moccasin dam, which is owned by San Francisco, is partially responsible for regulating the Hetch Hetchy water system that supplies water around the Bay Area.
The incident had no impact on the Bay Area’s water supply but some residents in the surrounding community, located 100 miles east of San Francisco, still do not have water.
“There are still a couple of residents that are downstream on the other side of Highway 120, they don’t have any water supply, so they are who we are trucking water to,” Ritchie said.
Damages to the surrounding community include erosion under nearly half of state Highway 49 and repairing potable water lines, according to officials.
In addition, 90 percent of the fish in the Moccasin Fish Hatchery were either washed out or killed, according to Ritchie.
“The Fish Hatchery is definitely out of commission for awhile,” he said.
The Moccasin dam was inspected five years ago, and questions remain about how an incident like Thursday’s could have happened.
“If the dam had been inspected and passed, where does the fault lie?” Ann Moller Caen, SFPUC commissioner, asked.
“We all are going to be looking at the whole situation,” Ritchie said. “Are there problems with the dam? Are there things that we need to do differently about how we manage it?”
Inspection of tunnels, pipelines and spillways for possible damage are currently in process and SFPUC staff said that it was too soon to estimate the costs of repairs.