Moccasin Dam at Hetch Hetchy Water & Power’s company town Moccasin remains in poor condition due to damage incurred during the March 22 megastorm that tore up Groveland, Moccasin and Highways 49 and 132, according to the state Division of Dams Safety.
The dam, called Moccasin Lower in a state report, has capacity of 554 acre-feet, it was built in 1930, and it’s owned by the San Francisco Public Utilities Commission. It’s one of 10 dams rated poor statewide out of 1,246 dams under state jurisdiction.
“Moccasin Lower Dam is rated poor in today’s report because of the incident that happened earlier this year,” Chris Orrock, a public information officer with the state Department of Water Resources, Division of Dams Safety, said Tuesday in a phone interview. “The repairs are not yet complete. The Division of Safety of Dams has received and approved the repair plans, and the dam owner is in the process of completing repairs.”
On March 22, heavy rains sent a surge of water and debris into Moccasin Reservoir, overwhelming its diversion system and nearly overtopping the dam, Hetch Hetchy public affairs staff said. Water seeped from the dam in places. Hetch Hetchy workers took steps to release as much water as possible from Moccasin Reservoir.
An evacuation order was issued for anyone potentially in harm’s way, including people at the Moccasin Fish Hatchery and Moccasin Point Marina and Campground. Hetch Hetchy crews drained the reservoir before it overtopped. At no point was there any threat to the Bay Area’s water supply.
“We anticipate the dam will be rated as ‘poor’ until we have the emergency repair and interim improvement projects installed at the end of this year or early next year,” Will Reisman, press secretary for the San Francisco Public Utilities Commission, said Tuesday. “At that point, we expect that the dam will be upgraded to ‘fair’ condition. The dam will like not be upgraded to ‘satisfactory’ until we make the longer-term improvements. We are currently evaluating what those long-term improvements entail.”
Reisman emphasized Tuesday that Moccasin Dam functioned properly under stress during the storm. The dam was rated satisfactory in last year’s Division of Dams Safety report.
According to Hetch Hetchy estimates, contractors are fixing $21 million in storm damage at Moccasin Dam and Reservoir.
March 22 damage to the state Fish and Wildlife hatchery just below Moccasin Dam totaled about $3.2 million, state Fish and Wildlife staff said.
Elsewhere in Mother Lode
A “Dams within Jurisdiction of the State of California” update released Tuesday lists information about 29 dams in Tuolumne County and 35 dams in Calaveras County.
New Melones, the fourth-largest reservoir in California with 2.4 million acre-feet capacity, is owned and operated by the federal Bureau of Reclamation and it is not under state jurisdiction. New Melones is not listed in the Tuesday report.
Some dams in Tuolumne and Calaveras have “extremely high” downstream hazard assessments in the event of failure of a dam when it’s holding back a full reservoir.
The assessments are based strictly on potential downstream impacts to life and property if a dam fails when the reservoir it holds back is full. The downstream hazard assessment is not related to condition of the dam or its related structures.
Donnells, Beardsley and Tulloch dams, all on the Stanislaus River and owned and operated by Tri-Dam, have extremely high downstream hazards in event of dam failure for different reasons, said Ron Berry, general manager for Tri-Dam.
“Tulloch shows extremely high hazard because with Tulloch there's more than a thousand people downstream,” Berry said. “For Donnells and Beardsley there is not a population of a thousand people. But it could be if it happened to be a very high recreation day.”
Tri-Dam has inundation maps and emergency action plans for Donnells, Beardsley and Tulloch, Berry said. Berry said he could not share them Tuesday.
“Tri-Dam Project takes dam safety very seriously,” Berry said. “We have been rated satisfactory by the Division of Safety of Dams, and continue to monitor our dams with public safety in mind.”
Statewide, the Division of Dams Safety is re-evaluating spillways on 93 dams similar to the one that came apart in February 2017 at Lake Oroville.
A list of those 93 dams includes O’Shaughnessy Dam on Hetch Hetchy Reservoir in Yosemite National Park, Cherry Valley Dam at Hetch Hetchy’s Cherry Lake reservoir, Donnells, Beardsley and Tulloch, owned and operated by Tri-Dam on the Stanislaus River, Don Pedro, owned and operated by Turlock Irrigation District, Strawberry Dam at Pinecrest, owned and operated by PG&E and Big Creek Dam, owned by Pine Mountain Lake Association.
Contact Guy McCarthy at email@example.com or 588-4585. Follow him on Twitter at @GuyMcCarthy.