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Responding to the colossal failure of the Oroville Dam spillway in February, state regulators have ordered a detailed assessment of the spillways at 93 dams, including three dams that hold most of the water that supplies central and southern Marin County.

Dams owned by the cities of Napa and St. Helena are also included in the “spillway re-evaluation program” announced last week by the state Division of Safety of Dams.

Warm Springs Dam at Lake Sonoma and Coyote Dam at Lake Mendocino, cornerstones of a system that delivers Russian River water to some 600,000 Sonoma and northern Marin residents, are under the jurisdiction of the U.S. Army Corps of Engineers and not the state agency, which is part of the Department of Water Resources.

The Army Corps conducts its own annual inspections of Warm Springs and Coyote dams, which will be done in the next two weeks, said Nick Malasavage, dam safety program manager for the corps’ San Francisco District.

The inspections, performed by a team of engineers, cover “anything integral to the safe operation of the dam,” including the embankment, gates, tunnel and spillway, he said.

Warm Springs and Coyote dams are designed differently than Oroville Dam, where excess water cascades over the lip of the dam, as happened during winter storms, prompting the evacuation of 188,000 people in harm’s way downstream along the Feather River.

Instead, the two local dams, designed primarily for Russian River flood control, open gates deep within the embankment to let water out through a tunnel near the dam’s base, keeping reservoir levels safely below the top of the dam, Malasavage said.

Warm Springs and Coyote dams do have auxiliary emergency spillways, reserved for extreme conditions rather than standard winter operations, he said.

Water has spilled over the concrete sill of Coyote Dam only once — in the epic flood year of 1964 — since it was built near Ukiah in 1958.

Warm Springs Dam near Healdsburg has never had water flow down its concrete-lined external spillway since it was completed in 1983, Malasavage said.

Meanwhile, the Marin Municipal Water District, which provides water to 186,000 people, has submitted to the state a plan for accomplishing the mandated assessment of spillways at the three largest of its seven dams, said Emma Detwiler, a district spokeswoman.

The Peters, Seeger and Soulejule dams impound nearly 83 percent of the district’s water, with the Seeger Dam holding back Nicasio Reservoir visible along the Point Reyes-Petaluma Road.

The district “currently has a comprehensive Dam Safety Program to ensure all of our dams and spillways are safe and functioning properly,” Detwiler said.

The marinwater.org website includes copies of the Division of Safety of Dams inspection reports for all seven dams in 2017 and 2016.

This year’s reports on the Peters, Seeger and Soulejule dams concluded: “From the known information and visual inspection, the dam, reservoir and the appurtenances are judged safe for continued use.”

Daniel Meyersohn, supervising engineer for the Division of Safety of Dams, said in an email the comprehensive spillway assessment just ordered “goes far beyond regular (state) maintenance inspections.”

Cost of the mandated assessments will “vary from dam to dam” and will be borne by dam owners, he said.

California engineers inspect 1,250 dams once a year, with “high hazard” dams receiving two annual inspections, Meyersohn said.

In developing the spillway evaluation program, engineers reviewed large spillways at more than 100 dams based on the spillway’s age, height and storage capacity of the dam and the “hazard potential downstream of the reservoir,” the dam safety agency’s official notice said.

Owners of 93 dams were subsequently notified of the demand for a spillway assessment, with work plans required by Sept. 1 for most owners.

The assessments are intended to evaluate the spillways’ “ability to perform satisfactorily during a flood event,” the notice said.

Meyersohn said the requirement was not unprecedented, noting that similar assessments were required following the unexpected failure of a spillway gate at Folsom Dam in 1995.

Among the 93 dams on the state list are the 585-foot Don Pedro Dam on the Tuolumne River, the ninth highest dam in the nation, and the O’Shaughnessy Dam, which impounds the Hetch Hetchy Reservoir in Yosemite National Park and supplies drinking water to about 2.4 million Bay Area residents.

Conn Creek Dam, owned by Napa, impounds Lake Hennessey in the mountains east of the Silverado Trail.

Phil Brun Jr., deputy director of public works, said the city will be “taking steps to comply with the (state) notice.”

Also on the list is Bell Canyon Dam, operated by St. Helena.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.

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