Months after the Oroville Dam crisis, state regulators are ordering sweeping inspections of aging dams throughout California to determine whether they, too, have vulnerabilities.
The move comes as state officials are still trying to determine precisely what caused both spillways at the Oroville Dam to fail, prompting the evacuation of numerous towns downstream of the reservoir.
The Oroville Dam was built five decades ago, but officials noted that other dams in the state are much older.
"Many of these dams are 50 to 100 years old, and it was a different era," said Daniel Meyersohn, supervising engineer for the California Department of Water Resources' Division of Safety of Dams.
The state wants local operators to review each structure's original design and building materials, its repair history for recurring issues, its drainage system, retaining walls and the geological makeup of its bedrock, among other elements, Meyersohn said.
More than 100 dams, including many in the Sierra Nevada foothills, are in the state's jurisdiction and probably will need to have these factors reassessed by local agencies, he said. More than 50 already have been identified.
Operators of various dams have been getting notices of the inspections in recent weeks.
In a letter received by the San Luis Obispo County Public Works Department on June 12, the chief of the Division of Safety of Dams ordered the county's flood-control district to complete a "comprehensive condition assessment" of the Lopez Dam's spillway.
"We completed a reconnaissance-level assessment of the spillway at Lopez Dam and have noted that structure may have potential geologic, structural, or performance issues that could jeopardize its ability to safely pass a flood event," the letter stated.
There had been rumors that a letter like this was going to be sent out by the state, so it wasn't a complete surprise when it arrived, said Mark Hutchinson, the county's deputy director of public works.
The dam was retrofitted from 2000 to 2003 for a potential earthquake, so the county has a head start on any issues, he said.
San Luis Obispo County is expected to return a work plan for the assessment by Sept. 1.
State regulators are prioritizing dams for further local assessment by age, the size of their concrete structures and the amount of water they're impounding, Meyersohn said.
Record rains this year in Northern California put tremendous strain on the state's waterworks, with several levees failing. The Oroville crisis received national attention, but there were other problems as well. Heavy rains pushed water over the banks of Santa Clara County's largest reservoir, sending massive amounts of water into the Coyote Creek, which runs through the heart of San Jose. Whole neighborhoods were flooded.
Though a full examination of the Oroville spillway failure hasn't been completed, a consultants' report prepared for the Department of Water Resources mirrored an independent assessment that identified several problems with the structure's design and construction.
The failure, the consultants said in one of two reports the state released in April, "likely occurred as a result of high velocity flow … penetrating under the slab, causing a strong uplift force and causing the slab to lift, eventually causing all or part of the slab to break away. Subsequent erosion of foundation material caused progressive failure both upstream and downstream."
The reports noted that numerous repairs had been made to cracks and spalls in the spillway. "Some of these holes were quite large and extended as deep as the reinforcing steel. The hole that triggered the failure was probably of the latter type," the report stated.
Moreover, "the thickness of the original concrete chute slab appears to vary widely from the specified thickness," the board reported, adding that the lack of water stops between the spillway slabs "was no doubt an important factor in the February failure."
The state has signed a $275-million contract to begin repairs on the main spillway and the eroded emergency spillway as soon as runoff from spring snowmelt declines.
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