State dam safety officials are ordering immediate spillway inspections at about 70 aging dams that it believes might not be sound enough to protect downstream communities in a flood.
The engineering and on-site reviews are part of stepped-up inspections following the February failure of both spillways at Oroville Dam. Authorities ordered about 180,000 people to evacuate in that crisis.
Since then, the Department of Water Resources Division of Safety of Dams has been reviewing its records on the 1,250 dams it monitors, focusing on 100 big, aging dams with people living downstream, supervising engineer Daniel Meyersohn said Wednesday.
The state has since written to owners of about 70 of the dams, ordering them to carry out a thorough review of the spillways' engineering and, if necessary, on-site inspections of the soundness of the spillways and the rock supporting it.
Those investigating the failure of Oroville Dam main concrete spillway and the hillside beneath an adjoining emergency overflow weir have focused on the presence of rapidly eroding incompetent rock beneath and near the structures.
Meyersohn declined to identify the dams that had received the orders for extra inspections, saying some of the owners may not yet have received their notices.
The city of San Luis Obispo, which operates the dam in conjunction with a nearby state prison and state university, received one of the letters this week.
In it, the state informs the city that the spillway at the dam -- which the Department of Water Resources built and completed in 1961 -- "may have potential geologic, structural, or performance issues that may jeopardize its ability to safely pass a flood event."
The state order mandates that dam operators fix any spillway problems they find before the next rainy season, which in California usually begins around November.
Many of California's reservoirs and rivers are at their fullest in years after heavy winter rain and snow. At Oroville, construction crews already are rushing to rebuild and anchor half-century-old spillways before November, as part of about $500 million in emergency response and repairs.
Oroville's main and then back-up spillways collapsed in February, despite years of inspection and maintenance reports that failed to warn of any catastrophic failure of the concrete main spillway in particular.
At Whale Rock Dam, the orders come despite dam operators' weekly visual inspections, said Noah Evans, supervisor for the city of San Luis Obispo's Whale Rock Reservoir.
About 2,000 people living in the coastal community of Cayucos are downstream from the reservoir, Evans said. The dam's operators have used the spillway a dozen times since the dam was finished more than half a century ago, Evans said, and have never encountered problems.
State officials also ordered inspections at another San Luis Obispo County facility, Lopez Dam.
The dam is upstream from about 5,000 people in the community of Arroyo Grande, said Mark Hutchinson, deputy director of San Luis Obispo County Public Works, which also received a letter from the state calling for an inspection.
Water first tumbled over the spillway a year after the completion of the dam in 1968. A detailed inspection in 1991 led officials there to perform significant repairs a decade later, Hutchinson said.
Inspectors check the Lopez Dam — a fraction of Oroville's size — each day, and Hutchinson said that he welcomes the opportunity to gain insights from the larger spillway's failure.
"If we were to build it today what would be different? If there is something that would be different, what are the implications of that?" he said. "There's some good stuff to sink your teeth into."