SAN FRANCISCO — California is ordering immediate spillway inspections at about 70 aging dams that it believes might not be sound enough to protect downstream communities in a flood, a state dam regulator said Wednesday.
The engineering and on-site reviews are part of stepped-up inspections following February's surprise failures of both spillways at California's 770-foot-high (230-meter-high) Oroville Dam, the nation's tallest. Authorities ordered nearly 200,000 people to evacuate in that crisis.
Since then, regulators at California's dam-safety division began reviewing their records on the 1,250 dams they monitor, focusing on 100 big, aging dams that have people downstream, supervising engineer Daniel Meyersohn said.
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.
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.
Operators at Whale Rock dam near San Luis Obispo on the Central Coast received one of the letters this week.
In it, the state informs dam operators that the state believes the spillway "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.
Many of the spillways the state is singling out for closer evaluations by dam owners are even older than Oroville's, including some dating back to California's 19th-century Gold Rush, Meyersohn said.
At Whale Rock dam, the orders come despite dam operators' weekly sight checks of the structure, said Noah Evans, supervisor for the city of San Luis Obispo's Whale Rock Reservoir.
Up to 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 built in the 1960s, the last time in 2005 with no problems, Evans said.
The Lopez 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 in size compared to Oroville — 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."