The Oroville Dam in Butte County cracked in February due to a wet winter, causing the evacuation of communities along the Feather River and a more than $275 million repair bill, which prompted the governor of California to call for the re-evaluation of dams across the state. Four of those dams help store water for San Luis Obispo County, but one could cause the most damage.
- Photo Courtesy Of San Luis Obispo Public Utilities Department
- DOWNHILL FLOW The Whale Rock Reservoir has a 46,000 acre-foot capacity and serves the city of SLO, Cal Poly, and the California Men's Colony. Cayucos would feel the impact if this dam failed.
There are about 1,400 dams that help collect and store water in the Golden State, and their average age is about 70 years old. Approximately 1,250 of those dams are reviewed by the Division of Safety of Dams (DSOD)—an agency within the California Department of Water Resources—as part of its safety program. Ninety-three of them are under scrutiny based on the age of the spillway, the height and storage capacity of the dam, as well as the hazard potential downstream of the reservoir.
Daniel Meyersohn, supervising engineer for DSOD, said the agency's engineers visually inspect the dams once a year. A feature of the dam that's constantly reviewed is the spillway, which allows high volumes of water to be released at one time when reservoir levels are full.
"Whenever possible our engineers walk the spillway chute and record the surface condition of the concrete chute, and approach the channel and control structure," Meyersohn said.
The Oroville spillway was approximately 3,000 feet long, and the DSOD reviewed it annually. When it was finally put to use earlier this year, gushing water displaced concrete and eroded hundreds of feet of dirt and debris, pushing it into the Feather River and causing concerns about whether the spillway would hold.
The damage to the community resulting from the spillway was enough to push concerns down the coast.
The DSOD-reviewed dams at Lake Nacimiento, Whale Rock Reservoir, Lake San Antonio, and Lopez Lake fall into the category of structures getting re-evaluated. They have two things in common with Oroville: They were built in or before the '60s and are earth-filled.
Of the four, Lopez (finished in 1969) would have the highest cost if it were to fail, affecting thousands of SLO County residents in Arroyo Grande, Grover Beach, Oceano, and Pismo Beach.
"If the dam ruptured at full capacity, approximately 10,000 to 12,000 residential and business occupants in the county could be affected," according to a dam and levee failure evacuation plan put together by the San Luis Obispo County Office of Emergency Services in 1987.
SLO County owns and operates the Lopez dam, which can store approximately 49,000 acre-feet of water. SLO County's Public Works Department Deputy Director Mark Hutchinson said Lopez has always been categorized as a "high hazard."
"It sounds terrible but what it means is if it all went bad [if the spillway gave out] how bad would it be," Hutchinson said.
If a spillway failure could potentially impact more than 1,000 people downstream, the structure is more than 25 feet high, and holds more than 5,000 acre-feet of water, it's given the high hazard label. But Hutchinson said he isn't worried about the structure of the dam.
"We've discovered that the Lopez spillway used all of the standard Army Corps details that were available in the 1960s. Oroville's spillway, interestingly, didn't—it was a custom design," Hutchinson said.
The Lopez dam was retrofitted in 2001 to increase its ability to withstand an earthquake. Lopez isn't crack-proof, though. It does have some small imperfections.
"If you go to Lopez, there are cracks that have been repaired, but they're hairline cracks. You can't get a toothpick in the crack, let alone your fingers," he said.
The recent state order to comprehensively inspect Lopez isn't a huge concern for Hutchinson. He sees it as an opportunity for improvements.
"For one, we want to make sure the spillway is safe. The other thing is the dam was built in the '60s and it had a detailed engineering analysis 25 years later. Now, it's been 27 years, and it's an opportunity to look at it again and put a microscope on it," Hutchinson said.
The dams on Nacimiento, constructed in 1957, and San Antonio, 1965, are operated by Monterey County. Officials from Monterey County didn't return New Times' request for comment before press time. Respectively, they have storage capacities at approximately 378,000 acre-feet and 348,000 acre-feet.
According to the evacuation plan, during a major flood event at Nacimiento, the community of San Miguel and surroundings areas could receive minor flooding. If the same were to happen at San Antonio, only a small portion of SLO County would be effected.
If Whale Rock's dam—built in 1960, owned by the Whale Rock Commission, operated by the city of San Luis Obispo—were to fail, it would impact about "1,500 residential, recreational, and small business occupants" in Cayucos, according to the county evacuation plan.
The reservoir has a holding capacity of 40,600 acre-feet of water, which it delivers to SLO, Cal Poly, and the California Men's Colony.
Whale Rock Supervisor Noah Evans said there are eyes on the structure constantly as staff is there every day. His department—the SLO Utilities Department—received a letter from the DSOD near the end of May, and Evans said his staff has already submitted inspection paperwork.
He said the city worked on an analysis of any structural or performance issues that could jeopardize the dam and its spillway in the event of a flood.
"Any day now we expect to get our work plan authorized from the Department of Water Resources or at the least a recommendation of what can be improved," Evans said.
For Evans and the rest of the SLO Utilities Department, this is an opportunity to take a second look at the structure and the maintenance of Whale Rock.
"We're going through this exercise to have a better understanding of the structure, its ability to function, and, mostly importantly, its design to keep the community safe," Evans said. Δ
Staff Writer Karen Garcia can be reached at