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Federal officials are working urgently to strengthen the spillway at Prado Dam near Corona to prevent it from failing in a major flood, which could imperil hundreds of thousands of people living downstream in Orange County.
According to the Los Angeles Times, “dozens of Orange County communities from Disneyland to Newport Beach” could be inundated if the dam fails.
After a May inspection determined the dam’s spillway could perform poorly in a major flood, the dam’s risk rating was changed from “moderate urgency” to “high urgency.”
Dena O’Dell, a U.S. Army Corps of Engineers spokeswoman, said Thursday that the agency is taking immediate measures to reduce the risk that the spillway will fail.
And she said the agency was preparing to launch a project in 2021 to bolster the spillway and raise it 20 feet.
O’Dell said the corps notified area congressional representatives, Orange County officials and flood-control agencies in Orange, Riverside and San Bernardino counties about the heightened risk.
Built in 1941, Prado is a flood-control dam on the Santa Ana River near the 71 and 91 freeways in western Riverside County. Its primary purpose is to shield 1.4 million people and $61 billion in property downstream in Orange County from flooding.
Dams are assigned risk ratings of 1 to 5, O’Dell said, with 1 being “very high urgency,” 2 being “high urgency,” 3 being “moderate urgency,” 4 being “low urgency” and 5 being “normal.”
The corps’ dam safety system defines “high urgency” as “dams where failure could begin during normal operations or be initiated as the consequence of an event. The likelihood of failure from one of these occurrences, prior to remediation, is too high to assure public safety.”
Lillian Doherty, chief of operations for the corps’ Los Angeles District, said the risk is the spillway might not safely pass water if the dam reaches capacity.
“We have identified a risk,” Doherty said. “I don’t think anyone can say definitively whether or not the spillway will or will not perform. No one has a crystal ball.”
In any event, she said, the corps expects to reduce the risk by the time the next rainy season arrives.
“We are definitely aware of the population that is relying on us,” Doherty said. “And we are really committed to finding a solution in the short and the long term.”
Doherty said the dam functioned properly this winter and spring, “when we saw a lot more water than we have seen in a long, long time.”
And she said water never has spilled over the top of the dam in its nearly-eight-decade history.
Prado benefits from a huge basin to the east. O’Dell said the dam is capable of holding back 217,000 acre-feet of water and ongoing improvements will boost capacity to 362,000 acre-feet. One acre-foot, about 326,000 gallons, is what two typical Southern California families use in a year.
The Prado Basin is expanding from 6,695 acres to 10,256 acres, O’Dell said.
It would take many days of heavy rain, Doherty said, to send water over the top.
Rep. Ken Calvert, R-Corona, said via email he would work to ensure the corps has money to cover short-term-fix costs.
“We also need to fully fund the planned modification of the Prado spillway sooner rather than later,” Calvert said.
Rep. Gil Cisneros, D-Yorba Linda, whose north Orange County district is bisected by the Santa Ana River, said he will continue to push for adequate funding.
“With this recent assessment, I stand ready to support the needs of the Army Corps of Engineers and will ensure that high priority projects like the Prado Dam are funded in order to protect our residents,” Cisneros said.
Herb Nakasone, executive director of the Santa Ana River Flood Protection Agency in Orange County, said the rating change may accelerate repairs.
“The sooner we can get that done, the better off we all will be,” Nakasone said.
Since 2002, O’Dell said, the corps has been working to raise Prado Dam, extend its reach, enlarge its reservoir and improve the outlet works and downstream channel. Early on, a dike was added along the 71 Freeway.
The plan to fortify the spillway is the last feature in the chain of improvements, she said.
Doherty, the operations chief, said experts are investigating to determine whether that plan needs to be enhanced.
“Public safety is our number one priority,” said Col. Aaron Barta, commander of the Los Angeles District, in a statement. “The primary objective of the Corps’ Dam Safety Program is to review our dams and ensure resources are prioritized to address the highest risks.”
