Federal engineers are raising alarms that a “significant flood event” could breach the spillway of Southern California’s aging Prado Dam and potentially inundate dozens of Orange County communities from Disneyland to Newport Beach, it was reported Friday.
After conducting an assessment of the 78-year-old structure earlier this month, the U.S. Army Corps of Engineers announced that it was raising the dam’s risk category from “moderate” to “high urgency,” the Los Angeles Times reported.
“Our concern right now is about the concrete slab of the spillway and how well it will perform if water were to spill over the top of the dam,” said Lillian Doherty, the Army Corps’ division chief, according to the newspaper. “We will determine whether or not it is as reliable as it should be.”
Located beside the 91 Freeway, on the border of Riverside and Orange counties, the dam contains little to no water for much of the year. During periods of heavy rain, however, the structure is intended to collect water and prevent flooding along the Santa Ana River.
Doherty said her agency is working with a national team of experts to develop interim and permanent risk-reduction measures at the dam, as well as public outreach strategies to alert the estimated 1.4 million people who live and work in 29 communities downstream.
The sudden downgrade in the structure’s evaluation comes after major problems have been identified in California dams.
In February 2017, a concrete spillway at the Oroville Dam disintegrated during heavy rains and triggered the evacuation of more than 180,000 people. The head of the California Water Resources Department, which operates the dam, was removed after an independent probe found the failure was the result of a lax safety culture.
That same year, the Corps of Engineers discovered that the 60-year-old Whittier Narrows Dam, about 40 miles to the west of Prado Dam, was structurally unsafe and posed a potentially catastrophic risk to more than 1 million people in working-class communities along the San Gabriel River floodplain.
In that case, engineers found that intense storms could trigger a premature opening of that dam’s massive spillway, swamping homes, schools, factories and roads from Pico Rivera to Long Beach. Engineers also found that the earthen structure could fail if water were to flow over its crest.
The Corps estimates it will cost roughly $600 million in federal funds to upgrade the Whittier Narrows facility, which has been reclassified as the agency*s highest priority nationally because of the risk of “very significant loss of life and economic impacts.”
Now, given concerns that Prado Dam poses a flood threat to much of Orange County, the agency is collaborating with Los Angeles, Orange and Riverside counties and several dozen municipalities to develop emergency plans that could be implemented before repairs to the dams are completed, The Times reported.