Federal engineers have cautioned that Southern California’s Prado Dam is vulnerable to a significant flood event that could breach the nearly 80-year-old dam and threaten dozens of cities downstream.
The U.S. Army Corps of Engineers reports that it has upgraded Prado Dam’s risk characterization from moderate urgency to high urgency as a result of a site-specific safety evaluation earlier this month.
Designed in 1930 and built in 1941, Prado Dam is a flood-risk management project located on the Santa Ana River near the intersection of Orange, Riverside and San Bernardino counties. It was constructed to capture water and prevent flooding along the Santa Ana River and the downstream communities. Today, more than 1.4 million people live and work in 29 cities downstream of Prado Dam, with more than $61 billion in property.
Engineers are concerned about how well the concrete portion of the spillway would hold up in a significant flood event. A spillway breach could lead to flooding in dozens of cities from north Orange County to Newport Beach, and leave major Southern California attractions, including Disneyland, Anaheim Stadium and the Honda Center, vulnerable.
“Public safety is our number one priority,” said Col. Aaron Barta, commander of the Corps of Engineers Los Angeles District. “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 Corps’ division chief. “We will determine whether or not it is as reliable as it should be.”
An $880 million improvement project is underway on the dam, but modification of the existing spillway is not expected to begin until 2021 and is currently the last feature of the project to be constructed. In the interim, the Corps is working with a national team of experts to implement risk-reduction measures in the interests of public safety.
Major problems have already been identified in other aging California dams. The concrete spillway at the Oroville Dam failed during heaving rains in 2017, forcing the evacuation of 180,00 people. The spillway recently reopened, after $1.1 billion in repairs. That same year, the Corps found that the 60-year-old Whittier Narrows Dam, about 40 miles to the west of Prado Dam, was structurally unsafe and could put more than 1 million people at risk.
“A troubling theme is emerging as the Corps reviews its portfolio of large flood control systems that were built a long time ago and are now showing signs of severe stress,” says UCLA climate scientist Daniel Swain. “Federal engineers are finding that these systems are not as resilient as they thought they were, and that the frequency of what were regarded as once-in-a-lifetime storms is increasing significantly.”