U.S. Army Corps of Engineers Reclassifies Prado Dam’s Risk from Moderate Urgency to High Urgency

By on May 20, 2019

A site-specific evaluation of Southern California’s Prado Dam, conducted earlier this month, found the potential for poor spillway performance, which could have adverse impacts to the downstream population should a significant flood event occur. The evaluation – and the risks identified – have prompted the U.S. Army Corps of Engineers to change the dam’s risk characterization from moderate urgency to high urgency.

Although the dam has historically operated without incident, it has not experienced a large enough storm to cause water to flow over the spillway. The Corps is now working with a national team of experts to reduce the risks associated with the spillway and is implementing interim risk-reduction measures.

“Public safety is our number one priority,” said Col. Aaron Barta, commander of the U.S. Army Corps of Engineers Los Angeles District. “The primary objective of the Corps’ Dam Safety Program is to review our dams and ensure resources are prioritized to address the highest risks.”

More than 1.4 million people live and work in 29 cities downstream of Prado Dam, with more than $61 billion in property. Prado Dam is a flood-risk management project and a major feature of the ongoing Santa Ana River Mainstem project. Designed in 1930 and built in 1941 Prado Dam is 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.

Although the dam is frequently dry, in the event of significant flood event that breaches the top of spillway could flood dozens of cities from north Orange County to Newport Beach. It its path are numerous Southern California attractions Including Disneyland, Anaheim Stadium and the Honda Center.

Prado Dam is operated and maintained by the Corps’ Los Angeles District. Modifications of the dam began in 2002 to provide additional capacity for storage of floodwaters and sediment by enlarging the existing Prado Dam and reservoir. Modifications includes raising the main dam embankment, replacing the outlet works, constructing other embankments (also known as dikes) and improving the downstream channel. Although the dam is undergoing improvements, it continues to be fully functional and operable during storm events.

Modification of the existing spillway is expected to begin in 2021 but is currently the last feature of the project to be constructed.

“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. “We will determine whether or not it is as reliable as it should be.”

“We value transparency and our relationships with our local, state and federal partners,” Barta said. “We will continue to work together to keep the public informed about Prado Dam.”