Army Corps of Engineers Raises Risk Assessment on Southern California Dam

May 20, 2019

The U.S. Army Corps of Engineers has changed Prado Dam’s risk characterization from moderate urgency to high urgency, based on a recent assessment.

More than 1.4 million people live and work in 29 cities downstream of Prado Dam, with more than $61 billion in property.

A site specific evaluation was conducted in May to assess conditions associated with the dam as part of a periodic review of its performance.

Risk factors identified indicate the potential for poor spillway performance in the event of a significant flood event, according to the review.

The dam, which is typically dry, has historically operated without incident. It has not experienced a large enough storm to cause water to flow over the spillway, according to the Corps.

The Corps is working with a national team to reduce the risks associated with the spillway. The agency is implementing interim risk-reduction measures. Modification of the existing spillway is expected to begin in 2021.

The agency also is coordinating with its partners and conducting public outreach as interim risk-management strategies.

Prado Dam is a flood-risk management project and a major feature of the ongoing Santa Ana River Mainstem project. It was constructed in 1941 and is located on the Santa Ana River in the city of Corona in Riverside County. It is operated and maintained by the Corps’ Los Angeles District and is in the process of being modified as part of the larger Santa Ana River Mainstem project.

Modification of the dam began in 2002 to provide additional capacity for storage of floodwaters and sediment by enlarging the existing Prado Dam and reservoir. This 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.

The Corps’ assessment of the dam doesn’t mean that failure is occurring, it means the agency has identified performance concerns that require attention to meet the Corps’ rigorous Dam Safety standards.

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