Prado Dam Failure Could Flood Dozens Of OC Communities

The 78-year-old earthen dam is all that stands between 1.4 million people and floodwaters during the next huge series of Pacific storms.

By Ashley Ludwig, Patch Staff | May 17, 2019 12:59 pm ET

https://patch.com/california/orange-county/prado-dam-failure-could-flood-dozens-oc-communities

ORANGE COUNTY, CA — Federal engineers have raised an alarm that a "significant flood event," such as a series of strong Pacific storms, could breach the spillway of the aging Prado Dam, in Orange County. Such an event has the potential to drown out dozens of downstream communities, from Anaheim's Disneyland to Newport Beach, the Army Corps of Engineers Los Angeles District reported.

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."

Over 1.4 million people live and work below Prado Dam, with property valued at over $61 billion, including Disneyland and many high-end resorts and properties in Newport Beach.

"Public safety is our number one priority," Col. Aaron Barta, commander of the U.S. Army Corps of Engineers Los Angeles District, said in a recent release. The Corps is currently reviewing all dams in the area and prioritizing the highest risks, of which Prado Dam tops the list. Any modifications to improve the spillway won't begin for at least two years, they said.

Prado Dam was authorized for development in 1936, to protect the populous communities downstream, and was still under construction during the flood in the spring of 1938, when two mighty Pacific storms swept through the region and dumped over a year's worth of rain in just a few days, according to the Orange County Historical Society. During that time, floodwaters swamped significant swaths of Orange County under six-feet of water for several weeks, including Anaheim and what is now Santa Ana.

During a 100-year flood, Prado Dam is the only stopgap between safety and catastrophe for hundreds of thousands of people. In 2005, a leak in the damn during a series of storms forced hundreds to evacuate their homes in nearby Corona. The latest look at Prado Dam has left the Army Corps of Engineers concerned about what another devastating storm would do to the dam.

"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, told the Los Angeles Times. "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.

>>6527143 lb notable

>>6528136 article continues

The sudden downgrade in the structure's evaluation comes after major structural 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."

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 to implement before repairs to the dams are completed.

"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," said Daniel Swain, a UCLA climate scientist told the LA Times. "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."