LOS ANGELES DISTRICT TAKES PROACTIVE APPROACH IN PRIORITIZING HIGH-RISK DAMS

BY DENA O’DELL AND LUCIANO VERA, Los Angeles District

When most people think about extreme weather events in Southern California, earthquakes and wildfires might come to mind.

Although both of these natural, and, sometimes, man-made disasters do happen frequently in the arid Southwest, another kind of disaster, often overlooked but with potentially devastating effect to the state, is from periodic flooding.

Whether in the coastal areas of California, inland, or in the valley, all 58 counties in California have experienced at least one significant flood event in the past 25 years, according to the California Department of Water Resources. These floods result in loss of life and billions of dollars in damages. With a large population living near or downstream of dams, these urban sprawls – coupled with the right mixture of heavy rainfall – can equal disaster.

That is one of the reasons the U.S. Army Corps of Engineers (USACE) Los Angeles District is being proactive by reassessing its inventory of dams.

“Public safety is our No. 1 priority,” said Col. Aaron Barta, commander of the 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.”

Dams are assigned Dam Safety Action Classification, or DSAC, risk ratings of 1 to 5, with 1 being “very high urgency,” 2 being “high urgency,” 3 being “moderate urgency,” 4 being “low urgency,” and 5 being “normal.”

USACE defines “high urgency” as dams where failure could begin during normal operations or be initiated as the consequence of an event. The likelihood of failure from one of these occurrences, prior to remediation, is too high to assure public safety.

In the last several months, the Los Angeles District’s Dam Safety Program has reclassified two of its dams – Whittier Narrows and Prado – and is reassessing several more.

WHITTIER NARROWS

Whittier Narrows Dam, located in Pico Rivera, California, is one component of a complex Los Angeles County Drainage Area flood risk management system that consists of five dams, 33 debris basins, and 180 miles of main and tributary channels.

USACE recently completed a draft for the Whittier Narrows Dam Safety Modification Study that recommends important modifications to the dam to ensure it continues to reduce flood risk for 1.2 million people living downstream from it.

U.S. Army Corps of Engineers Los Angeles District Whittier Narrows Dam Lead Project Engineer Doug Chitwood, left, and Whittier Narrows Dam Project Manager George Sunny, right, pose for a picture near the spillway gates of Whittier Narrows Dam during an interview with a Los Angeles Times reporter in January 2019 at the dam in Pico Rivera, California.

Photo By Dena O'Dell U.S. Army Corps of Engineers Los Angeles District Public Affairs

The dam, built nearly 60 years ago, continues to perform as it was designed; however, due to the combination of the potential loss of life with a very high likelihood of failure during a rare flood event, it was reclassified from a DSAC 2 to a DSAC 1, from high urgency to very high urgency.

The study showed should this rare flood occur, the dam could fail through erosion underneath or even overtopping, putting the population downstream at risk.

“The dam was designed consistent with the standards of the day and it has succeeded in reducing flood damages,” said lead project engineer Doug Chitwood. “However, floods larger than we have experienced since the dam was built will occur at some point. The proposed modification project and community preparation are key components in reducing the flood risk.”

The Los Angeles District continues to seek opportunities to communicate with the public and its partners about the importance of the study and the value of the dam to Southern California. Downstream communities have begun actively preparing evacuation plans to minimize risk, should a rare flood event occur in the future.

“There are no guarantees when it comes to Mother Nature,” Barta said. “Residents downstream from the dam should be aware of evacuation routes, local emergency communication procedures, and be prepared for any flood event, whether big or small.”

Whittier Narrows Dam is the only DSAC 1 dam in the nation not in the design or construction phase yet. Getting it to construction is a USACE priority. The dam modifications do not require congressional authorization, because it is already an authorized federal project. The study was completed in June 2019 and is being reviewed by USACE Headquarters. The goal is to get the study approved in 2019, initiate design, and start construction in 2021.

From 1982 to 2009, flood damages prevented by the dam were valued at $4.7 billion, with an average annual benefit of $169 million. Additionally, about 2.1 million people visit the dam’s recreation area annually.

PRADO DAM

Prado Dam is a flood risk management project located on the Santa Ana River near the city of Corona, California, and is in the process of being modified.

Constructed in 1941, the dam’s primary purpose is flood risk management, protecting 1.4 million people living and working in 29 cities in Riverside and Orange counties and more than $61 billion in property. Additionally, it also provides recreation and water conservation benefits.

Looking out toward the spillway at Prado Dam on June 17, 2019, near Corona, California. Phoebe Percell, USACE’s chief of the Dam and Levee Safety Branch and Deputy Dam and Levee Safety officer, and members from the Risk Management and Dam Safety Production Centers traveled to the Los Angeles District to visually inspect and assess the Prado, Whittier, and Sepulveda dams, meet with project managers, and learn more about each dam during their visit.

Photo by Dena O'Dell Los Angeles District Publications

Modifications to Prado Dam began in 2002 to provide additional capacity to store floodwaters and sediment. Modification of the spillway will be the last feature of the project to be constructed.

In early 2019, a site-specific evaluation was conducted to assess conditions associated with the dam as part of a periodic review of its performance. Risk factors identified at that time indicated the potential for poor spillway performance in the event of a rare flood event.

Although the dam is typically dry and has never experienced a large enough storm to cause water to flow over the spillway, because of the high population living downstream, USACE reclassified its rating from a DSAC 3 to a DSAC 2, from moderate urgency to high urgency.

The Los Angeles District is working with a national team of experts to reduce the risks associated with the spillway and implementing interim risk-reduction measures – structural and nonstructural – until modification of the existing spillway is complete. Construction is expected to begin in 2021.

“We are definitely aware of the population that is relying on us,” said Lillian Doherty, chief of the district’s Operations Division. “We are committed to finding a solution in the short and long term.”

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

“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.”

PUBLIC OUTREACH ACTIVITIES

Communicating flood risk to communities downstream of its 16 dams is a top priority for the district.

“The Los Angeles District Dam Safety Program, in partnership with emergency management, has been actively engaging our partner stakeholders to improve interagency coordination, community flood-risk awareness, and emergency preparedness,” said Kristen Bedolla, Los Angeles District Dam Safety Program manager.

During spring and summer 2019, the Los Angeles District hosted several outreach events, including two media press conferences at Whittier Narrows and Prado dams; a Santa Fe Dam Tabletop Exercise with local, state, and federal emergency management agencies; Whittier Narrows and Mojave River dam seminars with local, state, federal, and congressional officials; and congressional and city staff tours of Whittier and Prado dams. Additionally, the district is continuously conducting outreach events with local schools and communities through its new ranger program.

“This year’s program has rolled out an unprecedented amount of outreach programs,” Doherty said. “Our collective efforts will build the bridge to stronger communications, trust, and credibility, so we can all be better prepared should a large storm event or series of storms affect our Southern California area.”