California Floods: Dams in LA are at risk of overflowing

California Floods: Dams in LA are at risk of overflowing

  • US News
  • January 24, 2023
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With the shock of a string of January storms wearing off, Los Angeles County officials face a Herculean task: Five reservoirs along the south-facing slopes of San Gabriel Mountain are filled with so much debris and soupy mud that they pose a flood risk to the United States represent communities below.

Another violent storm, they say, could unleash new waves of debris, downed trees and boulders in ravines stripped of their cohesive vegetation by the 2020 Bobcat Fire, and chocolate-colored floods over the causeways and into the cities of Arcadia, Sierra Madre and Send Pacoima , Sonnental and Sonnenland.

A pressing concern is the emptying of the reservoir behind the 96-year-old Santa Anita Dam of about 600,000 cubic yards of mud at a depth of more than 80 feet. Two of the three valves that control the flow of rainwater from the 20-story dam are clogged with silt.

“It could flood, and we’re making every effort to make sure that doesn’t happen,” said Sterling Klippel, senior stormwater engineer at the LA County Department of Public Works.

Built in 1927, the structure is an important part of an extensive network of 14 dams, 487 miles of flood control channels, 3,330 miles of underground storm drains, and dozens of debris ponds and outflow areas built over the last century to prevent flooding and collect stormwater for recharge in local aquifers catch.

Just as the combined effects of atmospheric fluxes and global warming have manifested themselves in drought, extreme wildfire patterns and flooding across California, they are also testing the county’s flood control system.

“We have to keep up with these environmental changes and challenges,” said Mark Pestrella, director of the county department of public works.

It won’t be easy or cheap. The cost of removing an estimated 15 million cubic yards of debris and mud from all five reservoirs and transporting it to the foothill sediment deposits is about $550 million, Pestrella said.

That money is expected to come from county funds, state and federal grants and possibly an appraisal agreed by the property owners, officials said.

“We don’t have an endless bag of loose change,” Pestrella said. “As it stands, homeowners in the county are being charged $28 annually for these types of improvements — and that hasn’t changed since the 1980s.”

Logs, debris, and mud impede the flow of water in a dam.

Logs, mud and debris fill the reservoir behind Santa Anita Dam in the San Gabriel Mountains.

(Mel Melcon/Los Angeles Times)

The estimated volume of debris and mud to be removed from the other four facilities includes 3 million cubic meters from the Big Tujunga Dam, which protects Tujunga; 2 million cubic yards from Cogswell Dam, which controls flooding in the West Fork of the San Gabriel River; 4.5 million cubic yards from Pacoima Dam, which protects Olive View-UCLA Medical Center, Sun Valley and Sunland; and 5 million cubic feet from the San Gabriel Dam, which controls runoff from the 200-square-mile San Gabriel Canyon watershed.

Water from the San Gabriel Dam flows down the gorge to Morris Dam, a facility a few miles above Azusa that former President Hoover dedicated in 1934 in honor of Los Angeles consulting engineer Sam Morris.

The significant loss of vegetation due to the Bobcat Fire has increased the risk of flooding, debris flows and mudslides. Such events could prevent the dams from making controlled releases.

It took about four years to remove 330,000 cubic yards of sediment behind the Santa Anita Dam after the massive 2009 station fire.

Two workers are dwarfed by the towering wall of a dam.

Workers observe a water release at Santa Anita Dam.

(Mel Melcon/Los Angeles Times)

It could take three more years, said Klippel, to clean up the material that has accumulated behind the dam since the Bobcat fire roared through the heart of the watershed two years ago.

The Santa Anita Dam reservoir was “perfectly clean to increase storage capacity when the Bobcat fire hit,” he said. “Look at it now.”

The Bobcat Fire burned approximately 116,000 acres of forest between the San Gabriel and Antelope Valleys, destroying 160 homes and buildings. It was one of many wildfires that broke out in August and September — blazes that made 2020 the state’s worst fire year on record. More than 4 million acres burned and dozens of people were killed.

The Los Angeles County Board of Supervisors voted later that year to mobilize departmental resources to assist communities devastated by the fires.

Since then, Supervisor Kathryn Barger has overseen grant applications and proposed contingency actions, which still make their way through complex permitting processes required to start and complete repair and restoration projects.

“It is imperative that our crews complete this debris clearance work to maintain the integrity, safety and capacity of our flood defense system,” Barger said.

“The torrential rains we’ve been experiencing lately can quickly pose a flood risk to our local communities and lives could be at stake,” she added. “This is a daunting task – so I will continue to monitor progress to ensure the job gets done.”

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