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January storms leave LA County flood-control dams at risk of overflowing

Louis Sahagun, Los Angeles Times on

Published in Weather News

LOS ANGELES — Now that the shock of a series of January storms has worn off, Los Angeles County officials face a herculean chore: Five reservoirs along south-facing San Gabriel Mountain slopes are filled with so much debris and soupy mud that they pose a flood risk to the communities below.

Another intense storm, they say, could unleash new surges of dirt, toppled trees and boulders down canyons stripped of their binding vegetation by the 2020 Bobcat fire, sending chocolate-colored floodwaters over the dams and into the cities of Arcadia, Sierra Madre, Pacoima, Sun Valley and Sunland.

An urgent concern is emptying the reservoir behind 96-year-old Santa Anita Dam of about 600,000 cubic yards of muck more than 80 feet deep. Two of the three valves that control releases of stormwater from the 20-story-tall dam are blocked with silt.

“It could flood, and we’re busting our tails to make sure that doesn’t happen,” said Sterling Klippel, principal stormwater engineer at the L.A. County Department of Public Works.

Built in 1927, the structure is a critical component of a sprawling network of 14 dams, 487 miles of flood-control channels, 3,330 miles of underground storm drain channels, and dozens of debris basins and spreading grounds built over the last century to prevent flooding and to capture stormwater for groundwater recharge in local aquifers.

Just as the combined effects of atmospheric rivers and global warming have revealed themselves in drought, extreme wildfire behavior and flooding across California, they are also putting the county’s flood-control system to the test.

 

“We have to keep up with these environmental changes and challenges,” said Mark Pestrella, director of the county Department of Public Works.

That won’t be easy or cheap. The price tag for removing an estimated 15 million cubic yards of debris and mud from all five reservoirs and hauling it to sediment placement sites across the foothills is about $550 million, Pestrella said.

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

“We don’t have a never-ending pocket full of change,” Pestrella said. “As it stands, property owners in the county are charged $28 annually for these kinds of improvements — and that hasn’t changed since the 1980s.”

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