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Climate and Environment

Crews Race To Clear Debris Basins Before More Rain Comes To LA

A yellow backhoe pours dirt into the bed of a white dump truck in a debris basin.
After torrential rains in December and January, the Oakglade catchment area in Monrovia is about halfway full of sediment, trees and rocks. L.A. County Public Works cleared the area on Thursday, after the heavy rain from Monday and Tuesday.
(Jackie Fortiér
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With a new round of storms bearing down on Southern California, public works crews are racing the clock to clear tons of debris left behind by the heavy rainfall earlier this week.

A street lined with houses with the Bobcat Fire burn scar visible on the nearby mountain.
The Bobcat Fire burn scar is clearly visible from the streets of Monrovia.
(Jackie Fortiér
LAist )

Of particular concern are areas where wildfires wiped out most of the plants and material that absorb rain, like the trees and chaparral. That includes the Angeles National Forest, where the 2020 Bobcat Fire lasted over 82 days and burned almost 116,000 acres. The fire destroyed 169 structures. Thousands of people were forced to evacuate.

Burned vegetation coats soil with a wax substance that can cause soil to become hydrophobic, meaning it will repel water instead of absorbing it. It’s like rain falling on an asphalt parking lot — it just runs off.

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The first two years after a fire is when the worry is highest. But the flash flood risk often remains for much longer.

Below the Bobcat Fire burn scar, communities like Monrovia and Azusa are now highly susceptible to flash flooding. To combat that risk, Los Angeles County spent decades building more than 160 debris basins, kind of like a neighborhood dam. The Oakglade debris basin in Monrovia sits just above multimillion dollar houses, with swimming pools clearly visible.

After the recent torrential rains, the catchment area is about half way full of sediment, trees and rocks. Normally, it would take decades for it to become this full, but it sits within the Bobcat Fire burn scar.

Crews from L.A. County Public Works are racing the clock, trying to clear out more than 7,000 cubic yards of debris in less than 48 hours before the next expected storm cycle this weekend.

A cubic yard is the size of a washing machine, so their goal is to move the equivalent of 7,000 washing machines of debris. Crews use backhoes and bulldozers to fill up dump trucks, which drive the mud through winding neighborhood streets to a sediment placement area a few miles away.

This process is normally done twice a year at the Oakglade site since the Bobcat Fire, but recent rains and the threat of more storms mean emptying it is a top priority. The water is separated from the debris by drains, but if mud and trees plug up the basin it could spill out onto the streets and neighborhoods below.

Oakglade is one of three debris basins that have capacity issues and are being addressed before the next round of storms, said Steven Frasher, Public Information Officer with L.A. County Public Works.

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“Even if it’s a light storm, one cell can be very localized and that can have an impact that is out of proportion with the rest of the region,” Frasher said.

During the storms, crews will patrol the infrastructure and monitor areas like the Oakglade debris basin. With soils already fairly saturated, even light rain over a few days could bring some slopes to a tipping point.

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