Storm to test Southern California debris basins

With four days of rain forecast for Southern California beginning Sunday night, the best defense against mudslides and flooding in areas burned during the Station fire is a network of 29 debris basins scattered around foothill communities.

During most years, residents have little reason to think about the basins. But the Station fire, the largest in Los Angeles County’s recorded history, has left charred hillsides that federal officials say pose an extreme mudslide risk if the area gets sustained rains.

Officials say the man-made bowls have become crucial to catching boulders, timber and heavy mud that otherwise might head toward homes.

“The debris basins are our front line of defense,” said Arthur Vander Vis, a principal engineer in the county Department of Public Works’ Flood Maintenance Division.

Scientists with the U.S. Geological Survey identified Pacoima Canyon, Big Tujunga Canyon, the Arroyo Seco, the West Fork of the San Gabriel River and Devils Canyon as being at most risk of mudslides.

Under certain conditions, mud flows could go as far south as Foothill Boulevard in La Cañada Flintridge and La Crescenta, the scientists said in a report.

Officials said damage from mud is likely if debris basins in foothill communities and the mountains of the Angeles National Forest fill up.

There are more than 150 basins in Los Angeles County, and the 29 in the burn area hold a combined 1,206,900 cubic yards of material.

Typically, public works officials will empty a basin when it reaches 25% of capacity -- sometimes that takes one year, sometimes 10 years. But hoping to avoid flooding, officials plan to empty basins in the burn areas at 5% of capacity, a rate that will continue for the next five years while vegetation regrows on hillsides, Vander Vis said.

And because of the conditions, these basins can fill up very quickly.

A recent example: The Mullally debris basin in La Cañada Flintridge holds 9,400 cubic yards of material. In 30 minutes after a short but powerful burst of rain in mid-November, it was full. “It impressed me how quick it filled it up,” Vander Vis said. “I hadn’t seen that happen myself.”

Nine basins in the burn area have been cleaned, eight more are scheduled to be cleaned and six more are scheduled to get greater capacity with the installation of temporary walls to increase the height of the basin.

Cleaning a basin costs about $10 per cubic yard -- loads are hauled to nearby placement sites, a sort of sediment landfill -- but Vander Vis said it is well worth it.

“If the debris basins weren’t there, there would be even more mud coming down the street,” he said. “If it fills up and overflows, there’s still less debris in the street than it would be otherwise.”

Public works officials will keep watch on the basins during a rainstorm, sometimes checking every hour depending on the rain’s intensity.

This weekend’s rains are expected to arrive Sunday night, with a 50% chance then, and last through Wednesday. The greatest chance of rain (70%) is Monday, said Stuart Seto, a weather specialist at the National Weather Service’s Oxnard office.

Seto said forecasters aren’t yet sure what the intensity will be, but “if it rains half an inch in one hour, it could cause problems” in the burn area.

Public works spokesman Bob Spencer said foothill residents need to stay alert and be ready to evacuate if so ordered by responding agencies. He also said if the current forecast continues, the forest roads probably will be shut down as early as Sunday night.

Pat Anderson, president and chief executive of the La Cañada Flintridge Chamber of Commerce, was grateful for the Mullally basin that filled with debris in November.

If it hadn’t, she said, that debris may have come straight toward her home.

“It is engineered to do what it does, and it does it very well,” she said.

But Paul Dutton, head coordinator of Crescenta Valley’s Community Emergency Response Team, said he is still wary.

“My wife and I have a contingency plan to get out of the house and over the neighbor’s wall if we think the debris basin is going to flood,” he said. “It’s a scary thing.”

For foothill residents, living near one of the basins now presents a mixed bag of feelings. Some like the protections -- but other worry that if the basins overflow, their homes would be in the direct path of mud and water.

“It’s real nerve-racking,” said Dutton, who has one behind his home in La Crescenta.

baxter.holmes@latimes.com