Storms could test L.A. County’s aging flood-control system

With powerful storms expected to further soak hillside communities, crews worked Tuesday to remove muck and fallen trees from debris catch-basins perched in the fire-stripped mountains.

The storms could test the aging Los Angeles County flood-control system, which was developed in 1915. The system usually works, steering flowing debris away from homes and into steel and concrete traps at the entrances of drainpipes. The concern now is that rainwater will gather in steep canyons and surge down slopes stripped of binding vegetation.

Officials said that of the 30 catch-basins located along the south-facing slopes of the San Gabriel Mountains between Big Tujunga Canyon and Altadena, one was filled to the brim and nine others were a concern because of fast-rising debris levels. Crews worked around the clock with earthmovers and dump trucks to keep the basins flowing, but were slowed by intermittent hail, lightning and heavy rain. They were forced to halt operations several times Tuesday.

“The last place we want our people working is below a catch-basin filling with silt,” said Arthur Vander Vis, principal engineer for the county Department of Public Works’ flood-control division. “Our excavators would sink into a stew of quicksand.”

Surveying darkening skies and rain outside his office in Sun Valley, Vander Vis said: “It would be an overstatement to describe Wednesday as a window of opportunity. We were hoping to do more work today.”

Of particular concern was the Mullally debris basin above La Cañada Flintridge, which overflowed on Monday, contributing to a decision to temporarily evacuate neighborhoods at the base of steep, denuded slopes. On Tuesday, flood-control crews used earthmovers to scoop tons of silt, rocks and wood out of the basin, and to clear logs from the inlet of a storm drain channeling water to the Pacific Ocean.

“If the inlet gets plugged, the storm drain will back up, and there will be a lot of mud and logs flowing down neighborhood streets,” Vander Vis said.

A few miles to the west, the Pickens debris basin in Sun Valley -- which is the size of a baseball field and about 40 feet deep -- loaded up with mud 35 feet deep within four hours Monday. The basin was recently expanded to handle up to 125,000 cubic yards of silt and debris, officials said.

Flood-control and law enforcement authorities began preparing for winter storms in the area in September, “while the Station fire was still smoldering,” said Sheriff’s Capt. David Silversparre. “We’ve had a few practice runs with rain in September and on Monday and today.”

Under normal conditions, flood-control crews begin clearing debris basins when they are about 25% full. In the aftermath of the Station fire, however, “we lowered that threshold to 5% full in areas that were 100% burned,” Vander Vis said.

Most of that debris is being hauled to “sediment placement sites scattered across the foothills,” he said,

In the meantime, a contingent of 40 flood-control workers was hauling mud and installing concrete barriers to channel surges of water into streets and down local drains.

louis.sahagun@latimes.com