Debris basins form first line of storm defense for Los Angeles’ foothill homes

LA’s flood-control system holding up in storms

LA CANADA FLINTRIDGE, Calif. — Despite a week of heavy rain on mountains stripped dangerously bare by wildfires, thousands of homes have so far been spared from deluges of mud and rocks by a network of sprawling basins that act as huge bathtubs to safely catch debris.

Los Angeles County’s vast flood-control system has become a crucial line of defense in a region with a long history of devastating mudslides and flash floods.

“The infrastructure has given 1,000 percent and done its part,” county Public Works Director Gail Farber said as the week’s fourth storm headed east, trailed by occasional thunderstorms and hail.

With a dry weekend expected before the next possible storm, authorities lifted evacuation orders for about 2,000 homes along foothills and in canyons below the fire-scarred San Gabriel Mountains.

Small muddy streams of water coursed through some properties, but the basins kept debris that flowed out of the mountains from clogging storm drains or bulldozing through communities below the 250-square-mile burn area of the Station Fire, the largest wildfire ever in Los Angeles County.

Crews usually clear the basins when they are about 25 percent full. But after the Station Fire, basins in burn areas were cleaned out more often to keep them ready for winter storms.

A few also had capacity increased, and there are long-term plans for further expansion.

“We have crews out there now clearing the inlets and removing the debris so that we restore the capacity for the next major storm,” Farber said.

Some of that work is done by earth-moving machines, but the system depends on manual labor, too.

“They shovel it out, carry it down the hill by bucket and put it in a wheelbarrow. It’s a lot of hard work, but it’s got to be done,” said Robert Martinez, an 18-year veteran overseeing a team clearing one of the smaller basins behind a home in La Canada Flintridge.

The debris was dumped on the street and another team loaded it into a truck.

The 28 basins in the burn area are entry points for mountain runoff into a flood-control system that includes 14 dams, 500 miles of open channel, 2,800 miles of underground storm drains and 300 other basins.

Development of the system — designed to withstand a storm that might dump 5 inches of rain an hour — began in 1915 after a disastrous flood the year before caused more than $10 million in property damage.

By contrast, this week’s series of storm dropped a total of 4 to 8 inches of rain along the coasts and valleys, and 8 to 12 inches in the mountains.

Climatologist Bill Patzert with the Jet Propulsion Laboratory said the latest storms occur every three to four years.

“This was definitely not big in terms of rain rate and rainfall,” he said.

Without the drainage system, metropolitan sprawl could never have been accomplished at the foot of mountains that rise as high as 10,000 feet within eyesight of downtown Los Angeles.

The mountains have repeatedly disgorged destructive and even deadly debris flows, especially after wildfires.

On Dec. 31, 1934, short, intense rains on the 20th day of a storm unleashed a massive debris flow in the La Canada and neighboring areas, killing 30 people and destroying 483 homes. The U.S. Geological Survey said nearly 660,000 cubic yards gushed out of the mountains in that single event, including a 59-ton boulder.

Unlike mudslides, debris flows are fast-floods of water laden with soil, rocks and tree stumps that can plow through bridges and houses like battering rams. Among more recent disasters, a 1969 flow engulfed buildings and people trying to flee in cars.

When heavy rains pour down on Southern California, runoff descends thousands of feet from the mountains and rushes to the sea in a short 50 miles, turning usually dry flood channels into torrents. During the height of this week’s storms, water ran at speeds up to 40 mph, said Farber, chief engineer of the flood-control system.

The basins range in capacity from a few hundred cubic yards of material to a few hundred thousand cubic yards. Their designs capture soil, boulders, trees and other vegetation while allowing water to continue on through outlet channels.

Workers constantly tended basins during the stormy week.

“They’ve been at it 24-7,” said Dave McLaughlin, who lives across the street from a cul-de-sac where a minor mudslide occurred. “As soon as the mud comes, they scoop it up and get it out of here.”

If the basins are allowed to fill up, Martinez said, water will back up and eventually force debris over the basin and down on the homes below.

Overall, California emerged from the week of storms with only four storm-related deaths, although tornados toppled power lines and swept through neighborhoods, hundreds of trees fell and urban flooding hit coastal cities including Long Beach and the San Pedro area of Los Angeles.

AP Science Writer Alicia Chang reported from Los Angeles. AP writers Thomas Watkins and John Antczak contributed to this report.