Floods, mudslides among post-wildfire worries

updated 3:46 p.m. ET Sept. 7, 2009

LOS ANGELES - Southern California's huge wildfire has turned nearly a quarter of the 1,000-square-mile Angeles National Forest into a moonscape of barren mountains looming above thousands of homes that now face the threat of flash floods and mudslides.

Experts are already evaluating the extent of risk to lives and property as well as the impacts of the wildfire on a forest ecosystem that in some areas may not have burned in at least a century.

Sprawled across the San Gabriel Mountains, the Angeles is both a playground for millions in greater Los Angeles and a true wilderness ranging from arid desert to alpine forests and peaks topping 10,000 feet. Skiers dare its steeps in winter; bears wander out of its chaparral cloak in summer for dips in suburban pools.

The chief concern is the impact the 246-square-mile Station Fire is having on the watershed. Countless canyons, ravines and gullies funnel watercourses toward communities at the forest's edge.

Flood-control system
The Los Angeles County Department of Public Works maintains a legendary flood-control system including 14 major dams, 500 miles of open storm channel and a nearly 3,000-mile network of underground storm drains capable of carrying storm water and debris through the metropolitan region to the ocean.

The system also includes basins — 30 to 40 in the area impacted by the fire — that intercept debris-laden flows from the canyons and trap mud and vegetation before the water continues on.

"Our concerns are that we will have a larger quantity of debris than normal being captured by our flood control system and, primarily, that individual property owners may be impacted by mudslides or mudflows to their properties," said Mark Pestrella, public works deputy director.

An overall assessment to predict the water flow has already begun. The basins are being examined to determine how much they may need to be cleaned out to create capacity, and channels are being examined to make sure they are free of obstruction such as overgrowth, Pestrella said.

That work will be done by Oct. 15, which the department marks as the start of the storm season, he said.

Teams will also fan out to assess burned slopes to warn homeowners and determine if temporary structures need to be built.

‘This hillside is going to move’
In Big Tujunga Canyon, Joseph Stachura can already see the danger: The fire left boulders unsupported on a barren slope above his home.

"That's pretty scary," he said. "I'm going to have to send the wife and kids out again when it rains because there's a good chance this hillside is going to move."

Rocks have already fallen on forest roads.

Although the Station Fire is now the biggest in county history, each element of the flood-control system was engineered for its portion of the watershed and has been tested by previous fires, Pestrella said.

"The system is nearing 100 years old and it has quite a track record for performing during these kind of events," he said.

The biggest defense against disastrous flooding this winter may be the weather trend.

On June 30, most of southwestern California completed its fourth consecutive season of below-normal rainfall. Precipitation in downtown Los Angeles has been only 64 percent of normal in those years, according to the National Weather Service.

The region is in for more of the same, said Bill Patzert, the veteran Jet Propulsion Laboratory scientist who investigates how climate variation is linked to oceans, including the El Nino warming phenomenon that sometimes leads to dramatically heavy rains in California.

The current El Nino "is definitely wimping out on us," Patzert said.

"The dice are definitely loaded. When you have a weak El Nino or a disappearing El Nino, it's a below-normal rainfall year," he said.

‘Any kind of rain is going to be a mess’
Patzert cautioned that it doesn't take an El Nino to bring heavy rains and the full picture of the risk the region will face from winter rains won't be known until after the fall — the major fire season in Southern California. He is certain there's trouble enough already.

"Any kind of rain is going to be a mess," he said.

In the forest, the consequences of the fire range from loss of wildlife and habitat to an indefinite closure of a vast area used for hiking, fishing, hunting, mountain biking and even commuting.

For the thousands of hikers, much of the forest may no longer resemble the descriptions in "Trails of the Angeles," the bible for trekkers in the San Gabriels since the early 1970s.

"I think you have a hard time designing a more destructive fire from a hiker's standpoint," said Doug Christiansen, now co-author of the guide originated by John W. Robinson. "All that country that it took out contains some of the most heavily used ... and some of the oldest hiking trails on the mountain range."

Christiansen said he and his wife hiked in the Angeles a few days before the fire.

"I feel like that was probably my last glimpse of the mountain range as I knew it. It's going to be generations before it comes back," he said.

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