California Faces Threat Of Flooding, Mudslides

Washington Post

November 7, 2003

by William Booth

A few ground crews with shovels are still tamping down the last hot spots left over from the worst wildfires in California history. But now, all eyes are turned to the sky. Heavy rains are forecast for the weekend, and that means mudslides and flash floods. It could get downright biblical.

Emergency preparedness officials are rushing around four counties attempting to identify the most vulnerable and dangerous hillsides -- those charred and cleared of brush and trees, primed to come crashing down when the rain comes and turn the earth intoa potentially lethal slurry.

"If you have really intensive rainfall right away, the only thing you can do is get out of the way," said Ed Harp, a scientist with the U.S. Geological Survey's landslide research and prevention program.

Bill Schultz, deputy chief with the California Department of Forestry, warns that "there are going to be mudslides." He said: "Whenever you have major fires burn out an entire area, the following winter heavy rains can trigger those flows coming up to their own properties."

Newcomers to Southern California often say they miss the seasons of the year. But there are seasons here: the summer fog and smog, followed by the Santa Ana winds and the fires of autumn, and then the winter rains -- monsoons that can be real gully-washers.

The National Weather Service is forecasting moderate to heavy rain in the mountains here beginning Friday and continuing through Monday.

Southern California is especially vulnerable, too, because homes, roads and waterworks are laced throughout the burnt hills of the San Gabriel, San Bernardino and Laguna mountains.

"Nothing has ever occurred in California that moved this fast, burned this hot and caused this much devastation," said El Cajon Fire Chief Ed Jarrell, whose teams fought the Cedar blaze, which consumed 280,000 acres in 48 hours.

When the fires burn hot, as they did in the recent weeks, the plants of the chaparral -- the chamise, scrub oak and California lilac -- release a compound that mixes with the topsoil and makes the earth literally waterproof.

This residue, said Harp of the USGS, "you can't see it, you can't taste it. It has no color and it doesn't degrade in sunlight. You can toss a handful of dirt into a bucket of water and it doesn't get wet. It floats. You'd be amazed."

They call this "non-wettable soil," Harp said. "I had some of this soil in the lab, in water for three or four months, and it never got wet."

When rain -- and in some cases even a heavy mist -- falls upon a hillside that has been charred, its plants and roots burned and gone, and nothing left but water-repellant dirt, there is great potential for avalanches, debris flows, mud floods and other "mass movements."

The top six inches of the slope can literally float away, carrying with it boulders the size of Volkswagens. "Water is heavy, it adds weight to the slope. Water is also a great lubricant," said Jay Famiglietti, a professor of earth science at the University of California at Irvine. "In short, if you remove the vegetation and add water, the soil will flow downhill."

Landslides cause an estimated $1 billion in damage nationwide each year. Southern California has experienced repeated and deadly slides over the last century. Houses have been filled with earth up to their roofs. Buildings and cars have traveled hundreds of yards. The Los Angeles Times reminded readers recently that on New Year's Eve in 1934, a 20-foot-high wall of mud killed 49 people in the suburbs of La Crecenta and Montrose.

The area is so susceptible to mudslides that the counties, state and federal government have erected "debris basins" at the bottom of hillsides to protect homes, roadways and reservoirs. These basins essentially act as enormous filters, designed to stop falling rocks but allow water to flow through. Los Angeles County has 301 of them, the most of any locale. San Bernardino County has about 50. Workers are scrambling to empty the debris basins, because once they are full of rocks, they can become overwhelmed. The debris basins may help, but they are no panacea.

Cathleen Thompson, an information officer for the U.S. Forest Service, said "burned area emergency response teams" are spread across the region assessing the damage done by the wildfires. Teams have just seven days after a fire is contained to submit an initial report to qualify for federal funding for anti-flooding and mudslide measures. Treatments are expected to begin in the next day or two.

"We want to do our initial erosion control before the first large rains," said Shultz of the California forestry department. "The race is on to get that work done."

At the peak, there were 14 individual fires burning across Southern California, charring nearly 750,000 acres. At least 20 people died, including one firefighter, and more than 3,500 structures were lost.

Possible treatments include widespread grass seeding, clearing debris out of channels along roads and structures, or spreading hay and straw on top of heavily burned areas to protect the topsoil from being washed away. Burned trees may be cut down in some areas and lined up against hillsides to slow mudslides if heavy winter rains occur.

"It won't stop it completely but it will slow it down," Thompson said. "You can't prevent Mother Nature. What you can do is mimimize the damage that naturally occurs after a fire."

But the use of widespread grass seeding, for example, remains controversial. It can introduce nonnative grasses, and perhaps even slow regeneration. It also may not work.

Weather forecasters are working with scientists on the ground to predict which burned areas may be most vulnerable and are trying to improve their ability to warn local residents when and where to be most wary. Already, people in mudslide danger zones are being warned not to sleep in rooms that face hillsides during and afer rain showers.

Representatives from the National Weather Service have set guidelines for when flash-flood warnings will be issued. As little as a tenth of an inch of rain and hour or half an inch in six hours will be enough for a warning to be issued to residents. The guidelines were set Thursday and rain last Friday triggered a warning in Ventura County, where the Piru fire burned.

Of course, rain is not the only thing that can trigger a mudslide. "It's not exactly quiet here, tectonically speaking," said Famiglietti of UC Irvine. "Given a little shake from an earthquake, that can also set things off."