Storm Warnings : Rains Bring Threat of Mudslides to Wildfire Areas

As the season’s first big storm headed into Southern California, hydrologists studying soils denuded by last month’s wildfires identified the most dangerous mudslide areas.

Topping the list in the scorched Calabasas/Malibu fire zone is rugged Las Flores Canyon, a narrow, steep divide north of Topanga State Beach.

County and federal officials say residents of lower Las Flores Canyon are exceptionally vulnerable because of the large size of the basin, which was almost completely burned, and the fact that the channel at the bottom is small.

Other canyon areas considered potential trouble spots with the approach of rainy winter weather are Carbon Canyon, Big Rock and Piedra Gorda canyons on the Malibu side of the coastal range, and along the creek channels in Topanga Canyon, Red Rock Canyon and the Calabasas Peak area to the north.

“If we can get through this winter without anyone dying in a mudslide, that’s what we are hoping for,” said Michael Anderson, head of hydrology at L.A. County Public Works.

Today’s predicted storm is not expected to be catastrophic--one-half to one inch of rain is expected--mostly in northern Los Angeles County. But if the moisture falls fast enough, it could trigger slides on fire-blackened slopes.

Officials are quick to point out that just because an area is not on the list of trouble spots, that doesn’t mean there is no danger. Debris flows are highly unpredictable; no one in a canyon bottom in a burned-out area should count themselves safe when the rain starts falling.

“There isn’t much warning if a big storm comes in,” said Tom Benson, hydraulic engineer with the Soil Conservation Service. “If there is a storm coming, get out of those canyons . . . I wouldn’t wait in a creek bottom to see if something is going to happen.”

The county has already advised nearly a score of Topanga-Malibu residents whose homes may be in the path of debris flows to be ready to evacuate if more than one-half inch of rain falls in a half hour, or if debris appears to be building up in streams, Anderson said.

Mudslides can occur on any tilted soil plain, but what makes them a particular threat on rain-sodden slopes after fires is the convergence of several factors, of which the absence of vegetation is only the most visible.

County engineers and federal soil scientists have found widespread patches of “hydrophobic” soils, especially in Las Flores Canyon, which are formed by fire and are highly prone to mudslides.

Layers of hydrophobic soil form when organic matter on the ground is vaporized by the heat of a fire. The vapor penetrates the soil, cools and re-condenses further down, creating a waxy, water-resistant layer one to three inches below the surface, said Anderson. It is almost as if someone had laid down a sheet of Saran Wrap, and piled dirt on top of it.

With this layer in place, rain hitting the soil can no longer trickle down and be absorbed, said Denise Downie, a soil scientist with the Soil Conservation Service. Instead, it pools up in the topmost layer, which becomes steadily heavier and soggier, and eventually shears off and slides downhill.

The hotter the fire, the deeper the hydrophobic layer, and the greater the slide danger. In the Calabasas/Malibu fire area, a large buildup of dry shrubs and trees over many years made for a particularly hot burn.

How much rain falls, and how fast, are also key factors in calculating mudslide potential, said Anderson. UCLA meteorologist James Murakami said a period of moderate to heavy rain today is a possibility, although the brunt of the storm is expected to hit north of Point Conception.

But even half an inch of rain falling in an hour could start debris moving, because hard, pelting rain tends to break up the soil and cause it to shift. In burned areas, there are no leaves or branches, which collect water and allow it to pool up before hitting the ground, so the effect of a small amount of pelting rain is all the more intense.

The direction mud flows through the mountains roughly follows the contours of drainage basins or watersheds that feed creeks, said Mark Cocke, a civil engineer with the Soil Conservation Service.

A watershed is an area where all the rainwater drains into a single stream channel. The largest watershed in the fire area is also the most hazardous: Las Flores Canyon, 2,700 well-populated acres, the lower half of which is prime for debris flows.

Carbon Canyon is less populated, but is also a large basin, measuring 1,400 acres, that was nearly completely burned. The same conditions are replicated in Piedra Gorda, Rambla and Big Rock canyons.

Topanga Canyon poses a flood hazard because the creek channel is narrow and overflows easily. Debris pouring through the channel could endanger the road and homes along the bottoms of Old Topanga and lower Topanga canyons. Hot burns on Red Rock and Calabasas Peak could lead to floods of debris on the creek bottoms in those areas. Channel floods are also a possible danger in the Monte Nido area, Cocke said.

Homes at the bottom of slopes also are at risk this winter along Pea Road, Stunt Road, and in Hondo Canyon.

Road damage is almost inevitable throughout the fire areas, and it’s likely some areas accessible by only one road will be cut off at some point during the rainiest months of December, January and February, Cocke said.

“People are going to need to make plans to be isolated for a while,” he said.

The Threat of Sliding Canyons Canyons burned by the Calabasas / Malibu fire are at great risk for flooding and mudslides. Creeks running through the steep canyon slopes are likely to overflow and carry mud and debris with them. Contributing to the risk is the water-repellant soil created by the hot, fast-burning blaze. The greater danger follows fast-flowing, sustained storms. Overflowing Creeks The Soil Conservation Service divides the burn area into 23 watersheds. The larger and steeper the area, the more likely the chance for flooding. Homes along creeks and at mouths of canyons are at greater risk. Areas most threatened Los Flores: Because it is largest of the hardest-hit watersheds, covering 2,700 acres, it can hold a large amount of debris and water. Once filled to overflowing, water on the canyon floor and Los Flores Creek will flow from the high point of 2,700 feet through the narrow canyon mouth, down to the ocean. The canyon was 100% burned and has a higher potential for “hydrophobic” soil. Other high risk canyons: Carbon Canyon, Piedra Gorda Canyon, Red Rock Canyon, Calabasas Peak. Unstable Soil Fire can make hillsides hydrophobic, or water-resistant, leading to runoff and erosion. When chaparral burns it produces a gas that penetrates the ground, condensing as it cools, creating a waxy layer just beneath the surface. Funnel Phenomenon Steep at the top and narrow at the bottom, the canyons become channels for fast-moving debris flows. The larger the basin, such as Las Flores Canyoin, the greater the amount of debris that has to squeeze through a narrow path to the ocean. Ash: 1-2 inches thick 1. When it rains: water enters soil through root channels and animal and insect burrows. 2. Absorption is stopped: 2-6 inches below surface at hydrophobic layer. 3. Water runs off: unable to soak in, water slides downhill, carrying soil along. Sources: Soil Conservation Service; California Department of Conservation Division of Mines and Geology; Researched by JULIE SHEER