Flood control experts fear that wildfires have created potentially catastrophic landslide hazards in charred areas throughout Southern California -- especially in San Bernardino County, where as many as 50 catch basins built to block falling boulders, mud and trees may not be adequate.
Debris flows, as the deadliest form of the slides are known, can be ferocious, crashing down mountain slopes, overwhelming barricades and dropping tons of rubble on unsuspecting communities during heavy rains.
The San Gabriel and San Bernardino mountains are dotted with catch basins -- government's response to a long and violent history of sudden landslides. The basins are typically engineered to capture the muddy fallout from a 100-year flood -- a heavy rainstorm whose likelihood of happening in any given year is only 1%.
But in areas damaged by wildfires, the volume and velocity of material washing down can be 10 times greater than usual -- and exceptionally heavy even four to five years after a blaze.
As a result, many basins in fire-ravaged San Bernardino County could now be strained by a major storm, putting thousands of homes, schools and other buildings in harm's way, according to county flood control officials and other hydrologists.
"Most of these basins, if they get hit within a year or two of a good fire, they will not be big enough," said Pat Mead, an assistant public works director for San Bernardino County.
"In a normal fire year, we get maybe one or two canyons with watersheds in them burning. By the looks of things, these fires have burned every watershed in the north part of our county."
Last week, San Bernardino County officials said they would seek federal money to clear out and expand the basins, warn nearby residents about landslide dangers and erect walls of sandbags to minimize the threat.
Meanwhile, the U.S. Forest Service, which controls many of the wilderness areas hit hardest by the fires, has begun assembling a team to determine damage and look for ways to diminish erosion.
"We don't want to scare people because we don't think a disaster is about to happen, but they need to know that this is not normal," said Ted Golondzinier, another assistant county public works director. "We do think there are areas that are going to be getting some mud flows, and we're trying to figure out where those are most likely to happen."
Fire-scarred parts of Los Angeles, Ventura and San Diego counties -- including areas not typically prone to landslides -- also may face an increased chance of landslides because of the scope of this year's fires, among the worst in modern California history.
"Regionally, this is one of the worst potential flooding situations since this became a civilized place," said Douglas Hamilton, a flood control expert with Exponent Inc., an environmental consulting firm. "Everybody knows the San Gabriel and San Bernardino mountains have problems with debris flows. But even in San Diego, where debris has not been as big of a problem, you could now have a problem because of these fires."
Debris flows have caused dozens of disasters in Southern California over the last century, including a 20-foot-high avalanche of rocks and mud that swept over La Crescenta and Montrose just after midnight on New Year's Day in 1934, killing 49 people. A wildfire preceded the disaster. No debris dams were there at the time.
The dangers of debris flows were highlighted in the 1989 book "The Control of Nature" by John McPhee. One passage recounts the horrifying experience of the Genofile family, which nearly perished when a 6-foot wall of muck suddenly struck their home in Shields Canyon above Glendale in 1978 after a particularly intense rain.
"The house became buried to the eaves. Boulders sat on the roof. Thirteen automobiles were packed around the building, including five in the pool. A din of rock kept banging against them. The stuck horn of a buried car was blaring," McPhee wrote. "The family in the darkness in their fixed tableaux watched one another by the light of a directional signal, endlessly blinking. The house had filled up in six minutes, and the mud stopped rising near the children's chins."
If wildfires precede heavy rains, the threat of debris flows is exponentially greater, experts say. The fires consume the vegetation that coats hillsides and binds soils together, greatly exposing the areas to erosion. That erosion can deposit huge amounts of sediment downstream from burned areas during rainstorms in a matter of minutes.
"Wildfires remove the canopy that intercepts rainfall, the leaves and needles that are on the ground. And once you've removed that, the water is just going to run downhill, taking a lot of other things with it," said Susan H. Cannon, a researcher with the U.S. Geological Survey's landslide hazards program, which has been studying the link between fire and debris flows for years.