Fires Bring Hazard of Landslides
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.
Furthermore, in chaparral-coated Southern California, burning of the brush has been shown to harden surface soils, making the ground more water repellent than usual. That significantly increases the speed with which rainfall rushes down slopes, increasing its destructive power.
“It’s an amazing amount of water that can come out of those mountains when it rains,” said Chris Wills, a supervising geologist with the California Geological Survey, who vividly remembers his father taking him to see raging mountain waters that filled the Los Angeles River during floods in 1969.
One potential flashpoint is Deer Creek near Rancho Cucamonga. There, the capacity of a large debris basin below mountains that rise to nearly 9,000 feet was the subject of bitter controversy, long before last week’s wildfires. The stadium-sized basin lies in the mouth of a canyon at the foot of the San Gabriel Mountains in an alluvial fan molded over time by thousands of floods. Before the area was developed, the rushing mountain waters that spewed from the canyon during the short but strong seasonal rains traveled along a wide swath of the San Bernardino Valley and into the Santa Ana River.
Now that thousands of people live on the valley floor, the waters are corralled by a network of flood channels, and urbanization has been creeping ever closer to the foot of the mountains. The basin, built in 1983, was augmented by a levee that had long existed in the area, but a developer secured approval several years ago to breach the levee to build more homes above it, despite neighbors’ concerns that the debris basin alone could not withstand the torrent of muck the creek was capable of discharging.
John Cassidy, an engineering expert working for nearby Ontario International Airport, and Hamilton, of Exponent, who was hired by a citizens group, concluded that the basin, built by the U.S. Army Corps of Engineers, was too small to handle a 100-year flood.
“As constructed, the Army Corps’ debris basin would hold only a fraction of the debris that would come out of the watershed during a 100-year flood,” Cassidy, a former engineer for Bechtel Corp., said in a deposition. “Required storage would be deficient by 500 acre-feet or more. Five hundred acre-feet would be equivalent ... to some 20,000 truckloads of debris.”
Despite the experts’ criticisms, the Corps of Engineers has stood by the Deer Creek basin, and public elementary and high schools have since been built below it.
Joseph Evelyn, the supervisory hydraulic engineer for the corps’ Southern California office, said the basin had been built to withstand the largest debris flows the corps expects, and took into account that the flows could be made much worse by fires.
But last week, he stopped short of saying it could withstand anything rainwater could wash down. The reality of such structures, he said, is that they are built to reasonably minimize the risk of damage, within economic and even aesthetic constraints.
“It can happen, and has happened,” he said when asked if similar debris basins have been known to fail. “But the degree of damage has been within acceptable tolerance. We haven’t had an outcry from people asking for fewer teachers and police officers to build bigger debris basins.
“If you are going to assume the worst -- a huge storm situation after a huge fire -- you would have to build huge structures that would cost a tremendous amount and would not be very good to look at.”
Malissa McKeith, an attorney who lives just below the old levee and has spent tens of thousands of dollars of her own money in fighting to shore up the protections at Deer Creek, said she hoped the fires would lead local officials to reassess the flooding dangers.
“Everyone has known there was a problem; they just hoped it did not occur on their watch,” McKeith said. “Well, now the problem’s here. At this point, I’m just hoping that someone will take a look at these schools. It’s not too late to do something to protect them.”