Soil Experts Issue Warnings to Malibu’s Hillside Homeowners : Environment: Panelists at flood-erosion workshop also offer advice to residents on stabilization and landslide and mudflow mitigation.
The mudslides are coming to Malibu, and probably the most effective thing residents can do to minimize the damage and danger is hope and pray for a dry winter.
So say the experts, who spoke at a flood-erosion workshop in Malibu last Saturday, sending a worst-case scenario message to about 25 residents on slope stabilization, landslide and mudflow mitigation, flood insurance and tips on when and how to evacuate.
“Be advised that if you’re waiting till the storm to prepare, then you’re already too late,” warned Donald Nichols, a hydraulic engineer for the County Department of Public Works. “If you are at the bottom of a canyon (in a heavy storm), you really ought to save yourself.”
Nichols predicted that, as a result of the Nov. 2 wildfires, Malibu will be enduring about four years of heightened danger from floods, mudslides and debris flows before vegetation, which averts flooding by absorbing and dispersing rain, regenerates.
Not only did the fire kill plants holding dirt and rocks in place, U.S. Soil Conservation Service scientists added, it also released burning oils and gases from the plants that made the soil “hydrophobic” or water-repellent.
In an extensive area of hydrophobic soil, said Thomas D. Benson, an engineer with the federal agency, there is the possibility of a debris flow. This occurs in a heavy rain when the top three to six inches of soil gets so heavy that it cuts loose from the hillside, carrying loosened rock, burned vegetation and soil downhill at speeds only slightly slower than water.
“It’s a condition that you just have to live with because there are no laws of physics that control it once it starts,” Benson said. “We think if a big storm comes it is going to happen. I wouldn’t be in the canyons if it does.”
But where there is a disaster, there are experts with a multitude of solutions--sometimes conflicting. On Saturday, at least 16 different flood and landslide prevention tactics were discussed at the workshop in a head-spinning whirl.
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“I’ve had everyone come out to my house: a soil conservationist, a private civil engineer, a geologist,” said Pat Mudie, who lives on a Carbon Mesa Road hilltop. “And it seems to be an overwhelming problem with about 10 different experts. Where do you start? I want to know what’s the most logical way to go about this. I’ve had conflicting opinions. The soil scientist said don’t do anything. The geologist said everything looks OK (from a landslide perspective). And the civil engineer said, ‘I’m not really happy with what you’ve been told.’ . . . Then I was told to call a native plant specialist.”
There is so little agreement that one expert’s slide-prevention method is likely to be held up by other experts as an example of what not to do.
On a recent tour of the fire area led by Soil Conservation Service scientists, for example, the federal specialists pointed out a massive slope covered with jute netting above Pacific Coast Highway near the Malibu Lagoon as an example of poor erosion control.
“We don’t recommend jute netting on long, steep slopes,” said conservationist Rich Casale, who acknowledged that he had not actually analyzed the site. “The disturbance of the soil to lay it down and the soil disturbance of a fire . . . probably defeated the purpose. We also usually recommend that mulch be used in conjunction with jute.”
But the landscaper who covered the vast hillside with the netting, James H. Cowan and Associates Inc., defended the work.
“The conservationists say not to do anything and their school of thought may be well-taken (in light storms),” said Clark Cowan, the contractor on that job and son of company owner James H. Cowan. “But for the homeowner who assumes the liability (of rocks and mudslides rolling onto PCH) that’s not going to work. The first major rainstorm is a big gamble and a hazard to the highway.”
Cowan said he removed rocks and boulders from the slope, cut down burned vegetation leaving root masses, and stapled, overlapped and tucked in jute netting from top to bottom. Chain link fences and sandbags were placed at the base of the hill. The cost was about $100,000, he said. The owners of the hilltop home, William J. and Karen L. O’Connor, declined to comment on the project.
Casale of the Soil Conservation Service said the use of plastic sheeting or seeding with grasses was also risky on steep hillsides, because both tactics increase surface moisture--and weight--and are likely to cause slides during a rainstorm.
Another worry is improperly placed sandbags that drive water onto a neighboring property.
“I am personally alarmed at the incorrect application of sandbags,” said Casale, who added that there is no universal answer for homeowners, only site-specific recommendations. “Our streets are going to be rivers after the rains.”
Nichols, the county representative, encouraged residents to prepare as best they can, then watch the weather closely for signs that a storm is intensifying.
“If it starts raining, maybe you just ought to go stay with Aunt Tilley in the city,” he said. “In some cases you may not be able to evacuate by car, and you may have to leave as a pedestrian. If you’ve never seen a mudflow, I promise it will impress you.”
Malibu officials are recommending that residents consult the Soil Conservation Service and the county Flood Control District before implementing any preparation measures. The federal agency has already consulted with 1,500 homeowners, Casale said.
Woodward-Clyde, a consulting firm retained by the city, is working in conjunction with the Soil Conservation Service. It has produced a map of landslide and flood-prone areas available to residents through the city. The highest priority areas are Big Rock, Las Flores Canyon, Piedra Gorda, Rodeo Grounds Road and Pacific Coast Highway.
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Lest one thinks all this an overstatement of the threat, consider the effects of recent rains on Malibu. A storm last month that dropped only a half-inch of rain caused mudslides that closed all but one lane of Pacific Coast Highway. On Dec. 14, parts of Pacific Coast Highway were covered in three feet of mud from just an eighth-of-an inch of rain.
Benson of the Soil Conservation Service said three inches of rainfall in a 24-hour period could spell disaster.
He and other experts added that most storm preparation measures would probably be rendered useless by a heavy several-day storm. At that point the only advice is to get out.
Ivan Neaigus, who lives on Pacific Coast Highway across from the mouth of Las Flores Canyon where rainwater drains off, got the picture in spades.
“After learning that one inch of rain was 100,000 tons of water,” said Neaigus, whose basement filled with mud during the ’83 flood, “I feel that whatever you do is minimal compared to the force of nature.”
Information: The U.S. Soil Conservancy Service can be reached at (818) 222-4750. The city of Malibu also is planning more flood/erosion control workshops. Call City Hall at (310) 456-2489 for more information.
