Malibu Holds Its Own during Triple-Storm Offensive
• Scientists Set Up Local Monitoring to Determine How Burn Areas Handle Rain and Mud Problems
BY HANS LAETZ
BY HANS LAETZ
Creeks that hadn’t flowed in nearly three years were full of water—but no ashen debris flows appeared—as Malibu and the rest of the Santa Monica Mountains easily weathered three days of steady, moderate rain over the weekend.
Only a few rockfalls bedeviled motorists on canyon roads, county officials said. Most of them were outside the burn areas.
The series of three low pressure systems that brushed Southern California dropped less than half of the rain predicted as possible by forecasters, although it hit Northern and Central California as promised. Nevertheless, it was the largest rainfall event in Malibu since January 2005.
Spotters at Circle X Ranch, in the western peaks of the Santa Monicas, reported 5.8 inches of rain over the three storms, the National Weather Service said. Other storm totals included 4.4 inches in Agoura, 2.1 inches at Trancas, and 1.8 inches at Leo Carrillo State Beach.
There was some heavy rain Friday night, when 4.7 inches of rain fell at the Monte Nido fire station on Piuma Road. But despite the intense precipitation, the freshly burned slopes in a half-dozen canyons burned in the October and November fires held fast during the rains.
In advance of the triple-punch storms’ threat, U.S. Geological Survey catastrophe experts held a news conference in Malibu Thursday, and said they had rushed to activate new remote flood and mudflow-sensing equipment in three Malibu canyons prior to the big storms.
Scientists said they had calculated that torrential rains could wash down enough material from just three of the burned canyons—Winter, Sweetwater and Carbon—to cover a football field with 40 inches of debris. Add in the massive burned area in Corral, Escondido and Latigo canyons, and Malibu might have seen enough mud, ash and boulders to close Pacific Coast Highway in several locations, bury houses and claim lives, said USGS Southern California Multi-Hazards Coordinator Lucile Jones.
“We could have boulders the size of minivans, and minivans the size of boulders, down there,” Jones said at the news conference held under dark skies at Bluffs Park, overlooking the fire-scarred Civic Center area.
Debris flow from the Canyon and Corral fire zones is of such concern to National Weather Service and USGS that the agencies have set up remote monitoring equipment in several locations, including one in a narrow canyon north of the Pepperdine University tower.
Automated equipment, including a web camera that should begin operation soon, will measure ash and water flows in Winter Canyon, uphill from two schools, a small sewage plant and other key facilities on Civic Center Way.
Mudflows down Winter Canyon could cause major backup above a flood culvert below PCH, spilling out onto the beach just west of Malibu Colony. But the small arroyo draining past the schools was running with nearly clear water as the storms subsided, the flow having washed away soil dried by three years of drought that was then cooked by the Canyon Fire.
Another new USGS installation is a solar-powered stream flow measurement system at the Cross Creek bridge in Serra Estates. It will give emergency workers a real-time sense for how much water, and how much ash-laden sediment, is flowing out of the Santa Monica Mountains.
“Using rainfall thresholds from our rain gauge networks, we are able to monitor and keep track over the various burn areas that prompt the National Weather Service to issue a flash flood warning,” said Jayme Laber, an NWS hydrologist.
USGS and Weather Service scientists are using Malibu and other recent Southern California burn areas as a laboratory to research precise behavior by rainfall on freshly burned mountains. Some burned areas in Malibu have been laced with soil moisture probes, stream flow measuring devices and sediment flow gauges. Highly detailed topographical surveys have been undertaken as well.
Scientists at the press conference also unveiled a map that predicts in great detail which parts of burned-over Malibu are at greatest risk for mudflows. Sweetwater Canyon above Carbon Beach, and Palm Canyon north of Serra Retreat, were predicted to be the biggest risks in October’s Canyon fire burn zone.
Maps have not yet been finished for the area scorched in late November during the Corral Fire.
Susan Cannon, a USGS landslide expert, said Malibu provides a particular challenge because geology and rainfall totals can vary widely among the more than 25 different canyons and creeks that drain from the Santa Monica Mountains into the Pacific Ocean.
Cannon said scientists are hoping to avoid broad-brush predictions, and study how differing terrains shed water and debris in intensive rainfalls such as occurred this week.
A mobile Doppler radar truck, usually posted in the Midwest to chase tornados, was parked at Los Angeles Airport waiting for the storms to move in. “Our Smart-R truck gives us a gap-filling radar that can see the lower levels of clouds,” Laber said.
“It can see right down to the ridge tops, and information from the Smart-R truck is fed in real time to the Oxnard Weather Office for forecasters to analyze,” he said.
Laber said the total amount of rainfall is not as important as the intensity of the downpour. A cloudburst of two-tenths of an inch in 15 minutes, or a half-inch in one hour, will be enough to trigger ash flows in burn areas.
Jones, who has seen more than her share of catastrophes running the ISGS earthquake lab in Pasadena, said Malibu residents need to be aware that the debris flow threat is much greater than just mud flowing into a house.
“People get killed by these,” Cannon said. “The Christmas Day 2003 flood in San Bernardino moved four-foot boulders 20 feet per second.”
Jones said the installation of special monitoring equipment in the hills above Malibu, and in the Santa Ana Mountains near Irvine, is not designed specifically to trigger an improved warning system for those areas.
“We’ll not be issuing special warnings for Malibu, but we’ll be using Malibu as a guinea pig to improve future warnings,” Jones said.
After the storm, most of the guinea pigs in Malibu seemed to have come through without problem. One Los Angeles TV station mistakenly equated the Friday night flash flood warnings with “mandatory evacuation orders for Corral Canyon,” but the local sheriff’s office said conditions remained well below that level when the storms arrived, and no evacuations were ordered at any time.
Southern California Edison said there were no widespread or lengthy problems in the area, although a couple of traffic lights on PCH were left flashing after momentary outages Saturday at Point Dume and Sunday at Carbon Beach.
Beaches were covered with seaweed and debris to a high level, a result of eight-foot waves and a five-foot high tide Saturday. Waves trickled up to the sea wall at Zuma Beach for the first time in about two years, as the ocean once again chewed into beaches that had built up over the past few stormless years.
Motorists on Kanan Dume Road Monday were greeted with a 180-degree panorama of snowy mountains, as the Topatopas north of Ojai, the peaks north of Santa Clarita, and the San Gabriel Mountains above Pasadena looked like snow-capped ranges are supposed to look in the winter.