Scientists Monitor Malibu as Three Storms Head Its Way

DANGER ZONE­—Areas threatened by mud and ash flows in the Canyon Fire burn area between Malibu Canyon Road on the left and Las Flores Canyon on the east are mapped. The orange area—Sweetwater Canyon and Palm Canyon above Serra Estates—is the most threatened in this projection.

DATA—Jeffrey Agajanian, a USGS mudflow expert, examines the new automated stream-flow gauge just installed near the Malibu Creek bridge in Serra Estates. The solar-powered device will feed water-depth and mud percentage data to a satellite, which will relay it to National Weather Service computers and forecasters.

• Experts Warn that Dangers of Flooding Should Not Be Minimized •

By Hans Laetz

The U.S. Geological Survey rushed to activate the new remote flood and mudflow-sensing equipment it set up in three Malibu canyons this week in advance of the trio of big storms that threaten local flooding.
Scientists told reporters Thursday that torrential rains possible through Saturday could deposit enough material from Winter, Sweetwater and Carbon canyons alone to cover a football field with 40 inches of debris.
Add in the massive burned area in Corral, Escondido and Latigo canyons, and Malibu may see 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 a Thursday afternoon news conference held under dark skies at Malibu Bluffs Park.
Debris flow from the Canyon and Corral fire zones is of such concern to the 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 operations next week, 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.
Another new USGS installation is a solar-power 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 news conference unveiled a map that predicts in great detail, which parts of burned-over Malibu are at risk for mudflows.
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 were hoping to avoid broad-brush predictions, and determine how differing terrains shed water and debris in intensive rainfalls such as are expected this week.
A mobile Doppler radar truck, usually posted in the Midwest to chase tornados, is 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.”
Laber said the total amount of rainfall is not as import 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.