Malibu Weathers Wild and Wintry Water Woes Well
BY HANS LAETZ
A Caltrans geologist will examine an apparent crack in a cliff over Pacific Coast Highway at Latigo Beach; a cleft that partially gave way after a series of storms dropped prodigious amounts of rain on the coast.
The rockfall on PCH Sunday afternoon may have been the biggest consequence of the series of three storms capped by a weekend soaker that dropped 3.3 inches of rain at Trancas in 24 hours. Despite fears for the fire burn areas, little damage occurred.
U.S. Geological Survey officials said their studies show that soils in the fire areas are draining quickly, and shed much of their accumulated water within a few days of a heavy rainfall. But some of that groundwater migrates downward, possibly activating the large historic landslides lurking below numerous spots in the Malibu hills, scientists said.
All weekend, the City of Malibu stationed a bulldozer and plow at Latigo Beach for use on roads climbing fire-loosened hillsides in the burn area. But the contract crews were used only to clean up limited mudflows in the Canyon and Corral Fire burn areas.
“This is the most action they’ve had,” said the municipal public works superintendent, Richard Calvin, as he watched city crews remove a jumble of rocks that had cascaded onto PCH at 4:50 p.m. Sunday.
Malibu City Councilmember Andy Stern was driving north on the coast road when the rocks crashed down in front of him. “I stopped and used my car to keep other cars from hitting them,” Stern said, squinting into the setting sun directly behind the rocks.
The fallen rocks exposed a crack in the palisade above the state highway, and Caltrans said its geologists would check the hillside later this week. The danger zone is next to a section of historically unstable cliff that was cut back several years ago by Caltrans crews who reduced traffic to one lane in each direction for several weeks.
Another repeat geological problem was discovered last week on PCH above Broad Beach, where a slope beneath the road is slumping next to a culvert near Looschen Road. A five-inch-diameter water line broke and was repaired, but the replacement of drainage pipes may take a month, Caltrans said.
One northbound lane on Highway 1 is closed beyond Trancas Canyon Road, just to the west of where a similar failure in 2005 required the complete replacement of a culvert and fill area. That $5 million emergency project, funded by FEMA flood relief money, took nearly a year.
Along Malibu Creek, more than two inches of rain fell on most of the watershed Saturday, briefly submerging the deck of the Cross Creek bridge in Serra Estates. That bridge is designed to go underwater in floods. Its railings collected weeds and bushes as they washed downstream.
One large rock fell onto Malibu Canyon Road and struck a van, but no one was injured, the California Highway Patrol reported. City streets, patrolled by the Los Angeles County Sheriff’s Department, had one fender-bender in the rain Friday, and no reported crashes during the height of the weekend storm.
At Carbon Beach, silt and small gravel chunks washed off a fire-scorched hillside and into the highway next to the landside commercial center. The city’s trucks cleaned that up Saturday, as well as the normal muddy runoff that covers PCH at the Rambla Vista west intersection.
In the hills above, a small rockslide covered both lanes of Schueren Road east of Rambla Pacifico on Saturday. It was also quickly plowed away, this time by county road crews who made short work of numerous watery slides throughout areas of unincorporated Malibu.
In Menlo Park, near San Francisco, U.S. Geologic Survey scientists kept a close eye on satellite-delivered data from the network of cameras, rain gauges and surface-flow sensors that they have laced around Malibu’s burned-out hillsides and canyons, and reported that the fire-scarred hillsides appear to be shedding their water content quickly after storms pass through.
“We have some good real-time data from both the Canyon and Corral fires,” said Kevin Schmidt, “and we had somebody down there on the ground during the storm downloading data from the other gear.”
Schmidt said the burn areas appear to shed much of the water trapped in dirt and ash on the surface, but are not drying out completely between each pulse of rain.
“A lot of the ash has already washed away,” he said. “Most debris flows in a fire area will happen right away.”
Schmidt said the biggest fire-related danger zones are assumed to be wherever large amounts of silt and sand washed down last weekend, such as the flows onto PCH at Carbon.
“People who live at the base of the steeper slopes, who experienced heavy debris flows last weekend, are in general in places that are most susceptible to sudden slides,” Schmidt said.
The USGS study is focused on fire-related landslides, but Schmidt stressed that the big landslides that bedevil some areas of Malibu have nothing to do with heat-scarred hillsides. The big danger of large-scale slippage, he said, will result from migrating rainwater trickling down to strike zones beneath steep hills.
Those big slides will take several months to develop, and are independent of fire damage. “It takes a long time for the water to lubricate the landslide slip surface,” he said. “The highest surface displacements might take place in June or July.”