Rain Provides Data for Study of Mudslides

With El Nino-driven storms bearing down again on Southern California, state and federal geologists were hoping Thursday that the expected heavy rains would help them develop techniques to predict mudslides.

Since the beginning of the El Nino season last year, geologists with the United States Geological Survey and California’s Division of Mines and Geology have conducted a series of flights across the Los Angeles Basin in search of landslides that follow hard rains.

The idea is to find out how much rain it takes to trigger a mudslide, and where they happen. If successful, the geologists hope to create a first-of-its-kind mapping technology that would provide emergency officials with the knowledge to issue landslide alerts similar to the now-common flood and tornado warnings.

They hope the map would also be sufficiently detailed to allow individual homeowners to determine whether their house is in a landslide-prone area.

“This is truly plowing new ground,” said Doug Morton, the federal research geologist who conceived the $56,000 project. “There could be a handsome payout.”

And if weather forecasters are right, Morton and his colleagues will have plenty of soggy earth to study in coming days. The blustery Pacific front that slammed into California on Thursday promised to deliver several more storms, meteorologists said.

Meteorologist John Sherwin--who works for WeatherData Inc., which provides forecasts for The Times--said that as much as an inch of rain could fall at the Los Angeles Civic Center by dawn today, with about twice that much hitting some outlying communities.

Sherwin said skies over Los Angeles will begin to clear by this afternoon, with some sunny weather likely Saturday morning. But the clouds will roll back in Saturday night, he said, as yet another storm--the fourth, and maybe the last in the current series--approaches to drench Southern California on Sunday morning.

After that, California may get a welcome break from the rain that has left the ground soaked and in danger of precisely the sort of mudslides the geologists are studying.

“The ground is pretty well saturated by now,” Sherwin said. “It can still soak up about the first quarter-inch of rainfall, but after that, there’ll be a lot of runoff. There could be some more mudslides, especially in areas like the San Fernando Valley, where there already have been problems.”

And although Morton isn’t exactly cheered by the prospect--after all, people’s homes and lives are in danger--he is looking forward to the coming mudslides.

“I don’t mean it in a perverse way, but if these rains happen, we should take advantage of them to predict the future,” Morton said.

A private contractor hired by the geologists has already flown six flights for the study, which covers a 1,200-square-mile area from Ventura to Dana Point in Orange County and extends as far east as San Bernardino County.

The camera in the plane takes special photographs that allow the creation of three-dimensional maps. With successive shots of the same area, geologists will be able to pinpoint new mudslides and, by knowing how much rain had fallen since the previous photos, establish how much rain is needed to start a mudslide on a given slope, Morton said.

In Ventura County, Morton said he investigated one area that developed more than 1,000 mudslides--big and small--after receiving 3 to 4 inches of rain in recent storms.

One of those landslides, he said, was responsible for the natural gas pipeline rupture on Tuesday that sent up flames as high as a 30-story building.

The first maps from the study will come out in a few weeks and will continue to trickle out over the year as they are completed, Morton said.

But, he warned, if the El Nino season picks up and produces the maximum number ofslides--estimated at 40,000 in the study area--the maps will take much longer to develop.

“It’ll be a full-time job just keeping track of the slides,” he said.

Elsewhere in the state, snow fell in the High Sierra throughout Thursday, and officials said the snow level was expected to dip as low as 3,500 feet in the Tehachapi, San Gabriel and San Bernardino mountains of Southern California. Chains were required on most major routes through the Sierra--including Interstate 40 through Donner Pass--by midday Thursday.

Forecasters said that as the brunt of Thursday’s storm moves south, the snow probably will cause problems on Interstate 5 through the Tejon Pass near Gorman and Interstate 10 through the Cajon Pass north of San Bernardino, both of which peak at well above 4,000 feet.

Rain began hammering Northern California’s coastal valleys early Thursday. Two inches fell on the already soggy city of Petaluma by midafternoon, and the Petaluma River overflowed, flooding streets and forcing the evacuation of 190 homes.

In the Sonoma County town of Rio Nido, 140 homes remained empty, threatened by a mudslide. In nearby Guerneville, officials predicted that the Russian River would crest near the flood stage by sometime today. The level of Clear Lake, north of Santa Rosa, was expected to top the flood stage by more than 2 feet by this afternoon.

Traffic accidents soared to almost 400 in the nine-county Bay Area between daybreak and noon Thursday as windblown rain slowed the morning commute. Traffic across the San Francisco-Oakland Bay Bridge backed up for miles.

In San Mateo County, just south of San Francisco, emergency crews labored throughout the day in the community of La Honda to divert runoff from nine homes threatened by a mudslide. Three houses were tagged as uninhabitable, and officials said more would probably join the list.

The main body of the storm did not reach Southern California until nightfall Thursday, sparing the Southland’s evening commuters the sort of tie-ups that had plagued San Francisco’s morning rush-hour.

The Associated Press contributed to this story.