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Highway 50 Slide May Be Just a Start: Even Though the Rainy Season is Ending, the Ground is Still Giving Way. The Earlier Deluge is Sinking into Deeper Layers and Raising New Threats.

Posted on: Tuesday, 9 May 2006, 12:04 CDT

By M.S. Enkoji, The Sacramento Bee, Calif.

May 9--A 120-foot-wide wall of earth oozed onto U.S. Highway 50 near Kyburz late Sunday, burying the road in mud and debris a story high and ushering in what could be a month of slow-moving landslides all over Northern California.

"We've had that kind of season," said Richard Pike, a research geologist with the U.S. Geological Survey in Menlo Park. "This was simply a wet year."

A lot of rain soaking deep into bedrock that in California is relatively porous will mean mudslides when there isn't a cloud in the sky, said Pike.

Mudslides had closed six major roads in the state on Monday. Two were in Humboldt County on State Route 96, one was on State Route 144 in Santa Barbara County and another was on a mountain road in San Bernardino County. On Interstate 80 in the Bay Area, westbound traffic at Pinole has been narrowed to three lanes since an April 30 mudslide.

By Monday evening, workers on U.S. Highway 50 had removed enough dirt to open one lane about a mile east of Kyburz after a 45-mile stretch of the highway was closed to traffic for most of the day.

The layer of earth peppered with boulders and trees slid off a 40-foot embankment, said Mark Dinger, a spokesman for the California Department of Transportation. A fire in 2004 burned the same location, which is fed by an underground spring, he said.

Traffic to and from Lake Tahoe was diverted to Interstate 80. Starting about 11:25 p.m. Sunday and for most of the day Monday, eastbound traffic was turned around at Meyers, and westbound traffic at Sly Park, said California Highway Patrol Officer Rich Wetzel.

There were no injuries or property damage from what is the third slide on the same highway this year, he said.

The other two slides were during the rainy season on April 3, he said, near Missouri Flat Road and at White Hall.

Pike, who has published articles about California landslides, said there are two kinds in this state: those that slide suddenly during a pounding storm and the kind that are happening now.

A lot of rain over a longer period of weeks, not hours, soaks into the soil, he said.

As the rainy season expires, water is still sinking into the bedrock.

"Sometimes it takes a long time to get down there," he said.

Unlike other parts of the country, local bedrock is not as solid or rocky, which makes it more vulnerable to water and slipping, Pike said.

"It can be moving even when the sun comes out because it takes a long time to move," he said.

After an El Nino winter, a mile-long mudslide in 1998 slipped in an undeveloped part of Fremont. Called the Mission Peak Landslide, it was one of the Bay Area's biggest.

"Those are the kind to expect this year," Pike said.

They tend to move more slowly than the sudden, violent rush of mud off steep slopes where the top layer of soil is rain-soaked, he said.

The rain creates a slurry that runs downhill rapidly and can kill because you can't outrun those, Pike said.

After torrential rain in January 2005, the hillside over the Ventura County seaside village of La Conchita plunged into the settlement, killing 10 people caught at home.

California is particularly bedeviled by mudslides because of the climate, Pike said.

All the rain comes in one season.

But every state has landslide problems of some kind, such as in Ohio, where clay soil slides into rivers, he said.

Besides the rain, cutting into the land is a major factor in landslides, Pike said.

"Whether you cut into the landscape to build highways or roads, that's really destabilizing the terrain," he said.

"You change the slope and you're messing with Mother Nature and she doesn't like it."

As the demand to build spreads into California hillsides, more people and property are living with risk, Pike said.

Northern California went through a similarly wet winter in 1906, he said.

When the San Francisco earthquake struck in April that year, dampened earth slid - just as it is now, he said.

"There just weren't as many people and structures," he said.

The 1989 Loma Prieta earthquake couldn't have hit at a better time, at least for mudslides, he said: It was a dry year and it happened in the fall, the driest time of year.

"So if we have an earthquake this year," he said, "that could be really bad."

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Source: The Sacramento Bee

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