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The San Francisco Bay Area is falling into the Bay
WHEN IT RAINS, IT SLIDES -- BAY AREA ON THE ALERT
Michael Cabanatuan, Chronicle Staff Writer
Saturday, April 1, 2006
In like a lion, out like a lion. Chronicle Graphic Unstable ground. Chronicle Graphic A worker climbs under a tarp at a construction site on Bu...
The relentless rains that set Bay Area records in March now threaten to create big problems in April as soggy hillsides show signs they could give way.
"If we continue to have these continuous bursts of rainfall for another week or more, we could have hundreds of landslides -- easily,'' said Alan Kropp, a Berkeley geotechnical engineer.
Forecasters with the National Weather Service say the rainy weather is likely to continue after an expected pause today.
Showers lingering after the latest dousing Friday should end this morning, and the rest of the day should be dry and partly cloudy -- "the best day of the weekend,'' said meteorologist Suzanne Anderson.
More rain is expected Sunday through Tuesday. After a break on Wednesday, yet another storm is due Thursday or Friday, possibly extending into next weekend.
"It doesn't look like there will be any change in the pattern for the foreseeable future,'' Anderson said.
Saturated hillsides around the Bay Area already are beginning to move. In Sausalito on Thursday, a slide carried mud, a wooden deck and a 30-foot-tall tree downhill toward the city's main street. On notoriously unstable Highway 1 in San Mateo County, underground sensors detected movement Friday on Devil's Slide and switched on warning lights, though Caltrans officials said the movement was slight and did not pose a danger.
Crews worked Friday to shore up sinking roadway, undermined by a slide, on Highway 1 south of Pacifica at Shamrock Ranch, but they had to quit for the day when heavy rains inundated the area. Caltrans spokeswoman Lauren Wonder said the agency is keeping watch over those slides and will close the highway if they become dangerous.
Kropp had a burst of business in January helping homeowners cope with slides. He has seen business pick up again in the past couple of weeks, particularly in the East Bay hills.
"I've got lots of mud on my boots and pants in the laundry,'' he said.
They could get a lot muddier.
Heavy rains, combined with unstable soil conditions on slanting terrain, can trigger two types of landslides, said Gerald Wieczorek, a geological engineer and landslide specialist with the U.S. Geological Survey in Menlo Park.
Fast-moving, shallow landslides known as "debris flows" are often set off by intense rains -- perhaps 5 inches in 24 hours, he said. They strike quickly and can carry away houses, trees and rocks, and travel hundreds of yards. Particularly heavy rains in January 1982 touched off 18,000 landslides and debris flows that killed 20 people in the Bay Area, he said.
The rains that inundated the Bay Area between Christmas and New Year's Day also brought such landslides, particularly in the North Bay.
Continuous rains cause the other type of slide -- slow-moving, deep slides known as "earth flows" or "earth slumps," Wieczorek said. The steady soaking of rain into the soil -- even if the rains are not particularly intense -- can create enough water pressure beneath the ground to cause the earth to move, usually slowly, but in some cases not so slowly.
The rains in the month just ended -- which set the record in San Francisco and other Northern California locations for the March with the most rainy days -- generally were not very intense. So the deeper type of landslide is the biggest threat, barring an unanticipated deluge.
"At this point, (the rain) is building up water deeper in the soil,'' said Wieczorek, "and will be developing deeper slides.''
Where those slides will occur depends on the nature of the rainfall, the type of soil or rock, and the degree of the slope, he said. Debris flows typically happen on steep slopes, but the deeper, slower slides can occur on hills with as little as an 8 percent slope.
Parts of the Bay Area particularly prone to slipping and sliding hills include the areas in or near the Santa Cruz Mountains, Mount Tamalpais and the East Bay hills, Wieczorek said. The USGS has produced landslide maps that identify the areas of greatest risk, Kropp said, where 90 percent of all slides are expected to occur.
As the rains continued to fall Friday, some residents of the Bay Area's hillier neighborhoods were admittedly nervous.
In Oakland, residents on Wallace Street near Highland Hospital kept a wary watch. In 2002, three homes were red-tagged by the city after a mudslide.
On Friday, about 20 sandbags were lined outside the fence of a home shared by Carrie Ramirez, 33, and Evan Beckert, 38. Runoff from the rain was going down the hill on Wallace.
Last year, when people parked along the curb, the car tires sent water right into the basement.
Ramirez said, "I'm from the Seattle area, so when I came back here and saw the rain and the mud, that was one of my concerns."
As long as the water stays there," said Beckert, pointing to the gutter, "we're cool."
In Sausalito, Robert Taylor, a 46-year-old native of Australia who moved next door to a slide area three years ago, said the hillside had been stripped bare and trampled by construction workers over the past year. That, combined with constant pounding rain, he said, was asking for trouble.
"It seems anyone can build anything around here if they have enough money," said Taylor, a retired engineer, "until something goes wrong, and then the s -- hits the fan."
The record rains, mudslides and flooding are all part of a plan, said Alberto Alvarez, 84, as he walked confidently up a hill to his own home on Bulkley Avenue, where the hillside was still intact.
"Mother Nature is the boss, and sometimes she gets upset," said Alvarez, who has lived in Sausalito since 1962. "No, no, no, I'm not worried. We cannot be upset because that's just the way it goes."