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DANGER UNDERGROUND : The Downside to Life on the Hillside : A rainy winter, after two years of drought, may be a mixed blessing to some hillside residents of Orange County who know that with the water comes the possibility of major threats to their property--landslides and mudslides. :

January 25, 1990|PATRICK MOTT | Patrick Mott is a regular contributor to Orange County Life

Lying in their beds in the predawn darkness of Oct. 2, 1978, Dick Harley and his family thought that the popping sound was hail hitting their roof. They looked outside--no hail was falling. The sound began to take on a crackling quality, like fire.

"Then," Harley said, "we realized that the end of our driveway had curled up and a neighbor had tripped over it. We knew then that the earth was doing something strange."

Harley's entire neighborhood was sliding away. The crackling noise that he and his family heard above their heads was the sound of nails being ripped loose from rafters as the house shifted. In a few seconds, 22 homes in Laguna Beach's Bluebird Canyon were on their way to being destroyed or rendered worthless by the largest and most destructive landslide in recent Orange County history.

The Harleys, along with dozens of other confused and frightened residents, were quickly evacuated to City Hall, but they returned after daybreak to a point near the canyon where they should have been able to see their home.

"But it wasn't there any more," Harley said. "The whole block had dropped maybe 50 feet."

It had rained heavily during that late winter of 12 years ago, and most Southern Californians welcomed it. The region had suffered a long and costly drought that prompted restaurants to withhold ice water and many residents to cut back severely on home water use.

But the same water that brought relief to parched Southlanders began accumulating in large quantities in the ground beneath Bluebird Canyon, swelling the clay-type strata and acting as a kind of lubricant. And six months later, 3.5 acres of residential neighborhood crashed down the hill, leaving dozens of residents homeless.

Now, after two years of drought, weather forecasters say that Southern Californians once again may be facing a rainy late winter. And throughout the hillside neighborhoods of Orange County, residents again must deal with the possibility that living in the heights has a downside.

While Southern Californians know, from their experience with earthquakes, that the ground beneath their feet is far from static, hillside residents also know--or should--that the land on which they live has the potential to be even more unstable in nature, that gravity, water, soil and bedrock composition, geologic history and man-made changes can combine to damage or destroy their homes and property in the form of a mudslide or a landslide. Both earth movements can be dramatic and frightening.

A mudslide (geologists call it a mud or debris flow) involves only the top few feet of soil but sometimes can occur with startling speed. It happens when a barren or sparsely planted hillside, often with lightly compacted or clay-like topsoil, becomes saturated with water. The load becomes too much for the earth to bear and the soil flows downhill.

A landslide is more complex and can be massively destructive. It involves deeper strata, often well into the bedrock layers. Like mudslides, landslides often occur in clay-rich, unconsolidated strata that are prone to absorb and retain water.

When water seeps into the deeper clay layers, it generally stops there, since clay tends to be impermeable. The water, denser than the strata that are now above it, buoys up those strata, loosening them and creating a kind of lubricated layer. If the hillside is steep enough and the load above heavy enough, large sections of earth, sometimes many yards wide, can slide all at once. The hillside can look as if a huge hand scooped out a rather neat portion of it with a spoon. This is similar to what happened at Bluebird Canyon.

Landslides also can be triggered by water seeping into cracks in the ground that were formed after a long dry period that caused the ground to contract and split. The water applies pressure to the sides of the crack and can cause a bloc of land to break away.

Both landslides and mudslides can also be triggered by earthquakes.

Orange County is familiar with both phenomena. They have occurred with varying frequency in every hillside area of Orange County at various points in geologic time. They have damaged or destroyed property in such disparate areas as Fullerton, Brea, La Habra, Anaheim Hills, Orange, Laguna Beach, Laguna Hills, Laguna Niguel, Dana Point, San Juan Capistrano and San Clemente. However, it is in the South County where the most susceptible, clay-rich earth is concentrated in a wide-ranging stratification known to geologists as the Capistrano Formation, which takes in much of the coastal land from Laguna Beach to the San Diego County line.

This area has not been quiet in recent years. According to a study published in 1984 by the California Department of Conservation's Division of Mines and Geology, 71 landslides, rock falls or mudslides occurred in the South County's coastal area between 1977 and 1984.

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