Mother Lode Soils Are Rock Solid
To the untrained eye, the Mother Lode seems to have all the ingredients for a massive landslide: homes perched along hillsides, soil saturated by huge amounts of rainfall.
But experts say the likelihood of slides here is slim.
Differences in terrain, rainfall and vegetation all factor in, several area geologists and engineers said.
Most homes in Tuolumne and Calaveras counties rest on either granite or metamorphic bedrock. Those are stronger than the sedimentary bedrock that gave way this week in La Conchita, south of Santa Barbara, said Jeff Tolhurst, geology professor at Columbia College. And because the Sierra Nevada is a relatively young mountain range, there’s simply less soil to slide off its slopes, he added.
In Southern California, record rainfall has saturated much of the soil. And with less vegetation, the water there is not absorbed as quickly as it would be in the foothills, said Tolhurst. “They’re not used to getting rain like we are,” Bynum added.
Still, that doesn’t mean mudslides don’t occur in Gold Country. In 1987, in fact, a massive slide washed out Parrotts Ferry Road below Columbia. It was closed for several months for reconstruction.
Also, rockslides have regularly blocked Stockton Road below down-town Sonora.
“Anytime you have steep terrain, anytime you’re building on steep grades, there’s potential for slides,” said Chuck Patterson, a Sonora civil engineer.
Patterson and Tolhurst said the likelihood of a large slide increases each year as more roads and homes are built in the foothills.
Tolhurst noted that torrential rains during the El Nino winter from 1997 to 1998, caused a two-mile long slide near Dorrington off Highway 4, damming the Stanislaus River for over an hour. Another large slide occurred in the Carson-lceberg Wilderness near Sword Lake. And Yosemite National park has experienced several rockslides.
Angels Camp City Engineer Gary Ghio said uniform building standards have kept foothill homes, both new and old, safe from major slides. To prevent erosion, Ghio said, builders are often required to install retaining walls or horizontal drainage pipes, preventing water runoff from dragging dirt downhill. Home builders also must compact the dirt surrounding new homes, making that dirt 90 percent as solid as rock, he said. Builders and residents can add vegetation with a strong root system to further prevent erosion near homes, Ghio added.
Several minor rock and mud slides have inconvenienced motorists in the Mother Lode this week. Rocks fell onto southbound Highway 49 Monday morning north of New Melones Reservoir. Those rocks were cleared the same day. Near the town of Tuolumne, Buchanan Road was closed yesterday due to falling mud and boulders. The road is expected to reopen Friday afternoon, Bynum said. No injuries were reported in either slide.
Even with strict building codes and detailed engineering, no home is completely safe from forces of nature, Ghio said.
By Chris Nichols, Union Democrat, 2005
The Yosemite Gold Team
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