Preventing Slope Failure
Effective and ineffective measures to protect slopes from the effects of heavy rainfall were conspicuous in many places in southern California during this past storm season. Especially, hard-hit during El Niño was Orange County, where acres of plastic sheeting were spread to protect hundreds of steep slopes in the hillside residential districts. Adding to the problem is the siltstone and claystone underlying much of the region, which, due to its highly expansive clay content, is the most landslide-prone bedrock unit in Orange County.

The effectiveness of various approaches to slope-stabilization was dramatically demonstrated in San Clemente on streets such as Calle Familia. Here, the approximately 80-foot-high, west facing slopes had been graded at a slope angle of 1.5 to 1 (about 34 degrees).

Techniques used to protect the slope materials from erosion or slippage were easy to compare among neighboring properties. The above photo shows a system of well-constructed drainage ditches and channelways, combined with a dense ground cover, protecting the center property. In contrast, slopes on both sides planted only with grass were damaged by shallow landsliding and by erosion of the soil mantle.

 Photo by Siang Tan, California Department of Conversation, Division of Mines and Geology, CALIFORNIA GEOLOGY, September/October 1993.

Contrast the effective performance of bush-planted slopes vs. grassy slopes: In the above photo, the slope on the left was planted with grass along with a pattern of drought-resistant bushes with deeper root systems that held the slope firmly intact. The slope to the right, although planted with a few deciduous trees, is mostly comprised of shallow-rooted grass -- and it, too, lost much of the soil cover and grass. Trees on steep hillsides tend to aggravate rather than prevent slope failure.

 Photo by Siang Tan, California Department of Conversation, Division of Mines and Geology, CALIFORNIA GEOLOGY, September/October 1993.

 
Landslide