Mass Wasting in Southern California

More mass movement occurs in southern California than in any other region of the United States. Multiple factors contribute to this slope instability. Some of these include the area's steep hillsides, weak bedrock materials, adverse geologic structures, and heavy episodic precipitation. Recent landslides in Pacific Palisades, Palos Verdes, San Clemente, and Anaheim Hills have caused millions of dollars in damage. Moreover, as development continues to expand, southern California's landslide problem is only expected to worsen.

Mass earth movements are highly variable. Typically, they occur within the following spectrum: e.g., from movements which are dependent upon some fluid, to those which are not. In subaerial environments, this spectrum comprises falls, slides, and flows (Blatt et al. 176). Rockfalls consist of detached masses moving down steep slopes. With such movements there is little or no shear displacement between the two original materials. Rockfalls typically result in piles of scree (Campbell et al. 1-27).

In contrast, landslides occur when a sediment mass overcomes its own cohesion and internal friction. Once detached, slides remain in constant contact with the underlying slope materials. Relative movement within the mass may take place on "discrete shear planes (Blatt et al. 176)." These planes divide the mass into subunits. In fact, the main resistance to slide mass movements consists of frictional forces a

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tion can readily exceed soil conductivity. Soil saturation may the lead to slope failure. Moreover, when overland flow occurs, it may result in erosion (Minnich 91-100). Rainfall infiltration and slope stability may additionally be influenced by the physical presence of plants. The impact of vegetation tends to be variable. Because of differences in the physiognomy and dynamics of plant ecosystems, their different effects are difficult to isolate. Below about 1,900 meters, many of southern California's slopes are covered with chaparral. This type of ground cover generally consists of a contiguous, single layer of evergreen sclerophyllous shrubs. Various studies have shown that "denudation by fire temporarily exposes drainage basins to rapid sediment yield and debris flow (Minnich 91-100)." Further, such conditions may persist until a new layer of vegetation reestablishes itself. While southern California's general climate is certainly conducive to landslides, geomorphic processes such as slope failure are more typically related to "short-lived, extreme, precipitation events." These rainstorm triggered events commonly originate on slopes steeper than 3:1 (18 degrees); whereas, mass movement depositional processes oft
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Approximate Word count = 2833
Approximate Pages = 11 (250 words per page)

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