Soil slips on the San Dimas Experimental Forest

Includes bibliographical references (pages 77-79)Following maximum soil saturation by storms of November 1965 and\ud December 1966, large areas of chaparral watershed along the south front\ud of the San Gabriel Mountains were affected by soil slippage, a form of\ud landsliding. Soil slips damaged roads, trails, stream-gaging installations,\ud and produced substantial increases in debris production rates.\ud On some watersheds, up to 10 percent of the area was stripped of soil\ud and vegetal cover. Two hundred seventy-six acres on the San Dimas Experimental Forest\ud were compared in order to assess the separate effects of soil and vegetal\ud cover on the incidence and area affected by soil slippage. Soil\ud analysis was performed on sites selected from a larger area to further\ud isolate conditions peculiar both to sites. which slipped and those that\ud did not. It was found that the number and relative area in slips increased\ud with slope, soil surface rockiness and proximity to streams,\ud roads, or trails, and that slips decreased with increased soil depth,\ud hydraulic conductivity of subsoil, and brush density. Further, areas\ud which had been converted from brush to grass contained twice as much\ud slippage as those areas of natural vegetation. This difference, however,\ud was not in the number of slips but in area affected. Soil slippage is revealed as a significant geomorphic process in its function in\ud slope declivity control, drainage development, debris production, and\ud denudational rates