A FIELD STUDY OF THE PRESENCE OF ZONES OF ELEVATED SUBSURFACE WATER LEVELS AND PORE WATER PRESSURES IN HILLSLOPES AS A PREFERENTIAL LOCATION FOR SLOPE FAILURE IN UNBURNED HILLSLOPES IN SOUTHERN CALIFORNIA

Debris flows are a known hazard in southern California where growing numbers of people are moving into the urban-wildland interface, threatening lives and property. A common location to see a debris flow head scarp, particularly in areas of relatively shallow soils overlying bedrock, is on the upper one-third to one-half of an unburned slope at or near the head of a first-order catchment. A field study was undertaken to evaluate a possible mechanism for this preferential location. A pair of unburned, relatively steep slopes with gently rounded shoulders and thin soil over bedrock in southern California were investigated to determine if there is a position on these types of slopes where near-surface water levels and the associated pore pressures are relatively and consistently higher during and after rainfall events than the rest of the slope, resulting in an area of preferential shallow slope failure and debris flow initiation, and if that location corresponds to the location of the observed failure initiation points. It was hypothesized that this position of elevated water levels and pore pressures, if present, would be in the upper one-third to one-half of the slope near a change from a shallower slope to a steeper slope (the slope shoulder). It was further hypothesized that elevated subsurface pore pressures at this location would contribute to it being an area of preferential shallow slope failure. Near-surface water levels and subsurface pore pressures were monitored at the sites for three field seasons. While no slope failure occurred at the field sites during this time, analysis of the field data indicated areas of persistent higher water levels and subsurface pore pressures on the upper one-third to one-half of these slopes, supporting the first hypothesis. Based on this data, a conceptual model of the hydrologic processes and forces occurring in a hillslope which could lead to potential slope failure and debris flow initiation in the upper one-third to one-half of such slopes is proposed.