The Evaluation of Preferential Slope Failure and Debris Flow Initiation Locations in Areas of Shallow Soils Overlying Bedrock Using a Meso-Scale Articulated, Adjustible Slope and Rainfall Simulator

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. In a three-year, field-based study involving the monitoring of water level variation in selected slopes matching this description in southern California, the findings indicated that near-surface water levels and the associated pore pressures are relatively and consistently higher in these locations following rainfall events than the rest of the slope. It was hypothesized that this position of elevated water levels and pore pressures in the upper one-third to one-half of the slope near a change from a shallower to a steeper slope (the slope shoulder) was a preferential location for shallow slope failure which could lead to debris flow initiation. To further evaluate this hypothesis a meso-scale laboratory simulator was constructed to replicate conditions observed in the field. The simulator consisted of an articulated, adjustable artificial slope operated in conjunction with a rainfall simulator capable of producing variable raindrop sizes and rainfall rates. Experiments were conducted involving various combinations of upper and lower slope angles and rainfall rates encompassing both the actual slope conditions found at the field sites as well as a wider range of conditions. Consistent with the findings of the field studies, the laboratory simulations showed elevated water levels in the upper one-half to one-third of the simulated slopes. More significantly, in the laboratory simulations rapid slope failures consistently occurred at the hypothesized location in the upper one-third to one-half of the slopes in the area of the slope shoulder. The results appear to establish the connection between elevated water levels and pore pressures, slope change, and a preferential failure location for shallow slope failure and potential debris flow initiation. Work to further refine the governing conditions is ongoing.