MODELING TRANSIENT RAINFALL INFILTRATION AND SLOPE INSTABILITY FOR DEBRIS-FLOW INITIATION—UNSATURATED ZONE EFFECTS

Intense rainfall is the most common trigger of destructive debris flows. Although many debris flows result from erosive processes, many others begin as shallow landslides that result from direct infiltration of rainfall and storm runoff into hillside materials. Predicting the timing and location of debris-flow initiation is needed to assess the debris-flow hazard of an area. Theoretical models of rainfall infiltration into unsaturated hillside materials provide useful insights into the mechanisms and timing of rainfall-induced landslides. We modeled the infiltration process using a two-layer system that consists of an unsaturated zone above a saturated zone and implemented this model in a GIS framework by coupling analytical solutions for transient unsaturated vertical infiltration above the water table to pressure-diffusion solutions for pressure changes below the water table. The solutions are coupled through a transient water table that rises as water accumulates at the base of the unsaturated zone. Pore pressures computed by these coupled models are subsequently used in slope-stability computations to estimate the timing and locations of slope failures. Preliminary model results for an area in Seattle, Washington, indicate that the unsaturated layer attenuates and delays the rainfall-induced pore-pressure response at depth, thereby reducing the extent of instability predicted by our modeling of typical storms. Initial wetness of the colluvium affects the intensity and duration of rainfall required to trigger shallow landslides and the timing of their occurrence. Beyond aiding our understanding of the mechanisms and timing of shallow landslides, the model can be applied to forecasting landslide occurrence using real-time precipitation data and quantitative precipitation forecasts and can be used for deterministic modeling of rainfall thresholds based on topography, mechanical and hydraulic properties of hillside materials, and rainfall patterns.