ASSESSING SUSCEPTIBILITY AND TIMING OF SHALLOW LANDSLIDE AND DEBRIS FLOW INITIATION IN THE OREGON COAST RANGE, USA

Effective management of debris-flow hazard relies on accurate assessments of debris-flow susceptibility. Mathematical models of rainfall infiltration and slope stability can be applied to predict the temporal and spatial variation of debris-flow susceptibility. These models require high-resolution (<10 m) topographic data, as well as (ideally also high-resolution) data on initial groundwater conditions, physical properties of near-surface earth materials, depth to bedrock, and rainfall. A case study from the Oregon Coast Range, USA illustrates the use of generalized data from a soil survey, limited field measurements, and simple mod- els to parameterize a combined infiltration and slope stability model for predicting debris-flow timing and source-area locations. Although the model over-pre- dicts the extent of debris-flow source areas, results are consistent with mapping which shows channels to be the preferred source areas. Simulation of a November 1996 storm that produced debris flows in the study area indicates that instability probably developed near the end of the period of most intense rainfall; how- ever precise timing of debris flows in the study area is unknown. Model results also indicate that differences in rainfall interception between forested and logged areas may account at least in part for the observed dif- ferences in debris-flow susceptibility.