Geoscience and geotechnical engineering aspects of debris-flow hazard assessment

The results of recent studies of rainfall-induced pore pressures in potential debris now source areas show that positive pore pressures develop repeatedly during successive storms and, sometimes, even during a single storm. Failures occur when these pore pressure reach a critical level which depends on the hydrology, geometry, and soil conditions at the site. In particular, exfiltration from conductive bedrock into the soil mantle tends to generate local areas of destabilizing pore pressures. Stability analyses based on the results of isotropically and anisotropically consolidated undrained triaxial tests suggest that debris flows initiate in two stages: an initial movement under drained conditions, followed by undrained failure as a result of stress redistribution. This mechanism of failure is consistent with observed slow initial movement, followed by a rapid transformation to a viscous slurry, which is typical of debris flow failures. On a regional scale, debris flow hazard maps can be constructed using digital elevation data and variety of techniques ranging from simple correlations, to use of expert opinion, and to process-based modeling. The success of all these approaches depends strongly on the quality of the topographic data. A simple, essentially parameter free, model has been used in Pacific Northwest watersheds to delineate areas of debris hazard in order to reduce frequency of landsliding associated with timber harvesting. This model can be made more realistic if soil depth is predicted and used as part of a slope stability model which includes root strength.