Patterns of mountain precipitation and landslide hazard

Rainfall of high intensity and/or long duration is the primary trigger for shallow rapid landslides. Over mountainous regions, where most of these landslides occur, large spatial gradients in rainfall have been documented for individual storms and in the climatological mean. However, small-scale patterns of rainfall are seldom considered in studies and assessments of landslide hazard.

We quantify the effect of patterns in climatological rainfall on patterns of landslide hazard by forcing a physically-based model of slope stability (SHALSTAB) with the rainfall pattern produced by a high-resolution atmospheric model (MM5), over the western Olympic Mountains of Washington state. Our results suggest that 10 km scale variations in rainfall have a substantial effect on the assessment of landslide hazard. For our study region, the use of realistic rainfall patterns in hazard assessment results in as much as a 70% increase in the fraction of the landscape deemed unstable compared to when lowland climate data and the assumption of spatially uniform rainfall are used. Furthermore, sensitivity analysis using SHALSTAB reveals that the effect of spatial variability in rainfall on spatial variability in slope stability is comparable with the effects of large variability in soil parameters (such as 30% variations in soil thickness). At a practical level, these results imply that accounting for persistent patterns of rainfall may aid in discerning regions within the same watershed where similar land use practices will lead to differing degrees of landslide risk.