Every year, we respond to and (or) record hundreds to thousands of landslides. These data are periodically compiled and attributed in the DNR GIS statewide landslide database, accessible and downloadable through our online interactive geologic map. To help reduce losses from shallow landslides, we, in partnership with the National Weather Service, have developed a shallow landslide susceptibility model by analyzing precipitation data concurrent with and prior to shallow landslide initiation. This model is used to forecast landslide initiation thresholds based on cumulative and forecast precipitation. We use the term ‘shallow’ to denote landslides that occur within the rooting depth of vegetation – typically considered to be 10 feet. Landslides that penetrate beyond this rooting depth are considered deep seated and are not considered in this model.
The initiation thresholds are based on correlating historic landslides throughout western Washington with precipitation data. The majority of data collected is from 2006 to present; previous to this, landslide location data are not as reliable. The numerical model derived from this analysis is based on the precipitation reported by the NWS for the prior seven days and the forecast precipitation for the following two days. The most recent 24 hour period is weighted most heavily in the numerical model, followed by the 24 hours preceding that. Other preceding days and the forecast days are weighted equally.
This model will continue to be modified using future data and separated out by physiographic provinces (regions which are similar in geologic structure, climate, and landslide processes). This model is not a predictor that landslides will occur; it is intended to show relative susceptibility to shallow landsliding during a major storm. Landslides may occur in areas that have a low risk rating and may not occur in all or any areas at high risk.