Every year, we respond to and (or) record hundreds to thousands of landslides. These data are compiled and attributed in the DNR GIS statewide landslide database accessible and downloadable through our online interactive geologic map. To help reduce losses from 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 precipitation.
The cumulative precipitation threshold is 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 a one- to seven-day ratio of precipitation: P(0) = 3*P(1)-(P(7)-2.5), where P(0) is the threshold when the equation reaches or exceeds 0, P(1) is cumulative 1 day of precipitation, P(7) is cumulative seven day precipitation excluding the P(1) valued day. This tracks precipitation over the last 8 days and results in the capture of antecedent moisture and short-term 24-hour precipitation intensity. The P(0) value is further analyzed to find a specific value for a generalized estimate of probability of occurrence and frequency of landslides. Generalized frequency values have been determined to be when:
0-5 is low probability of occurrence and frequency
5-7 is moderate probability of occurrence and frequency
7-10 is high probability of occurrence and frequency
10 and above is extreme probability of occurrence and frequency
This model will continue to be modified by 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 landsliding during a major storm. Landslides may occur in counties that have a low risk rating and may not occur in all or any areas at high risk.