Rainfall thresholds for forecasting landslides in the Seattle, Washington, area — Exceedance and probability

Empirical rainfall thresholds and related information form a basis for forecasting landslides in the Seattle area. A formula for a cumulative rainfall threshold (CT), P₃=3.5–0.67P₁₅, defined by rainfall amounts (in inches) during the last 3 days (72 hours), P₃, and the previous 15 days (360 hours), P₁₅, was developed from analysis of historical data for 91 landslides that occurred as part of 3-day events of three or more landslides between 1933 and 1997. Comparison with historical records for 577 landslides (including some used in developing the CT) indicates that the CT captures more than 90 percent of historical landslide events of three or more landslides in 1-day and 3-day periods that were recorded from 1978 to 2003. However, the probability of landslide occurrence on a day when the CT is exceeded at any single rain gage (8.4 percent) is low, and additional criteria are needed to confidently forecast landslide occurrence. Exceedance of a rainfall intensity-duration threshold I=3.257D^–1.13, for intensity, I, (inch per hour) and duration, D, (hours), corresponds to a higher probability of landslide occurrence (42 percent at any 3 rain gages or 65 percent at any 10 rain gages), but it predicts fewer landslides. Both thresholds must be used in tandem to forecast landslide occurrence in Seattle.