MODELING RAINFALL CONDITIONS FOR SHALLOW LANDSLIDING IN SEATTLE, WASHINGTON

We present results from an application of a distributed, transient infiltration – slope stability model for a 25-km² area of southwestern Seattle, WA. The model (TRIGRS) combines an infinite slope-stability calculation and an analytic, 1-D solution for pore-pressure diffusion in a soil layer of finite depth in response to time-varying rainfall. This transient solution for pore-pressure response may be superposed on any steady-state groundwater-flow field that is consistent with model assumptions. Applied over digital topography, the model computes a factor of safety for each grid cell at any time during a rainstorm. Input parameters may vary from cell to cell and the rainfall rate can vary in both space and time. For Seattle, topographic slope derived from a Lidar-based 3-m DEM, maps of soil and water-table depths derived from geotechnical borings, and hourly rainfall intensities are used as model input. Material strength and hydraulic properties used in the model were determined from field and laboratory measurements, and a saturated initial condition is assumed. Because the solution to the groundwater-flow equations explicitly accounts for the response of pore pressure at depth to a flux at the ground surface with respect to time, the results from the TRIGRS study can be portrayed quantitatively to assess the potential landslide hazard based on rainfall conditions. Results calculated by means of TRIGRS for rainstorms typical of the Seattle area are presented as a map with three classes of landslide susceptibility defined by a destabilizing intensity and duration of rainfall. Rainstorms with known annual frequencies of 2.4, 0.38, and 0.08 destabilize 3.5 percent, 11 percent, and 19 percent, respectively, of the area with topographic slopes greater than 20°. Spatially, about 88 percent of the 242 historical landslides in the study area were located in areas classified as potentially unstable. This approach to landslide hazard mapping provides a physical, spatial, and temporal basis for empirical rainfall intensity-duration thresholds. Results can be interpreted in combination with real-time monitoring of soil-moisture conditions and quantitative precipitation forecasts to provide improved landslide-hazard forecasts and warnings.