A MODEL FOR SPATIALLY AND TEMPORALLY DISTRIBUTED LANDSLIDING TRIGGERED BY RAINFALL INFILTRATION

We have developed a model to simulate the effect of transient pore pressure response due to vertical infiltration of rainfall on the initiation of shallow landslides at a 3.03 m grid scale that can be applied over a broad region. The model implements a modified version of Iverson’s (2000) approximate analytic solution to Richards equation in one dimension coupled with a simple infinite-slope model. Applied over digital topography, the model computes a factor of safety for each grid cell at any time during a rainfall event. The study domain is a layered system in which relatively permeable sand overlies less permeable clay. The upper boundary condition is a rainfall infiltration flux into tension-saturated materials at the ground surface. The lower boundary is a no flow condition at the depth of the less permeable layer. The theory assumes tension-saturated conditions in the hillslope at the time of landslide initiation. The input values for saturated vertical hydraulic conductivity, hydraulic diffusivity, initial water-table depth, topographic gradient, soil depth, cohesion, total unit weight of soil, and angle of internal friction may vary from cell to cell throughout the study domain. The precipitation/infiltration rate can vary in both space and time. The results are a temporally and spatially varying factor of safety that can be easily viewed in a grid-based GIS.

The model was applied on a cell-by-cell basis over a 3.03 m DEM in an approximately 15 km2 area of southwestern Seattle, Washington. Parameter inputs were determined from field measurements of saturated hydraulic conductivity of hillslope materials, laboratory tests of material strength properties, and rainfall histories for several storms known to have triggered shallow landslides. The model simulates the progressive development of shallow landslides on steep slopes during rainfall events in Seattle and provides insights into the transient rainfall processes that trigger shallow slope failure.