Improving Spatial Prediction of Shallow Rainfall-induced Landslides

The increasing availability of high-resolution topographic surveys has raised the prospect of improved prediction of locations and sizes of shallow, rainfall-induced landslides. However, limitations of distributed slope stability models have hindered the realization of such improvement. Mathematical modeling commonly represents shallow landslides in digital landscapes as uniform slabs using the one-dimensional (1-D) infinite-slope stability analysis. This approach neglects the effects of irregular topography, variable thickness of slope deposits, and other conditions that violate the assumption of laterally constant stress. Model accuracy also decreases as the ratio of slab depth to length increases, or as topographic resolution increases, so that many isolated, small landslides are incorrectly predicted. These effects of variable geometry and depth-to-length ratio contribute to the over-prediction of unstable areas by distributed 1-D slope stability models. Use of 2-D and 3-D methods of slope stability analysis with gridded elevation models accounts for interaction between grid cells and improves the accuracy of predictions of landslide location, size, and shape. Whereas distributed 1-D methods compute factor of safety, F, cell by cell, 2-D and 3-D methods compute composite F values for rows (2-D) or contiguous groups (3-D) of cells. Although 1-D analyses commonly identify clusters of unstable grid cells (F<1) that roughly coincide with mapped shallow landslides, these analyses also identify isolated unstable cells and scattered small groups of unstable cells away from mapped slides. Many of these isolated cells and scattered groups are incorrect because they are supported by adjacent stable cells: 2-D and 3-D methods correctly predict F>1 in most of these non-landslide areas. Further, 2-D and 3-D analyses correctly predict larger landslides in observed landslide areas where 1-D analysis predicts unstable cells interspersed with stable, low F (<1.3) cells. Shallow landslides modeled in 2-D and 3-D are preferentially found within areas of straight or concave profile (hollows). These predicted landslides cannot cross into adjacent convex areas, because the line of thrust must remain within a modeled landslide mass to prevent interslice or intercell tension and numerical instability. Physically, tension would cause the mass to separate. Consequently, source areas of shallow landslides computed using 2-D or 3-D methods are likely to be confined to individual concavities, whereas observed source areas may be much larger as a result of simultaneous or progressive failure that cause neighboring slides to coalesce. Using 2-D or 3-D analyses to predict landslide size and location reduces spurious clusters of unstable cells and improves accuracy.