Regional test of a model for shallow landsliding

Landslides mapped in 14 watershed analyses in Oregon and Washington provide a regional test of a model for shallow landsliding. A total of 3224 landslides were mapped in watersheds covering 2993 km and underlain by a variety of lithologies, including Tertiary sedimentary rocks of the Coast Ranges, volcanic rocks of the Cascade Range and Quaternary glacial sediments in the Puget Lowlands. GIS (geographical information system) techniques were used to register each mapped landslide to critical rainfall values predicted from a theoretical model for the topographic control on shallow landsliding using 30 m DEMs (digital elevation models). A single set of parameter values appropriate for simulating slide hazards after forest clearing was used for all watersheds to assess the regional in ̄uence of topographic controls on shallow landsliding. Model performance varied widely between watersheds, with the best performance generally in steep watersheds underlain by shallow bedrock and the worst performance in generally low gradient watersheds underlain by thick glacial deposits. Landslide frequency (slides/km) varied between physiographic provinces but yielded consistent patterns of higher slide frequency in areas with lower critical rainfall values. Simulations with variable e€ective cohesion predicted that high root strength e€ectively limits shallow landsliding to topographic hollows with deep soils and locations that experience excess pore pressures, but that low root strength leads to higher probabilities of failure across a greater proportion of the landscape. # 1998 John Wiley & Sons, Ltd.