REGIONAL TEST OF A MODEL FOR SHALLOW LANDSLIDING

Landslides mapped in 14 watershed analyses in Oregon and Washington pr ovide a regional test of a model for shallow landsliding. A total of 3 224 landslides were mapped in watersheds covering 2993 km(2) and under lain by a variety of lithologies, including Tertiary sedimentary rocks of the Coast Ranges, volcanic rocks of the Cascade Range and Quaterna ry glacial sediments in the Puget Lowlands. GIS (geographical informat ion system) techniques were used to register each mapped landslide to critical rainfall values predicted from a theoretical model for the to pographic control on shallow landsliding using 30 m DEMs (digital elev ation models). A single set of parameter values appropriate for simula ting slide hazards after forest clearing was used for all watersheds t o assess the regional influence of topographic controls on shallow lan dsliding. Model performance varied widely between watersheds, with the best performance generally in steep watersheds underlain by shallow b edrock and the worst performance in generally low gradient watersheds underlain by thick glacial deposits. Landslide frequency (slides/km(2) ) varied between physiographic provinces but yielded consistent patter ns of higher slide frequency in areas with lower critical rainfall val ues. Simulations with variable effective cohesion predicted that high root strength effectively limits shallow landsliding to topographic ho llows with deep soils and locations that experience excess pore pressu res, but that low root strength leads to higher probabilities of failu re across a greater proportion of the landscape. (C) 1998 John Wiley & Sons, Ltd.