Forest clearing and regional landsliding

The influence of forest clearing on landsliding is central to long-standing concern over the effects of timber harvesting on slope stability. Here we document a strong topographic control on shallow landsliding by combining unique ground-based landslide surveys in an intensively monitored study area with digital terrain modeling using high-resolution laser altimetry and a coarser resolution regional study of 3224 landslides. As predicted by our digital terrain–based model, landslides occur disproportionately in steep, convergent topography. In terrain predicted to be at low risk of slope failure, a random model performs equally well to our mechanism-based model. Our monitoring shows that storms with 24 hr rainfall recurrence intervals of less than 4 yr triggered landslides in the decade after forest clearing and that conventional monitoring programs can substantially underestimate the effects of forest clearing. Our regional analysis further substantiates that forest clearing dramatically accelerates shallow landsliding in steep terrain typical of the Pacific Northwest.