Simulating effects of timber harvesting on the temporal and spatial distribution of shallow landslides

A distributed, physically-based slope stability model (dSLAM) was used to analyze the effects of different timber harvesting strategies on slope stability in a steep, forested drainage in the Oregon Coast Ranges. The simulated temporal distribution of landslide occurrence for a single 50% clearcut is consistent with previous empirical findings: most landslides are distributed in a period of about 3-15 yr after the clearcut. Of the 39,000 simulated rainstorms, 82 triggered landslides. Vegetation leave areas, management measures used to reduce landslide occurrence in unstable portions of proposed clearcuts, were evaluated as effective. The simulations indicate that the spatial distribution of failure potential is determined by patterns of timber harvesting and groundwater movement: areas with high failure potential were in sites where root strength is low and groundwater flow concentrates. Finally, results support the hypothesis that geomorphic hollows are positions that have highest potential of landslide occurrence in steep terrain.