The effects of timber harvesting and windthrow on landslide initiation, Southwestern Vancouver Island

A severe storm in November 2006 caused over 200 landslides on southwestern Vancouver Island. This thesis investigates 48 field truthed landslides and 233 GIS mapped landslides in unlogged and logged terrain. The impact of windthrow, clearcutting, soil properties and root reinforcement on landslide initiation were analysed within eight months after the storm event. The windthrow landslide density was 25 and 123 times the clearcut and natural landslide densities, respectively. Windthrow related landslides were concentrated on south to east aspects, near clearcut boundaries and recent clearcuts (<10 years old). Windthrow landslides occur on steeper, convex, well drained, colluvial slopes with thinner soils than clearcut landslides.This study used a root density and quality approach to define the loss of root reinforcement after logging. The results show a rapid decline over the first 11 years followed by an increase up to 50 years. Root quality shows a similar trend after logging.