The impacts of logging on landslide activity at Clayoquot Sound, British Columbia

Abstract

The objective of this study was to investigate the impacts of logging on landslide activity in Clayoquot Sound on the west coast of Vancouver Island, British Columbia. A total of 1004 landslides were documented in order to test the hypothesis that areas affected by logging activities show different density, frequency and magnitude characteristics of landsliding than areas unaffected by logging. The frequency of landslides in logged terrain was found to be nine times higher than in undisturbed forest. An exponential increase in landslide frequency within the area logged was observed on a large watershed scale. Failures in logged terrain occur on gentler slopes than in natural terrain, partly because fewer slopes steeper than 40° have been logged. Debris slides and debris flows are the most frequently occurring mass movements, initiating mostly from road fill failures and from within cutblocks. Most landslides initiated on SE-facing slopes, with the highest landslide frequency near the coast and diminishing exponentially further inland. This pattern of activity most likely reflects the greater impact of severe storms arriving from the Pacific with winds dominating from the southeast. Concave and straight slopes are most susceptible to landslide initiation. A preliminary examination of geologic control on the frequency of landslides suggests that the Quatsino Formation, the Island Plutonic Suite and the Sicker Group are significantly above the average rate. Further work is needed to identify and quantify factors confounding the trends and correlations identified in this overview study. Results obtained in this study will be used to implement changes in forest management.