ABSTRACT: The effects of timber harvesting and windthrow on landslide initiation on Vancouver Island, BC.

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Abstract

Coastal British Columbia experienced several large storm events during the winter of 2006/2007. One storm event, causing a majority of the landslides on November 15, 2006 in the Klanawa, Sarita and Nitinat River Valleys, was accompanied by high winds in excess of 100 km/hr and rainfall exceeding 200 mm in 24 hours. Approximately 150 landslides were initiated during the winter season, of these, 48 open slope cutblock and windthrow associated landslides were investigated. Terrain attributes and root density measurements for 24 cutblock and 24 windthrow landslides were collected in the field during the summer of 2007 and statistically analyzed. The windthrow related landslides were primarily adjacent to existing clearcuts. The clearcut related landslides were associated with concave slopes averaging 34o and morainal soils with an average depth of 1.1 m. They had an average length of 230 m and all were classified as debris flows, with a majority impacting streams. The windthrow related landslides were associated with steeper convex slopes averaging 39o and colluvial soils with an average depth of 0.6 m. They had an average length of 130 m, with 12 debris flows and 12 debris slides. Fewer of these landslides impacted streams. T-tests and chi-square tests indicate a significant difference (p < 0.05) between all of these variables for clearcut and windthrow related landslides To examine the effects of harvest age and root reinforcement several root density samples were taken at each landslide to measure the existing root density and quality. These measurements were statistically analyzed using bivariate fit tests and t-tests. Root densities in the 0-5 mm and 5¬10 mm diameter classes showed a decline during the first 12 years after timber harvesting with a more gradual increase between 12 and 50 years after harvest. The 10-15 mm and >15 mm diameter classes showed limited root density change over time. The percentage of good quality roots declined during the first 12 years after harvest in the 0-5 mm, 5-10 mm and >15 mm diameter classes with an increase in root quality between 12 and 50 years after harvest in the 0-5 mm diameter classes. Other root diameter classes showed no statistical variation in root quality. Coastal researchers have observed a basal organic soil layer associated with landslides. A basal organic soil layer, commonly overlying dense till or bedrock, was identified along the failure plane of 30 landslides. The basal organic soil layer averaged 11 cm thick in areas with morainal soils on lower concave slopes with poor drainage. Whereas it averaged 4 cm thick in areas with well drained colluvial soils on upper convex slopes. T-tests and chi square tests were carried out and showed a significant difference (p < 0.05) between areas with relatively thick and relatively thin accumaltions of a basal organic soil layer. Loss on ignition tests identified that the organic soil consisted of between 35 and 88% organic content with an average of 66%. Shear box tests, atteburg limits and x-ray diffraction will be used to further characterize the soil for numerical modelling of the failure mechanisms involved. The results of this research project are showing signifcant differences between attributes of landslides in windthrow and clearcuts as well as landslides in clearcuts of different ages.

Bibliography of Canadian Geomorphology