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Identification of natural and logging-related landslide in the Capilano River basin (coastal British Columbia) : a comparison between remotely sensed survey and field survey Brardinoni, Francesco


In the Pacific Northwest landslide inventories are routinely compiled by means of aerial photo interpretation. When examining photo pairs the forest canopy, notably in old-growth forest, hides a population of "not visible" landslides. The present study attempts to estimate how important is the contribution of landslides not detectable from aerial photographs, to the global mass of sediment production from mass failures on forested terrain of the Capilano basin. To achieve this, aerial photo interpretation has been coupled with intensive fieldwork for identification and measurement of all landslides. In order to minimise bias in the comparison and integration of field-collected and air photo-collected data it was decided to define a 30-year time window. Incidentally, it has been possible to prove how landslide scars that appear on a single photo set would date further back than 30 years. Results show that "not visible" landslides can represent up to 85 percent of the total number of failures and can account for up to 30 percent the total volume of debris mobilised. Rates of sediment production differ greatly (one order of magnitude) between two sub-basins of the study area, suggesting that such figures should be generalised with care within a physiographic region. The difference in denudation rate is explained qualitatively by GISbased analysis of slope frequency distributions, drainage density and spatial distribution of surficial materials. Fieldwork has demonstrated that gully-related failures have a greater importance than one could expect from air photo interpretation. ANOVA and nonparametric tests indicate that careful logging in East Cap Creek has produced no detectable effects on mass wasting. Similarly, Sisters Creek, where timber harvesting stopped about 20 years before the start of our 30-year time window, has apparently recovered from the signs of past extensive logging. The existence of "not visible" events affected in a minor way conclusions about the impact of logging on slope stability in terms of land use (management) effects. It had a major impact on the nature of landslide magnitude-frequency relations and, finally is demonstrated to have implications for British Columbia Terrain Stability Classification from the terrain sensitivity point of view.

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