Articles | Volume 4, issue 3
04 Aug 2004
04 Aug 2004

Magnitude and frequency of landslides triggered by a storm event, Loughborough Inlet, British Columbia

R. H. Guthrie and S. G. Evans

Abstract. One hundred and one landslides were documented across 370km2 following a rainstorm that swept the British Columbia coastline on 18 November 2001. Despite the regional nature of the storm, the landslides were spaced close together, even within the study area. Landslide clustering is attributed to high intensity storm cells too small to be recorded by the general hydrometric network. The evidence nicely corroborates previous historical studies that reached similar conclusions, but against which there was no modern analog analyzed for coastal British Columbia. Magnitude-cumulative frequency data plotted well on a power law curve for landslides greater than 10000m2, however, below that size several curves would fit. The rollover effect, a point where the data is no longer represented by the power law, therefore occurs at about 1.5 orders of magnitude higher than the smallest landslide. Additional work on Vancouver Island has provided evidence for rollovers at similar values. We propose that the rollover is a manifestation of the physical conditions of landslide occurrence and process uniformity. The data was fit to a double Pareto distribution and P-P plots were generated for several data sets to examine the fit of that model. The double Pareto model describes the bulk of the data well, however, less well at the tails. For small landslides (<650m2) this may still be a product of censoring. Landscape denudation from the storm was averaged over the study area and equal to 2mm of erosion. This is more than an order of magnitude larger than the annual rate of denudation reported by other authors for coastal British Columbia, but substantially less than New Zealand. The number is somewhat affected by the rather arbitrary choice of a study area boundary.