Climate and hydrological circumstances antecedent to selected landslide events in southeastern British Columbia

Debris torrents and debris avalanches in southeastern B.C. are usually associated with snowmelt. The hydroclimatic circumstances and possible influence of forest harvesting on 6 landslide cycles that occurred over a 10 year period are investigated in this paper. Slides appear to occur when a rain storm or a hot spell coincide with peak snowmelt at the particular elevation zone above an unstable slope. Melt may be due to either rain-on-snow or radiation and case studies of slides illustrating each circumstance are examined. The event that caused the largest amount of impact was a 90 mm rain-on-snow event on July 12, 13, and 14, 1983. This storm caused hundreds of debris torrents in watersheds with a significant area above 2000 m, the approximate snow line at the time of the slides. In 1988 at least 10 debris avalanches were associated with a radiation event in mid April, when the snow line was at a much lower elevation. Snowmelt data collected from a nearby lysimeter indicated peak snowmelt of 22 and 40 mm respectively in a forested and clearcut site.