A SYSTEMATIC APPROACH TO ESTIMATING THE 300-YEAR RUNOUT FOR DENSE SNOW AVALANCHES

摘要

For residential zoning near a snow avalanche path in Canada, three zones are usually identified: Red (no new construction), Blue (restricted development) and White (unrestricted development). The Red-Blue boundary depends partly on the runout from a 300-year dense snow avalanche. This runout is estimated along the centerline of the path using up to four sources: historical records, trim lines in vegetation, statistical runout models, and dynamic runout models. The confidence in the estimated runout distance and return period for these sources vary. For example, the extreme runout position in a forested path can often be identified with high confidence based on obvious trim lines, however, even if the date of the last extreme event was known (e.g. 60 - 63 years ago), the confidence in the return period is low when it is based on a single event only. Traditionally, these estimates with different levels of confidence and return periods are combined with the consultant's expert knowledge. In the proposed approach McClung's (2000) recently validated Space-Time model is used to adjust the statistical runout estimate to a 300-year runout. The other runout estimates are extrapolated to 300-years with expert knowledge. The 300-year runout for a dense flow avalanche is then calculated as a confidence-weighted average. To compensate for high uncertainty, sometimes because runout estimates from less than four sources are available, the red-blue boundary can be extended down the path. An example is presented to illustrate the proposed method.