Estimating snow avalanche runout distances from topographic data

Abstract

To protect rural areas and roads from disasters caused by snow avalanches, avalanche hazard maps are being developed. In Japan, when determining snow avalanche runout distances, empirical values are normally used. These assume that the line-of-sight angle at the starting point of an avalanche, viewed from its runout point, is less than 18° for surface-layer avalanches and less than 24° for full-depth layer avalanches. This rule of thumb ignores topographic features of the slope carrying the large mass of snow. We have made analyses to find a method that can obtain the avalanche runout distance, taking into consideration the longitudinal profiles of the sites, using regression analyses on data from 66 avalanches. The line-of-sight angle can be expressed sufficiently by functions, such as line-of-sight angle between the starting point of an avalanche and a point on its course of flow 10° in gradient, an angle at the starting point of an avalanche, and the curvature of an approximate quadratic curve of topography. Based on statistics for the above relationship, we have established four different ranking categories to classify the degree of hazard. If the starting points of avalanches are known, hazardous areas can automatically be ranked on a basis of topographic data.