The return period for land-use planning in snow avalanche terrain

The return period or average time between events as a function of position along an avalanche path is a primary determiner for zoning in snow avalanche terrain. Areas with long times between the occurrences are sought to enable permissions for occupied structures and placement of infrastructure facilities. Generally, both the avalanche frequency and potential destructive effects decrease proceeding down into the runout zone where applications reside. Both the avalanche frequency and the estimated destructive effects are used in zoning methods. In typical zoning problems, the allowable return period exceeds 25 years. In most cases, there are not enough prior data on avalanche occurrences for an avalanche path to specify return periods accurately which implies that models must be used. Both avalanche dynamics and empirical, probabilistic models are used to predict zones for rare avalanche occurrences. However, the return period is a probabilistic quantity which places emphasis on empirical models. In this paper, the common methods for determining return period of avalanches are reviewed. The main emphasis in this paper is on verification, accuracy and quantification of the methods for determining return period through use of case histories. From a scientific perspective, the results here suggest some deficiencies in commonly used zoning strategies.