Dendrogeomorphologic approach for snow-avalanche activity reconstruction in a maritime cold environment (upper Erdalen, Norway)

This paper proposes a first reconstruction of snow-avalanche activity using dendrogeomorphic analyses to deliver a one-year-resolution chronology of snow-avalanche winters in upper Erdalen valley, Nordfjord, Western Norway, at the scale of a colluvial cone partially covered with Downy birches (Betula pubescens Ehrh). Reconstruction of the avalanche history is conducted using tree-ring analyses. Determination of the most reliable growth disturbance (growth eccentricity) and applying a tempering index value with threshold 10% of trees responding in the same year, avalanche-activity years are highlighted, resulting in 17 avalanche winters from 1930s. Amongst those, seven winters have activity index over 25 %, indicating major years of activity. Calculation of frequency of similar growth disturbances at each tree provides a return period ranging from 4.4 to 33 years. Inferred spatial extent of snow-avalanche events induces flow-like snow avalanches with limited extent around the tree-less parts of the cone with a return period under 6 years, the cone is totally covered and the distal tree-limit overpassed with a return period of 16 to 33 years.