Snow-avalanche impact landforms, deposits and effects at Urdvatnet, southern Norway: implications for avalanche style and process

An unusual assemblage of landforms and deposits is described from upper Norangsdalen, Sunnmøre region, southern Norway, and interpreted as the product of snow‐avalanche events that vary in magnitude, frequency and debris content. An avalanche impact plunge pool, proximal scar and distal mound are associated with a coarse gravel deposit covering part of the valley floor. Landforms in this debris spread include gravel ridges, boulder lines, beaded ridges, fine sediment banked against and covering large boulders, and gravel clumps. Many of these landforms are aligned, indicating across‐valley transport radiating from the plunge pool. Features were mapped in the field and samples analysed for grain size and heavy‐mineral content. The debris spread is attributed to deposition by high‐energy, debris‐rich snow‐avalanche events that collect debris from large areas of the valley side, lower slopes and plunge pool. Aligned landforms develop through sediment transport in a basal shear zone, and randomly distributed gravel clumps represent melt pits following debris transport in the avalanche body. Air displacement ahead of larger avalanches is thought to have felled and tilted trees on the lower slopes of the distal valley side. Approximate ages of damaged trees allowed estimation of the frequency of snow‐avalanche events: (1) small, frequent events (several per annum) carry debris to the lower valley slopes and the plunge pool; (2) moderate events with an annual to decadal frequency maintain the pool–scar–mound complex; and (3) large, debris‐rich events with a decadal to centennial frequency add material to the debris spread.