Avalanches overflowing a dam: Dead zone, granular bore and run-out shortening

Abstract

The influence of a dam on granular avalanches was investigated. Small-scale laboratory experiments were designed to study the effectiveness of dams built to protect against large-scale dense snow avalanches. These experiments consisted of releasing a granular mass that first flowed down an inclined channel, then hit and overflowed a dam spanning the channel exit and finally spread out on an inclined unconfined run-out zone. First, we measured the volume retained upstream of the obstacle and the overrun length downstream of the obstacle. In the avalanche regime studied here, no simple relation was found between the volume retained and the run-out shortening resulting from the obstacle. The results highlighted that the avalanche run-out was also shortened by complex local energy dissipation. Second, we report the study of the granular deposit propagating upstream of the dam. We show that there was a change in behaviour from an overflow-type regime for low dam heights to a bore regime for higher dam heights. Finally, we show that this change in behaviour directly influenced the local energy dissipation and the resulting avalanche run-out shortening downstream of the dam.