Interplay of mass detrainment and the braking effect of forests on small to medium avalanches - An experimental study
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Small-scale testing was conducted on a laboratory avalanche model built to quantify the deceleration of avalanches flowing through alpine forests. The inclination of the chute is given as 35◦, the inclination of the runout section as 10◦. Two materials were tested with regard to avalanche representativity: alumina powder and ballotini. Four parameters were varied: relative humidity, material type, tree density, and front/surface velocity. A total amount of 62 experiment runs were executed, of which 48 were successful. The latter include 28 test runs with forest and 20 reference test runs without forest. Avalanches modeled with both materials showed deceleration when traveling through the forest. The most significant cause of this deceleration was determined to be the loss of mass. It was observed that the ballotini move approximately twice as fast as alumina powder, during both reference trials and forested trails. Avalanches modeled with ballotini reached a farther run out distances - approximately 45cm for reference trials and 10-15cm for forested trials- then the avalanches modeled with alumina powder. Humidity was found as the major control factor for avalanche behavior. A relative humidity of approximately 65% was determined as recommended for this experimental set-up. Grouping of trees had the largest effect on the avalanche volume release, producing the largest deceleration effect within this experimental study. Avalanche rheology was found to be greatly influenced by alternating forest densities. It can be concluded that alumina powder does best represent wet avalanches, whereas ballotini are more representative for dry avalanches.