The Braking Effect of Trees on Snow Avalanches - An Experimental Study

• Main Author: Kahrs, Katharina
• Format: Dissertation
• Language: English
• Subjects: Bygg- og miljøteknikk, Geoteknikk

Mountain forests can protect people and infrastructure from snow avalanches in a cost-effective way. In this thesis, we have carried out a series of small-scale chute experiments to study the braking effect of trees on snow avalanches. We assume that the avalanche releases above the forest and that the trees are not broken or bent upon impact of the avalanche. The chute consists of three straight sections: an upper or acceleration section 0.2 m wide and 1.5 m long, a middle or forest section 1.0 m wide and 2.0 m long consisting of three aluminium plates with a rectangular grid of pre-drilled holes, and a lower or runout zone 1.0 m wide and 2.0 m long. The inclination of both the upper and the middle section is 35°, while the lower section is inclined at 10°. The avalanche is released from a container at the top of the upper section. The snow is modelled by glass beads (ballotini) with a diameter of 0.1 mm. The trees are modelled by metal pins that are inserted into the aluminium plates in computer-generated random patterns. The experiments were filmed from above and from the side with a frame rate of 60 fps. A total of 54 experiments have been carried out: twelve reference experiments without a forest, of which six were carried out at the beginning and six at the end of the study, and 42 experiments with a forest. We have varied the following three parameters: the velocity at which the avalanche hits the forest (incident front velocity), the stem diameter of the trees, and the forest density. The incident front velocity was varied by varying the mass of the avalanche. Three different masses of ballotini were used: 2 kg, 4 kg, and 6 kg. We used two different stem diameters. The metal pins with a diameter of 3.2 mm were used as the thinner trees. For the thicker trees, plastic tubes with an outer diameter of 6 mm were threaded onto the metal pins. The forest density was varied by varying the number of metal pins inserted into the aluminium plates. The forest density is 100% if there is a metal pin inserted into every pre-drilled hole. Four different densities were used: 10%, 25%, 50%, and 90%. We have measured the front velocity and the flow depth of the avalanche along the forest section of the chute by analyzing the films manually frame-by-frame. Further, the runout distance was measured with a ruler straight after each experiment. We have given a detailed phenomenological description of the flow of the avalanche throughthe forest. We have observed two main ph