Avalanche hazard is an important factor to consider not only for community planning in mountainous areas, but also for linear infrastructure where avoidance is often impossible or impractical. At locations affected by avalanche risk the threat can be reduced through an on-going avalanche control program or by the installation of avalanche protection structures. Avalanche control programs mitigate avalanches by artificially inducing smaller avalanches periodically (i.e. with explosives) to avoid large avalanches. Such an approach requires the high cost of on-going professional expertise. Avalanche protection structures are engineered solutions that can be considered a one-time cost. Though such structures may carry a higher initial cost, the reduction of ongoing maintenance and professional expertise involved yield long-term savings as well as improved operations. Engineered structures can be installed in the avalanche initiation zone or avalanche track to reduce the probability of larger avalanches starting and threatening the element at risk.
Static Defense Structures
Static defense structures are mitigation measures installed in the initiation zones in order to prevent avalanches in their first stage of occurrence. Two examples are snow nets and flexible-net snow rakes. They consist of nets strung with steel wire ropes between posts and anchored to the ground. Fence systems are installed in a series, completely covering the initiation zone. During the winter, the net retains the accumulated snow. This method provides structural support to the snowpack, therefore minimizing the risk of avalanche initiation.
Mitigating avalanches in this manner has many benefits, including but not limited to:
System design and layout is carried out in conjunction with avalanche experts and follows Austrian or Swiss standards (e.g. ONR 24805-24807, WSL). Installation is most often in difficult to access terrain and so ease of installation is of great importance. Individual components of the system can be assembled into packages prior to installation. This greatly reduces the amount of helicopter or crane time required during the erection of the fence and the amount of manpower required. Connections between components are designed to be as simple as possible to aid in the construction and the redistribution of forces throughout the system.
Dynamic Defense Structures
Dynamic defense structures are those that interact with an avalanche that is already in motion. There exists very little research on the use of flexible net structures for this type of mitigation, with most knowledge coming from the experience of rockfall catchment fences being impacted by avalanches. Trumer Schutzbauten, in conjunction with the Austrian Department of Natural Hazards, set up a test site for specifically this purpose where artificial or natural avalanches impact an instrumented system.
Dynamic avalanche defense structures are linear systems like rockfall protection systems, which extend across the runout zone. While rockfall fences are designed to retain rock, these systems help minimize the run out distance, thus reducing the the extent of the hazard zone.
The application of dynamic systems has some advantages over static defense structures:
System design and layout is carried out in conjunction with avalanche experts on a site-to-site basis. A characteristic loading scenario is developed based on the understanding of past avalanche events and flow modeling. This information is then used to dimension the system.
ASI (2010) ONR 24805: Permanent technical avalanche protection – Terms, definitions, statistical and dynamic load assumptions . Austrian Standards Institute, Vienna, Austria.
ASI (2010) ONR 24807: Permanent technical protection against avalanches – Monitoring and maintenance. Austrian Standards Institute, Vienna, Austria.
ASI (2011) ONR 24806: Permanent technical avalanche protection – Design of structures. Austrian Standards Institute, Vienna, Austria.
Gleirscher et al (2012) SNOWCATCHER: A new snow avalanche protection measure. Proceedings, 2012 International Snow Science Workshop, Anchorage, Alaska.
Rammer, Stelzer and Kern (2009) Investigation on the effectiveness of the catch-fence “Snowcatcher” as avalanche protection system. Proceedings, 2009 International Snow Science Workshop, Davos, Switzerland.
WSL (2007) Defense structures in avalanche starting zones. Swiss Federal Institute for Forest, Snow and Landscape, Bern, Switzerland.