World of Science | Review ISSW2018: Avalanche protection measures

This time: Protection measures: Risk management and engineering solutions (Session 2)

A session to bring together engineers and scientists to validate real protection structures in numerical simulations to find general solutions from case studies. In short, another excellent example of how theory and practice can be combined. The topics of the session can be roughly summarized in four categories: Pressures and forces of snowpack and avalanches on protective structures, design and construction of protective structures in alpine terrain, hazard zone plans and validation of protective structures, and snow loading as a hazard basis for avalanches and influencing visibility conditions on traffic routes.

Three of these four topic blocks deal with protective structures, i.e. the structural response of engineers to existing hazards caused by snow or avalanches. There are two main types of avalanches: Structures in the avalanche start area and in the run-out area. Both types can be further subdivided into two classes. Blowout protection as a measure against snow deposition by wind in potential avalanche start areas and avalanche start protection, which is intended to prevent the spontaneous release of avalanches. A distinction is made between interception or braking structures and deflection or transfer structures at the outlet of avalanches. Examples of these are catchment dams, braking humps and avalanche breakers, as well as deflection dams, avalanche galleries, tunnels and pipe bridges.

Excursus: Pipe bridges are certainly interesting structures, but are or were rarely built. A prominent example can be found in the Großer Gröben pipe bridge, where a bridge runs relatively flat over a ravine that is prone to avalanches. The idea behind it is that the road virtually passes through an artificial tunnel and is therefore not affected by a possible dust avalanche and there is no closure (Rohrbrücke Großer Gröben).

Construction

The design and construction of protective structures is always a trade-off between costs and benefits: For a major benefit, the structures are designed for up to 300-year events, but then the municipality may not necessarily pay for that either. As mentioned above, different experts can also arrive at different design values.

The size of such structures can be shown in article P2.7 (Avalanche barrier in East Tyrol) about an avalanche barrier that was built in a ravine above a village. The article reports in great detail on the design, construction and implementation. Incidentally, a similar avalanche breaker can be seen in the Mühlauer Klamm near Innsbruck.

From New Zealand, there is an article P2.18 (Design and construction of an avalanche deflecting berm ...) about the extension of a deflecting dam to protect a small settlement. The original 2 m high dam was enlarged to a height of up to 10 m using coarse stones.

From Svalbard comes article P2.19 about the construction of snow fences and bridges in permafrost (The challenges of mitigation measures in Longyearbyen, Svalbard). Between summer and winter, a layer of permafrost up to 3m thick thaws and freezes, meaning that the foundations of the shoring have to be cemented in 4m deep. Likewise, such deep holes could only be drilled in winter, i.e. during the cold polar night.

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Danger zones

Danger zone plans are primarily spatial planning tools for maintaining an overview of avalanche hazards at a national level. They also form the basis for planning new buildings and renovations at municipal level. For example, no construction is allowed in the red zone and only under certain conditions in the blue or yellow zones, such as buildings without windows on the mountain side. In general, such hazard zone plans are based on historical avalanche events, expert assessments and simulations.

Contribution O2.2 (Rezoning after installing avalanche mitigation measures: Case study of the Vallascia avalanche in Airolo, Switzerland) describes the renewal of the hazard zone plan for the municipality of Airolo in Ticino. After several major avalanches, including a Galtür-like disaster in 1951, upgrades were made from 1984 onwards and in 2012 one of the largest Swiss shoring projects was completed with 10km of snow fences, 2 containment dams and 54ha of reforestation. The change in the hazard plan is sobering: The new red zone is even larger than before the former plan, the assessment basis of which cannot be traced due to a lack of documentation (and this in Switzerland!). According to the authors, new findings from e.g. avalanche simulation mean that such cases occur more frequently. Instead of comparing the new zoning plan with the old maps, the authors suggest first updating the old plan without taking the protective structures into account and only then incorporating the protective effect into this plan. But even with this trick, the result from Airolo is sobering: the red zone has slid uphill by around two rows of houses.

Two further articles deal in equal detail with two avalanche paths and the effectiveness of the protective structures: article P2.2 (Braking mounds in avalanche simulations - a samosAT case study) examines the Arzler Alm avalanche of January 2018 using aerial images, mass balances and detailed simulations. Contribution O2.6 (Effectiveness of avalanche protection structures in run-out zones: The Taconnaz avalanche path case in France) simulates different avalanche scenarios with variations in avalanche volumes and friction parameters. The simulations are still particularly useful for dry and cohesionless snow, but most of them fail at the low velocities and special deposition effects of wet snow avalanches.

Conclusion

In general, snow-related natural hazards vary greatly depending on the slope, avalanche path and hazard potential. Appropriate measures from spatial planning strategies, such as hazard zone plans, temporary measures such as road closures or situational avalanche releases, together with structural avalanche barriers, form the technical avalanche protection. Although there are (country-) specific guidelines for the entire area of technical avalanche protection, very specific solutions for individual avalanche paths are being developed time and again. It is precisely such individual solutions that lead to changes and updates to the catalogs of guidelines and enable the continuous further development of technical avalanche protection.