Avalanche barriers
An avalanche barrier is a structure that is intended to prevent avalanches from occurring and sliding and to protect infrastructure such as settlements, traffic routes and sports areas. When humans penetrate into alpine areas in which the mountain forest or the protective forest are no longer sufficient or non-existent, artificial means must be used to stabilize the snow cover and prevent the slipping of snow masses.
Types of construction
There are mainly four different types of avalanche barriers. The construction and maintenance of the first two types, which are intended to prevent an avalanche from occurring, is very complex and expensive. Therefore, one likes to use the other two installation options to limit damage:
Drift barriers
The drift barriers influence the deposition of the snow so that it does not break loose. Among other things, the snow fence , scour panels (see also: Windkolk ) and nozzle roof serve this purpose .
The drift shoring prevents unfavorable deposition of snow by the wind at possible break points. This can limit the risk of avalanches. Even small-scale buildings have a major impact here. Such structures are snow fences. They are erected in places exposed to the wind in order to reduce the wind speed there. Even at half the wind speed, only an eighth of the original amount of snow is transported. On the leeward side, the snow is deposited before it is blown into an avalanche breakout area and creates a dangerous snow drift deposit there. Snow fences are therefore mainly used on flat mountain ridges above an avalanche slope. The efficiency of snow fences depends on their height and the degree of filling. A fence height of 3 to 6 meters is typical. The degree of filling indicates the ratio of the closed to the total fence area. The best effect is achieved when the snow fences are positioned perpendicular to the wind direction and have a filling level of 0.5 to 0.7. Retaining structures are often used in combination with scour panels and tuyeres. Scour panels are usually positioned on ridges. They do not hold back the snow, but cause a cheaper deposition by causing wind turbulence. Years of observation of wind direction and snow distribution are necessary to achieve an ideal effect with drift structures.
Support shoring
The support shoring holds the snow in the avalanche slope. Well-known types are the steel snow bridge, wooden snow rake and snow nets.
The support shoring takes effect in the area where it begins and thus prevents an avalanche from breaking loose by supporting the snow cover. Small snow movements that can occur in the shoring are absorbed and slowed down. This avoids a larger tearing area. The snow stability is maintained by a damming effect. The creeping and sliding corners of the snow exert static pressure on vertical support surfaces anchored in the ground. This static snow pressure is the basis for the dimensioning. The snow pressure depends on the snow depth, the snow density and the snow sliding. Forces of 50 to 100 kN / m can occur. The forces are introduced into the ground by means of anchor, micropile foundations, base plates or concrete foundations. Another technique for the foundation of the avalanche barriers is the use of spinning anchors.
- Steel snow bridges are the most common avalanche protection measure. They have heights of up to four meters and can hold up to 2.5 tons of snow per square meter. In comparison, steel snow bridges are long-lasting and cause only few maintenance costs. They are assigned a lifespan of up to 100 years. However, the assembly of the snow bridges is complex. The great weight and the inaccessible areas cause many problems here. The snow bridges are therefore prefabricated and flown to the desired location, where they are only founded and assembled.
- Wooden snow rakes are snow bridges made of wood. Their service life is considerably shorter in comparison. They are mainly used in the afforestation area, as the forest gradually takes over the avalanche protection.
- Compared to snow bridges , snow nets are less sensitive to falling rocks because they are extremely flexible. This allows high dynamic energies to be absorbed without damage. Another advantage is the low transport weight. As a result, the construction is also more gentle on the landscape and has little impact on the landscape. The filigree construction is therefore particularly suitable for protection in tourist and recreational areas.
Snow nets are currently more expensive than snow bridges. However, assembly is more cost-effective even in difficult topographical conditions. It should be noted that with large mesh sizes the snow retention is insufficient in loose snow cracks. Freezing temperatures and melt water represent a further burden for the galvanized wire ropes and snow nets therefore have a shorter service life.
Brake shoring
The braking system stops the avalanche in the run-out area in good time. You can use brake bumps or a catch dam for this purpose.
A brake shoring has the task of reducing the speed of an avalanche. The avalanche can then be initiated in a containment dam, where it is then completely stopped. The braking of the snow masses is effected by offset dams and humps. The distances between the bumps should be kept small. The advantages of an application with brake bumpers are, on the one hand, the shortening of the discharge route, and on the other hand, the more favorable dimensioning of the collecting dam. The parameters for calculating the dam height are the speed of the approaching avalanche, the flow height of the avalanche and the height of the old snowpack in the storage space. The height of rise is also taken into account in the dimensioning. Heights of rise reach 3 meters at speeds of 10 m / s and a considerable 15 meters at 25 m / s. The dam must not be overflowed by the avalanche. Here you can see that dam heights of at least 20 meters are required.
Brake shoring can only be used in deep, shallow and sufficiently large avalanche deposition areas, since the avalanche snow can be distributed and deposited here without damage.
Since brake barriers are often also located in the ditches of the watercourses that form the avalanche line, they are then linked to the function of a bed load barrier for the torrent control .
Deflection shoring
The deflection shoring directs the avalanche away from the endangered object. They form the last stage of the building measures, directly on the threatened property.
If avalanches cannot be prevented from developing, deflection or deflection structures are used. These guide the fall path or its discharge area in a desired direction or limit the lateral expansion of the avalanche. In this way, the avalanche cannot cause any damage or only a justifiable minor damage. This is where guide combs, deflection dams and riving knives are used. Deflection structures achieve their maximum efficiency when they run parallel to the avalanche axis. Large deflection angles (> 20 ° –30 °) should be avoided, as the required building height would have to be dimensioned very large or the flow of the avalanche could no longer be guaranteed. Deflection structures are therefore only used on steep terrain. However, deflection structures are almost ineffective against loose avalanches.
- Avalanche gallery : deflection structures are often used in combination with galleries. Avalanche galleries are roof structures made of reinforced concrete, which are supposed to protect traffic routes from snow masses. These galleries are to be dimensioned according to the avalanche forces. The shear forces occurring in the back wall of the building are diverted into the ground via anchors.
- Riving knife : A riving knife has the task of dividing the avalanche that has broken loose at a certain point. The avalanche is then steered in a desired direction. This means that objects at risk of avalanches can be protected over the long term.
Before and after an avalanche decline:
Protective structures of the Gotthard Railway near Wassen
After the decline of the Entigtal avalanche train
See also
literature
- Falser, Michael: Alpine landscapes of defense: On modern-vernacular avalanche protection systems in the Swiss Alps. In: GJ Schenk (ed.): Historical disaster experiences. Heidelberg 2017, 399-422. In the German version see: Falser, Michael: Historische Avalanche Protection Landscapes, in: kunsttexte.de (3.2010), online
- Edlinger, Staude-Stock: Avalanche protection. Geographical exercise, 2003 ( pdf , eduhi.at).
- Thomas Egli; Building insurance canton Zurich, building department canton Zurich, AWEL office for waste, water, energy and air (publisher): Guideline for property protection against natural hazards.
- T. Johannesson, P. Gauer, P. Issler, K. Lied (Eds.) Et al .: The design of avalanche protection dams - Recent practical and theoretical developments. In: Research project SATSIE - Avalanche Studies and Model Validation in Europe, Climate Change and Natural Hazard Research , Series 2, European Commission, Brussels 2009 (Directorate-General for Research, Publication EUR 23339).
- S. Margreth: Avalanche protection in the starting area. Technical guideline as an implementation aid. Environmental Implementation No. 0704. Federal Office for the Environment , Bern, WSL Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, 2007 ( bafu.admin.ch ).
Norms:
- ONR 24805–7 Permanent technical avalanche protection (AT)
- ASTRA 12 007 Effects of avalanches on protective galleries , Federal Office f. Strassen (ASTRA), Federal Department for the Environment, Transport, Energy and Communication (DETEC) in cooperation with SBB AG Infrastructure Engineering 2007 V2.00 (Swiss guideline)
- BAFU-WSL-SLF-Guideline Avalanche Control in the Starting Area Ed .: Federal Office for the Environment FOEN / WSL, Federal Institute for Snow and Avalanche Research SLF, Bern 2007 (Swiss guideline)
- SLF guideline construction manual for sliding snow protection and temporary support shoring , publisher: SLF (Franz Leuenberger)
