Defense structures in avalanche starting zones - SLF

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$N e w s l e t t e r N a t u r g e f a h r e n Schneeverfrachtung - SLF$

Technical Guideline for Defense Structures in Avalanche Starting Zones FOEN / WSL 2007 22 3.3 Structure types 3.3.1 Rigid structures Where the creep and sliding motion of a snow layer is arrested by a supporting surface that is subject to only slight elastic deformation, it is referred to as a rigid supporting surface or a rigid supporting structure (e.g. snow bridge with steel crossbeams, see Fig. 1). 3.3.2 Flexible structures If the supporting surface is to a certain extent able to follow the movement of the snow layer, the surface or supporting structure is said to be flexible (e.g. snow nets, see Fig. 3). 3.3.3 Loading of a supporting structure As explained in Section 3.2.2, a supporting structure must withstand both the snow pressure and the dynamic forces. Whereas dimensioning of the structures is based on the static snow pressure (Section 5), the magnitude of the dynamic forces may be influenced by suitable arrangement of the structures (see Section 3.7) to ensure that they suffer no or very little damage. 3.3.4 Choice of structure The structure should be chosen in accordance with the requirements of the objects to be protected (Section 3.2.3) and in relation to the local snow, terrain and ground conditions. Snow nets are less sensitive to creep movement and rockfall (cf. Section 7.4.3.1), but more difficult to anchor in loose ground.
3 > Planning of supporting structures 23 3.4 Extent and positioning of a supporting structure 3.4.1 Slopes to be controlled by structures Supporting structures are generally required for slope inclinations between 30° and 50° (58 % and 119 %). In exceptional cases, flatter or steeper terrain in a starting region may need to be controlled, e.g. flatter shoulders above steeper slopes, or flatter sections of the slope. 3.4.2 Positioning of the uppermost structures 3.4.2.1 General Supporting structures should mainly be installed below the highest observed or anticipated fracture lines of snow slab avalanches (Section 3.1.1), in such a way that these still lie within the actual back-pressure zone of the structures. As explained in Section 3.2.2, this is the case when the structures are installed not more than 2–3 HK below the fracture lines. 3.4.2.2 Cornices Where the slope to be controlled is bounded by a ridge known to form a heavy cornice, the uppermost structures should be positioned as near as possible to the foot of the cornice, without, however, coming to lie within the cornice itself. The structures should be dimensioned very generously to accept the large volume of snow and withstand falling sections of the cornice. In many cases, the mass of the cornice can be reduced by anti-drifting structures. If appropriate, these should be installed prior to erection of the supporting structures. 3.4.2.3 Rocky terrain Where the upper edge of the slope to be controlled is bounded by very steep, rocky, terrain, the uppermost structures should likewise be very generously dimensioned. Furthermore, where there is a danger of rockfall, they should be provided with a supporting surface having the highest possible resistance to rockfall (for example: snow nets, massive steel grates or timber covering). Where there is a danger of damage to the supporting structures from snow, rock or ice falls from higher ground that cannot be secured, this may be reduced with the help of deflecting or retaining structures (earth dam or rockfall protection net). 3.4.2.4 Secondary starting zones Supporting structures should mainly be located at the highest observed or anticipated starting zones of snow slab avalanches. Depending on the situation, a check should be made whether avalanches could be triggered in secondary starting zones further above,
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Defense structures in avalanche starting zones - SLF

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