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Identifying Avalanche Terrain Avalanche Mitigation Avalanche Zoning Acceptable Risks CGS Bulletin 49 Climate Change & Avalanches Avalanche Dynamics & Modeling Publications

Structural Defenses

Structural defenses are justified where people may be exposed for long periods of time in avalanche areas (e.g., in buildings or houses), where valuable objects are exposed, or where avalanche risk is too high or cannot be otherwise avoided. If possible (or justified based on high property values) buildings and areas that concentrate human use should be located outside the limits of the “design avalanche,” an event with a return period of 100 to 300 years. If this is not possible or is undesirable for political, legal, or economic reasons, structural protection must be used. This can take the following forms:

direct protection of buildings, valuable objects, highways, railroads;

deflecting avalanches and/or channelizing flowing snow;

arresting snow and storing it,

supporting snow in the starting zone (preventing release or reducing the released slab size)

wind fences and wind baffles alter wind-deposited snow areas and can reduce avalanche frequency, volume and runout

Options (a) through (c) require that the design avalanche velocity, impact pressure and flowing height be calculated in advance. Option (d) requires knowledge of the snowpack depth and density, ground roughness and the state of stress in the snowpack in the starting zone. Option (e) is generally used with other mitigation measures.

The following photos are examples of structural protection:

(a) Direct Protection

These structures have been reinforced for snow impact and static forces:

Residential Structures, Sun Valley, Idaho	Residential Structure, Ketchum, Idaho
Transmission Line Tower, Ophir, Colorado	Gemmi Pass
Alta, Utah	Davos, Switzerland
Alta, Utah	Lammern Hut, Gemmi Pass, Switzerland

(b) Deflecting and/or Channelizing Avalanches

Deflecting Berm, Park City, Utah	Earthen Splitting Wedge, Galtur, Austria
Steel Splitting Wedge, Snettisham, Alaska	Deflecting Structures, Galtur, Austria
Stóri-boli Earthen Deflection Dam, Siglufjörður, Iceland	Masonry Deflection Wedge, Val d'Aran, Spain

(b) Snow Sheds

Snowsheds or galleries are a type of deflecting structure

East Riverside Snowshed, Red Mtn. Pass, SW Colo.	Snowshed, Davos, Switzerland
Snowshed, Snoqualmie Pass, Washington	Snowshed, Wolf Creek Pass, Colorado
Snowshed, Lech, Austria	Lanark Shed, Rogers Pass, Canada
Snowshed, St. Anton am Arlberg, Austria	Snow Shed, Rogers Pass, Canada

(c) Arresting and Storing Snow & Debris

These earthen mounds reduce runout at a road maintenance facility and parking lot, Crested Butte, Colorado	Avalanche Dam, Galtur, Austria
Stone & Concrete Mounds, Pas de la Casa, Andorra	Avalanche Dam, Arinsal Valley, Andorra In February 1996, a large avalanche impacted several building shortly after authorities completed an evacuation. No fatalities occurred, but property damage was significant. This dam and other protective measures were completed after the 1996 avalanche. The dam is over 1000 long and 52 feet high.
Earthen Mounds, Hoback Canyon, Wyoming	These steep-faced mounds use mechancally stabilized earth (MSE) to achieve greater energy dissipation and a reduced footprint compared to earthen mounds, Iceland

(d) Supporting Structures in the Starting Zone

Several generations of avalanche prevention structures above Davos Dorf, Switzerland are shown in this photo. Early structures were earth and masonry terraces. Modern structures are mostly rigid steel sections or flexible steel nets.	These recently constructed wood and steel avalanche prevention structures were selected for aesthetics. Wood structures have a shorter design life than steel and are sometimes used with re-foresting.
Snow nets, Mt. Crested Butte, Colorado	Steel snow bridges were pioneered in Austria in the mid-1950s, Galtur, Austria
Snow nets, Pas de la Casa, Andorra An avalanche in 1970 damaged several buildings and killed one person at this ski resort village. Risk has been reduced with protective structures and a tunnel under the pass.	Combined types of Starting Zone Structures The upper structures are continuous "woven" flexible steel cable nets that use a swivel post to transfer part of the snowpack stress to the ground. The lower "umbrella" style structures are discontinuous and each structure has a single ground tension anchor point.

(e) Snow Fences and Wind Baffles

Relatively low cost wind fences can be used to alter wind erosion and deposition of snow. Experience has shown that wind fences and wind baffles can reduce avalanche size and runout, but supplimental measures are usually needed to achieve a high level of protection.

Bethel avalanche path near Berthoud Pass, Colorado.

Several rows of structure are intended to reduce wind-loading above State Route 88 in the Sierra Nevada mountains, California