Avalanches

Avalanches

“Avalanches are masses of snow which, when falling from steep slopes, ditches and the like, can cause dan­ger or dam­age as a con­se­quence of the ki­netic en­ergy or the wave of air pres­sure caused by them or by their de­po­si­tion.”

This de­f­i­n­i­tion under the 1975 For­est Act (sec­tion 99(2)) de­scribes avalanches as slid­ing, flow­ing or rolling masses of snow or as swirled clouds of snow which are char­ac­terised by mor­pho­log­i­cal and ki­netic prop­er­ties. 

The catch­ment area of an avalanche is di­vided into the start­ing zone, the avalanche track and the de­po­si­tion zone.

De­pend­ing on the shape of the break­ing point, a dis­tinc­tion is made be­tween

• loose snow avalanches, which start from a point and fan out­ward as they de­scend and
• slab avalanches, which de­velop as a re­sult of plates of snow slid­ing off on a smooth or un­sta­ble layer of snow due to the ex­ceed­ing of the sta­bil­is­ing shear stress and sub­se­quent col­laps­ing. 

De­pend­ing on the type of move­ment one dis­tin­guishes be­tween

• flow­ing avalanches (flow­ing move­ment of the snow masses) and
• air-borne pow­der avalanches (a scat­ter­ing of an air-sow mix­ture).

Other dis­tin­guish­ing fea­tures of avalanches

De­pend­ing on whether a slab avalanche starts in the snow cover or on the ground, it is re­ferred to as sur­face avalanche or as ground avalanche. So while the dif­fer­ent start­ing mech­a­nisms lead to the dis­tinc­tion be­tween loose-snow avalanches and snow slab avalanches, an­other dis­tinc­tion is made be­tween un­con­fined avalanches in cases, where snow masses glide ex­ten­sively, and con­fined avalanches, where the avalanche comes down in a chan­nel (gully).
Dry snow causes dry-snow avalanches, wet snow causes wet-snow avalanches. A flow­ing move­ment of the snow masses will lead to a flow­ing avalanche; in the case of scat­ter­ing move­ment, an air-borne pow­der avalanche will de­velop. If the flow­ing avalanche has a high water con­tent, it may reach down to the ground and pick up its com­po­nents. In this case it be­comes a ground avalanche - a type of avalanche usu­ally ex­pe­ri­enced in spring.

Avalanches are trig­gered by heavy new snow fall, rapid rise in tem­per­a­ture, the trans­port of snow by wind, the steep­ness of the land­scape, and the im­pact of veg­e­ta­tion. An im­por­tant phe­nom­e­non is the meta­mor­pho­sis of the snow crys­tals in the snow cover which de­pends on the dif­fer­ent weather con­di­tions. Apart from self-trig­ger­ing, also game move­ments and hu­mans may ini­ti­ate avalanches (ski tourism, ski moun­taineer­ing).

In Aus­tria about 5,800 avalanches are ad­vanc­ing to­wards the per­ma­nent set­tle­ment area. Areas par­tic­u­larly prone to the risk of avalanches are lo­cated in Vo­rarl­berg, Tyrol, East­ern Tyrol, Salzburg, Upper Carinthia, Upper Styria as well as in the south­ern part of Upper Aus­tria. Many more avalanches pose a risk to the Alpine ski areas.

Avalanche con­trol

For­est-bi­o­log­i­cal mea­sures

Forests are con­sid­ered the most ef­fi­cient pro­tec­tion against avalanches. An un­even-aged, multi-storey and closed stand is the best pro­tec­tion against the de­vel­op­ment of avalanches below the for­est line and the best nat­ural “brake” to stop snow events from areas above the for­est line. Forests lift the wind field, have a bal­anc­ing ef­fect on the snow de­posit and pre­vent the for­ma­tion of co­her­ent areas of ten­sion in the snow pack, with the lat­ter also be­com­ing very com­pact in the area sur­round­ing trees.

Tech­ni­cal (en­gi­neer­ing) mea­sures

Where the pro­tec­tive ef­fect of forests is poor, avalanche de­fense works in steel or wood or snow nets are con­structed in avalanche-prone areas. In ad­di­tion, con­trol mea­sures to pro­tect against glid­ing snow (wooden sup­ports, ar­rays of posts) re­duce snow move­ment and per­mit af­foresta­tion at the tim­ber line. For the man­age­ment of pro­tec­tion forests in avalanche catch­ment areas some spe­cific for­est tech­niques, like the trans­verse felling of trees, have been de­vel­oped.
In the run-out zone of avalanches de­flect­ing dams and catch­ment basins force flow­ing avalanches to de­posit out­side the threat­ened res­i­den­tial area. To pro­tect high­ways and ski areas also tem­porar­ily ef­fec­tive tech­niques for the ar­ti­fi­cial (tar­geted) trig­ger­ing of avalanches are ap­plied. Among them are avalanche-blast­ing rope­ways, gas ex­ploders or “avalanche or­gans”.

Per­ma­nent avalanche con­trol in­clude also land-use plan­ning mea­sures (Haz­ard Zone Plan­ning) as well as the in­for­ma­tion of those con­cerned or in­ter­ested in snow- and avalanche-re­lated processes.