A v a l a n c h e

D y n a m i c s

a n d

D e f e n ce

Community 5-15%.Because of the high costs supportingline 1890 m, well above the houses. No damages havebeen
structures are usedmostly for the protection of settlements.recorded. The heightof the snow deposits was less than 1
In the SwissAlps supporting structures in thestartingzonem (Fig.1).
have been used foravalanchecontrol forabout 120 years.On March 30, 1964, an avalanchewith exactly the same
Until today supportingstructures withatotal lengthof somestartingzone like1994had beenobserved.Atthattime
470 kmhave been built.there were no structures in this starting zone.The snow
height was 140cm.The new snow height in the starting
2.CASE STUDIES^{zonefrom27thto29thwasabout110cmwithamean}
Theinteraction ofsupportingstructureswiththe snow-^{density of 77 kg/m3 . The snow fell on the icy surface of an}
pack has been studied forseveral winters in selected ar-^{old snowpack only 0.6 m deep. The avalanchestopped on}
eas.Aftercritical avalanche periods monitoring flights to^{the main road of Pontresina inbetween the houses.The}
controlledareashavebeenmadebySFISARandlocal^{runout was 350 m longer than1994. The snow deposit was}
responsibles.If possible field visitswere added. Two ex-^{several meters high.}
amples are given to illustrate avalanching in defenseareas^{How can this difference be explained? The main start-}
composed of modern supporting structures:^{ing zone is between the contourlines 2620m and 2500 m.}
Thewidthoftheunconfinedslopeis290mandthe
Pontresina - Val Giandains, 8/9.1.1994^{inclinationyis37º.Atfirstavalanchedynamiccalcula-}
Avalanchedefensework above the village of Pontresina in^{tions with the Voellmy-Salm model without any support-}
the Engadinvalley beganat the endof the last century with^{ing structuresare made(situation A, 1964). The slab thick-}
stone terraces and afforestation.In the main starting zone^{ness d is 0.9 m.}
of the Val Giandains avalanche 1360m of 3 mhigh steel^{Then calculations are made fora width of 200 mwith}
constructionshavebeenbuiltbetween1982and1989.^{structures andfor a width of 90 m without structures (situ-}
About25%oftheentirestartingzoneof8hectaresis^{ation B,1994).The slope distance among the 6 lines is 19}
protected. In the northern gully and above the controlled^{m (Fig.2).It is assumed that the slab avalanche started si-}
area are secondary starting zones.^{multaneously on the whole area. In the controlled area the}
The Giandains avalanche occurred during the night of^{slabisseparatedbythelinesofstructures.Thesnow}
January 8/9,1994,aftera snowfall from the 7th to the 9th^{impacts the next line before it reachesfull speed. Through}
of about 105 cm on a weak snowpack. The mean density of^{the impact a loss of energy and mass occurs. This is taken}
the new snow was 111kg/m³ .The snowheight was esti-^{into account by varying the turbulence coefficientyfrom}
matedto120cm,thatmeansonethirdofthestructure^{1000to 500 m/s2and by reducing the flow depth d from}
height.During the period of snow fallheavy winds blew^{0.9mto0.5m.Thespeedvinrelation tothecovered}
from the south.The Swiss avalanche bulletin indicated a^{distance x can be expressed (Voellmy, 1955):}
high degree of danger.^{v2= dxsiny -m cosy( (1-e-(2 gª x/ dx).}
Picturesshow thatatfirsta smalldryslabavalanche^{The flow rate Q flowing through the bottom of the con-}
started at 2800 m in a leeward slope. Afterwards the whole^{trolled area is calculated with the smallerspeed and the}
starting zone fractured, in the controlled area (inclination^{reducedflow depth. Because of the smaller speed and the}
38º, slope distance betweenthe lines 19 m) too. The crown^{smaller volume of the avalanchingsnow the dynamic fric-}
surface occurred along thesupportsofthe structures.In^{tion coefficientyis increasedfrom 0.25 to 0.27 in the track}
the averageabout 20-50%of the sliding snow was stopped^{and runout.}
bythestructures.The propagation oftheshearfracture^{The calculations show that the structures reduced the}
could not beavoided.The avalanche stopped at contour^{speedoftheavalancheattheendofthestartingzone}
from19.1m/sto10.0m/sand theflowratefrom4983

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