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

Fig.7: Avalanche path

• Winter and snow conditions
  
  Aftersupportingstructuresare completedinan area
  snowdepositsand snowmovementsshouldberegu-
  larlyobserved,especially inordertojudge theeffec-
  tiveness of the structures duringmajor avalanchesitua-
  tions.Sometimesthestructuresarelocallyoverfilled
  with snow drifts and additional measures against snow
  drift have to be taken.
• Topography of the track below the controlled area
  
  Very important fora safetyanalysisis the position of
  the objectto be protectedin thetrack. The highestsafety
  is achieved if the structures are combined with an af-
  forestation (Fig.7.1) and the object to be protected is at
  the bottom ofa valley.As the protecting forest grows,
  the level of safety increases. Avalanchesreleasedin the
  controlledareas arelimitedtoaharmlessmagnitude
  by the forest. If there is a runoutzone between the track
  and the objectsto be protected (Fig.7.2), then often ava-
  lanches released in thecontrolledarea can not reach
  them.Theseavalanches havea muchshorterrunout
  thanan extreme avalanchewithout anyprotection work.
  Thehazar dcanbedeterminedwithr unout
  calculations.If the objects to be protected are located in
  asteeptrackbelowthecontrolledarea oratistend
  (Fig.7.3) without any retardationzone, also small slabs
  can reach thiszone. The hazardcan be estimated by the
  calculation of avalanchepressures exerted on obstacles
  and frequency analysis.

5. RECOMMENDATIONS FOR THE USE
   
   OF SUPPORTING STRUCTURES
   
   Supporting structures are suitable to prevent extreme de-
   structiveavalancheswithlongreturnperiodsandlong
   runouts.In Switzerland the expensivesupportingstruc-
   tures arewidely used to protect zones or objects which are
   difficulttoevacuateortocloseduringhighavalanche
   hazard. These are mostly settlements that have been built
   a long time ago in the runout of extreme avalanches. Gen-
   erally during catastrophicsituations the supporting struc-
   turesstoodthe test.If possiblethestructureshave tobe
   combined with an afforestation to increase safety.
   Theiruse forthe protection of roads is justifiable if the
   startingzone hasalimitedaltitudinalrange. Thecloser
   the road is situated tothe slope the higherare the safety

208

requirements forthe controlled area. If there is no runout
infrontoftheroadtheloweststructuresshouldhave a
densesupportingplaneoradditionaldepositionspace
shouldbeestablishedascloseaspossible.Snowsheds
normally povide a higher degree of safety.
In ski areas supporting structures are generallynot recom-
mended for the protection of skiruns, but they are suitable
toprotectforexampleterminalstationsorpylons.As
shownbeforeinareas controlledwithsupportingstruc-
tures,smallslabscan startand killa skier.The arrange-
ment and design of supporting structuresaccording to the
Swiss Guidelines (1990) are not made for the protection of
persons in or directlybelow the controlled area. Toachieve
thisthespacebetweenthe linesshouldbesmaller,the
supporting plane denser and the entire starting zone has
to be protected. Some protected areas aboveski runs which
donotfulfillthesestrongrequirementshavetobe
controlled additionally with artificial release.The use of
supporting structures forthe protection of skiruns seems
only justifiable if artificial release is not possible.
Theeffectivenessofsupportingstructureswillbein-
vestigatedinmoredetailinafutureSFISARresearch
project.

6. REFERENCES
   
   Frutiger,H.(1966):Behaviorof avalanches in areas control-
   led by supporting structures, IAHS Publication No.69, p.243-
   250
   
   Frutiger, H. (1988): Ueber die Wirksamkeit der Stützverbauung
   von Lawinen,Interpraevent 1988, Graz,p.137-188
   
   Ry chetnik ,J.(19 8 5):Lawinenak tiv itätim
   Lawinenanbruchgebiet mit und ohne Lawinenstützverbau und
   Aufforstung,Mitt.forstl.BundesversuchsanstaltWien,156.
   Heft, p.197-208
   
   Salm,B.(1994):Snowslabrelease,its mechanismandcon-
   clusion for the arrangement of supporting structures, Defence
   Science Journal,New Dehli,Vol 45, No 2, April 1995, p.125-
   129
   
   SalmB.,BurkardA.,GublerH.U.(1990):Berechnungvon
   Fliesslawinen, eine Anleitung für den Praktiker mit Beispielen,
   Mitt.desEidg.InstitutfürSchnee- undLawinenforschung
   Nr.47, Juli 1990
   
   SwissGuidelines.O riginaltitle:R ichtlinienfürden
   LawinenverbauimAnbruchgebiet.BUWAL,WSL,Eidg.
   Institut für Schnee- und Lawinenforschung, 1990
   
   Voellmy, A. (1955): Ueber die Zerstörungskraft von Lawinen,
   Schweiz.Bauzeitung73. Jg,Hefte 12, 15, 17, 19und 37