Destructive mass movements in high mountains: Hazard and management

The remote va1le.v~ and high rnountain areas of western Canada have experienced accelerated econotnic development only in recent years. Alrhough there have been occasional mass movements of destructive impact in this region in the past, the short histon. of human settlement n~akes ir dificulr to properly evaluate the potential hazard and appropriate countermeasures during future development of steep mountainsides and narrow valleys. A large body of dochmenred experience from long-inhabited mountain regions, notably the Alps of Europe, suggests that major misrakes can be avoided if this experience is used properly. Most destrucrive mass movements are co~nplex and related to mountain torrent systems and ir~stabilities of steep bedrock slopes. They can be cor~verrierrrly grouped info ( I ) debris flows from surficial deposits. (2 ) debris ,flows from bedrock failures. (3) mass movements on volcanoes, (4) glacier-relared mass movements, and ( 5 ) rockfalls and rock avalanches. Each of rhese requires a djfferent ype of hazardappraisal and particular set of remedial or preventive measures. The range of possible destrucri~*e inlpacts and appropriate countermeasures are illusrrated with 137 case histories from the Alps which are presented in a coherent geological-climatic framework transce~lding the tradiriorlul political boundaries. Analysis of these case histories, combined with insighrs gained elsewhere, is used to outline the application of active measures ( j o r e s n , control works, protective works), passive measures ( ior~ ing, planning), monitoring, and acceptance of risk. The decisions regarding which active or passive measures sholtld be applied and what level of risk is acceptable are based on ( I ) information on recurrence and magnitude of mass movements and ( 2 ) a broad social-economic consensus. Re'sume Dans les valle'es P1oignPe.s et les haures re'giorls motrtug~reuses de I'Ouest ca~ludierl. 1'e.rpan. siorl P(.onomiqrre ne st1 poursrrir a un r-\.rIime accPIPrP que depuis ces derniires an~le'es. MCme si, a I'occasior~, rles mouvcJments de masse orit fair serrtir leurs effers de.srrucrjfs darls ces re'gions d une e'poque fort ar~cienrrc. Iu courte histoire de la c~olonisrrrion humaine rend drflcile toure ir~rrluatiorr precise des risqrres Cverrtuels et des mesures pre'venrives appropriies d prkvoir au coltrs des prochaines phases de formation de versanrs escarpe's et de valle'es htroires. Cependant, rout ltrr ensemble d'expiriences note'es, acylrises rlat~s les re'gions monrcrgnelcses depuis longtemps peuple'es, comme Ies Alpes tl'Europe, permerrent de croire que de graves erre1rr.s polrrraient i t re iviries en mettat~t d profit ces e.rpPriences. Les nlouvements de masse Ies plus desrrucreltrs .rant de nature comple.re et sorrt lie's arlx rksearrx de rorrenrs de rnontagnes et a I'insrabilite' des perltes abnrptes de la roche en place. Ces mouvements peuvent, d tolrres fins pratiqltes. &re group6s duns I'rtr~e des core'gories suivantes: ( 1 ) co~tle'es de dibr is proverrant de de'pbrs de surface. ( 2 ) cortlPes de dibr is gine're's par des Pbo~tlements rochercx. (3 ) mo1rvemerrrs de masse sur Ies volcans, (4 ) rnouvemer~ts de masse lie's aux glaciers. et (5 ) glissen~ents d'e'boulis rocheu et trvalarlches de pierres. Chacl~te care'gorie de mortve~trerrt necessite UII mode diffe'rent d'evalltarion des risqrtes er ltrr jeu particrtlier de mesltres correcrives ou priventives. La gamn~e possible des effets destrrrcreurs et Ies rneslrres privenrives appropriees est illrrstre'e par 137 cas sunpenus dans Ies Alpes et est prisente'e dons rtn cadre climnrique er ge'ologiqlre cohirent qlti dipasse les frontieres politiqrres tradirionnelles. Une anolyse de ces cas oitlsi que le savoir acqlris ~ l i l le~rrs servent ci ditermirrer I'c~pplicatior~ de mesrtres octives (joresterie, travarrx de surveillance, rravoux de protectiotl), de mesures passives (zonnge, plor~ification), cle srtrveilla~~ce et d'acceptation des risques. Les rle'cisior~s quarlr au choir errrre les mesures actives er passives er a la de'termitration tle niveaux de risque acceptables sonr prises en fonctiot~ ( I ) de I'irlformarion fournie sltr la fi-e'qlrence et I'imnportance de.7 mouvements de masse et ( 2 ) dlr conserrslts qlri se fair arr niveau socio-6conomique.