Peculiar debris-flow event of June 2013 in Livingstone mountain range (Alberta, Canada)

B. Rudaz

Dans les comptes rendus d’articles de la conférence: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: GEOHAZARDS - V Climate ChangeFloods & Landslides

ABSTRACT: Intense precipitationsoccurred in western Alberta in mid-June 2013(176 mm in 72 hours), which lead to several floods.In particular, this event produced many debris-flows in the area of Mount Livingstone,Alberta, Canada. The area ismostly composed by folded carbonates, and has a mountainous continental climate. The peculiarity of the June 2013event lies in the fact that the initiations of the~70debris-flows were located high in scree slopes, with reducedcontributing area.No debris-flowactivityanterior to that event were visiblein 2012.Fieldwork was carried out less than one month after this event.Two debris flows were extensively documented bymeans of field mapping,photographic,and Terrestrial Laser Scanner (TLS) data acquisitions. In-line grain-sizedistribution, fine matrix sampling and cross-sections of the debris flow channelswere performed at the initiation zone, thepropagation zone andthedeposition area.Grain Size Distribution (GSD) analyses were performed up to 0.1 •m.Morphologic characterization wasperformed throughfield mapping,pre-event HR-DEM (1m cell size) and TLS point-cloud comparison, along with cross-sections. Volumeswerecalculated. Field observations and GSD analysispointed outthe anisotropic composition of the scree slopes. Moreover, indurated layers and preferential water circulation paths havebeen witnessed. In some locations, thecomplete thickness of thescree deposit was mobilized,revealingthe bedrock.The conditionsof initiation of the debris flows were compared with literature values, in term of slope gradient andcontributing area.Our resultsindicatethat these debris-flows were triggered at unusually low slope angles in regard tothe contributing area.This study, thanks to its extensive dataset and combination ofseveralmethods, sheds a new lighton a poorly known debris-flow initiation context. It points out the role of single high-magnitude pulse events as adominant factor in term of mass wasting, after an extended period of scree accumulation.R†SUM†Des pr‡cipitationsde forte intensit‡(176mm en 72 heures)sesontabattuessur l…ouest de l…Alberta — la mi-juin 2013,causant de multiples inondations. Cet ‡v‡nement a ‡galement produit de nombreuses coul‡esde laves torrentiellesdans la r‡gion du Mont Livingstone,Alberta, Canada. Cette r‡gion se compose principalementde couches carbonat‡espliss‡esetposs–de un climat montagneux continental. La particularit‡ des ‡v‡nements de juin 2013 r‡side dans led‡clenchement des~70coul‡es dans les zones sommitales des talus d…‡boulis, avec unesufacecontributive tr–s faible.Les versantsne pr‡sentaient aucune trace de coul‡esen 2012.Des investigations de terrains ont ‡t‡ conduites moins d…un mois apr–s les‡v‡nements. Des donn‡es photographiques‡tendues, ainsi que des acquisitions topographiques par scanner laser terrestre ont ‡t‡ produites pour deux chenaux delaves torrentielles. Des lev‡s granulom‡triques en ligne, des pr‡l–vements de matrice fine et desprofils en travers ont‡t‡ mesur‡s sur ces mƒmes chenaux, tant dans les zones de d‡part, de propagationquede d‡p⁄t. Des analysesgranulom‡triques en laboratoire ont ensuite ‡t‡ conduites jusqu…— 0.1 ‹m.LeMNT hauter‡solutionavant ‡v‡nement(r‡solution de1m) et les nuages depoints obtenuspar lesscans LiDARsont compar‡s, ainsiqueles profils en travers.Les volumesmobilis‡s en sontd‡duits.Tant les observations de terrain que les analyses granulom‡triques ont soulign‡lacomposition anisotropique destalusd…‡boulis. Des niveaux indur‡s et des circuits de circulation d…eau ont ‡t‡observ‡s. Dans certaines zones, les ‡boulis ont ‡t‡ mobilis‡s dans leur totalit‡, laissant appara›tre la roche en place.Les conditions de d‡part des coul‡es ont ‡t‡compar‡es aux valeurs figurant dans la litt‡rature, en terme de pente etdesurface contributive. Cette analyse montre que les coul‡esse sont d‡clench‡es — des pentestr–sfaibles en regard — lazone contributive respective. Cette‡tudeapporte un regardnouveau sur un contexte d…initiation peu commun, gr−ce auxdonn‡es ‡tendues r‡colt‡es ainsi qu…— la combinaison desm‡thodes utilis‡es.Elle met en ‡vidence l…effet d…ununique‡v‡nement courtcomme agent principal du transfert de masse, apr–s une p‡riode‡tendue d…accumulation de d‡p⁄ts depente.1INTRODUCTIONThe study siteis located on both sides of the BeaverCreekriver(GPS UTM: 0685918/5555290) within theLivingstone Range(Fig. 1), around 70 km north of themunicipality ofCrowsnest Pass, AB. Beaver Creek cutsthe north-northwest trending Livingstone-Riveranticlineand exposes folded Devonian to Mississippian carbonaterocks (Banff to Mount Head formations),ranging inelevation between 1820 and 2420 m.a.s.l.The lithologiesare mainly constituted by grainstone to mudstonelimestone and dolostone.This area wasfirst visited in2012. No recentdebris flow activitywas observed, and

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