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An unusually intense atmospheric river affected the southern coastal region of British Columbia in mid-November 2021. Flooding and several ground failures such as landslides and riverbank erosion caused significant damage and interrupted the main transportation corridors, including major highways and railroads, severing land connectivity between Metro Vancouver and the rest of Canada. In this short note, we summarize and describe slope and riverbank ground failures in the Lower Mainland and connecting corridors to the interior such as the Fraser Canyon and the Coquihalla Valley, with focus on those that produced damage on highways. Field observations are discussed in the context of potential future similar disasters in the region due to more frequent extreme climatic events associated with climate change. [ABSTRACT FROM AUTHOR]

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Two large landslides in overconsolidated glaciolacustrine clay and silt, located in river valleys in British Columbia, were found to be quite similar in their main aspects. However, their failure behaviour was very different. One continues to exhibit intermittent, mainly ductile deformations with limited mobility which are typical of compound landslides in stiff clay. The other suddenly developed into an extremely rapid flow slide of 6.4 Mm[sup 3] , damming a large river in the course of a few minutes and projecting a wave onto the opposite bank. The flow slide followed a period of strong infiltration. A comparison of the two cases explains the difference in their failure behaviour. Three possible mechanisms for brittle flow slide formation are proposed, the first based on the undrained brittleness of soil at the rupture surface, the second on the high internal strength of the sliding body, and the third on macroscopic brittleness developed by pre-failure ductile deformation and softening. Both latter mechanisms are facilitated by the presence of nonplastic units in the sliding mass.Key words: overconsolidated clay, silt, landslide, flow slide, brittleness, glaciolacustrine deposits.On a observé que deux grands glissements dans les argiles et silts glacio-marins surconsolidés situés dans des vallées de rivières en Colombie-Britannique étaient tout à fait semblables sous leurs principaux aspects. Cependant, leurs comportements en rupture ont été très différents. L'un continue de montrer des déformations intermittentes principalement ductiles avec une mobilité limitée, ce qui est typique des glissements composés dans les argiles raides. L'autre a soudainement développé une coulée extrêmement rapide de 6,4 Mm[sup 3] , damant une rivière importante en l'espace de quelques minutes et projetant une vague sur la berge opposée. La coulée s'est produite après une période de forte infiltration. Une comparaison des deux cas explique la différence dans leur comportement en rupture. On propose trois mécanismes possibles de formation de coulée: le premier basé sur la fragilité non drainée du sol le long de la surface de rupture, le second sur la forte résistance interne de la masse en mouvement, et le troisième sur la fragilité macroscopique développée par une déformation ductile et un ramollissement avant la rupture. Le deux derniers mécanismes sont facilités par la présence d'unités non plastiques dans la masse en mouvement. Mots clés : argile surconsolidée, silt, glissement, coulée, fragilité, dépôts glacio-marins.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]