A slope failure caused by drainage cutoff through the advancement of seasonal frost, Nelson River, northern Manitoba

D.P. van Zeyl, L.A. Penner

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Landslides and Geohazards

ABSTRACT: A failure occurred between December 14th and 17th, 2008 in the upper part of a 45 m high, northwest facing bank of the Nelson River in northern Manitoba (56.687°N, 93.777°W). The slope failure occurred at a spring site in a bay associated with a buried valley. The sediment input to the river from this event is roughly 20,000 to 25,000 m3. The source zone is made up of 25 m of water-bearing sand and gravel confined between ice-rich silty clay at the top of the bank and laminated to rhythmically bedded silt and clay at the base of the section. The collapse was confined to the material above the basal silts and clays and was associated with a perched groundwater flow system. A strong argument for drainage cutoff by the advancement of seasonal frost has been demonstrated through the correlation of the bank collapse with the timing of an extreme cold snap recorded at two nearby weather stations. RÉSUMÉ Une erreur s'est produite entre le 14 Décembre et le 17, 2008 dans la partie supérieure de 45 m de haut, au nord-ouest banque face de la rivière Nelson, dans le nord du Manitoba (56,687° N, 93,777° W). La rupture de pente est survenu sur un chantier de printemps dans une baie associée à une vallée enfouie. L'apport de sédiments de la rivière de cet événement est d'environ 20.000 à 25.000 m3. La zone source est constituée de 25 m de sable aquifère et de gravier confiné entre riches en glace d'argile limoneuse au sommet de la banque et stratifié à lits rythme limon et d'argile à la base de la section. L'effondrement a été limité à la matière au-dessus des limons et des argiles basales et était associé à un système d'écoulement des eaux souterraines perché. Un argument de poids pour le drainage coupure par l'avancement de gel saisonnier a été démontrée par la corrélation de l'effondrement de la banque avec le calendrier d'une vague de froid extrême enregistrée à deux stations météorologiques à proximité. 1 INTRODUCTION A significant volume of sediment was delivered into the Nelson River by a bank failure that occurred in December 2008 at a spring site in a small bay (Figs. 1 and 2). The failure appears to have been caused by drainage blockage through the advance of seasonal frost during an extreme cold snap, a mechanism not often described but that should be expected in northern regions. The landslide was studied during an erosion assessment for the potential Conawapa hydropower project being planned by Manitoba Hydro. This case record illustrates the important roles that stratigraphy, groundwater and extreme low air temperatures can play in controlling bank erosion in certain geological settings in this region. 1.1 Previous work During reconnaissance work from Split Lake to Hudson Bay, Piteau (1972) found that actively failing slopes were a common feature along the banks of the Nelson River. Dredge and Nielsen (1985) described bank failures along the Nelson estuary. Upstream of Horseshoe Bay, Donnelly et al. (1990) described minor mudslides initiating from water-bearing, granular deposits near the top of the bank, and they documented a failure mechanism involving creep-like deformation at the base of a till bank related to a weak varved clay deposit. Penner et al. (1975) provided descriptions and interpretations of mass wasting processes in their work on the Nelson River. Figure 1. Location of the study site in northern Manitoba. Lake Agassiz and Tyrrell Sea limits after Teller et al. (1983)

RÉSUMÉ: pe failure caused by drainage cutoff through

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D.P. van Zeyl; L.A. Penner (2014) A slope failure caused by drainage cutoff through the advancement of seasonal frost, Nelson River, northern Manitoba in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper127,author = D.P. van Zeyl; L.A. Penner,title = A slope failure caused by drainage cutoff through the advancement of seasonal frost, Nelson River, northern Manitoba,year = 2014}