Surficial Geology and Geomorphology of the North Slide, Thompson River Valley, British Columbia, Canada: Application of Fundamental Geoscience Information to Interpretations of Geospatial Monitoring Results

Abstract Our study focuses on a slow-moving landslide in the Thompson River valley, south-central British Columbia, Canada, that has posed a hazard to the national railway transportation corridor since 1880. Real-time kinematic global navigation satellite systems, unoccupied aerial vehicles, and satellite synthetic aperture radar interferometry time-series show significant displacement encroaching on railway infrastructure. In this paper, geospatial relationships between landslide distribution and specific terrain features, and the environmental conditions triggering instability are determined from field-based geological observations. We describe how earth material stratigraphy, textures, and penetrative planar structures are important controls on sub-surface drainage, and how these factors influence the style, timing, and rate of slope displacement. West of the railway tracks, slide scarps extend across the toe slope, corresponding to narrow zones of high displacement, presence of perennial springs and seepage, and cutbank erosion along the river channel. Fluvial incision exposes weak, failure-prone units at the base of the fill sequence, and with ongoing channel migration promotes instability by altering landslide toe geometry. Currently, the zone of potential displacement does not extend upslope into the inactive (1880) main slide body, east of the tracks. Seasonal variations in hydrogeological conditions influence the spatial and temporal patterns of surface water and groundwater flow, in turn controlling the distribution of translational-rotational displacement of slide blocks, and rates of movement on reactivated shear surfaces that extend under Thompson River. Slope failure occurs along weak, sub-horizontal shear zones within poorly drained glaciolacustrine clay and silt units, overlain by rapidly drained glaciofluvial outwash gravel, and imperfectly drained till diamicton. River levels exert a complex control on landslide stability, influencing hydraulic gradients within the basal glaciolacustrine unit, particularly along rupture surfaces within it. Ground displacement occurs while river levels are at their lowest between February and March, before peak flows in June, or after July until December while storm-fed river levels progressively lower to the next winter minimum. Groundwater levels remain elevated in the slide body throughout the year, contained in porous gravel and sand beds, and along brittle fractures and sub-horizonal shear zones in silt-clay varve beds. Geospatial and temporal change-detection monitoring of active landslides and at-risk infrastructure, when benchmarked with terrain and hydrogeological observations, is a cost-effective hazard management practice that provides important geoscience information to help develop appropriate early warning, mitigation, adaptation, and risk reduction measures.

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