The 28 November 2020 Landslide, Tsunami, and Outburst Flood – A Hazard Cascade Associated With Rapid Deglaciation at Elliot Creek, British Columbia, Canada

Abstract

We describe and model the evolution of a recent landslide, tsunami, outburst flood, and sediment plume in the southern Coast Mountains, British Columbia, Canada. On November 28, 2020, about 18 million m3 of rock descended 1,000 m from a steep valley wall and traveled across the toe of a glacier before entering a 0.6 km2 glacier lake and producing >100‐m high run‐up. Water overtopped the lake outlet and scoured a 10‐km long channel before depositing debris on a 2‐km2 fan below the lake outlet. Floodwater, organic debris, and fine sediment entered a fjord where it produced a 60+km long sediment plume and altered turbidity, water temperature, and water chemistry for weeks. The outburst flood destroyed forest and salmon spawning habitat. Physically based models of the landslide, tsunami, and flood provide real‐time simulations of the event and can improve understanding of similar hazard cascades and the risk they pose.

Kinematic Analysis of the 2020 Elliot Creek Landslide, British Columbia, Using Remote Sensing Data

“…The landslide site is located approximately 150 km northwest of Vancouver, British Columbia (Canada) (Figure 1A) in the Pacific Ranges of the Coast Mountains (Holland, 1976). On 28 November 2020, an 18 million m 3 landslide detached from the western slope of Elliot Creek, in proximity of the retreating West Grenville Glacier tongue, and rapidly moved downslope, entering the Elliot Creek glacial lake, and producing a wave with a maximum runup of 114 m (Geertsema et al, 2022). The wave, with an estimated volume of 13.5 million m 3 of water, scoured the moraine at the southern end of the Elliot Lake and incorporated debris from the bed and lower slopes of Elliot Creek.…”

“…The debris transported by the wave was then deposited at the intersection with the Southgate River, which flows from east to west 10 km south of the landslide site into the Bute Inlet. The Southgate River was temporarily dammed by the resulting debris fan, and the turbidity of the water current downstream of the debris dam was significantly increased for several weeks after the event (Geertsema et al, 2022).…”

“…InSAR data analysis conducted on pre-failure datasets evidenced that the slope was deforming at an average rate of about 5.5 cm/y (measured along the satellite line-of-sight, LoS) between June 2018 and October 2020. Between June and October 2020, a cumulative LoS displacement of 15 cm was measured, corresponding to a 0.3 m/y LoS velocity over the summer 2020 (Geertsema et al, 2022).…”

“…The slope from which the 2020 Elliot Creek landslide detached was previously affected by another landslide of unknown age, but older than 1951 (Geertsema et al, 2022), which left a landslide scar that is visible in the pre-failure satellite and aerial imagery (Figure 1B). Prior to the 2020 event the older landslide scar appears to be relatively fresh and bounded by sharp, sub-vertical cliffs devoid of vegetation.…”

“…The behavior and displacement of the 2020 Elliot Creek slide prior, during, and after the slope detachment, as well as the impacts produced along the Elliot Creek have been summarily described in the work by Geertsema et al (2022). However, the environmental, structural, geomorphic factors that controlled the detachment of the 2020 Elliot Creek landslide have until now not been studied.…”