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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.

The shape of a wave generated by a landslide, snow avalanche, or fluid flow greatly influences its size and speed as it propagates away from the source region, which are critical parameters needed to estimate the impacts of these waves on coastal communities. In this study, laboratory data are produced from waves generated by the impact of water into a wave flume akin to the impact of a fluidized, highly mobile, and neutrally buoyant slide into a reservoir. Water surface observations are made using wave probes that remain at fixed positions, while the water depths and source volumes of slide material are varied. The wave shape is quantified by calculating the asymmetry about the vertical axis at each wave probe. The experimental results indicate that waves with positive or near-zero asymmetry in the near field have a small influence on the maximum wave amplitude along the flume. However, waves with negative asymmetry in the near field change rapidly in shape and amplitude due to breaking until a stable state with symmetrical shape and wave breaking limit of 0.6 is reached. The length scale at which the breaking waves reach this state is quantified based on the initial asymmetry. An enhanced mathematical framework is developed using horizontal-scale coefficients to modify the solitary wave equation such that it can be used to generate asymmetrical waves. This new method might be used in combination with predictions of the maximum wave amplitude to create time series needed to account for the shape of the tsunamis.Plain Language Summary The shape of a wave generated by an avalanche or landslide greatly influences its size and speed as it propagates away from the source region, which are critical parameters needed to estimate the impacts of these waves on coastal communities. In this study, laboratory data are produced from waves generated by the impact of water-impact slides into a reservoir that propagate and evolve along a wave flume. Water surface observations are made using wave probes, and the water depths and source volumes of water are varied in the experiments. The wave shape is quantified by calculating the "asymmetry," and the experimental results indicate that waves with positive or near-zero asymmetry in the near field have small changes in the maximum wave amplitude along the flume. However, waves with negative asymmetry in the near field change rapidly in shape and amplitude due to breaking until a stable state with symmetrical shape is reached. A novel framework is developed to mathematically describe asymmetrical waves, and this new method can be used to create time series needed to account for the shape of the tsunamis.

Tsunamis are generated when landslides transfer momentum to water, and these waves are major hazards in the mountainous coastal areas of lakes, reservoir, and fjords. In this study, the influence of slide mobility on wave generation is investigated using new: (i) experimental observations; (ii) theoretical relationships; and (iii) non-hydrostatic numerical predictions of the water surface and flow velocity evolution. This is accomplished by comparing landslides with low and high mobility and computing the momentum flux from landslides to water based on data collected in laboratory experiments. These slides have different materials, different impact velocities, different submarine runout distances, and generate very different waves. The waves evolve differently along the length of the waves' flume, and the experimental results are in close agreement with high-resolution phase-resolving simulations. In this short communication, we describe new research on landslide generated waves conducted at Queen's University, Canada, and presented at Coastlab18 in Santander, Spain.

<p>On 28 November 2020, about 18 Mm<sup>3</sup> of quartz diorite detached from a steep rock face at the head of Elliot Creek in the southern Coast Mountains of British Columbia. The rock mass fragmented as it descended 1000 m and flowed across a debris-covered glacier. The rock avalanche was recorded on local and distant seismometers, with long-period amplitudes equivalent to a M 4.9 earthquake. Local seismic stations detected several earthquakes of magnitude <2.4 over the minutes and hours preceding the slide, though no causative relationship is yet suggested. More than half of the rock debris entered a 0.6 km<sup>2 </sup>lake, where it generated a huge displacement wave that overtopped the moraine at the far end of the lake. Water that left the lake was channelized along Elliot Creek, deeply scouring the valley fill over a distance of 10 km before depositing debris on a 2 km<sup>2</sup> fan in the Southgate River valley. Debris temporarily dammed the river, and turbid water continued down the Southgate River to Bute Inlet, where it produced a 70 km turbidity current and altered turbidity and water chemistry in the inlet for weeks. The landslide followed a century of rapid glacier retreat and thinning that exposed a growing lake basin. The outburst flood extended the damage of the landslide far beyond the limit of the landslide, destroying forest and impacting salmon spawning and rearing habitat. We expect more cascading impacts from landslides in the glacierized mountains of British Columbia as glaciers continue to retreat, exposing water bodies below steep slopes while simultaneously removing buttressing support.</p>