Vous êtes tous invités à assister à la conférence présentée par :
Monsieur Michael Tilston
(réf. : candidat pour le recrutement d'un membre du corps professoral - Poste de professeur en processus côtiers)
Titre de la conférence : The impacts of climate change on Canadian coastlines : a case study of the 2020 Elliot Creek hazards cascade at Bute Inlet, British Columbia
Mardi 5 septembre 2023
Conférence : 13h30 – 14h15
Période de questions : 14h15 – 14h30
Participer à la réunion Zoom : https://INRS.zoom.us/j/81367970972
ID de réunion : 813 6797 0972
Canada is facing some especially complicated coastal management issues since it has the longest coastline in the world spanning a range of climatic and geologic zones, each with their own unique set of challenges. For example, the reduced temporal coverage of winter sea-ice and the anticipated uptick in the frequency and intensity of seasonal storms mean that sandy beaches and deltas are more prone to coastal erosion and flooding. In Northern Canada where the rate of climate warming is expected to be strongest, coastal erosion manifests in the form of retrogressive slides and slumps due to the liquefaction of soils as permafrost thaws. Conversely, the rapidly melting glaciers in the coastal mountains of British Columbia are projected to cause a three-fold increase in the volume of water stored in glacial lakes, and the inevitable outburst floods that they generate have the potential to trigger a cascading set of geohazards spanning from the mountain tops to the deep oceans. But despite these vastly different environments, the overarching questions from a management perspective remain the same: what are the relative rates of sediment being imported to, stored within, and exported from the coastline?
My research program utilizes experimental, numerical and in situ studies to address this question. Experimental studies provide the ideal setting to study fundamental processes relating to sediment transport and bedform development, and the fact that journals like Nature are still publishing revisions to Shield’s (1936) seminal work on sediment entrainment in simple, clear-water currents demonstrates that this remains an active topic of scientific inquiry. Unsurprisingly, sediment transport processes in flows with high suspended loads like debris floods, storm surges and turbidity currents are substantially more complex as they require either novel equipment (e.g.: CT scanners) or new analytical techniques (acoustic backscatter inversion) to simultaneously measure their velocity and concentration profiles. The latter is especially important as my research hasdemonstrated that high sediment loads alter the forces acting on the bed. Consequently, widelyused diagnostic tools like the bedform stability diagram cannot be confidently used to reconstruct paleoflow conditions from the depositional record. Indeed, this fundamental experimental work on sediment transport processes guides the numerical modelling aspect of my research. Here, the goal is to use stochastic modelling techniques to estimate sediment fluxes from the landscape to the seascape over decadal to geologic timescales, and given the uncertainty associated with the model inputs in the distant past or the near future, probabilistic estimates on coastal sediment fluxes are vital for assessing adaptation strategies. I anticipate much of my research at the INRS would focus on experimental research designed to further constrain these uncertainties. However, I’m also keen to continue collaborating with my colleagues on in situ studies of complex geohazard cascades events driven by climate change in Western Canada, which is the basis of my most recent research endeavor outlined below.
Glacial Lake Outburst Floods (GLOFs) devastate the downslope regions in terrestrial environments, but little is known about how they alter the seaward section of the system, where turbidity currents rather than rivers are the main agent of geomorphic change. Normal turbidity currents are typically triggered by the re-mobilization of sediments in the near-shore environment, but gigantic turbidity currents require much larger sediment inputs by rare, catastrophic events like earthquakes. Quantifying the impacts of these events is challenging as submarine canyons are the planet's least monitored sediment transporting systems, and the probability of a catastrophic current occurring at one of the few existing monitoring stations is small. Here we show that a GLOF in November 2020 triggered a catastrophic turbidity current at Bute Inlet, British Columbia, that became progressively more erosive as it travelled downfjord, increasing its sediment load by a factor of ~2.5×. These observations support previous theoretical work indicating that turbidity currents have an intrinsic density limit and become catastrophic once exceeded, whereupon they self-accelerate, becoming faster, more erosive, and denser downslope. These findings also appear to contradict conventional wisdom, where the lobe is built primarily by the largest events; instead, the very largest events may be catastrophic in nature, where they grow as they travel downslope and reach far beyond the lobe. Moreover, the risk posed by these truly gigantic, self-accelerating turbidity currents may increase in some glaciated margins as the number of glacial lakes increases as a result of climate change.