GM1.5

Late-breaking Session: Hazard cascades from source to sink – the Elliot Creek and Chamoli events

Two significant flow hazard cascades have been captured with unprecedented detail, with events in Elliot Creek and Bute Inlet (Canada) and the Chamoli and Uttarakhand (India) both occurring with the past few months. These events both have a suite of background observations and baseline datasets on which to contextually place and explore these flows end events in a depth and breadth of detail that is unprecedented, potentially unlocking new understanding of hazard cascades from source to sink.
We welcome contributions that (i) investigate the processes of production, mobilisation, transport, and deposition of sediment in these two events, (ii) explore the feedbacks between erosion and deposition of the flows through these systems, (iii) consider how these flows shape new understanding of hazards cascades through the source to sink linkages. We invite papers that are observational, analytical or modelling based in their approach, across a variety of temporal and spatial scales. We particularly welcome new and innovative methodologies that show potential to unlock new understanding.

Co-organized by CR5/NH3/SSP3
Convener: Dan Shugar | Co-conveners: Peter Talling, Sanem Acikalin, Gwyn Lintern, Kristen Cook, Anand K Pandey
vPICO presentations
| Wed, 28 Apr, 13:30–15:00 (CEST)

vPICO presentations: Wed, 28 Apr

Chairperson: Dan Shugar
13:30–13:35
13:35–13:37
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EGU21-16587
Anand Kumar Pandey, Kotluri Sravan Kumar, Virendra Mani Tiwari, Puranchand Rao, Kirsten Cook, Christoff Andermann, Michael Dietze, Marco Pilz, and Niels Hovius

The slope instability and associated mass wasting are among the most efficient surface gradation processes in the bedrock terrain that produce dramatic landscape change and associated hazards. The wedge failure in periglacial Higher Himalaya terrain on 7th February in Chamoli, Uttarakhand (India) produced >1.5 km high rock avalanche, which amalgamated with the glacial debris on the frozen river bed produced massive debris flow along the high gradient Rishi Ganga catchment. The high-velocity debris flow and a surge of high flood led to extensive loss of life and infrastructures and issuing the extreme event flood warning along the Alakananda-Ganga river, despite there was no immediate extreme climatic event. The affected region is the locus of extreme mass wasting events associated with Glacial Lake Outburst Flood (GLOF) and Landslide Lake Outburst Flood (LLOF) in the recent past. We analyzed the landscape to understand its control on the 7th February 2021 Rishi Ganga event and briefly discuss other significant events in the adjoining region e.g. 1893/1970 Gohna Tal/Lake LLOF and 2013-Uttarakhand events in Chamoli, which have significance in understanding the surface processes in Higher Himalayan terrain.

How to cite: Pandey, A. K., Kumar, K. S., Tiwari, V. M., Rao, P., Cook, K., Andermann, C., Dietze, M., Pilz, M., and Hovius, N.: Extreme mass wasting during 2021 Dhauli Ganga event in the Higher Himalaya: insight from the landscape, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16587, https://doi.org/10.5194/egusphere-egu21-16587, 2021.

13:37–13:39
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EGU21-16591
Martin Mergili, Ashim Sattar, Johnathan Carrivick, Adam Emmer, Kyle T. Mandli, Mylène Jacquemart, Scott Watson, Matt Westoby, John J. Clague, Umesh K. Haritashya, and Dan H. Shugar

On 7 February 2021, a 25 million m³ rock/ice avalanche in Uttarakhand, northern India, developed into a far-reaching and devastating debris flow/debris flood, which we refer to as the ‘Chamoli event’. Based on an extensive remote sensing-based geomorphological mapping campaign, the key mechanisms and characteristics of this process chain are largely, but not yet fully understood. Numerical mass flow simulations can help confirm or reject hypotheses regarding spatiotemporal aspects of flow evolution, its magnitude and dynamics, and therefore contribute to a better process understanding. More broadly, geomorphological mapping and numerical modelling of the Chamoli event help us to gain insights of the extent to which we are able to accurately simulate complex high-mountain geohazard process cascades. Such an understanding is invaluable for predictive modelling efforts targeted at informing disaster risk reduction strategies.

In the present work, we back-calculate the flow dynamics of the Chamoli event with three state-of-the-art simulation models operating at different levels of complexity: (i) the one-phase mixture model RAMMS; (ii) the two-phase model GeoClaw, and (iii) the three-phase model r.avaflow. Input parameter sets are optimized against detailed reference data such as mapped trimlines and boulder locations, flow velocities and discharges obtained from video recordings, and erosion/deposition patterns derived by differencing pre- and post-event digital terrain models. The main aims of the study are to: (i) better understand the mechanisms of flow evolution of the Chamoli event; (ii) evaluate the level of model complexity that is necessary for accurating reproducing specific known characteristics of the process chain; and (iii) learn more about the sensitivity of model outputs to differences in initial conditions and model parameters, where these remain uncertain. The findings will facilitate the design of predictive modelling campaigns for hazard mapping purposes.

How to cite: Mergili, M., Sattar, A., Carrivick, J., Emmer, A., Mandli, K. T., Jacquemart, M., Watson, S., Westoby, M., Clague, J. J., Haritashya, U. K., and Shugar, D. H.: Exploring the flow evolution of the Chamoli event (Uttarakhand, India) of 7 February 2021: From geomorphological mapping to multi-model computer simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16591, https://doi.org/10.5194/egusphere-egu21-16591, 2021.

13:39–13:41
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EGU21-16583
Marco Pilz, Fabrice Cotton, Kristen Cook, Michael Dieze, Niels Hovius, Rajesh Rekapalli, Venkatesh Vempati, Ravi Prakash Singh, N. Purnachandra Rao, Davuluri Srinagesh, and Virendra M. Tiwari

Debris flows and corresponding floods are a significant natural hazard for downstream communities in vulnerable regions, as yet unpredictable triggers and remote source locations might cause dynamics which are difficult to measure and quantify. Continuous observational coverage offered by seismic monitoring is one potential avenue for addressing this problem. Displacement of mass at Earth’s surface generates elastic seismic waves, which carry information about the temporal and spatial variability of the source and which can be recorded by seismometers at high temporal resolution across large spatial scales. Here, we report on seismic observations of the destructive 2021 Uttarakhand (India) debris flow and flood events. By means of a dense regional seismic network, we track and quantify the spatial and temporal evolution of the flood. Using continuous time-stamped seismic observations, a coherent signal of the flood movement is observed in a limited frequency band which can be tracked down the valley during the flood duration. Our analysis highlights potential benefits of using a network-wide seismic monitoring systems.

How to cite: Pilz, M., Cotton, F., Cook, K., Dieze, M., Hovius, N., Rekapalli, R., Vempati, V., Singh, R. P., Rao, N. P., Srinagesh, D., and Tiwari, V. M.: Seismic observations of the 2021 Uttarakhand landslide/debris flow and flood events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16583, https://doi.org/10.5194/egusphere-egu21-16583, 2021.

13:41–13:43
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EGU21-16593
Michael Dietze, Himangshu Paul, Anand Kumar Pandey, Rajesh Rekapalli, Puranchand Rao, Srinagesh D., Kirsten L. Cook, Marco Pilz, Niels Hovius, and Virendra Mani Tiwari

The 7 February Chamoli, Uttarakhand singularity imposed a severe geomorphic crisis. While remote sensing imagery quickly identified a major rock avalanche as its origin, there is a fundamental lack in high precision temporal information on the kinetics of this event about when, how, and why it evolved from a slope failure into a channel-confined mass wasting process, and ultimately into a debris laden flood. Furthermore, while the initial rock slide could be detected and located by global seismic networks, it was the flood which caused most of the destruction and fatalities. Yet, that part of the process cascade remained elusive in global seismic data sets.

Here, we present a detailed anatomy of the hazard cascade, with emphasis on the flood part. Using information from a dense seismic network, we explore the limits of detection and constrain its propagation velocity. By jointly inverting two physical models that predict spectral signal properties of floods, we estimate important hydraulic and sediment transport metrics. These information are key for designing any future early warning infrastructure.

How to cite: Dietze, M., Paul, H., Pandey, A. K., Rekapalli, R., Rao, P., D., S., Cook, K. L., Pilz, M., Hovius, N., and Tiwari, V. M.: Anatomy of a cascading hazard: the flood part of the 7 February Uttarakhand event, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16593, https://doi.org/10.5194/egusphere-egu21-16593, 2021.

13:43–13:45
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EGU21-16597
Maximillian Van Wyk de Vries, Shashank Bhushan, David Shean, Etienne Berthier, César Deschamps-Berger, Simon Gascoin, Mylène Jacquemart, Andreas Kääb, and Dan Shugar

On the 7th of February 2021, a large rock-ice avalanche triggered a debris flow in Chamoli district, Uttarakhand, India, resulting in over 200 dead or missing and widespread infrastructure damage. The rock-ice avalanche originated from a steep, glacierized north-facing slope with a history of instability, most recently a 2016 ice avalanche. In this work, we assess whether the slope exhibited any precursory displacement prior to collapse. We evaluate monthly slope motion over the 2015 and 2021 period through feature tracking of high-resolution optical satellite imagery from Sentinel-2 (10 m Ground Sampling Distance) and PlanetScope (3-4 m Ground Sampling Distance). Assessing slope displacement of the underlying rock is complicated by the presence of glaciers over a portion of the collapse area, which display surface displacements due to internal ice deformation. We overcome this through tracking the motion over ice-free portions of the slide area, and evaluating the spatial pattern of velocity changes in glaciated areas. Preliminary results show that the rock-ice avalanche bloc slipped over 10 m in the 5 years prior to collapse, with particularly rapid slip occurring in the summer of 2017 and 2018. These results provide insight into the precursory conditions of the deadly rock-ice avalanche, and highlight the potential of high-resolution optical satellite image feature tracking for monitoring the stability of high-risk slopes.

How to cite: Van Wyk de Vries, M., Bhushan, S., Shean, D., Berthier, E., Deschamps-Berger, C., Gascoin, S., Jacquemart, M., Kääb, A., and Shugar, D.: Resolving pre-collapse slope motion at the February 2021 Chamoli rock-ice avalanche via feature tracking of optical satellite imagery, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16597, https://doi.org/10.5194/egusphere-egu21-16597, 2021.

13:45–13:47
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EGU21-16580
Shuai Li, Hui Tang, Chong Peng, and Hui-Cong An

On 7 February 2021, a massive flood occurred in the river Dhauliganga that damaged two hydroelectric stations, five bridges and trapped 100 to 150 casualties who are feared dead. Some evidence has been indicated that caused by a landslide, an avalanche, or a portion of the Nanda Devi glacier that broke off early in Uttarakhand's Joshimath area Chamoli district. The magnitude of the flood caused by the collapse was so large that it far exceeds the collapse itself. Two potential explanations were proposed to explain: the frictional heating of the avalanche may result in high temperatures in the sliding face, which is sufficient for ice and frozen sediments melting to occur in the path. The high-water content generated debris flows that enhanced the mobility of flowing. Another explanation is that it could be related to a glacial lake outburst flood or a temporary lake that eventually broke through its debris dam and poured down the valley. In any case, the collapse materials hold very high moisture content and fast mobility. In this study, a three-dimensional Smoothed Particle Hydrodynamics (SPH) method is adopted to model the flow-like Uttarakhand slides and to explore the physical processes during this event. The SPH is an adaptive, mesh-free, Lagrangian method that simulates free surfaces, moving interfaces, and large flow deformations. A non-Newtonian debris flow model, the Bingham rheological relationship, was incorporated into the SPH framework to describe source materials' characteristics. Besides, the whole flow processes of the flow-like Uttarakhand slides across the 3D terrain are represented. The time history of the velocity, acceleration, and forces were obtained from modelling to analyze the landslide dynamics.

How to cite: Li, S., Tang, H., Peng, C., and An, H.-C.: Three-dimensional modelling of Uttarakhand slides using smoothed particle hydrodynamics (SPH), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16580, https://doi.org/10.5194/egusphere-egu21-16580, 2021.

13:47–13:49
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EGU21-16586
Stuart Dunning, Simon Gascoin, Dan Shugar, and Wolfgang Schwanghart

The 7th February Chamoli hazard cascade originated from a 25 million m³ rock/ice avalanche slope failure that transformed into a destructive, far travelled debris flow / debris flood. There has a been necessarily a significant science focus on the proximal and immediate part of the hazard cascade. Here we report on the larger spatial and temporal scale: the sediment plume that progressed over the following days and weeks along the Ganga (Ganges) River. At the time of submission this was still recognisable over 900 km from the landslide site and had passed through hydro and nuclear power schemes. Beyond the initial plume, which has implications for rapid sedimentation in hydropower schemes and water / aquatic habitat quality, the subsequent (or not) mobilisation of event sediments over future years is a possible medium term chronic-threat to some hydropower projects. We show spectral ‘recipes’ and semi-automated methods for tracking the mass movement sediment plume and quantifying celerity using Sentinel 2 imagery, infilled using high-temporal repeat optical imagery from Planet Labs. data. The plume averaged ~60 km/day and, as expected has begun to slow as the river gradient decreases, as well as becoming less distinctive as some sediment is deposited, and as other sediment-rich water joins the Ganga.

The tracking of sediment plumes from these hazard cascades can be extended over inventories of similar events using both Sentinel 2 and Landsat archives. Such approaches allow us to provide insight into the possibilities of automated detection of hazard cascade sediment plumes to identify previously unknown events from remote source regions, as plumes have a far larger spatial-temporal footprint than the initial event.

How to cite: Dunning, S., Gascoin, S., Shugar, D., and Schwanghart, W.: Tracking the sediment plume from the 7th February 2021 Chamoli (Uttarakhand, India) hazard cascade, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16586, https://doi.org/10.5194/egusphere-egu21-16586, 2021.

13:49–13:51
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EGU21-16592
Sansar Meena, Akshansha Chauhan, Kushanav Bhuyan, and Ramesh P. Singh

The Himalayan rivers are glacier-fed and are vulnerable to devastating flash floods caused by damming of landslides and outbreak of glacial lakes. On 7 February 2021, around 10:30 am IST, a huge block of glacier mass broke from the Nanda Ghunti glacier. It is evident from the multi-temporal satellite imageries from Planet Scope that snow dust deposited in the affected area. During the course of the event, a huge amount of debris along with broken glacial fragments flooded the Rishi Ganga river and washed away the Hydropower plants; Rishi Ganga and Tapovan, more than 71 people were killed, and about 100 people are still missing. Detailed analysis of optical and radar data has been carried out to show the impact of the rockslide, changes in the surface characteristics of the source region, flood plains of the river and water quality of the Himalayan rivers (Alaknanda and Ganga). We have used five different indices Modified Normalized difference water index (MNDWI), Normalized difference vegetation index (NDVI), Enhanced vegetation index (EVI), Normalized difference turbidity Index (NDTI), and Normalized difference chlorophyll index (NDCI), that show pronounced changes in water quality and flood plain at the four different sections of the river. The spectral reflectance and backscattering coefficients derived from high-resolution Planet scope and Sentinel 1 SAR data show characteristics behaviour of the flood plain and water quality. Further, we have also found changes in the water quality of several canals after the Chamoli disaster event as the flood gates were closed to stop the deposit of sediments in the canal.

How to cite: Meena, S., Chauhan, A., Bhuyan, K., and Singh, R. P.: Impact of the Chamoli disaster on flood Plain and water quality along the Himalayan rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16592, https://doi.org/10.5194/egusphere-egu21-16592, 2021.

13:51–13:53
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EGU21-16589
Wolfgang Schwanghart, Ugur Öztürk, Sumit Sen, Ankit Agarwal, and Oliver Korup

The 7 February Chamoli flood once again unveiled the vulnerability of Himalayan hydropower. On its destructive path downstream, the flood inflicted the loss of two nearby hydropower projects and damaged at least two more projects further downstream.

The flood is the third in a series of events with severe impact on the Himalayan hydropower sector. Uttarakhand was among the Indian states affected most by the 2013 Indian floods. Heavy rain, snow melt, and a glacial lake outburst flood damaged and partly destroyed more than 20 hydropower projects. The Gorkha Earthquake in 2015 led to damages to >30 projects, leading to a temporary loss of 34% of the hydropower generated in Nepal.

Analysis of these events reveals that neither flood discharge nor ground shaking were the primary processes responsible for the losses. Instead, the majority of damage was caused by geomorphological processes including landslides and rockfall, debris flows and extreme sediment discharges.

Only 20% of the ~500-GW hydropower potential is currently tapped in the Himalayas. This share is likely to increase given the high energy demands in the rapidly growing economies of the Himalayan countries.

With many opportune sites along large rivers being already occupied, there is a trend towards developing hydropower further upstream at higher elevations and closer to glaciated areas.

We argue that these developments and the past events highlight the need for a reappraisal of the Himalayan hazardscape. Risk analysis should increasingly incorporate processes such as glacial lake outburst floods and extreme sediment discharge events, and particularly aim to better understand hazard cascades which originate in glaciated and steep headwater catchments.

How to cite: Schwanghart, W., Öztürk, U., Sen, S., Agarwal, A., and Korup, O.: Hydropower in the Himalayan hazardscape, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16589, https://doi.org/10.5194/egusphere-egu21-16589, 2021.

13:53–13:55
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EGU21-16599
Marten Geertsema, Brian Menounous, Dan Shugar, Tom Millard, Brent Ward, Göran Ekstrom, John Clague, Patrick Lynett, Jonathan Carrivick, Pierre Friele, Andrew Schaeffer, Davide Donati, Doug Stead, Jennifer Jackson, Bretwood Higman, Chunli Dai, Camille Brillon, Derek Heathfield, Gemma Bullard, Ian Giesbrecht, Katie Hughes, and Mylène Jacquemart

On 28 November 2020, some 18 Mm3 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. Pre-slide optical and radar remote sensing data indicated some slope deformation leading up to failure. More than half of the rock debris entered a 0.6 km lake, where it generated a 115 m displacement wave that overtopped the moraine at the far end of the lake. We estimate that some 13.5 Mm3 of water left the lake from the combined impact of the landslide as well as erosion of the dam. The water that left the lake was channelized along Elliot Creek, scouring the valley more than 40 m in some places over a distance of 10 km before depositing debris on a 2 km2 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.

How to cite: Geertsema, M., Menounous, B., Shugar, D., Millard, T., Ward, B., Ekstrom, G., Clague, J., Lynett, P., Carrivick, J., Friele, P., Schaeffer, A., Donati, D., Stead, D., Jackson, J., Higman, B., Dai, C., Brillon, C., Heathfield, D., Bullard, G., Giesbrecht, I., Hughes, K., and Jacquemart, M.: Terrestrial overview of a landslide-tsunami-flood cascade at Elliot Creek, British Columbia , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16599, https://doi.org/10.5194/egusphere-egu21-16599, 2021.

13:55–13:57
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EGU21-16594
Ian Giesbrecht, Suzanne Tank, Justin Del Bel Belluz, and Jennifer Jackson

Rainforest rivers export large quantities of terrestrial materials from watersheds to the coastal ocean, with important implications for local ecosystems and global biogeochemical cycles. However, the impact of episodic disturbance on this process is a critical knowledge gap in our understanding of land-sea connections. Fjords represent a global hotspot for terrestrial carbon burial in marine sediments, yet the relative importance of typical riverine fluxes vs. mass wasting fluxes is uncertain and dynamic. Similarly, mass wasting events can generate both an instantaneous pulse and a sustained shift in the material export regime. Riverine sediment regimes also have important implications for freshwater ecosystems and fisheries resources. A recent mass wasting event in Bute Inlet – Homalco First Nation traditional territory and British Columbia, Canada – presents an important opportunity to quantify the sustained impact of such an infrequent large disturbance on the source-to-sink linkages between glacierized mountains, rivers, and fjords.

On November 28, 2020, a landslide in the headwaters of the Elliot Creek watershed (118 km2) triggered a glacial lake outburst flood (GLOF) that eroded 3 km2 of forested land and exported large volumes of water and terrestrial materials to the lower reaches of the Southgate River watershed (1986 km2) and ultimately to the head of Bute Inlet. Here we assess river and ocean surface turbidity over four winter months following the event, in comparison to pre-event measurements taken across all seasons in recent years. River turbidity was measured on the Southgate River above and below the confluence of Elliot Creek, beginning in December 2020, and at the mouth of the Southgate and nearby Homathko Rivers prior to November 2020. Bute Inlet turbidity was measured (every month to two months) starting in May 2017.

Prior to the GLOF event, Southgate River turbidity ranged from a low of 3.3 ± 0.4 FNU in the winter to a high of 71.4 FNU in the summer meltwater period. Since the event, Southgate River turbidity has been consistently elevated ≥6 times background levels recorded above Elliot Creek. At the extreme, on January 13, 2021, seven weeks after the GLOF, Southgate River mean turbidity (105.2 ± 3.3 FNU) was 32 times the background (3.3 ± 0.4 FNU), equating to a sustained increase in wintertime turbidity that sometimes exceeds the historical summertime peak. Given the typical coupling of turbidity with discharge, we expect further increases in turbidity with the coming freshet of 2021; the first meltwater season following the GLOF. These results suggest the potential for a sustained shift in the seasonal turbidity regime of the Southgate River and the estuarine waters of Bute Inlet. The elevated turbidity signals broader changes to: sediment export and carbon burial, the depth and seasonality of light penetration, river water quality, and spawning habitat quality for anadromous fish. Ongoing monitoring will be used to characterize the duration, dynamics, and potential recovery of elevated turbidity regimes across the land-to-ocean aquatic continuum in Bute Inlet.

How to cite: Giesbrecht, I., Tank, S., Del Bel Belluz, J., and Jackson, J.: Sustained Impact of a Glacial Lake Outburst Flood on Winter Turbidity Regimes across the Land-Ocean Aquatic Continuum, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16594, https://doi.org/10.5194/egusphere-egu21-16594, 2021.

13:57–13:59
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EGU21-16595
Michael Tilston, Dan H. Shugar, Michael Clare, Maarten Heijnen, Sanem Acikalin, Matthieu Cartigny, Peter Talling, Daniel Parsons, Gwyn Lintern, Cooper Stacey, Stephen Hubbard, Sophie Hage, Daniel Bell, John Hughes Clark, Ian Giesbrecht, Jenifer Jackson, and Brian Menounos

Submarine systems where the canyon head is directly connected to the river mouth arguably provide the best setting for in situ studies of turbidity currents since the sediment supply propelling them arrive in periodic pulses linked to fluvial freshet events. Consequently, the frequency of, and similarity between, the turbidity currents flowing through these systems make it easier for their channel morphology to evolve towards a state of dynamic equilibrium. Therefore, if an extreme event occurs that dramatically alters the system’s sediment supply, it is reasonable to assume that submarine channels will undergo a period of rapid adjustment. This is the present scenario occurring in Bute Inlet following the recent Elliot Creek hazard cascade. Bute Inlet is one of the most actively monitored sites for turbidity currents in the world, and the extensive historical dataset that has been amassed at this site along with the rare Elliot Creek event provides the unique opportunity to study the impacts of extreme allogenic forcing mechanisms on the morphodynamics of submarine channels.

Preliminary measurements indicate that the turbidity in Elliot Creek has increased by ~40x compared to pre-slide measurements, and oceanographic measurements within a few days of the event show very high turbidity in ocean bottom water to a distance of almost 70 km from the delta. While the bathymetric survey since the landslide is so far constrained to the proximal region of the inlet, early results show that channel morphology was rapidly altered. Specifically, the submarine channel fed by Southgate River, which supplied water and sediment from the landslide and glacial outburst flood, was lowered by about 3m across the width of the channel bed. Conversely, the morphology of the channel fed by Homathko River has remained static between the 2020 and 2021 surveys. Below the confluence of these two submarine channels, the cyclic steps that once dominated the bed morphology appear to have been largely infilled by a 1-2m thick drape of sediment along the inner half of the channel bend, whereas the outer banks have laterally eroded by upwards of 50m at some points. This trend of channel widening and lateral migration appear to be propagating down the system. Importantly, the nature of the slide suggests that sediment delivery will remain elevated with respect to background conditions for decades into the future, suggesting that the submarine channel may be in the process of adapting to an entirely new flow regime rather than reacting to a singular extreme flow event.

How to cite: Tilston, M., Shugar, D. H., Clare, M., Heijnen, M., Acikalin, S., Cartigny, M., Talling, P., Parsons, D., Lintern, G., Stacey, C., Hubbard, S., Hage, S., Bell, D., Clark, J. H., Giesbrecht, I., Jackson, J., and Menounos, B.: Effects of extreme events on the morphology of submarine channels: the case of the Elliot hazard cascade, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16595, https://doi.org/10.5194/egusphere-egu21-16595, 2021.

13:59–14:04
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EGU21-16596
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solicited
Sophie Hage, Sanem Acikalin, Lewis Bailey, Matthieu Cartigny, Michael Clare, Ye Chen, Valier Galy, Maarten Heijnen, Kate Heerema, Stephen Hubbard, Jennifer Jackson, Gwyn Lintern, Dan Shugar, Stephen Simmons, Cooper Stacey, Peter Talling, Michael Tilston, Daniel Parsons, and Ed Pope

It is often assumed that particles produced on land (e.g., sediment, pollutants and organic matter) are transported through watersheds to a terminal sediment sink at the seashore. However, terrestrial particles can continue their journey offshore via submarine channels, accumulating in abyssal plains of the oceans. Offshore sediment transport processes are key controls on the burial of organic carbon and the distribution of benthic food, yet they are challenging to study due to the difficulty of capturing usually short duration events within large-scale systems at great ocean depths. Fjords are sufficiently small scale to enable their submarine channel systems to be studied from river source to terminal sink on seafloor fans. Bute Inlet is an up to 650 m deep fjord in British Columbia, Canada. The Homathko and Southgate rivers both feed Bute Inlet with freshwater and terrestrial sediment. A large landslide occurred on 28th November 2020, which caused a Glacial-Lake Outburst Flood (GLOF) which breached a moraine-dam and transported huge volumes of material through the Southgate valley and into Bute Inlet. The impact of this recent event on the submarine system in Bute is, for now, poorly constrained but ongoing work is exploring the impact of this major event on the Inlet. Bute Inlet is one of the most studied fjords worldwide, with a range of offshore campaigns that have been conducted during the last seventy years, providing an unprecedented background dataset and thus opportunity to explore what impact a large magnitude, low frequency terrestrial event had on the submarine system. This presentation will provide an overview of the past research conducted on the Bute submarine channel system, under more usual river discharge conditions and compare this background context to the recent GLOF event.

Previous studies have revealed that the floor of the Inlet is characterized by a 40 km long submarine channel formed by submarine avalanches of sediment (turbidity currents) that can be up to 30 m thick and reach velocities of up to 6.5 m/s. Based on time-lapse bathymetric mapping over 10 years, the evolution of this channel is known to be controlled by the fast (100 to 450 m/yr) upstream migration of 5 to 30 m high steps (called knickpoints) in the channel floor. Sediment cores reveal that the channel floor and proximal lobe are dominated by sand and up to 3 % of total organic carbon in the form of young woody debris. Research in Bute Inlet has thus allowed submarine flow processes, seafloor morphology and deposits to be linked in unprecedented detail. Using those past results as a baseline, new data collected after the GLOF will be crucial for testing the impact of high-magnitude catastrophic events on a marine system and the ultimate sink for the terrestrial material. Understanding what impact the GLOF had on the usual seafloor processes has direct implications for the preservation of benthic communities living in the fjord and for the global carbon cycle.

How to cite: Hage, S., Acikalin, S., Bailey, L., Cartigny, M., Clare, M., Chen, Y., Galy, V., Heijnen, M., Heerema, K., Hubbard, S., Jackson, J., Lintern, G., Shugar, D., Simmons, S., Stacey, C., Talling, P., Tilston, M., Parsons, D., and Pope, E.: A field-scale laboratory to study particulate transport from river source to marine sink: Bute Inlet (Canada), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16596, https://doi.org/10.5194/egusphere-egu21-16596, 2021.

14:04–15:00