GM1.5

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.

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.

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.

On the 7^th 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.

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.

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 km²  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.

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.

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.

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 28^th 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.