GM1.1 | Frontiers in Geomorphology
EDI
Frontiers in Geomorphology
Including GM Division Outstanding ECS Award Lecture
Convener: Kristen Cook | Co-conveners: Laure Guerit, Aayush SrivastavaECSECS, Philippe Steer
Orals
| Thu, 18 Apr, 10:45–12:30 (CEST)
 
Room D3
Thu, 10:45
Plenary geomorphology division session and ECS award lecture. This session will consist of the Geomorphology Early Career Scientist Award winner’s lecture and additional invited talks on related topics. More information when the ECS award winner is announced.

Orals: Thu, 18 Apr | Room D3

Chairperson: Kristen Cook
10:45–10:50
10:50–11:20
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EGU24-6103
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ECS
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solicited
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Highlight
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GM Division Outstanding ECS Award Lecture
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On-site presentation
Fiona Clubb

Quantifying landscape form can provide crucial insight into the interactions between tectonics and climate. River long profile morphology, quantified by metrics such as channel steepness, is the most commonly used tool to investigate topographic form, with many studies relating long profile morphology to uplift rate, precipitation, sediment properties, or lithology, for example. River long profiles record the signal of external forcing over large spatial scales (i.e. tens of kilometres). This has many advantages: for example, it is a convenient scale for analysing variations in large-scale processes, such gradients in tectonic uplift. It also means that high resolution digital elevation models (DEMs) are not required and therefore river long profiles can be extracted globally. However, analysis of river long profiles over tens of kilometres can also result in signal smoothing and subsequent loss of finer scale tectonic or climatic signatures encoded into the landscape.

Tectonic and climatic processes do not only leave their fingerprint in the long profiles of rivers. Hilltops, hillslopes, and valleys make up the majority of Earth’s landscapes by area, yet their morphology has received much less attention than that of rivers. This is in part due to the difficulty in accurately extracting hilltops and valley morphology from DEMs, especially on a global scale. Here, I show that we can now extract hilltop and valley metrics from high-resolution (< 15 m) DEMs over orogenic to continental scales using new topographic analysis techniques and high-performance computing facilities. I argue that by combining hilltop, hillslope, and valley metrics, we can obtain more information about tectonic and climatic processes than from river profiles alone.

How to cite: Clubb, F.: Going beyond the river long profile, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6103, https://doi.org/10.5194/egusphere-egu24-6103, 2024.

11:20–11:30
Solicited presentations - 2022 and 2023 OSPP Award winners
11:30–11:40
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EGU24-849
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ECS
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On-site presentation
Davide Tognin, Alice Puppin, Massimiliano Ghinassi, Andrea D'Alpaos, and Alvise Finotello

Tidal channels are key players in ecogeomorphological dynamics of coastal wetlands, controlling the exchange of water, sediment and nutrient fluxes between low-lying coastal areas and the open sea. Traditionally seen as stable features, the importance of migrating and abandoned channels has been often overlooked. Yet, the constant evolution of tidal channels significantly influences sediment reworking in coastal wetlands and reduced flow velocities within abandoned channels promote particle settling thus rapidly storing large volumes of sediment. Moreover, reach abandonment may lead to the reorganization of the channel network, thus leading to sudden changes in flux dynamics at the basin scale. Hence, the characterization of channel-fill deposits is a critical step for a better understanding of the mechanisms that lead to channel abandonment and consequent network reorganization.

To this aim, we conducted a detailed analysis of the sedimentary features and the related depositional processes in abandoned tidal channels in the microtidal Venice Lagoon, Italy. We collected undisturbed sedimentary cores within abandoned channels identified from aerial images taken in the last 70 years. Cores were longitudinally cut and photographed for classical sedimentary facies analysis and identification of the main depositional environments. Cores were then subsampled at 5 cm intervals and prepared for different laboratory analyses, including organic matter estimation through Loss-On-Ignition and particle size distribution using laser diffractometry.

By combining facies and laboratory analyses, different depositional environments can be readily identified and characterized. The deposits accumulated during the channel abandonment phase are usually floored by a layer rich in shell and shell fragments related to the channel lag, which in most cases overlies sandy-laminated tidal flat deposits. Instead, channel-fill deposits consist of dark grey, organic-rich mud without any visible sedimentary structures. Massive mud can be locally mixed with very fine sand, and close to the lag they tend to become richer in coarser fractions. Moreover, channel-fill deposits are characterized by abundant submillimetric vegetation debris and by an almost constant organic matter content all along the deposit thickness. Our analysis provides a distinctive characterization of the features of channel-fill deposits and is foundational to the comprehension of the processes that lead to channel abandonment and infill.

How to cite: Tognin, D., Puppin, A., Ghinassi, M., D'Alpaos, A., and Finotello, A.: Characterization of channel-fill deposits in coastal wetlands, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-849, https://doi.org/10.5194/egusphere-egu24-849, 2024.

11:40–11:50
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EGU24-6586
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ECS
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On-site presentation
Dominique Townsend, Julian Leyland, Hachem Kassem, Charlie Thompson, and Ian Townend

There has been limited exploration of the nearshore zone of mixed sediment beaches despite being widespread globally.  Our work aims to build a more complete picture of coastal change by looking at seabed evolution from the very upper reaches of the swash zone, down to the edge of the nearshore zone, at a variety of timescales. Bathymetry surveys, completed using both traditional and automated surface vessels collected single and multibeam sonar data over a 19-year period, were complimented by a shorter period of weekly radar sea surface roughness images which are indicative of nearshore bed morphology. Additionally, grab samples were collected from across the nearshore to show the bed sediments composition variation over a year, providing valuable insight on bed response to varying hydrodynamic conditions. Process based analysis gave long and cross shore drift rates to help understand the observed changes. 

We found that the nearshore zone experiences significantly larger volumetric bed changes in comparison to the upper beach but were limited to approximately +/-0.3 m, which is equivalent to the upper error limits of the surveyed data. The depth of closure, a term used to mark the offshore point of no change over a defined time scale, varied across the mixed sediment bay at both seasonal  (ranging between –4.7 and –8.4 metres Ordnance Datum(mOD)) and decadal (ranging between –7.3 and –8.2 mOD) timescales, yet was consistently shallower than all predictive equations of this depth. Moreover, our results indicate a loss of volume in the nearshore zone over time which is coupled with a simultaneous steepening of the upper beach for two thirds of the frontage.  The observed steepening poses questions for the current ‘hold the line’ management strategy, which is achieved through active beach management works, and accommodation space.  

Overall, our observations highlight the substantial sediment transport occurring within the predominantly sandy nearshore zone and demonstrate evidence of impacts to the upper shingle beach. Consequently, to ensure the sustainability of management practices in the upper beach, a comprehensive understanding of the nearshore dynamics becomes imperative. The presented findings emphasize the necessity of integrating nearshore considerations into coastal management strategies, providing a more holistic and effective approach to making sustainable management decisions. 

How to cite: Townsend, D., Leyland, J., Kassem, H., Thompson, C., and Townend, I.: Observations from a constantly changing nearshore zone in a mixed sediment bay , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6586, https://doi.org/10.5194/egusphere-egu24-6586, 2024.

11:50–12:00
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EGU24-15791
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ECS
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On-site presentation
Anne Guyez, Stephane Bonnet, Sebastien Carretier, Clare Wilkinson, Tony Reimann, Kevin Norton, and Jakob Wallinga

Luminescence is a powerful dating technique that is increasingly being used as a new tool to investigate surface processes. In the past decade, it has been used successfully for instance to estimate virtual sediment velocity in rivers, sediment storage time in floodplains, and to trace sediment sources (McGuire & Rhodes, 2015; Gray et al., 2018; Sawakuchi et al., 2018; Guyez et al., 2022; Guyez et al., 2023). As part of ongoing development, here we quantify sediment transport and catchment-wide erosion rates in natural systems using luminescence. For this purpose, single-grain post infrared-infrared (pIRIR) equivalent doses of felspars from modern floodplain deposits were measured in several catchments in the Southern Alps of New Zealand and compared to catchment-wide erosion rates derived from 10Be cosmogenic nuclide concentrations measured in fluvial quartz grains.

The fraction of grains that were well-bleached before their burial in the modern floodplain was calculated, as well as the fraction of grains with a saturated luminescence signal. Signal distribution was characterised using the central age model.

Our findings indicate that the luminescence signal is characterized by few well-bleached grains and lots of grains with a high luminescence signal where erosion rates are high. On the other hand, in catchments with lower erosion rates, bleaching appears to be more pervasive, resulting in an overall lower luminescence signal. Therefore, we hypothesize that bleaching efficiency is related to erosion rates.

To test the relationship between luminescence, bleaching, erosion, and transport processes, we include the luminescence signal of individual grains in a landscape evolution model that already takes into account the concentration of cosmogenic nuclides (Carretier et al., 2023). By tracking both signals in an evolving stream basin, the model helps better understanding the relationship between erosion and luminescence signal, on longer time scales. Single-grain pIRIR equivalent holds promise as a new method for measuring erosion and sediment transport.

How to cite: Guyez, A., Bonnet, S., Carretier, S., Wilkinson, C., Reimann, T., Norton, K., and Wallinga, J.: Using single-grain feldspar luminescence signal and modelling to study landscapes erosion rates and sediment transport, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15791, https://doi.org/10.5194/egusphere-egu24-15791, 2024.

12:00–12:10
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EGU24-15955
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ECS
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On-site presentation
Amalie Skålevåg, Oliver Korup, and Axel Bronstert

Suspended sediment poses a risk to human and natural systems in terms of compromising water quality, flood hazard, hydropower production, and aquatic habitats. In many rivers the bulk of annual suspended sediment yield is mobilised and transported during (extreme) episodic runoff. Understanding such sediment-discharge events, including their drivers, may inform management strategies aimed at mitigating potential detrimental effects. Event-based analysis of local time series of suspended sediment transport has become a common approach to infer the dominant drivers and processes of sediment dynamics at the catchment scale. The increasing availability of detailed and continuous monitoring time series data enables us to use machine-learning techniques to identify groups of similar events, i.e. event types, and test whether and how these groups reflect similar catchment conditions and hydro-meteorological drivers. 

We present an approach which automatically detects, characterises and clusters sediment-discharge events. Hydrograph separation is used to automatically detect events, which are then filtered based on suspended sediment magnitude. The detected events are subsequently characterised with a selection of metrics, which are transformed into uncorrelated event characteristics with principal component analysis. Based on these characteristics events are clustered using a Gaussian mixture model. Finally, the identified event types are interpreted using catchment metrics describing antecedent conditions, hydrometeorological forcing, and catchment freezethaw state and snowcover.

Applying our approach to a high alpine, glaciated catchment we find that the event regime in the catchment is mainly defined by event magnitude, hysteresis and event shape complexity. However, for the clustering suspended sediment and streamflow magnitude, and event shape complexity are the most important factors, whereas sediment discharge hysteresis is less relevant. The four identified event types are attributed to (1) compound rainfall-melt extremes, (2) glacier and seasonal snow melt, (3) freezethaw-modulated snow-melt and precipitation events, and (4) late season glacier melt. 

Our approach enables event-based analysis of riverine sediment fluxes, by detecting and grouping similar events together, which can in turn be interpreted to understand under which conditions episodic sediment fluxes occur in the target catchment.

How to cite: Skålevåg, A., Korup, O., and Bronstert, A.: Identifying sediment-discharge event types with a data-based clustering approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15955, https://doi.org/10.5194/egusphere-egu24-15955, 2024.

12:10–12:20
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EGU24-20657
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ECS
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On-site presentation
Janbert Aarnink, Florent Rouge, Aldo Fornari, and Virginia Ruiz-Villanueva

Floods in mountain rivers cause significant geomorphic changes and may entrain and transport large quantities of sediment and wood (uprooted trees). Although large wood contributes to healthy river ecosystems by providing and forming habitats for various species, it can also pose additional risks. In particular, the wood can block critical infrastructure. Governed by its density, the buoyancy of wood allows it to float and generally move at a velocity similar to the river flow. However, wood movement is complex due to interactions with the flow and the influence of turbulence and drag forces. Moreover, contact with other wood pieces can alter the forces on the floating wood. Observations of wood transport during floods are very rare, and this study aims to address this by monitoring the motion of wood during large-scale experimental floods in the Spöl River. 

The Spöl River, partially situated in the Swiss National Park, is controlled by two upstream dams, the Punt dal Gal and Ova Spin. As part of a restoration project, the company responsible for the dams releases environmental floods annually. This study monitored the movement of floating wood pieces entrained during these releases using a combination of methods. 

During the flood in June 2023, three drones were flown simultaneously. Each drone flew a maximum of 15 minutes, which was done five times between 09:30 and 11:45 in the morning. Each drone captured 24 frames per second with a resolution of 3840x2160 pixels. At an average height of 60 meters, this resulted in an average pixel size of 2.3 centimetres. Also, 6 topographical cross-sections, distributed over the observed river section were taken before and after the flood. 

The flood caused important geomorphic changes along the riverbed. In some of the surveyed sections, erosion was significant, whereas in others, aggradation was the dominant process. The video footage included approximately 90 pieces of floating large instream wood with and average length of 3 meters. When comparing the wood movement with the cross-sections, we observed that the wood pieces did not necessarily follow the river thalweg or deepest part of the channel. Local morphology, flow velocity and turbulence played a large role in the wood trajectory. In addition, the size, trajectory and rotation of wood are being analysed and compared with the flow field and local topography. 

 

This study provides unique insights into the behaviour of individual floating wood pieces transported during a flood. The findings will be valuable for comparison with numerical models and will help improving our understanding of large wood dynamics in rivers. 

 

This work is funded by the Swiss National Science Foundation project PCEFP2_186963, and supported by the University of Lausanne, the Swiss Academy of Sciences, the Swiss National Park, and the Engadiner Kraftwerke.

How to cite: Aarnink, J., Rouge, F., Fornari, A., and Ruiz-Villanueva, V.: UAV-based tracking of floating wood during a flood , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20657, https://doi.org/10.5194/egusphere-egu24-20657, 2024.

12:20–12:30