GM2.9 | From the field to the laboratory to better understand environmental flow processes

GM2.9

EDI

The Earth's surface is shaped by many processes occurring over a wide range of time and length scales, all of which are interdependent with each other. Unraveling this complex system is challenging, especially because of the wide range of scales involved, which makes observation difficult. However, in recent years, major advances in understanding are being driven by new methods (e.g. by using fibre optic cables, or via environmental seismology) as well as the use of simplified and controlled experiments, which is widely used to monitor isolated processes or their interactions. Field observations may provide new opportunities to design and adapt the laboratory scale experiments, while laboratory experiments will help in better interpreting field observations. Together results from field and laboratory will provide insights to test and refine numerical models.

Thus, this session aims to bring together researchers from different communities that on one hand are working in the laboratory to reproduce and on the other hand are using novel field methods to monitor the natural processes in various systems and on various scales.

We welcome contributions but not limited to:
- fluvial and coastal systems
- aeolian processes and arid environments
- systems associated with melting, dissolution and precipitation
- gravity-driven flows
Finally, we particularly encourage participation from students and early career scientists.

Co-organized by NP3
Convener: Pauline Delorme | Co-conveners: Jakob HöllriglECSECS, Katrina Kremer, François MettraECSECS, Cyril Gadal, Anne BaarECSECS, Andrew Gunn
Orals
| Mon, 24 Apr, 14:00–15:45 (CEST)
 
Room G1
Posters on site
| Attendance Mon, 24 Apr, 16:15–18:00 (CEST)
 
Hall X3
Orals |
Mon, 14:00
Mon, 16:15

Orals: Mon, 24 Apr | Room G1

The oral presentations are given in a hybrid format supported by a Zoom meeting featuring on-site and virtual presentations. The button to access the Zoom meeting appears just before the time block starts.
Chairpersons: Pauline Delorme, François Mettra, Cyril Gadal
14:00–14:05
Lab experiments
14:05–14:25
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EGU23-1787
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GM2.9
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solicited
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Highlight
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On-site presentation
Douglas Jerolmack and Nakul Deshpande

It is now well established that many lansdcapes are organized to be close to the threshold of sediment motion: rivers, wind-blown dunes and hillslopes. 

Whether explicitly or implicitly, this threshold is almost universally treated as a Mohr-Coulomb failure criterion, which is an opaque barrier that prevents us from viewing and understanding motion beneath the yield point. Below-threshold motion is creep, and the dynamics are creepy indeed: typical continuum descriptions break down, and observed behaviors can be counterintuitive. 

In this talk I present two experiments, using two different optical techniques, that study very slow particle motions below the threshold of motion. Experiments in a scaled-down river use refractive-index matched scanning to image the interior of a sediment bed sheared by a fluid, and track particles over many orders of magnitude in velocity to show that creep is activated deep into the sediment bed. This creep hardens the bed and drives segregation. Tracking creeping grains becomes impractical, however, as it takes several months to measure the slowest particle motions. 

To overcome these simplifications and expand our study of creep, we examine an apparently static sandpile that is isolated from external disturbance. Instead of particle tracking, we use an optical interferometry technique called Diffusive Wave Spectroscopy (DWS) that allows us to measure creep rates as low as nanometers/second. Viewed through the lens of DWS, the model hillslope is alive with motion as internal avalanches of grain rearrangements flicker throughout the pile. We observe similar dynamics to those observed in the river experiment -- albeit over much shorter timescales -- even though the only significant stress is gravity. What causes these grains to creep below their angle of repose? Observations suggest that minute mechanical noise may play a role, but reducing the noise floor beyond our fairly quiescent conditions is very challenging. Instead, we raise the driving stresses through heating, tapping and flow. 

The observations lead to new view of sediment creep as relaxation and rejuvenation of a glassy material, where mechanical noise plays a role akin to thermal fluctuations in traditional glass materials. Sub-yield deformation is a new world to explore, for those patient enough to look for it. 

How to cite: Jerolmack, D. and Deshpande, N.: The seedy underbelly of yield: how measuring verrrrrry slow grain motion changes our view of landscapes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1787, https://doi.org/10.5194/egusphere-egu23-1787, 2023.

14:25–14:35
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EGU23-12076
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GM2.9
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ECS
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Highlight
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On-site presentation
Eric Deal, Jeremy Venditti, Santiago Benavides, Ryan Bradley, Qiong Zhang, Ken Kamrin, and Taylor Perron

Predictions of bed load sediment flux are notoriously imprecise despite widespread occurrence and importance in contexts ranging from river restoration to planetary exploration. Natural variations in grain size, shape and density are possible sources of inaccuracy in sediment transport, as well as mixtures of different grain sizesand time-dependent bed structure. While many of these effects have been studied in depth, the effects of grain shape have rarely been quantified, even though shape has long been hypothesized to influence sediment transport.

During bed load transport, the granular bed is sheared by the flow passing over it. Aspherical grains and rough surfaces generally increase the resistance to such shearing, enhancing frictional resistance, and reducing the efficiency of bed load transport. However, aspherical grains also experience higher fluid drag force than spherical grains of the same volume, enhancing transport efficiency under the same flow conditions. These two competing effects generally get stronger as grain shape deviates from spherical, making it challenging to predict the net effect of grain shape on sediment transport. We disentangle these competing effects by formulating a theory that accounts for the influence of grain shape on both fluid-grain and grain-grain interactions. It predicts that the onset and efficiency of transport depend on the average coefficients of drag and bulk friction of the transported grains. Because we use the average statistics of drag and friction to characterize the effect of grain shape, our approach is also applicable to materials like natural gravel that have many different shapes in the same sample.

Using a series of flume experiments with different granular materials of distinct shapes, we show that grain shape can modify bed load transport rates by an amount comparable to the scatter in many sediment transport data sets. Our data also demonstrates that, although bed load transport of aspherical grains is generally inhibited by higher bulk friction and enhanced by higher fluid drag, these two effects do not simply cancel each other. This means that the effect of grain shape on sediment transport can be difficult to intuit from the appearance of grains, with the possibility for grain shape changes to lead to either a reduction or an enhancement of sediment transport efficiency.

How to cite: Deal, E., Venditti, J., Benavides, S., Bradley, R., Zhang, Q., Kamrin, K., and Perron, T.: Grain shape effects in bed load sediment transport, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12076, https://doi.org/10.5194/egusphere-egu23-12076, 2023.

14:35–14:45
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EGU23-8707
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GM2.9
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ECS
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On-site presentation
Jack Moss and Romeo Glovnea

Granular material is nearly ubiquitous in nature.  Some examples include sand, soil, snow, rocks; even the interactions of ice burgs and floes can reasonably be considered as large-scale particle interactions.  It is well accepted that continuum-scale behaviour of a granular body is determined by the grain-scale interactions of its constituent particles, but there is still much to learn regarding those grain-scale interactions and their relationship with continuum-scale inputs.  Vibrating granular beds are a good case study for examining this, since differing flow regions generally form within the bed – depending on both the nature of the vibrations and granular material – and the test conditions can be repeated accurately in a laboratory. 

In this experimental study, various beds of spherical glass beads were subjected to sinusoidal horizontal vibrations of various amplitude and frequency combinations.  The granular beds were framed as quasi two-dimensional: the particles were three-dimensional, contained within a thin transparent tank such that phenomena could only occur in two dimensions.  The tests were designed to provide insight into the grain-scale interactions within granular materials.  That is: how do various load inputs and granular compositions affect general grain-scale response, and in turn, how does this grain-scale response affect the continuum-scale behaviour of the material?

Grain-scale interactions were compared between differing granular beds undergoing equivalent vibrations.  The results are used to discuss how behavioural response of granular material to macro-scale inputs is ultimately tied to the geometric complexity of the internal packing structure and the corresponding network of contact forces that packing structure lends itself to.  The concept of ‘geometric compatibility’ between particles within any granular medium is discussed as an explanation for large behavioural differences between grain-scale, and by extension continuum-scale, responses to vibration – or indeed any mechanical work a granular material is subjected to.

How to cite: Moss, J. and Glovnea, R.: Grain-scale geometry and force networks in general granular materials, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8707, https://doi.org/10.5194/egusphere-egu23-8707, 2023.

14:45–14:55
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EGU23-15044
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GM2.9
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ECS
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On-site presentation
Joon-Young Park, Enok Cheon, Seung-Rae Lee, Jinhyun Choo, Hwan-hui Lim, and Ye-eun Nam

A centrifuge model test platform was designed and developed to verify the critical continuous rainfalls triggering shallow landslides in natural slopes. Based on literature reviews, in-situ dimensions of shallow landslides on natural slopes were determined to 40 m (Length) × 16 m (Width) × 2 m (Depth) on average. In consequence, considering the model mounting space of the centrifuge test facility, a gravity level was decided (N = 40g) so that the length of a model slope equals 1 m according to scaling law. The width and depth of the model slope were hence determined to 0.4 m and 0.05 m, respectively. On the other hand, a rainfall simulator comprised of a series of air-atomizing spray nozzles was designed and developed considering scaling laws of rainfall infiltration and subsurface water flows. As a simulation result in a 40g condition, rainfall dispersions reduced and its trajectory bending induced by Coriolis’ force was almost vanished. After the development of centrifuge model test platform, several 1g performance tests of the rainfall simulator were conducted to test the spatial uniformity of rainfall distributions and fit the conditions of applying water and air pressures to rainfall intensities. The study also presents preliminary test results of shallow landslides in a 1g condition conducted to find and solve errors and unexpected problems before mounting the platform to the centrifuge test facility.

How to cite: Park, J.-Y., Cheon, E., Lee, S.-R., Choo, J., Lim, H., and Nam, Y.: Centrifuge model test platform for rainfall simulation triggering shallow landslides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15044, https://doi.org/10.5194/egusphere-egu23-15044, 2023.

14:55–15:05
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EGU23-17200
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GM2.9
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ECS
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On-site presentation
Abigaël Darvenne, Sylvain Viroulet, and Laurent Lacaze

Impulse waves are waves generated by subaerial landlsides impacting the free surface of a lake or a sea. These waves differs from earthquake tsunami, even if often associated, as the generation mechanism and the scale of influence are not the same. Although they can travel over much shorter distance than other tsunamis, waves generated by landslides can be locally more dangerous [1]. Consequently, predicting the wave amplitude, and particularly its maximum during the generation remain crucial. Even if several studies have been devoted to the prediction of the wave amplitude at the laboratory scale, the mechanisms involved during the generation and particularly the role of the granular material to mimic landslide are still poorly understood [2, 3]. In this context, the presented study aims to better understand the interaction between the landslide and the generated waves, by understanding the physical mechanisms at the origin of the deformation of the free surface and the dry-wet transition of the granular flow. A laboratory model is used consisting of a 2m long chute of varying slope angle ending in a 4m long water tank. More specifically, the landslide is modelled by a monodisperse granular flow of 1mm spherical glass beads.
A picture of the experiment is represented in Figure 1a. The dynamic of the slide when crossing the air/water interface as well as the spatio-temporal structure of the wave are caracterised as a function of the properties of the impacting granular flow. Figure 1b shows the spatial and temporal evolution of the water free surface elevation during the wave generation process. This figure also highlights that the wave crest is stronlgy correlated to the granular front at early stages, while freely propagates in the far field. Based on physical mecanisms during generation, this study allows to discuss existing models relating the maximum wave amplitude to a so-called impulse parameter [4].

  

                                                                                                                                                                                            
Figure 1: (a): Picture of the granular flow penetrating water, (b): Space-time representation of the free surface elevation, compared with granular flow front position.

References:
[1] Fritz H. M., Mohammed F. & Yoo J. Lituya Bay landslide impact generated mega-tsunami 50th anniversary., Pure and Applied Geophysic 166, 153–175 (2009).
[2] Viroulet S., Sauret A. & Kimmoun O. Tsunami generated by granular collapse down a rough inclined plane., Europhysics Letters. 105, 34004 (2014).
[3] Robbe-Saule M., Morize C., Henaff R., Bertho Y., Sauret A. & Gondret P. Experimental investigation of tsunami waves generated by granular collapse into water., J. Fluid Mech. 907, A11 (2021).
[4] Heller V. & Hager W. H. Impulse Product Parameter in Landslide Generated Impulse Waves., Journal of Waterway, Port, Coastal, and Ocean Engineering. 136, 145–155 (2010).

How to cite: Darvenne, A., Viroulet, S., and Lacaze, L.: Laboratory modelling of landslide-generated impulse wave, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17200, https://doi.org/10.5194/egusphere-egu23-17200, 2023.

15:05–15:15
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EGU23-7952
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GM2.9
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ECS
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On-site presentation
Yi-Yun Liang, Chiun-Chau Su, and Hervé Capart

When rivers with high suspended sediment load plunge into lakes and reservoirs, the resulting density currents often cause the formation and progradation of hyperpycnal deltas. Suspended load can contribute to delta progradation through two different mechanisms: (1) indirectly, by increasing the excess density of the underflows, thus enhancing the basal shear stresses that drive along-bed transport; (2) directly, by settling out of suspension onto the evolving bed. In this work, we conducted laboratory experiments designed to investigate the relative importance of these two mechanisms, aided by a conceptual model that includes both processes. The experiments are conducted in a narrow tank of constant slope, supplied with prescribed water, sediment, and/or saline influxes. Both suspended sediment load and salinity can therefore contribute independently to the excess density of the inflow. Simultaneous measurements of delta profile evolution and suspended sediment concentration are then acquired using imaging methods. To interpret the results, we construct a simplified one-dimensional model of delta progradation in which along-bed transport is modelled as a diffusion process, and suspended sediment settling as an advection-deposition process. We then examine the influence of process coefficients on the morphology and rate of evolution of the delta fronts and compare simulations with the experiments. It is found that the evolution of the bed profile alone is not sufficient to distinguish between the two mechanisms, hence the importance of simultaneously measuring suspended sediment concentration. Although obtained in a simplified setting and at reduced scale, the results should provide useful guidance for the modeling and monitoring of reservoir sedimentation at field scales.

How to cite: Liang, Y.-Y., Su, C.-C., and Capart, H.: Influence of suspended sediment concentration on hyperpycnal delta progradation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7952, https://doi.org/10.5194/egusphere-egu23-7952, 2023.

Field measurements
15:15–15:25
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EGU23-966
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GM2.9
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ECS
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On-site presentation
Daniela Vendettuoli, Michael Strupler, Flavio S. Anselmetti, Stefano C. Fabbri, Anastasiia Shynkarenko, and Katrina Kremer

Large lacustrine mass movements and delta collapses are increasingly being considered as potential tsunamigenic sources. They are therefore hazardous for the population and infrastructure along lakeshores. In most studies of slope stability and triggered tsunamis, however, subaqueous deltas have largely been excluded as we lack information on their morphodynamic evolution. Thus, a holistic assessment of tsunami hazards in the lacustrine environment is required for a better understanding of how delta lakes evolve through time and space.
Within a study funded by the Federal Office of the Environment, we aim to understand what types of deltas are susceptible to slope failure within the perilapine Swiss lakes. To achieve our goal, we primarily focus on those deltas that present an increased potential for subaqueous erosion and analyse their morphological, morphometric and sedimentological characteristics taking advantage of the existing and publically available datasets. In this contribution, we present the designed approach and preliminary results, using Lake Lucerne as a case study. This approach will then be applied to all lakes with a surface area > 1 km2, for which high-resolution bathymetric data are available. The outcomes of such a study will be summarized in a geodatabase of the different delta-types for the perilpine Swiss lakes and it represents an important milestone for the assessment of tsunami hazard with regard to the lakes of Switzerland.

How to cite: Vendettuoli, D., Strupler, M., Anselmetti, F. S., Fabbri, S. C., Shynkarenko, A., and Kremer, K.: What controls deltas failure in the Swiss perialpine lakes?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-966, https://doi.org/10.5194/egusphere-egu23-966, 2023.

15:25–15:35
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EGU23-7709
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GM2.9
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ECS
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Highlight
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On-site presentation
Irena Schulten, Cecilia Clivati, Aaron Micallef, Simone Donadello, Davide Calonico, André Xuereb, Alberto Mura, and Filippo Levi

Sediment gravity flows are common processes in the submarine environment. They are important for the global sediment transport, but can destroy offshore infrastructure and may even contribute to tsunami generation. These flows, however, remain poorly understood. There is a lack of direct observations due to difficulties with deploying appropriate instruments and predicting the occurrence and route of these flows, especially on open continental slopes. Deployed instruments are further often destroyed as a result of the gravity flows. Submarine fibre cables are present along almost all continental margins worldwide. They are economically important for telecommunication and internet data transfer. Historic records, however, have shown that submarine gravity flows affect and even severe these cables. 


Recent studies successfully tested the usage of fibre optic cables to detect earthquakes and other processes such changes in the wave height associated with storm events. The aim of this study will be to test whether fibre optic cables can also detect submarine gravity flows using laser interferometry. The study is based on a cooperation between the University of Malta and the Istituto Nazionale di Ricerca Metrologica (INRiM) in Italy and is part of the European funded project “Modern and recent sediment gravity flows offshore eastern Sicily, western Ionian Basin (MARGRAF, ID 101038070)”. The University of Malta has been granted permission to collect data from a 260-km long optical fibre cable that connects Malta and Sicily through the western Ionian Basin. INRiM provided the measurement system and technical support needed to carry out the experiment. The western Ionian Basin is an ideal study site, as it is characterised by many earthquakes, tsunamis and submarine sediment gravity flows. The cable crosses known pathways of these gravity flows and thus provides a high possibility to detect modern sediment flows. The laser interferometry data will be analysed to detect disturbances (e.g., twists, expansions, contractions) on the cable. Any detected disturbances will be compared with oceanographic and seismometer data, both from onshore stations and Ocean Bottom Seismometers (OBS). This comparison will allow us to infer the source of the cable disturbance. In addition, we plan to collect gravity cores in vicinity of the event to assess whether the event was based on a gravity flow or not. Initial results showed earthquakes and various storm events recorded by the cable. 


The findings are expected to improve our current understanding of gravity flows in the region in terms of potential trigger mechanisms and reoccurrence rate. Eastern Sicily is densely populated and hosts touristic and industrial infrastructure, which makes it important to constrain the geohazard implication of these flows. A successful test will further allow to use this application on cables in other regions worldwide. 

How to cite: Schulten, I., Clivati, C., Micallef, A., Donadello, S., Calonico, D., Xuereb, A., Mura, A., and Levi, F.: Testing the potential of a submarine fibre optic cable to detect sediment gravity flows using laser interferometry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7709, https://doi.org/10.5194/egusphere-egu23-7709, 2023.

15:35–15:45
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EGU23-9236
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GM2.9
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On-site presentation
Ton (A.J.F.) Hoitink

Sediment transport in rivers and estuaries is typically monitored infrequently and discontinuously, which is a key reason why sediment budget estimations are often poor. On the contrary, water discharge is often monitored with high accuracy, and continuously, which requires periodic ship measurements for recalibration of rating curves. Even in tidal rivers, continuous flow measurements can be obtained by upscaling transect flow measurements to cover the entire cross-section, which rarely occurs for sediment transport (Kästner et al., 2018). This contribution discusses how existing discharge measurement schemes can be extended to yield continuous measurements of sediment transport, separating between suspended load and bedload sediment fluxes. A new approach is outlined, which relies on repeated cross-river transect measurements, using multiple acoustical and optical instruments. Innovative suspended load measurements make use of acoustic profilers with multiple sound frequencies and a spectrometer, which can measure suspended sediment mean particle size and carbon content from light absorbance (Sehgal et al., 2022). Inference of bedload transport from bedform tracking improves when taking secondary bedforms into account, which can migrate fast and persist in the lee of primary dunes, contributing significantly to the total bedload transport (Zomer et al., 2021). For sand-bed rivers in particular, a generic approach to upgrade existing discharge monitoring programmes to include continuous sediment transport may be feasible with limited additional ship survey time.

Kästner, K., Hoitink, A. J. F., Torfs, P. J. J. F., Vermeulen, B., Ningsih, N. S., & Pramulya, M. (2018). Prerequisites for accurate monitoring of river discharge based on fixed‐location velocity measurements. Water resources research54(2), 1058-1076.

Sehgal, D., Martínez‐Carreras, N., Hissler, C., Bense, V. F., & Hoitink, A. J. F. (2022). Inferring suspended sediment carbon content and particle size at high frequency from the optical response of a submerged spectrometer. Water Resources Research58(5), e2021WR030624.

Zomer, J. Y., Naqshband, S., Vermeulen, B., & Hoitink, A. J. F. (2021). Rapidly migrating secondary bedforms can persist on the lee of slowly migrating primary river dunes. Journal of Geophysical Research: Earth Surface126(3), e2020JF005918.

How to cite: Hoitink, T. (A. J. F. ).: Quantifying sediment transport from periodic transect measurements in rivers and estuaries, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9236, https://doi.org/10.5194/egusphere-egu23-9236, 2023.

Posters on site: Mon, 24 Apr, 16:15–18:00 | Hall X3

The posters scheduled for on-site presentation are only visible in the poster hall in Vienna. If authors uploaded their presentation files, these files are linked from the abstracts below, but only on the day of the poster session.
Chairpersons: Anne Baar, Pauline Delorme
X3.36
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EGU23-2733
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GM2.9
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ECS
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Cyril Gadal, Matthieu Mercier, and Laurent Lacaze

Most gravitational currents occur on sloping topographies, often in the presence of particles that can settle during the current propagation. Yet, an exhaustive exploration of associated parameters in experimental devices is still lacking. Here, we present an extensive experimental investigation on the slumping regime of turbidity (particle-laden) currents in two lock-release (dam-break) systems with inclined bottoms. We identify 3 regimes controlled by the ratio between settling and current inertia. (i) For negligible settling, the turbidity current morphodynamics correspond to those of saline homogeneous gravity currents, in terms of velocity, slumping (constant-velocity) regime duration and current morphology. (ii) For intermediate settling, the slumping regime duration decreases to become fully controlled by a particle settling characteristic time. (iii) When settling overcomes the current initial inertia, the slumping (constant-velocity) regime is not detected anymore. In the first two regimes, the current velocity increases with the bottom slope, of about 35% between and 15°. Finally, our experiments show that the current propagates during the slumping regime with the same shape in the frame of the moving front. Strikingly, the current head (first 10 centimeters behind the nose) is found to be independent of all experimental parameters covered in the present study. We also quantify water entrainment coefficients E, and compare them with previous literature, hence finding them proportional to the current Reynolds numbers.

How to cite: Gadal, C., Mercier, M., and Lacaze, L.: Slumping regime in lock-release turbidity currents, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2733, https://doi.org/10.5194/egusphere-egu23-2733, 2023.

X3.37
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EGU23-9662
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GM2.9
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ECS
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Highlight
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François Mettra, Rafael Sebastian Reiss, Ulrich Lemmin, Valentin Kindschi, Benjamin Graf, and David Andrew Barry

During calm cooling periods, differential cooling can induce winter cascading which is an important process for littoral-pelagic exchange and deep water renewal in large, deep lakes (Fer et al., 2001; Peeters et al., 2003). Generated in the shallow near-shore regions, such cold-water density currents travel down the sloping lakebed until they reach their depth of neutral buoyancy. The latter is strongly dependent on the entrainment of warmer ambient water, often expressed by the entrainment coefficient (i.e., the ratio of the entrainment velocity to the bulk velocity of the density current, e.g., Legg, 2012). Fer et al. (2001, 2002) studied density currents in Lake Geneva and showed that they occur in the form of cold-water pulses that last 1-2 hours, with a typical thickness of 10 m, a mean velocity of ~5 cm s-1 and an entrainment coefficient of ~0.03.

With recent advances in instrument capabilities, our recent investigations in Lake Geneva reveal also the presence of shorter, but still strong, temperature fluctuations of O(10) min in those density currents. To investigate further the mechanisms of entrainment in cascading flows, we designed a turbulence platform that was deployed on the sloping bed of Lake Geneva at 25-m depth. The platform is equipped with (high frequency) temperature and current velocity sensors which collect data over 3 meters vertically. A connection to the shore via a cable laid on the lakebed enables to control the platform’s vertical position and ensures continuous long-term measurements at high frequency. The background variables, such as velocity and temperature profiles, characterizing the nearshore zone in the surrounding of the platform are measured continuously using lower resolution sensors.

Here, we briefly expose the design of the platform, present a case of cascading flow and give small-scale hydrodynamic details of eddies that are observed intermittently within the density currents. Indeed, instantaneous unstable profiles (warm water intruding below cold water) within the dense cold flow show the presence of large eddies with spatial scales similar to the thickness of the mean current. The preliminary results shed light on the mechanism of warm ambient water entrainment into the cold-water density current. The high intermittency of occurrence of large eddies, i.e., those that contribute the most to entrainment, contrasts with the classic concept of a bulk entrainment coefficient.

Fer, I., Lemmin, U., & Thorpe, S. A. (2001). Cascading of water down the sloping sides of a deep lake in winter. Geophysical Research Letters, 28(10), 2093–2096. https://doi.org/10.1029/2000GL012599

Fer, I., Lemmin, U., & Thorpe, S. A. (2002). Winter cascading of cold water in Lake Geneva. Journal of Geophysical Research: Oceans, 107(C6), 13-1-13–16. https://doi.org/10.1029/2001JC000828

Legg, S. (2012). Overflows and convectively driven flows. In E. Chassignet, C. Cenedese, & J. Verron (Eds.), Buoyancy-Driven Flows (pp. 203-239). Cambridge: Cambridge University Press. doi:10.1017/CBO9780511920196.006

Peeters, F., Finger, D., Hofer, M., Brennwald, M., Livingstone, D. M., & Kipfer, R. (2003). Deep-water renewal in Lake Issyk-Kul driven by differential cooling. Limnology and Oceanography, 48(4), 1419–1431. https://doi.org/10.4319/lo.2003.48.4.1419

How to cite: Mettra, F., Reiss, R. S., Lemmin, U., Kindschi, V., Graf, B., and Barry, D. A.: A moored profiling platform to study turbulent mixing in density currents in a large lake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9662, https://doi.org/10.5194/egusphere-egu23-9662, 2023.

X3.38
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EGU23-12918
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GM2.9
Katrina Kremer, Stefano C. Fabbri, Daniela Vendettuoli, Carlo Affentranger, Stéphanie Girardclos, and Flavio S. Anselmetti

Deltas represent transfer zones where sediment is moved from terrestrial to the subaquatic domains. They are depositional areas and a source for sediments simultaneously. One of the aspects in this highly dynamic environment that has experienced so far little attention are slope failures in deltas. Such failures are, however, mentioned as potential cause for large (up to m-scale), graded deposits in the sedimentary record, often referred to as megaturbidites or homogenites. In some cases, they may have generated tsunamis in the near-shore area. These delta failures can be triggered, amongst other causes, by spontaneous slope collapses (e.g. Muota delta 1687 in Lake Lucerne, Switzerland). To better understand the controlling factors of slope stability in deltas, we need to comprehend the interplay between deltaic deposition and erosion through time and monitor their evolution.

Repeated bathymetric mapping has been used as powerful tool to better understand the short-term processes occurring in deltas. In this contribution, repeated bathymetric mapping is used to better characterize, which short-term processes may shape subaqueous delta fronts. Using the dataset acquired in recent years in Swiss lakes, we seek to answer (1) what processes can be visualized based on repeated bathymetric mapping?; (2) which areas are prone to depositional/erosion processes?; and (3) what type of delta is more prone to slope failures? We present the first datasets of differential maps from four deltas in Switzerland that show different processes of erosion and deposition on short and long time scales. In addition, we will present the design of a planned multi-method monitoring campaign for delta processes in a sublacustrine delta in a peri-alpine lake in Switzerland. 

How to cite: Kremer, K., Fabbri, S. C., Vendettuoli, D., Affentranger, C., Girardclos, S., and Anselmetti, F. S.: Monitoring sediment processes in different delta systems in Swiss peri-alpine lakes through 4D bathymetric mapping, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12918, https://doi.org/10.5194/egusphere-egu23-12918, 2023.

X3.39
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EGU23-2362
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GM2.9
Francois Metivier, Olivier Devauchelle, and Pauline Delorme

We study the effect of an increase in flow discharge on the shape and growth of an experimental alluvial fan. The fan is built by a single-thread channel in which the flow occurs near the threshold of sediment motion. We first define a criterion that predicts the conditions under which a change in discharge leaves an inprint on the morphology of a fan. We then report on experimental runs which allow us to establish the relevance of this criterion. Experiments are carried out during which climatic changes are applied to the feeding channel of a fan. By playing on the initiation time of climate change, on the duration of the rise in flow, or on the total variation in discharge, we scan a range of configurations that allow us to qualitatively and quantitatively test our incision criterion. Qualitatively, we note that the dynamics of the fan seems altered only for values of the criterion which exceed the critical value of 1.5. In these situations, the channel stops moving and entranches. Quantitatively, we extract a characteristic time by autocorrelating spatio-temporal channel migration diagrams and show that this time correlates with the value of the incision criterion.

How to cite: Metivier, F., Devauchelle, O., and Delorme, P.: Adaptation of an experimental alluvial fan to climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2362, https://doi.org/10.5194/egusphere-egu23-2362, 2023.

X3.40
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EGU23-14185
|
GM2.9
|
ECS
Pauline Delorme, Stuart McLelland, Brendan Murphy, and Daniel Parsons and the EvoFlood Team

There is now a clear consensus that climate change will lead to an increase in the frequency and intensity of extreme rainfall events in many parts of the world, which, in turn, will lead to increased flood flows and thus flooding of large areas. Numerical simulation is one way to improve our understanding of flooding processes, especially through Global Flood Modelling (GFM). Current GFMs represent the morphology of river channels and floodplains in a very simplified way. In particular, GFM assumes that the channel morphology remains unchanged over time. However, rivers are dynamic, their morphology evolves by erosion and deposition of sediments carried by the flow. These morphological changes can radically alter the conveyance capacity of the channel and therefore the flood risk. Integrate these morphological changes in the new GFM framework is one of the main objectives of the NERC-funded EVOFLOOD project. 

Here we present the results of the experimental part of the project. We designed a controlled laboratory experiment to identify the factors controlling the morphodynamic response within river channel. In this experiment, we generate a succession of flood events characterised by different magnitudes and durations, and we quantify the evolution of the flooded area and channel width as a function of the duration, intensity and flood history. 
We find that the main parameters controlling morphological changes are flood intensity and flood history. The duration of the flood does not have a significant impact on the morphological changes because the main changes occur during the first period of the flood event. Finally, we show the importance of the upstream sediment discharge on the modification of the conveyance capacity.

 

How to cite: Delorme, P., McLelland, S., Murphy, B., and Parsons, D. and the EvoFlood Team: Experimental evidences of the influence of flood magnitude and duration on the morphological evolution of a river: Initial results from the EVOFLOOD project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14185, https://doi.org/10.5194/egusphere-egu23-14185, 2023.

X3.41
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EGU23-14356
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GM2.9
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ECS
Jakob Höllrigl, Koen Blanckaert, David Hurther, Guillaume Fromant, and Florian R. Storck

At present, SSC fluxes in rivers are typically estimated by multiplying the river discharge with the
average suspended sediment concentration (SSC). The latter is typically obtained from optical turbidity
measurements in one single point of the river cross‐section. The optical turbidity is converted in
average SSC based on a relation that is derived from the laboratory analysis of regular SSC samples.
This method has the disadvantages that it is based on a one‐point measurement and that it is
expensive.

The SSC distribution in an entire profile – vertical or horizontal – can also be derived from the
backscatter of single‐frequency echosounders. The disadvantage of this method is that the particle size
of the suspended sediment needs to be known in order to convert the profile of backscatter into a
profile of SSC.

Here we present a hydro‐acoustic method based on multi‐frequency echosounding. Operating on
multiple acoustic frequencies allows estimating the mean particle size directly from the backscatter at
the different frequencies. The method based on multi‐frequency echosounding is illustrated with
measurements on the Rhône River just upstream of Lake Geneva in Switzerland. The results are
compared to measurements based on optical turbidity measurements and to measurements based on
single‐frequency echosounding.

How to cite: Höllrigl, J., Blanckaert, K., Hurther, D., Fromant, G., and Storck, F. R.: Measurements of sediment flux in rivers with a multi-frequency echosounder, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14356, https://doi.org/10.5194/egusphere-egu23-14356, 2023.

X3.42
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EGU23-8385
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GM2.9
|
ECS
Benjamin Dedieu, Philippe Frey, and Julien Chauchat

Vertical size segregation or sorting of particles in bedload transport, strongly impacts the sediment rate and the river bed morphology. To better account for this process in sediment transport models, it is essential to understand the mechanisms acting at the grain scale (Frey, 2009). Following the work of Rousseau (2021), focus is made on the behaviour of a single large particle segregating upwards in a monodisperse mixture of smaller beads during bedload transport. Experiments are carried out in a narrow flume and the bead dynamics is recovered through image analysis. A great number of repetition is performed for different size ratios (large to small bead diameter) in order to conduct statistical analysis. This work confirms the measurements from Rousseau (2021) and suggests that the time for the large particle to reach the bed surface is minimum for a size ratio of 2. This result supports previous research which, using simpler granular configurations, evidenced a similar tendency in terms of segregation force (Guillard, 2016, Jing, 2020) or segregation velocity (Golick, 2009). Other observations are made on the spatial trajectory of the intruder, which have been previously reported to be linear with a repeatable slope independent of the size ratio. These observations offer interesting insights to understand the mechanisms governing size segregation and could provide new closures to upscale the phenomenon at the continum scale.

Illustration: A 5 mm intruder in a 2 mm bed (size ratio = 2.5), flow from right to left, dimensionless bed shear stress (Shields number) = 0.25.

Frey, P. and Church, M. (2009). “How River Beds Move”. Science, 325(5947), pp. 1509–1510.
Golick, L. A. and Daniels, K. E. (2009). “Mixing and Segregation Rates in Sheared Granular Materials”. Physical Review E, 80(4), p. 042301.
Guillard, F., Forterre, Y., and Pouliquen, O. (2016). “Scaling Laws for Segregation Forces in Dense Sheared Granular Flows”. Journal of Fluid Mechanics, 807.
Jing, L., Ottino, J. M., Lueptow, R. M., and Umbanhowar, P. B. (2020). “Rising and Sinking Intruders in Dense Granular Flows”. Physical Review Research, 2(2), p. 022069.
Rousseau, H. (2021). “From Particle Scale to Continuum Modeling of Size Segregation in Bedload Transport : A Theoretical and Experimental Study.” PhD thesis. Université Grenoble Alpes.
Rousseau, H., Frey, P., and Chauchat, J. (2022). “Experiments on a single large particle segregating in bedload transport”. Physical Review Fluids, 7(6), p. 064305.

How to cite: Dedieu, B., Frey, P., and Chauchat, J.: Experimental investigation of the segregation of a large intruder in bedload sediment transport, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8385, https://doi.org/10.5194/egusphere-egu23-8385, 2023.

X3.43
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EGU23-6809
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GM2.9
|
ECS
|
Highlight
Anne Voigtländer, Vadim Sikolenko, Jens M. Turowski, Luc Illien, Jonathan Bedford, and Gunnar Pruß

Prior to earthquake ruptures and slope failures, accelerated surface deformations can sometimes be observed. To anticipate rupture processes, these deformations are interpreted in terms of a strain budget and its stressors. If the budget exceeds an assumed critical value, rupture happens. But not all components of the budget can readily be inferred from the bulk deformation. For example, elastic strain build-up and other ‘silent’ contributions challenge the predictability of these potential natural hazards. We present preliminary experimental results, focussing on deformation by compaction. We report an analogous experiment of loading and unloading to constrain compaction behaviour, elastic strain-build up, and release to understand their ‘silent’ contributions to the strain budget. As analogue material, we use sand to assess emergent bulk behaviour. Using natural quartz crystals allows to apply in-situ neutron diffraction to measure elastic strain during loading and unloading stages. We find that while compaction and remnant compaction scale linearly with load magnitudes, elastic strain build-up seems to be independent of stresses ≥60 MPa. In addition to the in-situ neutron diffraction experiments, we conducted mechanical compaction tests at ramped load stages and analysed the post-compaction changes of the grain size distribution. With increased loading, the mean grain size decreased, leading to increased bulk density in the compacted portion. Based on these observations, we reason that the linear elastic bulk compaction of our samples is due to non-linear local brittle deformation. There is only limited elastic strain built up during the compaction, which is likely released due to local crushing. Localized failure produces a denser material in which strain can build up more homogeneously, causing rupture at its bulk elastic limit. Our experiments show that deducing or simply converting loading and displacement to stress-strain relationships to establish a strain budget may be inadequate. Silent components that are likely due to non-linear and emergent processes can in the short term lead to local elastic strain energy release or bulk dynamic ruptures. Conceptually, to especially anticipate the timing of slope failures and the magnitude of earthquake ruptures, the hidden costs, e.g. due to localized failure, and internal changes, concerning density or elastic properties, are crucial components that need to be constrained while compiling a strain or energy budget of these processes.

How to cite: Voigtländer, A., Sikolenko, V., Turowski, J. M., Illien, L., Bedford, J., and Pruß, G.: Tales of compacting sand to anticipate strain budget of rupture processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6809, https://doi.org/10.5194/egusphere-egu23-6809, 2023.

X3.44
|
EGU23-7051
|
GM2.9
|
ECS
|
Mathieu Souzy

Geomaterial are complex porous material presenting a wide diversity of structures, which set how a fluid will flow through it. The understanding of the mechanisms controlling the flow kinematics at the pore scale is however decisive to predict and control transport processes (dispersion and mixing) and to relate them to the macroscale behaviour of porous materials. Because of the opaque nature of porous media, the flow visualization and characterization of the velocity fields within a porous media is particularly challenging in three-dimensional (3-D) porous media. However, recent development of experimental techniques including index matching, allow to develop transparent porous media to perform direct visualization of the flow in these artificial material.

I will here discuss about how such approach have already been successfully implemented to study porous media composed of randomly packed solid monodisperse spheres, allowing to directly visualize the flow within the bulk of the 3-D media, and to investigate how a blob of dye stretches and get mixed when injected within such 3-D porous media. Using Particle Image Velocimetry techniques (PIV), these promising techniques also allow to perform successive scans of the velocity field, providing highly resolved experimental reconstruction of the 3-D Eulerian fluid velocity field in the bulk of the porous media. This approach is therefore promising to further investigate flow kinematic in more complex porous media, or to directly visualize other crucial mechanisms in such media, like for instance erosion, clogging, or the effect of strong heterogeneities on the overall flow behavior.

How to cite: Souzy, M.: Direct flow visualization and transport processes in transparent 3D porous media, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7051, https://doi.org/10.5194/egusphere-egu23-7051, 2023.

X3.45
|
EGU23-17594
|
GM2.9
Determining the interactions between extrinsic and intrinsic drivers of failure in soft cliffs using physical modelling
(withdrawn)
daniel parsons, Serena Teasdale, Chris Hackney, Georgina Bennett, and David Milan
X3.46
|
EGU23-3735
|
GM2.9
|
ECS
Yi-Fan Hung, Hervé Capart, and Colin P. Stark

In some landslides, collapse is accompanied by the upslope retreat of a well-defined scarp whose speed controls the rate of mobilization of debris. Here we examine the evolution of such scarps in an idealized laboratory setting. We conduct tilted channel experiments involving retrogressive dry granular landslides over an erodible substrate. After first tilting up a deep sand layer to close to the angle of repose, then imposing an abrupt base-level drop, granular flow is induced at the downstream outlet. This flow generates an upstream-traveling wave with a well-defined scarp at the upstream tip. Downstream of the moving scarp, sand flows as an avalanching layer of finite depth over the erodible but stationary substrate, and outflows over the lowered outlet sill. A series of such experiments were conducted to determine the influence of channel width and base-level drop height on the speed of scarp retreat and other flow properties. Measurements included the time-evolving profile of the free surface, surface velocities acquired using particle tracking velocimetry, and the time-evolving mass outflow rate at the downstream outlet. Dimensional analysis clarifies the physical mechanisms governing the rate of scarp retreat. These results will help guide and validate numerical models of granular landsliding over erodible substrates.

How to cite: Hung, Y.-F., Capart, H., and Stark, C. P.: Kinematics of scarp retreat in idealized tilted channel experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3735, https://doi.org/10.5194/egusphere-egu23-3735, 2023.

X3.47
|
EGU23-2913
|
GM2.9
Samuel Pelacani, Francesco Barbadori, Federico Raspini, Francois G. Schmitt, and Sandro Moretti

River flows and associated suspended sediment (SS) transport are intermittent processes possessing fluctuations over a large range of time scales and space, making it challenging to develop predictive models that are applicable across timescales and rivers. A concept of “effective timescales of connectivity” has been used to define the timeframe over which sediment (dis)connectivity occurs, whereby parts of the catchment are “switched on and off” as a response of events with varying frequency-magnitude relationships and antecedent soil moisture. These concepts provide excellent frameworks to understand temporal variability and identify relevant timescales for sediment transport, but do not help in the knowledge of mechanisms for temporal variability in SS transport. The complexity and scale dependency of processes driving SS transport stress the need to detect how sediment generation, storage, and transport are linked across different timescales. Furthermore, the mechanisms that produce travel time distributions over many orders of magnitude are not known precisely. To this end, in this study we have considered SS transport as a fractal system. By approaching SS transport dynamics as a fractal system, it is assumed that patterns of variation in SS transport exist over different timescales, while linkages across those temporal scales are expressed as fractal power-laws.

This work aims to defines the link between (i) sediment transport and deposition and (ii) fractal geometry and fractal storage time distributions in streams.

Here, we present case study where fractals are used to describe and predict patterns over different spatial or temporal scales of dynamics in SSCs. We have considered in these studies the statistics and the dynamics of streamflow, SSCs and associated grain size distribution at event based by considering respectively their probability distribution function and Fourier power spectra.

We set up a natural experiment site of a first-order mixed bedrock and alluvial stream channel by using LISST instrument coupling with LIDAR remote sensing measurement. Here we obtain high-resolution observations of streambed topography and continuously long-term measurements of suspended sediment in natural experimental site located in an agricultural watershed of a Chianti area (Florence, Italy).

The LISST is a submersible laser diffraction particle size analyzer for measuring suspended particle size (range from 2.50 µm to 500 µm), its volume concentration at different time step and depth. We set up at time interval equal to 5 minutes of sample rate.

Preliminary results obtained indicate large fluctuations with heavy tails, and long-range properties, characterized by extreme events much more frequent than what is found for a Gaussian process.

Hence, insights into the degree of fractal power of a SS transport system may provide a useful basis to evaluate and develop the most appropriate predictive models and management strategies.

How to cite: Pelacani, S., Barbadori, F., Raspini, F., Schmitt, F. G., and Moretti, S.: Fractal characteristics of suspended sediment transport in rivers: natural experiment site, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2913, https://doi.org/10.5194/egusphere-egu23-2913, 2023.