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.

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.

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.

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.

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.

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.

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.

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.

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 research, 54(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 Research, 58(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 Surface, 126(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.