Avalanches of dry granular materials, such as rocks, snow, and ice, are chief contributors to hazardous geophysical flows in nature. A key problem hampering progress in predicting the destructiveness of such hazards is the poorly understood dependence of the flow velocity on the physical properties of the grains constituting a given material. In particular, their usually irregular, non-spherical shapes prevent application of rigorous theories, which were derived for spherical grains. In addition, we do not have a good empirical grasp of the issue, as evidenced by the failure of existing scaling laws across flows of different granular materials when applied to measurements and numerical simulations for idealized flow geometries. Here, we report a scaling law for the steady-state velocity of homogeneous granular flows down rough inclines. It holds for granular materials consisting of irregularly-shaped but relatively uniformly-sized grains descending rough slopes. Laboratory chute experiments and numerical simulations for a diverse range of granular materials corroborate its validity and generality. It exhibits a power-4/3 dependence on the flow thickness, as opposed to the power-3/2 dependence suggested by previous scaling laws. It is also unique in the aspect that it depends only on a single parameter characterizing the granular material: the dynamic angle of repose. This suggests that, quite surprisingly, most of the physical complexity associated with the composition and shape of a material's grains boils down to its bulk ability to resist externally-driven shearing.
How to cite: Pähtz, T.: General scaling law for the velocity of steady, homogeneous granular flows down rough inclines, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9678, https://doi.org/10.5194/egusphere-egu25-9678, 2025.
We report on chute measurements of the free-surface velocity $v$ in dense flows of spheres and diverse sands and spheres-sand mixtures down rough inclines. These and previous measurements are inconsistent with standard flow rules, in which the Froude number $v/\sqrt{gh}$ scales linearly with $h/h_s$ or $(\tan\theta/\mu_r)^2h/h_s$, where $\mu_r$ is the dynamic friction coefficient, $h$ the flow thickness, and $h_s(\theta)$ its smallest value that permits a steady, uniform dense flow state at a given inclination angle $\theta$. This is because the characteristic length $L$ a flow needs to fully develop can exceed the chute or travel length $l$ and because neither rule is universal for fully-developed flows across granular materials. We use a dimensional analysis motivated by a recent unification of sediment transport to derive a flow rule that solves both problems in accordance with our and previous measurements: $v=v_\infty[1-\exp(-l/L)]^{1/2}$, with $v_\infty\propto\mu_r^{3/2}\left[(\tan\theta-\mu_r)h\right]^{4/3}$ and $L\propto\mu_r^3\left[(\tan\theta-\mu_r)h\right]^{5/3}h$.
How to cite: Wu, Y., Pähtz, T., Guo, Z., Jing, L., He, Z., and Zhang, J.: Unified flow rule of undeveloped and fully-developed dense granular flows down rough inclines, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10237, https://doi.org/10.5194/egusphere-egu25-10237, 2025.
We investigate the submerged cohesive collapse of cohesive granular columns, as a function of packing densities and cohesive force strength, via grain-resolving direct numerical simulations. We not only obtain the randomly packed granular columns but also the regular densely packed columns by Hexagonal close-packed (HCP) structure. The cohesive forces act to reduce the final runout distance of the collapsing column, which will no longer collapse when the cohesive force is larger than a critical value. This critical value decreases with the increase of the packing density. The cohesive forces significantly accelerate the contraction for loosely packed columns and decelerate the dilation for densely packed columns, resulting in a larger positive excess pore pressure and a smaller negative excess pore pressure, respectively. The collapsing column has distinct straight-like failure surfaces at the initial time, whose angle with the horizontal plane increases with the packing density. The force-chain network analysis indicates that the strong cohesive force chains form more easily in the failure region and have a larger size with increasing the cohesive force and packing density, which induces a larger macroscopic cohesive resistance. The cohesive force has a canceling effect on the normal contact force, which results in a smaller size for the contact force chains.
How to cite: Zhu, R., He, Z., and Meiburg, E.: Submerged granular collapse: different cohesion strength and initial packing densities, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20382, https://doi.org/10.5194/egusphere-egu25-20382, 2025.
CFD (computational fluid dynamics)-DEM (discrete element method) model has been widely applied in the simulation of the multiphase flow involving granular materials, but it’s time-consuming for the calculation of a large number of particles with different sizes in DEM. In this study, a model based on the computational micropolar fluid dynamics and discrete element method, viz. a CMFD-DEM model, is proposed to describe the coupling system that consists of gas-liquid two phases and discrete particles with different sizes. In this model, micropolar fluid model is employed to describe the mixture of the pure fluid with fine particles, while discrete element method is used to calculate the motion of the larger particles. In addition, VOF (volume of fluid) method is adopted to track the free surface of the liquid. The implementation of the CMFD-DEM model is based on the open source software, OpenFOAM and LIGGGHTS, and is validated in single particle sedimentation and particles pouring into quiescent water cases. Then, the simulation of debris flow is carried out. The results show that specific dynamic behaviors of debris flow can be reproduced by CMFD-DEM model. The average velocity and runout of debris flow are decreased with the increase of micropolar parameter N/L. Through the comparisons to the exiting results, it suggests that CMFD-DEM model is capable to describe the multi-size effect of the granular materials in debris flow.
How to cite: Wang, L., Chu, X., and Sun, H.: An extended CFD-DEM model based on micropolar fluid for debris flow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3598, https://doi.org/10.5194/egusphere-egu25-3598, 2025.
Debris flows, as a type of large-scale geological disaster, are a global focus regarding their formation boundary, kinematic properties and deposit morphology. In small-scale laboratory simulations, factors such as water content, equivalent grain size, grain size ratio and aspect ratio significantly influence the formation boundaries and flow regime. Quantifying the effects of these numerous variables is a crucial prerequisite for advancing research on geological disasters represented by debris flows. We conducted simulations of the debris flow triggering process within a horizontal chute and used the proposed centroid vector displacement method to quantitatively assess the kinetic characteristics from an energetic perspective. By integrating the influence of water content into the traditional Bond number, we were able to clearly differentiate three distinct collapse regimes. Through modulation of the size and density ratios, we explored the distribution of intensity for various mechanisms along the flow direction. To characterize the relative strength of diffusion and buoyancy effects on the length scale, we introduced a dimensionless parameter λ. This parameter enabled us to define the boundary conditions necessary for the formation of core-band patterns.
How to cite: Yang, H., Chi, Z., Chen, Q., and Xu, Y.: Energetic kinetic of debris flow in a horizontal chute using centroid vector displacement method, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14133, https://doi.org/10.5194/egusphere-egu25-14133, 2025.
Non-hydrostatic dispersive models can better describe the landslide motion. Following a dispersive wave equation and a mechanical erosion model for mass flows, here, we develop a novel dynamically coupled dispersion-erosion wave model that combines these two very essential complex processes. The newly developed model for landslide recovers the classical dispersive water waves and dispersive wave equation for landslide as special cases. We present several exact analytical solutions for the coupled dispersion-erosion model. These solutions are constructed for the time and spatial evolution of the flow depth. Solutions reveal that the dispersion and erosion are strongly coupled as they synchronously control the landslide dynamics. The results show that the wave dispersive wave amplifies with the increasing particle concentration, decreasing earth pressure, higher gravitational acceleration, increased slope angle and increased basal friction. The important novel understanding is that the intensity of the dispersive wave increases when erosion and dispersion are coupled. The results indicate the essence of proper selection of the initial and boundary conditions while solving applied and engineering problems associated with the dispersive - erosive mass transport. This provides the foundation for our understanding of the complex dispersion and erosion processes and their interplay.
How to cite: Kafle, J., Dangol, B. R., and Pudasaini, S. P.: Dispersion - Erosion Coupling in Landslides, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17293, https://doi.org/10.5194/egusphere-egu25-17293, 2025.
Due to various destabilizing factors such as hydro-thermo-mechanical degradation, and earthquakes, the strength of the Earthsurface material may decrease, leading to increased slow earthflow events. Earthflows often cause extensive damage to infrastructure and permanently change the landscape pattern. However, the earthflows have received much less attention compared to their fast-moving counterparts, like avalanches, landslides, and debris flows. Here, we present some novel laboratory experiments simulating slowflows to understand their initiation, movement, and long-term morphological evolution by using a highly viscous material, the molten jaggery, locally found in Kathmandu. The tremendously slowly deforming and moving jaggery is assumed to represent earthflows. Experimental results demonstrate some key aspects of slowflow dynamics of earth materials and seminally contribute to the systematic understanding of earthflow processes. We simulate the slowflow propagation process by using a dynamic earthflow model. Simulation results capture some essential features of the massively viscous, exceptionally slowly deforming, and moving earth surface materials.
How to cite: Kattel, P., Tiwari, C. N., and Pudasaini, S. P.: Slowflows: Experiments and numerical simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14812, https://doi.org/10.5194/egusphere-egu25-14812, 2025.
Progressive slope steepening can trigger episodic dry sand avalanches, resembling landslides commonly observed in natural environments. Similarly, gradual river incision can induce periodic slope instability and failures. To thoroughly investigate the impact of gradual river incision on catchment topography and slope dynamics, we conduct a series of idealized dry sandbox experiments. This simple setup is expected to provide a deeper understanding of the patterns and dynamics of landslides in mountainous regions.
In the experiments, dry sand is removed by applying negative suction pressure through a nozzle traversing prescribed paths over the topography. This process simulates river channel incision into the sand substrate and triggers avalanches on adjacent slopes. The experimental setup consists of a simple box filled with dry sand, equipped with a suction mechanism inspired by the extrusion nozzles used in 3D printing. Unlike 3D printing, where material is added, negative pressure at the nozzle is used to extract material instead.
To validate the system, we first employ a vertically descending suction nozzle at a controlled rate to produce an expanding conical pit. This simple setup allows us to test the suction mechanism and ensure consistent material removal. Subsequently, we simulate river incision by utilizing an idealized curved path designed to mimic the geometry of an incising river. Initially, the nozzle was manually guided along this path to replicate the incision process. In later experiments, a computer-controlled traversing system is implemented to ensure greater precision and reproducibility.
We then explore imposed motions along the main river channel and incorporate tributaries to explore the river incision processes. The results, including the formation of ridges, avalanches, and slope adjustments, are analyzed and compared with computational predictions derived from an eikonal model. This comparison provides valuable insights into the behavior of slopes under conditions of gradual river incision and elucidates the mechanisms driving slope instability and morphological evolution in natural catchments.
How to cite: Chang, E. and Capart, H.: Experimental Analogue Modeling of Slope Dynamics Induced by Gradual River Incision Using a Controlled Suction Nozzle in Dry Sandbox Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12716, https://doi.org/10.5194/egusphere-egu25-12716, 2025.
Assessing particle-scale interactions and transport phenomena is essential yet complex within geophysical flows found in both natural and artificial settings. This research introduces the design, validation, and calibration of a spherical inertial sensor particle meticulously engineered to achieve full kinematic equivalence with a solid sphere. By employing Micro-Electro-Mechanical Systems Inertial Measurement Unit (MEMS-IMU) technology, this low cost 40 mm particle can measure triaxial acceleration up to ±16g and triaxial angular velocity up to ±2000°/s, operating at a high sampling rate of 1000 Hz over a duration of one hour. The sensor particle possesses a dual-layered spherical configuration deliberately crafted to ensure alignment in shape, density, center of mass, moment of inertia, and elastic modulus with that of a solid sphere. Its performance is rigorously assessed, validated, and calibrated through a series of physical experiments. Furthermore, a data enhancement technique grounded in lubrication theory is invented to mitigate technical challenges associated with accelerometer saturation and temporal resolution. This method enables our sensor particle to accurately capture particle collision processes within liquid environment, which proves challenging with conventional approaches. This investigation offers a foundational instrument for large-scale particle motion studies, such as those related to debris flows, facilitating, for the first time, the precise measurement of the dynamic behavior of individual particles within a substantial ensemble.
How to cite: An, Y., Jiao, J., and Zhang, L.: Spherical Inertial Sensor for Measuring Particle-Scale Interactions in Geomorphic Flows with Full Kinematic Equivalence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15433, https://doi.org/10.5194/egusphere-egu25-15433, 2025.
We have investigated the modeling of collisional bed-load transport with a focus on continuum approaches for granular flow. A frictional-collisional framework, combining the Coulomb model and the kinetic theory of granular flows, is proposed to address the limitations of classical kinetic theory, which fails to accurately reproduce results from coupled fluid–discrete simulations. These discrepancies are attributed to assumptions of negligible interparticle friction and the absence of a saltation model in continuum formulations.
To guide model development, the fluctuating energy balance obtained from discrete simulations is systematically compared with kinetic theory predictions. The analysis reveals that interparticle friction significantly affects the radial distribution function and increases energy dissipation, aligning with previous findings. Additionally, a saltation regime is identified, causing deviations from the viscosity and pseudo-thermal diffusivity laws of kinetic theory in dilute regimes.
Building on these insights, the two-fluid model is modified to incorporate interparticle friction and coupled with a saltation model. The results demonstrate that for inelastic, frictional particles, interparticle friction primarily governs energy dissipation, and the macroscopic granular flow behavior is independent of microscopic particle properties. The enhanced model successfully reproduces the 𝜇(𝐼) rheology in the dense regime of granular flow. Experimental validation confirms significant improvements in predicting granular flow behavior, highlighting the model’s effectiveness in capturing key physical processes.
How to cite: Chauchat, J., Chassagne, R., and Bonamy, C.: A Continuum Framework for Modeling Frictional-Collisional Interactions in Bed-Load Transport: Insights from Discrete Element Simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16152, https://doi.org/10.5194/egusphere-egu25-16152, 2025.
In steady, fully developed flows over erodible beds, the average bed shear stress is generally the dominant factor governing sediment flowrate. However, fluctuations induced by turbulence can play a significant role in altering sediment transport dynamics. This study investigates the effects of such turbulence by conducting flume experiments with flow disturbances created by various cylinder arrays placed in the flow. To measure the turbulent flow field, Laser Doppler Velocimetry (LDV) was employed, while bed shear stress was quantified using a shear plate. The bedload motion was analysed using Particle Tracking Velocimetry (PTV), which allowed for the quantification of key variables such as sediment concentration, velocity, and sediment flowrate. A descriptive model was developed to capture the relationship between these primary variables and both the average and fluctuating components of the flow. Our results show that with increasing turbulent fluctuations, both sediment concentration and velocity rise at a fixed mean shear stress. Notably, turbulence influences concentration more strongly than velocity.
How to cite: Rebai, D., Koll, K., Radice, A., Aberle, J., and Ballio, F.: Turbulence increases sediment transport, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9842, https://doi.org/10.5194/egusphere-egu25-9842, 2025.
Bedload transport, critical in various natural and engineering systems, involves the complex interaction between particles and flowing water. Predicting bedload transport rates has long been a focal point of interest due to its significance in understanding river dynamics. Pioneering contributions from Einstein and Bagnold have led to substantial progress in this field derived from extensive laboratory and in-situ observations, which are yet to achieve the desired accuracy when validated against real-world hydrological data. The discrepancies in predictions can partly be attributed to the difficulties in accurately capturing the movements of near-bed particles and the flow field characteristics.
This paper presents a numerical investigation via Computational Fluid Dynamics-Discrete Element Method into detailed observations on particle movements and flow characteristics of bedload transport. It provides a thorough review of the assumptions and theories prevalent in current bedload models. Simulations have been conducted covering flow velocities ranging from below the generally accepted critical Shields number to the onset of bedform formation. We analyze particle trajectories and statistical behaviors under various conditions, focusing on both the motions of individual particles and the collective evolution of bedforms, and our primary results include: 1. The incipient motion of particles is a gradual process that can occur before reaching the generally accepted critical Shields number. 2. The emergence and development of bedforms under varying conditions. 3. Observations on the relationship between particle movement characteristics and the shear conditions. These findings enhance our understanding of particle-scale dynamics in bedload transport, providing a foundation for evaluating and improving existing models for predicting transport rates.
How to cite: Li, X., Zhao, T., and Xu, B.: Flow Characteristics and Particle Kinematics in Bedload Transport: a CFD-DEM investigation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15234, https://doi.org/10.5194/egusphere-egu25-15234, 2025.
Transport of granular materials on Earth and planetary surfaces are at the heart of landscape dynamics and geohazards. These transport phenomena are controlled by particle-scale mechanisms, including particle motion, collisions, and interactions with the ambient fluid, which highlights the importance of particle-resolved measurements in physical experiments. However, despite recent progress in particle tracking velocimetry (PTV) for spherical (and regularly shaped) particles, there still lacks a robust technique in tracking and analyzing the motion of non-spherical particles, particularly because conventional PTV cannot identify moving objects of an arbitrary shape. This limitation largely compromises our particle-scale understanding of the transport of natural granular materials with a wide range of shapes and sizes. To tackle this issue, we propose a novel deep learning-based PTV framework for arbitrarily shaped and sized particles, which consists of a real-time computer vision algorithm called YOLO (you only look once) and an accurate inter-frame matching algorithm based on Kalman filtering. The proposed PTV framework is validated in various granular flow and sediment transport scenarios, using high-resolution data obtained from discrete element method simulations and small-scale physical experiments. Using this new technique, we are able to precisely analyze the kinematics information of spherical, non-spherical, and mixed particles with different concentrations in a series of open channel bedload transport experiments. Scaling relations are obtained between the sediment flux and bed shear stress to reveal the effects of particle shape and composition on the sediment transport dynamics across bedload and sheet flow conditions. The proposed PTV technique and its potential applications are expected to provide a new avenue for future research on the micromechanical aspects of geophysical granular flow and sediment transport.
How to cite: Su, W., Jing, L., and Xu, M.: Deep learning-based particle tracking velocimetry (PTV) for spherical and non-spherical particles: Application to granular flow and sediment transport, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7708, https://doi.org/10.5194/egusphere-egu25-7708, 2025.
This study employs remote sensing technology to thoroughly analyse sediment dynamics in expansive aquatic environments, with a specific focus on the Ganga River basin. The investigation spans from 2007 to 2011, utilizing Medium Resolution Imaging Spectrometer (MODIS) MYD09A1.061 Aqua Surface Reflectance 8-Day Global data to assess Suspended Sediment Concentration . By integrating ground-based silt data with satellite data, the study captures temporal variations in suspended sediment levels. The Google Earth Engine (GEE) platform was employed to process sensor imagery and calculate reflectance data, enabling accurate computations for specific time intervals. To further analyse the data, Support Vector Regression (SVR) model was developed. This model analyse changes in reflectance data to corresponding observed silt measurements, providing insights into sediment behavior. The results from this model are presented using 2D graphs, highlighting the effectiveness of remote sensing technology in understanding the sediment dynamics in large river systems. This research offers significant advancements in methods for monitoring and maintaining water quality in aquatic environments.
How to cite: Bhoopathi, S., Pal, M., and Choubey, H.: Suspended Sediment Concentration Analysis Using Remote Sensing and Machine Learning Approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15011, https://doi.org/10.5194/egusphere-egu25-15011, 2025.
The formation and evolution of sediment ribons over a uniform sediment bed in an open-channel flow was investigated via a stereo-photogrammetric system to measure the bed evolution in combination with a stereo-PIV system to measure the three-component velocity field in a cross-sectional plane above the bed. The formation of ribbons is observed to be triggered by the initially meandering low and high-speed streaks sharing the same spanwise wavelength as the fully-developed ribbons. When the ribbons are fully developed, the streaks are locked in place with low-speed streaks over the ridges and high-speed streaks over the troughs with strong secondary flows. The lateral stabilization appears to be facilitated by the stable streaks near the wall.
How to cite: Eiff, O. and Trevisson, M.: Formation and evolution of sediment ribbons in open-channel flow, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13625, https://doi.org/10.5194/egusphere-egu25-13625, 2025.
Step-pools are common bedforms in mountain streams and have been utilized in river restoration or fish passage projects around the world. Step-pool units exhibit highly non-uniform hydraulic characteristics which have been reported to closely interact with the morphological evolution. Further understanding towards these interactions builds the basis for better prediction of channel evolution and more advanced design of artificial step-pool system. However, detailed information on the flow-morphology interactions has been limited due to the difficulty in measuring the flow structures or the flow forces in a step-pool unit.
To fill in this knowledge gap, we established an approach combining physical experiment and computational fluid dynamics (CFD) simulation for a step-pool unit made of natural grains at six flow conditions. Structure from motion (SfM) was used to capture the detailed 3D reconstructions of the bed surfaces with various conditions of pool scour. The hydraulic measurement was applied both as input data at the inlet boundary and also in the validation for the CFD model. The high-resolution 3D flow structures for the step-pool unit were visualized, as well as the distributions of flow forces from both pressure and shear stress.
The results illustrate the segmentation of flow velocity downstream of the step, i.e., the integral recirculation cell at the water surface, streamwise vortices formed at the step toe, and high-speed flow in between, resulting from the complex morphology of the step-pool unit. Both the recirculation cells at the water surface and the step toe perform as energy dissipaters to the flow with comparable magnitudes. Pool scour development during flow increase leads to the expansion of the recirculation cells until step-pool failure occurs. Significant transverse variability of the flow forces from both the shear stress and pressure has been revealed. The flow forces in both streamwise and transverse directions are closely related to the flow structures and morphology in the unit. The ratios between skin and form drag have large variations at low flows while show a relatively limited range of 0.05-0.1 at high flows, suggesting a small proportion occupied by the skin resistance in the total flow resistance in the step-pool channel. The drag coefficient of the step-pool unit is around 0.3 at high flows. Our results highlight the feasibility of the approach combining physical and numerical modeling in investigating the complex flow-morphology interactions of step-pool features.
How to cite: Zhang, C., Hassan, M., and Xu, Y.: Investigating interactions between flow and morphology in a step-pool unit combining physical and numerical modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7900, https://doi.org/10.5194/egusphere-egu25-7900, 2025.
Understanding the morphodynamics of tidal dunes is essential for improving predictions of sediment transport and seabed evolution in coastal and estuarine environments. This study advances our understanding through a combined experimental and numerical investigation into the short-term morphodynamic evolution of laboratory-scale tidal dunes under controlled conditions.
Building on earlier flume experiments examining hydrodynamic interactions of reversing currents with fixed-bottom, sand-coated asymmetric compound dunes, we incorporated a cm-thick layer of unimodal sediment over the rigid dune models to simulate mobile-bed conditions. High-resolution Particle Image Velocimetry (PIV) was employed to capture detailed spatial and temporal dynamics of turbulent flows and the concurrent evolution of dune surfaces.
Complementary numerical modelling utilised the oceanographic circulation model CROCO, incorporating its non-hydrostatic solver and the USGS sediment transport module. The lab-scale model application was calibrated and validated against the laboratory measurements, demonstrating exceptional agreement in the short-term evolution of dune morphology. Key findings include the accurate replication of observed boundary layer dynamics, sediment transport mechanisms, and morphodynamic changes under reversing tidal currents. These experiments establish a solid benchmark for validating non-hydrostatic models of tidal dune morphodynamics.
This work underscores the transformative potential of integrating detailed physical experiments with advanced numerical models to refine our predictive capabilities for morphodynamic processes in tidal environments. The insights gained are particularly significant for coastal engineering and seabed mobility studies, with direct applications to the design and optimisation of offshore wind farm infrastructures.
How to cite: Porcile, G., Mouazé, D., Weill, P., Gangloff, A., and Bennis, A.-C.: Quantifying Tidal Dune Morphodynamics at the Laboratory Scale: A Combined Measuring and Modelling Approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19135, https://doi.org/10.5194/egusphere-egu25-19135, 2025.
Alluvial fans develop at the base of mountain fronts, where rivers emerge from the constrained mountain area onto the plain. Acting as a transition zone between mountain streams and alluvial rivers, the fan-river system is typically characterized by a slope break in the bed profile, a significant discontinuity in bed surface sediment fining from gravel-sized to sand-sized, and a sudden increase in channel width. In large rivers with great morphological diversity and strong human interference, the shift between upstream and downstream river morphology and sediment dynamics within the alluvial fan-river system exhibits a more complex process. However, this phenomenon remains insufficiently documented and lacks comprehensive analysis.
Here, we take the middle Yangtze alluvial fan as an example and use field observations and numerical modeling to improve the understanding of the large-scale alluvial fan-river system. The result shows that, in contrast to other alluvial fan-river systems, the Yangtze alluvial fan downstream of the Three Gorges Valley had no obvious breaks in the recent bed profile. In addition, the channel width showed an abrupt increase at Zhicheng, followed by a narrowing trend beginning at Chenjiawan. After the Three Gorges Dam (TGD) operation in 2003, the erosive water released from the TGD induced significant erosion, however, the spatial pattern of the bankful width remained stable. The bed profile exhibited increasing variability but continued to lack a distinct slope break; The transition in surface material from gravel to sand was observed throughout approximately 60 kilometers and the location migrated 40 kilometers downstream in the post-TGD period, with gravel and sand patches alternating randomly; Zhicheng and Chenjiawan are two characteristic locations marking the shifts in the mode of sediment transport in the middle Yangtze alluvial fan-river system. For sand transport mode, the reach upstream of Zhicheng had sand transported in suspension, whereas the downstream reaches were dominated by mixed-load transport. For gravel transport mode, gravel from upstream, mostly in the 25–50 mm grain size range, was selectively transported downstream of Zhicheng and deposited at Chenjiawan; The sediment dynamics in the Yangtze alluvial fan-river system were controlled by the width variability and distributary streams. The deposition of fine sand upstream of the gravel smoothed the previously deposited gravel fan profile, resulting in the absence of a slope break in the bed profile. Since 2003, the pattern of the sediment transport mode remained stable despite some local adjustments. This stability is attributed to the stable fan-river morphology induced by the strong resistance of riverbank lithologies and the Jingjiang Great Levee constraints.
How to cite: He, Z., Sun, Z., Li, Y., Luan, H., and Qu, G.: Large-scale alluvial fan-river system of the middle Yangtze River: morphological diversity, grain size discontinuity, and sediment dynamics complexity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14540, https://doi.org/10.5194/egusphere-egu25-14540, 2025.
Posters on site: Wed, 30 Apr, 16:15–18:00 | Hall X3
We present the results of laboratory experiments investigating the intense transport of bimodal bed load under high bed shear conditions in a tilting flume. Particles of two lightweight sediment fractions, differing in size, tend to separate during transport above the plane surface of an eroded mobile bed. Coarser fraction particles are predominantly present in the collisional layer above the bed, while finer fraction particles are primarily concentrated in the interfacial layer, which develops between the eroded bed and the collisional layer. This observed stratification of transported fractions influences their respective contributions to the total bed load discharge through the flume. Vertical distributions of local velocity and volumetric concentration were measured across the flow depth for each fraction separately, allowing the determination of each fraction's proportion in the total discharge. The experimental results were combined with a previously collected dataset to compare the discharges of bimodal and unimodal sediments under hydraulically similar conditions. Additionally, the experimentally determined discharges were evaluated against predictions from transport models designed for intense unimodal and bimodal bed loads.
How to cite: Matousek, V., Krupicka, J., Picek, T., and Svoboda, L.: Quantification of intense transport of fractions of stratified bimodal bed load based on measured distributions of velocity and concentration, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8700, https://doi.org/10.5194/egusphere-egu25-8700, 2025.
Sediment transport dynamics are of great importance in understanding geophysical flows, where determining the threshold conditions for the initiation of sediment motion presents a complex challenge. In a pioneering work, Shields (1936) established the Shields’ criterion to assess the critical shear stress (τc) required for sediment motion in non-turbulent flows. Although this approach has significant advantages, including a robust empirical foundation and the implementation of non-dimensional critical shear stress, it is valid for limited conditions since it oversimplifies vital aspects such as sediment heterogeneity and complex flow interactions.
In turbulent flows, the effective critical shear stress acting on a grain may become higher than that measured in the case of laminar flows (i.e., the average critical stress, τc, defined by Shields, 1936) as a result of fluctuations in the shear stress (τ′). Owing to this, in geophysical turbulent flows near the threshold of motion, neither the driving nor the resisting parameters of sediment motion have crisp values; instead, they may be considered probabilistic parameters. The reliability-based approach is applied here in to handle the complex nature of the initiation of sediment motion.
This study aims to present preliminary results of research that aims to enhance the knowledge of incipient motion by applying a reliability-based analysis of Shields’ criterion based on the theory and empirical equations adopted by Zanke (2003). In this analysis, the turbulence parameter (n) and angle of repose (ϕ) are introduced as key parameters regarding the initiation of sediment motion. These parameters are generated as random variables by means of Monte Carlo Simulations, introducing various probabilistic distributions (e.g., normal, log-normal, triangular, gamma) and statistical moments (e.g., mean, standard deviation).
By simulating a wide range of angles of repose and turbulence parameters with Monte Carlo Simulations, the inherent uncertainties in sediment transport and the complexity of hydrodynamic models are incorporated. In this work critical shear stresses of thousands of grains are assessed for different grain Reynolds numbers. As a result, threshold of motion curves are probabilistically derived, indicating confidence for grain entrainment, and establishing a model that enables risk assessment and decision-making for a wide range of scenarios. Comparisons of model results with empirical data show that the model captures the complex physical process.
How to cite: Baysal, S., Kırca, V. Ş. Ö., and Valyrakis, M.: Reliability-Based Analysis of Initiation of Sediment Motion on Movable Bed, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8745, https://doi.org/10.5194/egusphere-egu25-8745, 2025.
We employ data about a dry granular flow down a 19º smooth-walled chute, partially obstructed at the downstream end, to verify the solution of a shallow-water continuum model. The system of conservation equations is based on depth-averaging the ensemble-averaged mass, momentum and fluctuating kinetic energy equations:
(1) $\partial_t \left(\phi h \right) + \partial_x \left(\phi h u \right) = - \partial_t z_b$
(2) $\partial_t \left( \rho h u \right) + \partial_{x} \left( \rho h u^2 \right) = -\partial_{x} \left( \rho g h^2 / 2 \right) - g \rho h \, \partial_{x} z_b - \tau_b$
(3) $\partial_{t} z_b = - \left( E(x,t) - D(x,t) \right)$
(4) $P = f(\phi) f(e,k,\phi_c) \rho_g T$
(5) $-Q^\prime + \frac{1}{2}\tau_b u/h - \Gamma = 0$
where $x$ is the distance, $t$ is time, the conservative variables are the elevation of the granular bed, $z_b$, the equivalent depth of flowing granular material $\phi h$ and flow momentum $\rho \phi h$, where $\phi$ is the solid fraction, $h$ the granular depth and $u$ the depth-averaged longitudinal velocity, $\tau_b$ is the wall stress, $E$ and $D$ are the rates of particle pick-up and deposition, respectively, $e$ is the normal coefficient of restitution, $k$ is particle stiffness, $\phi_c$ is the critical solid fraction, $\rho_g$ is the density of the solid particles, $\rho = \rho_g \phi$, $\Gamma$ is the rate of dissipation of fluctuating kinetic energy and $Q^\prime$ is the flux of fluctuating kinetic energy at the bottom wall. The solid fraction is determined from (4) as a function of the granular pressure $P$ (considered hydrostatic) and the granular temperature $T$.
Preliminary results of simulations with borosilicate spheres ( g/cm3 and coefficient of restitution ), with and as tuning parameters, indicate that the celerity of the jamming wavefront is well-reproduced. The jump strength and the head losses are not in full agreement, requiring adjustments in the equation of state (4).
Acknowledgements
Portuguese Foundation for Science and Technology (FCT) through the PhD scholarship PD/BD/150693/2020, project PTDC/ECI- EGC/7739/2020 and CERIS funding UIDB/04625/2020.
How to cite: Ferreira, R. M. and Mendes, S.: Shallow-water continuum modelling of dry granular flows in partailly obstructed chutes , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13548, https://doi.org/10.5194/egusphere-egu25-13548, 2025.
Landslides in narrow valleys may block adjacent rivers and dam the incoming water flow. The collapse of these landslide dams may lead to catastrophic flooding downstream. The measurement and early warning of dam failures is an important issue in geomorphic processes. However, Optical and radar-based monitoring methods are not suitable for deep internal probing of a dam, which is necessary for dynamic measurement and early warning. In this study, the acceleration of a smart rock in the simulation dam was measured using inertial navigation method. It is found the acceleration response of smart rocks is detected more than 20 seconds before external observations of dam failure. Buried at different positions within a dam, smart rocks exhibit distinct temporal and data form responses to dam failure. Smart rocks located deeper within the dam show multiple acceleration fluctuations before the actual failure occurs. We hope that the measurement data provided by smart rocks will assist in developing multi-scale models of dam failure.
How to cite: Li, R., Li, Y., Mu, T.-T., and Dai, H.: Experimental investigation on the formation and failure of landslide dam using inertial navigation method, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14061, https://doi.org/10.5194/egusphere-egu25-14061, 2025.
Debris flows, prevalent in mountainous regions, exhibit distinct dynamics depending on whether they occur over bedrock (rigid bed) or accumulated deposition (erodible bed). Understanding the transition between these bed types is essential for hazard prediction and mitigation. This study improves an existing unsteady, non-uniform debris flow model to more accurately simulate the evolution of flow depth and velocity under varying boundary conditions. The improved model is grounded in mass, momentum, and kinetic energy conservation principles, incorporating a linearized μ(I) rheology to describe granular flow behavior and Coulomb friction along sidewalls, ensuring a realistic representation of debris flow mechanics.
To validate the improved model, granular dam break experiments were conducted in a narrow glass channel (3.5 m long, 0.04 m wide) with varying downstream deposit depths to establish different basal boundary conditions. High-speed camera footage and Particle Tracking Velocimetry (PTV) were employed to capture granular motion and generate velocity fields. The model exhibited good agreement with experimental results, accurately predicting the flow depth and velocity evolution during the transition between rigid and erodible beds.
Furthermore, the model was applied to field-scale debris flows at the PuTunPunas River in southern Taiwan, a site that has experienced several debris flow events over the past decades. Channel width variations at this site were incorporated into the model to assess erosion potential and flow behavior under real-world conditions. Comparisons with field observations confirmed the model’s capability to simulate debris flow transitions and erosion processes in natural channels, offering valuable insights for hazard assessment and mitigation in mountainous regions.
How to cite: Liao, C.-Y., Tsai, Y.-L., and Hung, C.-Y.: Modeling Debris Flow Transitions: Experimental Validation and Field-Scale Application, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14717, https://doi.org/10.5194/egusphere-egu25-14717, 2025.
Geophysical granular flow is ubiquitous in nature and plays a crucial role in shaping the landscape (hillslope creep, riverbed evolution) and causing geohazards (landslide, debris flow). Small-scale models are an effective way to understand these natural phenomena at large scales. However, finite-size effects inevitably occur due to the multi-scale nature of granular materials, hindering integration of mechanisms obtained from small-scale investigations and continuum models (e.g., granular flow rheology) for large-scale applications. Here we use granular column collapse as a model case to address finite-size effects in granular flows from a novel rheological perspective. We computationally simulate column collapse of varying system-to-particle size ratios using the discrete element method and extract detailed local rheological information during the flow via a coarse-graining technique. We find a disproportional increase in the dimensionless runout distance with the system-to-grain size ratio and a significant difference in the dynamic process. This discrepancy is reflected in the μ(I) curve as non-collapsed data at low inertial number regimes, but casting the data into a non-local rheology framework proposed by Kim and Kamrin (2020) leads to data collapse onto a single master curve for all simulations. This indicates that the finite-size effect is controlled by velocity fluctuations at the grain scale and is a manifestation of the non-locality of granular materials. As a result, the introduction of an intermediate length scale that reflects velocity fluctuations is expected to enable accurate modeling of geophysical granular flows with varying system and particle sizes in a unified continuum framework. It also provides a new perspective for continuum modeling of polydispersity, size segregation, hysteresis, and other size-dependent phenomena in geophysical granular systems.
How to cite: Xia, J., Jing, L., and Peng, M.: Finite-Size Effects in Geophysical Granular Flow from a Nonlocal Rheology Perspective, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7958, https://doi.org/10.5194/egusphere-egu25-7958, 2025.
Dunes are ubiquitous in river, marine, desert and Martian environments. The flow of fluid
over mobile bed results in evolution of dunes of different sizes and shapes. The shape of dune
has critical role in sediment transport and interacting with flow. Earlier studies assumed the
dune shape as a triangle (2-Dimensional) to study the flow field over dunes. However, dunes
are highly three dimensional and their 3D patterns can increase the form drag compared with
equivalent 2D dunes in similar flows. Pearson correlation, 2D spatial correlations are used to
describe three dimensionality of dune in previous studies. A robust methodology to quantify
3D bed forms and linking it to the flow needs to be developed. In this study, experiments are
conducted to form 3D dunes on plane bed with non-uniform fine sand (d50 = 0.395 mm, σg =
1.56) under sub critical flow conditions. The bed morphology is continuously monitored
using ultrasonic ranging probes (URS) placed 5 cm c/c distance in 1 m wide flume.
Experiments are performed till equilibrium state is achieved and continued further (2 hrs) to
observe the bed changes. The equilibrium bed is measured at 2 cm resolution with a laser
distance meter. The 3D velocity components and suspended sediment concentration are
continuously measured using down looking Accoustic Doppler Velocimeter (25 Hz). Signal
processing techniques are used to remove outliers, to smoothen the local fluctuations and
identification of dune crest and troughs. In addition to 2D correlation and Pearson correlation
coefficient, Fractal dimensions and topological metrics are also used to asses three
dimensionality of the sediment bed. Roughness of the sediment bed is quantified using
standard deviation of bed elevation. From the experiments, it was observed that three
dimensionality is reduced with an increase in discharge. The spatial data is transformed into
frequency domain. Periodicity of the process is analyzed from harmonics and spatially
averaged spectrums. The height and length of dunes is modelled using exponential fits and
observed a nonlinear growth of dunes. The flow measurements showed that the flow velocity
in lobe region and turbulent kinetic energy in saddle region are increased. The mean sediment
flux in the flow direction is directly proportional to the depth. Whereas, the turbulent fluxes
exhibit an increasing trend up to 0.36–0.38 times the flow depth and then decrease with
further increases in flow depth.
How to cite: Bodapati, S. S. P. and Chandra, V.: The Fractal and Topological Metrics for Assessing Three-Dimensionality in Dune Morphology , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18148, https://doi.org/10.5194/egusphere-egu25-18148, 2025.
Posters virtual: Mon, 28 Apr, 14:00–15:45 | vPoster spot 2
EGU25-2032 | ECS | Posters virtual | VPS25
Towards Enhancing River Bed Stability Assessment: A Comparative Study of LSTM, GRU, and Transformer Predictive ModelsMon, 28 Apr, 14:00–15:45 (CEST) | vP2.13
As decision support methods (including as Artificial Intelligence supported decision making) progress, new ways and frameworks are emerging to enhance our understanding of sediment transport processes (via improved monitoring), but also better modeling those phenomena. This study offers a preliminary view of how deep learning models can link with real-time data from instrumented sediment particles, to predict the risk of bed surface destabilization in channels and rivers, which can lead to infrastructure scour.
Specifically, three deep learning models are analyzed, herein: a) Long Short-Term Memory (LSTM), b) Gated Recurrent Units (GRU), and c) Transformers. These models were compared according to their accuracy, computational efficiency, and suitability for real-time applications.This study integrates data from specifically designed sediment stability monitoring sensors [1-3], with three deep learning models to predict the possibility that sediment is transported along the bed surface of the river [4], in real time. This is important for a series of applications, such as flood risk management, assessment of hazards to hydraulic infrastructure and water resource management, helping achieve resilient and sustainable development under a changing climate change. Future studies can explore further improving the efficiency of sensor enabled novel hydroinformatics approaches.
References
[1] Al-Obaidi, K., Xu, Y., & Valyrakis, M. (2020). The design and calibration of instrumented particles for assessing water infrastructure hazards. Journal of Sensor and Actuator Networks, 9(3), 36.
[2] AlObaidi, K., & Valyrakis, M. (2021). Linking the explicit probability of entrainment of instrumented particles to flow hydrodynamics. Earth Surface Processes and Landforms, 46(12), 2448-2465.
[3] Al-Obaidi, K., & Valyrakis, M. (2021). A sensory instrumented particle for environmental monitoring applications: Development and calibration. IEEE Sensors Journal, 21(8), 10153-10166.
[4] Valyrakis, M., Diplas, P., & Dancey, C. L. (2011). Prediction of coarse particle movement with adaptive neuro‐fuzzy inference systems. Hydrological Processes, 25(22), 3513-3524.
How to cite: Mavris, I. and Valyrakis, M.: Towards Enhancing River Bed Stability Assessment: A Comparative Study of LSTM, GRU, and Transformer Predictive Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2032, https://doi.org/10.5194/egusphere-egu25-2032, 2025.
EGU25-2045 | ECS | Posters virtual | VPS25
Resilience of Mediterranean Mussels to Hydrodynamic Stresses: Insights for Climate Change AdaptationMon, 28 Apr, 14:00–15:45 (CEST) | vP2.5
The increasing frequency and magnitude of extreme weather events across the Earth's surface results in increasing pressure for living organisms and their habitats, including those in aquatic ecosystems. The main focus of this study is on the resilience of Mediterranean mussels (Mytilus galloprovincialis) against pronounced hydrodynamic stresses that may be experienced more frequently compared to the past. These mussels can be typically found in Mediterranean coasts and estuaries (such as in Greece, Spain, Italy, and Portugal), and they are also extensively farmed in the open sea using aquaculture practices. As such, they are of particular interest given their economic significance for Mediterranean countries, as well as their ecological role (offering significant ecosystem services as "ecosystem engineers", such as coastal protection).
The hydrodynamic stress of Mediterranean mussels is herein assessed indirectly using appropriately designed wave-flume experiments and analyzing video observations of the effects of wave motions of different characteristics on the Mediterranean mussels. For these experiments we embed specialised sensors to these mussels so they can record even minute displacements and changes in their orientation [1, 2]. Specifically, small, medium, and large mussels are exposed to two different configurations (similar to earlier studies [3]) on the surface of an artificial seabed, over which different wave fields are traversing. The movement of individual mussels was visually evaluated under varying wave intensities, transitioning from high to low energy and vice versa. These observations aim to determine the conditions and orientations under which these organisms drift relative to the wave flow direction or remain practically undisturbed. In the context of climate change and its impact on marine environments, this study may provide valuable insights into efforts to protect endangered marine species and enhance strategies for safeguarding aquaculture crops against damage caused by storms or significant wave fields.
References
[1] AlObaidi, K., & Valyrakis, M. (2021). Linking the explicit probability of entrainment of instrumented particles to flow hydrodynamics. Earth Surface Processes and Landforms, 46(12), 2448-2465.
[2] Al-Obaidi, K., & Valyrakis, M. (2021). A sensory instrumented particle for environmental monitoring applications: Development and calibration. IEEE Sensors Journal, 21(8), 10153-10166.
[3] Curley, E.A.M., Valyrakis, M., Thomas, R., Adams, C.E., & Stephen, A. (2021). Smart sensors to predict entrainment of freshwater mussels: A new tool in freshwater habitat assessment. Science of the Total Environment, 787, 147586.
How to cite: Karagianni, E. and Valyrakis, M.: Resilience of Mediterranean Mussels to Hydrodynamic Stresses: Insights for Climate Change Adaptation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2045, https://doi.org/10.5194/egusphere-egu25-2045, 2025.
