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.

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.

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.

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.

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.

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.