Ocean surface waves and currents can interact to produce strong seabed shear stress and mobilization of sediments that can significantly impact the seabed stability and benthic habitats on continental shelves. Modelled waves, near-bottom tidal current and circulation current data for a 3-year period were used in a widely applied sediment transport module to simulate the seabed shear stresses and the mobilization of observed sediment grain size on the Scotian Shelf of eastern Canada.

The Scotian Shelf is affected by strong waves and tidal currents. These waves, currents and/or their interaction cause maximum mean bed shear velocities of 5 – 10 cm s⁻¹. Observed sediments on the Scotian Shelf can be mobilized by tidal currents at least once during the modelled 3 year period over 28% of the shelf area while waves can mobilize sediments over 60% of the shelf area suggesting much stronger sediment mobilization by waves. Interaction between waves and currents can produce enhanced combined wave-current shear velocity that is capable to mobilize sediments over 74% of the shelf area. The spatial variation of the relative importance of sediment mobilization frequency by component processes was used to classify the Scotian Shelf into six disturbance types. In comparison with previous studies using depth-averaged tidal currents, the present study based on near-bottom tidal currents has resulted in reduced sediment mobilization frequency by tidal currents, smaller extent of high mobility areas and significant changes of the spatial pattern of disturbance type distribution on the Scotian Shelf. The universal Seabed Disturbance Index and Sediment Mobility Index have also been applied to quantify the seabed exposure to physical processes and sediment mobilization on the Scotian Shelf by accounting for both the magnitude and frequency of these processes. The results of this modelling study are important for environmental assessments and for the spatial planning and management of the Scotian Shelf bioregion. 

How to cite: Li, M., Wu, Y., Ma, Y., and Wang, Y.: Modelling seabed shear stresses and sediment mobilization on the Scotian Shelf, eastern Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2864, https://doi.org/10.5194/egusphere-egu23-2864, 2023.

Studies have categorized turbulent bursting events into outward interactions, ejections, inward interactions, and sweeps. Among these events, ejections and sweeps contribute notably to the time consumption, momentum flux, and sediment flux. Studies have reported that the distribution of turbulent coherent structures is uniform. However, research has revealed that the distribution of bursting events is nonuniform at different bed elevations. Although the nonuniform distribution of turbulent bursting events has been investigated, their influence on sediment transport has yet to be examined. This study established an improved stochastic diffusion particle tracking model (SD-PTM) using the stochastic Lagrangian method to describe sediment particle movement. This model integrates turbulent characteristics determined using a direct numerical simulation data set for comprehensively analyzing the sediment particle motion during turbulent flow. We developed a modified SD-PTM that considers the nonuniform spatial distribution of ejection and sweep events and the particle movement direction during these events. Particle trajectories were obtained using this model, and the anomalous diffusion during sediment transport was analyzed by calculating the variance in the particle trajectories. The performance of the proposed model was evaluated by comparing the flow velocities and sediment concentration profiles obtained using it with those measured in previous studies. Therefore, the sediment particle motion during turbulent flow was comprehensively investigated under extreme flow conditions.

How to cite: Tsai, C. and Wu, M.-J.: Stochastic Modeling for Anomalous Suspended Sediment Transport, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10870, https://doi.org/10.5194/egusphere-egu23-10870, 2023.

Landscape Evolution Models (LEMs) are powerful tools for simulating erosion and accumulation patterns over a landscape. To accurately capture the complexity of the system, several parameters are used to describe the flow of water on a surface and thereby calculate a change in landscape. However, due to the need for reasonable computing power, simplification is often necessary. The CAESAR-Lisflood LEM simplifies the transition from precipitation to runoff with a cell storage-based system, where water is retained in each cell and released at later iterations, simulating infiltration and forming the hydrograph. This reduced complexity works well for surface processes, but can lead to issues in catchments, where ground water flow and infiltration are unknown parameters, and an unknown proportion of sediment transport is connected to sub-surface processes.

Located in the Northern Franconian Jura, Germany, the Weismain River catchment (~125 km2) is a particularly challenging area to model the sediment dynamics, due to its karstic geology. The Weismain river and its tributaries are deeply incised into a limestone plateau forming small, well-defined valleys opening to wider floodplains in the lower parts of the catchment, where sandstone is dominant. To gain a better understanding of the sediment dynamics and the evolution of this catchment, the impact of the karstic environment needs to be evaluated. With CAESAR-Lisflood we are looking at the spatial distribution of alluvial sediment from different model inputs and how differences in discharge calculations affect model results. With this model setup we can gain insight into the sediment dynamics of the catchment and increase confidence in future results.

How to cite: Ringleb, B. and Fuchs, M.: The impact of geology on sediment dynamics. A modelling approach for the Northern Franconian Jura, Germany., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7781, https://doi.org/10.5194/egusphere-egu23-7781, 2023.

Repeated alpine glaciations during the Quaternary partly reached far into the foreland and caused profound landscape changes beyond glacial margins in northern Switzerland. Climate-driven glacier growth and decay and commensurate variations in water and sediment delivery caused river systems to aggrade and to incise, leading to the widespread occurrence of glacio-fluvial deposits (Deckenschotter) and associated terraces. Mapping and numerical dating of these depositional complexes increasingly offer insights into the spatial patterns and timing of Quaternary glaciations, and associated changes in (glacio-)fluvial dynamics. An improved understanding of how fluvial systems respond to glacial-interglacial cycles will help to assess the erosion potential around repository sites of nuclear waste over the next one million years. In this study, we contribute to close this research gap using numerical landscape evolution modelling (LEM).

We use EROS, a numerical landscape evolution model, which implements a particle-based approach to simulate water and sediment fluxes that interact with topography through erosive and depositional actions. Unlike LEMs based on the stream-power incision law, the method solves the 2D shallow water equations with both basal and lateral erosion and deposition, which allows for variations in width and lateral mobility of rivers; these variations induce changes in the transport capacity of the sediments that cause specific patterns of deposition and erosion to emerge. We adjusted the model so that it is capable to run over 1 Myrs, and imposed boundary conditions that - informed by estimates on longterm erosion rates – reflect rock uplift and plausible variations in water and sediment fluxes following a 100-kyrs glacial cycle. Our model relies on digital elevation models and sediment thickness data with 60 m spatial resolution and is applied to the Hochrhein river between Stein am Rhein and Basel and the Aare river downstream of the area, where Limmat and Reuss enter.

Our simulations show that the model reproduces widespread aggradation, reworking of the sediments by highly laterally mobile, braided river systems and incision during periods of increased runoff and low sediment availability.

Our model setup and parametrization features several uncertainties. For example, the capacity of rivers to laterally erode strongly determines the thickness and extent of depositional complexes lining the Hochrhein and Aare system. Also, our model is sensitive to temporally varying boundary conditions of water and sediment input about which precise estimates are lacking. Regardless, the more detailed and realistic representation of hydraulic and sediment transport processes by EROS compared to conventionally used landscape evolution models at this spatial and temporal scale provide the opportunity to test different hypotheses using numerical experiments and link the results to field evidence. Further sensitivity analyses and uncertainty quantification will enable us to use our model as simulation tool to hindcast and investigate the behaviour of fluvial system in response to different tectonic and climatic scenarios, thus helping to better understand potential spatial patterns of and sediment assemblages within widespread glacio-fluvial deposits.

How to cite: Mey, J., Schwanghart, W., Landgraf, A., and Davy, P.: Fluvial response to glacial-interglacial cycles - modelling the evolution of the Hochrhein using EROS, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2327, https://doi.org/10.5194/egusphere-egu23-2327, 2023.

Records of incision history such as topographic data and landform dating can be gathered in inversion schemes to reconstruct base-level fall and uplift history. Here, we use a non-linear inversion scheme and the stream power incision model to study the landscape evolution of a mountainous area to quantify whether it responds to an uplift or a capture-induced local base-level fall. Our inversion is constrained by ¹⁰Be cosmogenic nuclide data. The probabilistic approach yields an ensemble of solutions made by a combination of model parameters. We apply our model to the Draa Canyon located in the Anti-Atlas of Morocco at the outlet of the Ouarzazate Basin which was internally drained during the Miocene. We show that incision rates in the Ouarzazate Basin and the southern margin of the High Atlas are compatible with a Pleistocene age for the opening of the Basin. The forcing to drainage integration may be due to capture by regressive erosion in the proto Draa river or tilting to the south of the High Atlas, Ouarzazate basin, and Anti-Atlas as a whole in response to mantle-related continental-scale uplift, or a combination of both. The southern border of the High Atlas in this region displays a transient landscape previously interpreted as evidence for recent shortening and rock uplift. Our results suggest that the rejuvenation of the southern Central High Atlas, in the northern margin of the Ouarzazate Basin, mainly occurred in response to the opening of the Ouarzazate Basin, with only several hundreds of meters of rock uplift localized along the South Atlas front during the late Cenozoic.

How to cite: Babault, J., Owen, L. A., Arroucau, P., Charco, M., Bodet, L., Van Den Driessche, J., and Caffee, M.: Pleistocene opening of the Ouarzazate Basin, and incision rate history of the Draa canyon in the Anti-Atlas of Morocco revealed by 10Be cosmogenic nuclide dating and non-linear river profile inversion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14510, https://doi.org/10.5194/egusphere-egu23-14510, 2023.

Any tectonic forcing acting on a mountain range consists of vertical uplift motion and horizontal advection motion. In landscapes with a high advective component, such as those affected by low angle normal faults, advection can form a substantial portion of the total tectonic forcing. Questions therefore remain relating to the role of advection in shaping mountain range topography, surface drainage patterns and erosion rate distributions. We ask, how does advection influence mountain range topography and drainage? How do drainage basins and divides respond to advection? And how does advection influence the spatial distribution of erosion rates? Through numerical modelling with the Channel-Hillslope Integrated Landscape Development (CHILD) model and comparison to a natural landscape, the Sierra de la Laguna (Mexico), we test the extent to which advection affects mountain range evolution. Advection is shown to alter surface drainage patterns by promoting catchment elongation and reducing outlet spacing at the mountain front. The mountain range’s Main Drainage Divide (MDD) exhibits faster erosion on the distal flank’s headwaters; relative to the proximal flank’s headwaters, leading to a migration of the MDD towards the fault. Steady-state is achieved when the MDD migrates towards the fault at a rate equal to the rate at which the footwall is advected away from the fault. This pattern of erosion rates is found across all spatially fixed geomorphic features. Cosmogenic radionuclide analysis for catchment-averaged erosion rates in the Sierra de la Laguna demonstrates a difference in erosion rates across the MDD that is also indicative of a migration of the MDD towards the fault. Topographic, drainage and erosion rate observations for the Sierra de la Laguna are consistent with the numerical modelling when advection is included, suggesting that advection induces continual drainage migration whilst maintaining mountain range flank widths. We suggest that highly elongate catchments are indicative of substantial advection, and we also identify a mechanism through which divide migration can occur without altering the size of adjacent drainage basins.   

How to cite: Hoskins, A., Attal, M., Mudd, S., and Castillo, M.: Drainage Divide Migration in Steady-State Landscapes Experiencing Horizontal Tectonic Advection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7602, https://doi.org/10.5194/egusphere-egu23-7602, 2023.

How to cite: Gailleton, B., Steer, P., Davy, P., Schwanghart, W., and Bernard, T.: GraphFlood: a fast stationary solution for 2D hydrodynamics in landscapes evolution models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3421, https://doi.org/10.5194/egusphere-egu23-3421, 2023.

Most of the alluvial rivers in the globe are suffering and necessitate critical management regarding sand and gravel extraction and other related river ecosystem protection. Sediment mining is the activity of extracting useful materials such as sand, gravel, and aggregate from a river bed, banks, and flood plains. Sediment mining activity causes hydro-morphodynamic variations on the river bed, impacting the river plan and hydraulic structures, and also can harm flora and fauna within the river ecosystem. The period and magnitude of the mining rate with the discharge and channel bed material properties directly impact channel bed mobility and bed-level equilibrium conditions. The flow field, bedload transport, and morphological evolution in the pit's vicinity are varied in space-time, multifaceted, and three-dimensional. Most of the preceding researchers on sediment mining characteristics were concerned with the physical aspects, scouring study, and degradation rate. Very few researches are available on the flow regions and morphological bathymetry of a river under sediment mining. On the bases of this area of gaps, the present study aimed to understand the flow field and bathymetry of the channel under sediment mining. The experimental work is conducted at the Hydraulics Laboratory of the Civil Engineering department, IIT Roorkee, India, on a trapezoidal sediment mining pit constructed of uniform cohesionless bed material in an open channel flow. The average streamwise velocity was measured at five various sections near the sediment mining pit using a three-dimensional acoustic doppler velocimeter. The numerical simulation was conducted in a Flow-3D solver using the Reynolds Averaged Navier–Stokes equations (RANS), standard k-ɛ, the volume of fluid, and FAVOR, and the results were compared to the experimental observations. Both approaches depict an increase in the average longitudinal velocity at the upstream and downstream nick points. The observed longitudinal velocity from the experiment at the downstream nick point is higher than the velocity simulated using CFD at the downstream nick point. Both approaches indicated a significant increase in the longitudinal velocity downstream of the pit center, especially in the flume center. CFD simulations depicted the decreasing velocity at the upstream nick point due to more degradation and increment of flow depth at the section. The detachment and degradation of the upstream nick were observed in the initial stage of the experimental work but not at the downstream nick point. Because the pit is used as a bedload trap, the likely sediment-free water compelled out of the pit causes bed erosion and flattening in the downstream section of the sediment mining pit. The degradation at the upstream nick was 4.6% and 9% for the experimental observations and the CFD simulations, while it was 21.3% and 3.1% for the downstream nick, respectively. The findings of this study can help authorities and experts in the effective maintenance and supervision of river ecosystem balance by supplying cost-effective sediment resources.

How to cite: Abdullahi, N. H. and Ahmad, Z.: Experimental and CFD Studies on the Flow Field and Bed-Morphology in the vicinity of a Sediment Mining Pit, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-446, https://doi.org/10.5194/egusphere-egu23-446, 2023.

Recently, several extreme flood hazards with active sediment and driftwood transport have occurred in mountainous areas of Japan, as typically observed in the Akatani river flood disaster in 2017. Therefore, in order to mitigate such hazards, it is required to develop methods to evaluate flood, sediment, and driftwood runoff from a basin during heavy rainfall. This research proposes integrated methods to evaluate flood, sediment and driftwood runoff from a basin during heavy rainfall using numerical methods. The proposed methods are applied to the Terauchi Dam basin, where large amounts of sediment and driftwood were discharged during the 2017 Northern Kyushu heavy rainfall event, to discuss the applicability of the methods.

The methods are composed of a distributed rainfall-runoff model, slope stability analysis, sediment and driftwood transport in the slope based on mass system equation, and sediment and driftwood transport in the river channel by unit channel model. These models are integrated as Rainfall-Sediment Runoff (RSR) model to evaluate sediment and driftwood runoff from a basin. As a result of its application to the Terauchi Dam basin, we found that whether or not the debris flow enters the river channel, i.e., the definition of the upstream end of the river channel, has a significant impact on the results. Therefore, we investigated whether the sediment supplied by debris flow enter the river channel during the actual event. The results show that the upstream end of the river channel in the computation should be defined as approximately 4% to 10% slope, and in case the mesh size is sufficiently fine, the debris flow inflow into the channel is sufficiently evaluated. In conclusion, this research proposes an integrate methods to evaluate flood, sediment and driftwood runoff from a basin, and discusses its applicability to the disasters such as the 2017 Northern Kyushu heavy rainfall event.

How to cite: Harada, D. and Egashira, S.: Method to evaluate sediment-driftwood transport processes with flood runoff in a basin during heavy rainfalls, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4275, https://doi.org/10.5194/egusphere-egu23-4275, 2023.