HS9.4

Introduction: Erosion has become an urgent problem to society due to the increasing intensity and frequency of disturbances, e.g. storms, wave energy and rainfall. Yet, a universal model to predict erosion thresholds for cohesive sediment is still missing. Short range interaction of clays is recognized as the source of cohesion and adhesion of cohesive sediment. The interaction of negatively charged (i.e., montmorillonite (MMT) and beidellite (BD)) and neutral clay particles (i.e., kaolinite (KL)) are traditionally simulated through DLVO theory and van der Waals interaction[1]. However, the applicability of DLVO theory at short range (i.e., at distance less than 3 nm) has been increasingly challenged in molecular dynamics simulations[2]. A suitable description of short-range clay particle interaction is crucial for the prediction of cohesive sediment erodibility. The aim of this study was to determine how clay mineralogy and water chemistry influence clay particle interactions at short range to affect inter-particle attraction and stability under imposed forces.

Methods: Molecular dynamics models of clay minerals and water were created using LAMMPS to simulate the interactions between water, clay and dissolved salt to investigate the forces determining clay cohesion, i.e. attractive force between clay particles [3]. A 2-layer model was used in this study, which is a simplification of the multi-layer particles found in nature. A multifactorial design was used with two factors: mineralogy and salinity. For clay mineralogy, kaolinite (KL), beidellite (BD) and montmorillonite (MMT) with sodium (Na) counter ions were tested. Three types of salt were considered, i.e., KCl, NaCl and CaCl2, with concentration ranging from 1% to 4%. Clay particle interactions with bulk water containing salt solution were simulated for 5 ns. Clay swell, i.e. the increase in the interlayer distance (d the distance between the mass center of two adjacent layers) and the underlying forces were quantified. The resistance of clay particles to imposed force was also investigated. 

Results and Discussion: First, for most negatively charged MMT and BD treatments, positively charged cations act as a bridge to hold clay layers together, which contrasts with the swelling predicted by DLVO theory. Second, Na-MMT with -0.375, -0.5, and -0.625 e/unit swelled in pure water, induced by the breakdown of cation bridges rather than osmotic swell pressure. Third, low concentrations of dissolved salt (i.e. KCl, NaCl or CaCl2) inhibit the swelling of MMT, by increasing the cation bridge strength. Fourth, non-charged KL did not swell because of strong van der Waals interaction. Finally, stable clay particles were more resistant to external pull and shear forces. These novel molecular dynamics simulations are helping to uncover the mechanisms controlling clay cohesion to support new formulations to predict the erodibility of cohesive sediment. 

Acknowledgements: The support of the UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/T001100/1. 

References: [1] Grabowski et al. (2011) Earth Sci Rev 105:101-120; [2] Shen and Bourg (2020) J. Colloid Interface Sci. 584:610-621 [3] Chen et al. (2022) ACS omega (accepted).

How to cite: chen, W., Grabowski, R., and Goel, S.: Modelling short-range interaction of clay particles to improve erodibility prediction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1172, https://doi.org/10.5194/egusphere-egu22-1172, 2022.

We investigate the submerged collapse of weakly polydisperse, loosely packed cohesive granular columns, as a function of aspect ratio and cohesive force strength, via grain-resolving direct numerical simulations. The cohesive forces act to prevent the detachment of individual particles from the main body of the collapsing column, reduce its front velocity, and yield a shorter and thicker final deposit. All of these effects can be accurately captured across a broad range of parameters by piecewise power-law relationships. The cohesive forces significantly reduce the amount of available potential energy released by the particles. For shallow columns, the particle and fluid kinetic energy decreases for stronger cohesion. For tall columns, on the other hand, moderate cohesive forces increase the maximum particle kinetic energy, since they accelerate the initial free-fall of the upper column section. Only for larger cohesive forces do the peak kinetic energy of the particles decrease. Computational particle tracking indicates that the cohesive forces reduce the mixing of particles within the collapsing column, and it identifies the regions of origin of those particles that travel the farthest. The simulations demonstrate that cohesion promotes aggregation and the formation of aggregates. They furthermore provide complete information on the temporally and spatially evolving network of cohesive and direct contact force bonds. While the normal contact forces are primarily aligned in the vertical direction, the cohesive bonds adjust their preferred spatial orientation throughout the collapse. They result in a net macroscopic stress that counteracts deformation and slows the spreading of the advancing particle front.

How to cite: Zhu, R., He, Z., Zhao, K., Vowinckel, B., and Meiburg, E.: Grain-resolving simulations of submerged cohesive granular collapse, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1888, https://doi.org/10.5194/egusphere-egu22-1888, 2022.

Microplastics have been reported in environmental media for decades, but gaps in our knowledge about them still remain. We investigated the third biggest freshwater lake in China – Taihu Lake – and the 30 major rivers around it. Microplastics were detected in lake water and sediment, and in river water, at abundances varying from 1.7 to 8.5 items/L, 460 to 1380 items/kg and 1.8 to 18.2 items/L, respectively. Inflow rivers were more polluted with microplastics than outflow rivers. The most common shape was fragment. Microplastic sizes of < 100 μm dominated in inflow rivers, 100–200 μm dominated in lake water and outflow rivers. The average size of microplastics in outflow rivers (200.4 μm) was larger than that in inflow rivers (166.2 μm). Microplastics of < 100 μm only accounted for 28% in the lake surface water but were as high as 70% in the sediment, indicating that smaller microplastics may more easily settle in the lake. The main components of the microplastics were identified as being polyvinyl chloride and polyethylene. There were about 1.2× 10⁶ items/s microplastics entered Taihu Lake. Four main rivers located at northwestern lake accounted for 79% of the total inflow microplastic fluxes.

How to cite: zhang, Q. and qian, X.: Distribution and Sedimentation of Microplastics in Taihu Lake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3306, https://doi.org/10.5194/egusphere-egu22-3306, 2022.

A large-scale water erosion and sediment transport model is introduced and applied to predict continental-scale hydrological transport processes at the Yellow River Basin in China. Our model couples the Atmospheric and Hydrological Modelling System (AHMS) with the CASCade 2 Dimensional SEDiment (CASC2D-SED), by considering a scale-adaptive water erosion parameterization and eight possible flow directions of the channel routing model. Here, the AHMS-SED is applied to simulate the water erosion processes in the Yellow River Basin over 10 years with a spatial resolution of 20 km. The simulated daily sediment fluxes from four major hydrological stations along the Yellow River (namely, Tangnaihe, Lanzhou, Toudaoguai and Huayuankou) are compared with corresponding observations. There is a quantitative agreement between these observations and modelling results at all stations. Our results demonstrate the good performance of the new scale-adaptive parameterization and the integrated AHMS-SED, paving the way for the studies of water erosion and sediment transport at large scales. We also show how of our numerical simulations can be used to predict the evolution of sediment transport in the Yellow River Basin under consideration of specific climate change scenarios.

How to cite: Jiang, C., J. R. Parteli, E., and Shao, Y.: A model for continental-scale water erosion and sediment transport and its application to the Yellow River Basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7976, https://doi.org/10.5194/egusphere-egu22-7976, 2022.

Norway's authorities are delayed in implementing the European Water Framework Directive (EU-WFD). A common challenge for the implementation of EU-WFD is finding natural reference conditions in water bodies, which can be challenging for lakes that have been regulated and used for hydropower production before any physical variables were made. Hydrological modelling of unregulated lakes can be a solution.  Modelling water level fluctuations in unregulated lakes allow us to determine the ecological functioning of the lake and the water storage that could be used for different sectors such as hydropower, agriculture and others.

Previous studies showed that lakes had a strong influence on the performance of models when using the Hydrological Predictions for the Environment (HYPE) model. This study aims to develop model strategies for improving lake dynamic modelling with natural flow conditions in terms of discharge and water stage in HYPE. We modelled seven lakes in Norway with areas more than 5 km² and a gauging station at the output. Each lake was calibrated independently, and each model was set up from an existing one for the mainland of Norway. Stepwise calibration was implemented to create separate discharge and water stage models. Rating curves for lakes were calculated and introduced to the model for water discharge and stage calibration following the equation Q=k(w-wo)^p. Where w is the observed water level, w_o is the reference water level, k is the rate, and p is the exponent. The model performance was evaluated in terms of Kling–Gupta efficiency (KGE). Preliminary results showed improvement of model performance for water stage modelling when employing a pre-calibrated model with discharge time series data. Also, improved model performance in discharge was found when using rating curves for calibration

How to cite: Saldana Espinoza, C. I., Schönfelder, L., and Seidel, J.: Study of unregulated flow conditions in Norwegian rivers- Strategy for improving lake outflow using HYPE model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8118, https://doi.org/10.5194/egusphere-egu22-8118, 2022.