HS9.4

In recent decades, computational hydraulics and sediment modelling have a great development due to compute technology. Applying a finite-volume Godunov-type hydrodynamic shallow water model with hydro-sediment-morphodynamic processes, this work demonstrates and analysis the ability of single-host parallel computing technology with algorithmic acceleration technology. This model is implemented for high-performance computing using the NVIDIA’s Compute Unified Device Architecture (CUDA) programming framework, using a domain decomposition technique and across multiple cores through an efficient implementation of the Open Multi-Processing (Open MP) architecture, and using an algorithmic acceleration technology named local time stepping scheme (LTS), which is capable of obtain much efficiency improvement via different time step sizes for different grid sizes. The model is applied for three cases, through which we compare the effectiveness of CPU, Open MP, Open MP+LTS, CUDA, and CUDA+LTS, demonstrating high computational performance across CUDA+LTS which can lead to speedups of 40 times with respect to CPU and high-precision results across CUDA +LTS.

KEY WORDS: Hydro-sediment-morphological modeling; local time step; Open MP; CUDA.

How to cite: Zhao, Z., Hu, P., Li, W., Cao, Z., and He, Z.: Research on the single-host parallel computing with the local time step scheme for modeling of hydro-sediment-morphodynamic processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7027, https://doi.org/10.5194/egusphere-egu21-7027, 2021.

With the increasing computational power of today's supercomputers, geometrically fully resolved simulations of particle-laden flows are becoming a viable alternative to laboratory experiments. Such simulations enable detailed investigations of transport phenomena in various multiphysics scenarios, such as the coupled interaction of sediment beds with a shearing fluid flow. There, the majority of available simulations as well as experimental studies focuses on setups of monodisperse particles. In reality, however, polydisperse configurations are much more common and feature unique effects like vertical size segregation.

In this talk, we will present numerical studies of mobile polydisperse sediment beds in a laminar shear flow, with a ratio of maximum to minimum diameter up to 10. The lattice Boltzmann method is applied to represent the fluid dynamics through and above the sediment bed efficiently. We model particle interactions by a discrete element method and explicitly account for lubrication forces. The fluid-particle coupling mechanism is based on the geometrically fully resolved momentum transfer between the fluid and the particulate phase. We will highlight algorithmic aspects and communication schemes essential for massively parallel execution.

Utilizing these capabilities allows us to achieve large-scale simulations with more than 26.000 densely-packed polydisperse particles interacting with the fluid. With this, we are able to reproduce effects like size segregation and to study the rheological behavior of such systems in great detail. We will evaluate and discuss the influence of polydispersity on these processes. These insights will be used to improve and extend existing macroscopic models.

How to cite: Rettinger, C., Eibl, S., Rüde, U., and Vowinckel, B.: Numerical study of sheared mobile polydisperse sediment beds with a coupled lattice Boltzmann - discrete element method, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1464, https://doi.org/10.5194/egusphere-egu21-1464, 2021.

The dynamics of cohesive sediments under various flow conditions are of special interest in the framework of aquatic ecosystems. Being one of the main sources of transport for minerals and organic matter, the constituents of cohesive sediments are the source of food for many aquatic organisms. Due to the additional complexity of physical mechanisms, there are only a few simulation techniques for cohesive sediments, which do not cover all spatial scales. The primary cohesive clay particles are platelets smaller than 2 μm, which is small enough to experience Brownian motion. Composed together under the influence of van der Waals forces, they shape rounded aggregates also known as microflocs that are rather stable. These microflocs can form fragile, larger macroflocs with complex shapes exceeding 100 μm in size. Owing to the huge difference in the spatial scales, it is almost impossible to simulate macroflocs as the assembly of primary clay particles in the context of cohesive sediment transport modeling. In contrast to separate sediment grains, microflocs represent porous aggregates. To perform direct numerical simulations of microflocs transported in a viscous fluid flow, we are developing a computational model for immersed porous particles. The model resolves the flow outside and inside porous aggregates and accurately computes the hydrodynamic forces on the microflocs. The simulation of macroflocs is also attainable by employing cohesive forces between microflocs, which assembles them into bigger aggregates with the propensity of breaking up under high shear rates. Our computational model solves the system of Navier - Stokes equations directly with an additional Darcy term inside the porous aggregate. Using this approach, it becomes feasible to consider the influence of the flow inside porous media, so that we can study its impact on the mean flow characteristics depending on the properties of the porous flocs. The hydrodynamic forces are calculated implicitly based on the pressure and shear stress distribution. By comparison with methods that use Stokes-based drag coefficients, our approach allows estimating the influence of local flow conditions and the presence of neighboring aggregates on the resulting fluid force.

How to cite: Metelkin, A. and Vowinckel, B.: A numerical method for simulations of cohesive, porous sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2438, https://doi.org/10.5194/egusphere-egu21-2438, 2021.

Braiding is among the most dynamic landscape on Earth. It provides diverse habitats for freshwater creatures. Unfortunately, the number of braided rivers is reducing affected by human activities in the Anthropic period. The increase of the vegetation cover within the river corridor is one important factor, which is induced by flow regime change, land-use change, or alien vegetation invasion. Vegetation clearance could be a promising measure to mitigate vegetation overexpansion. Several previous research suggested vegetation clearance may induce geomorphological metamorphosis. However, quantitative prediction of the morphological change resulted from the vegetation clearance is still an open question to date. We simulated the river morphological response of vegetated braided river with gravel bed to the vegetation clearance using the Nays2DH model combined with a vegetation module. Except for vegetation removal, the developed conceptual model considered vegetation colonization and the destruction induced by floods. Multiple scenarios have been tested, considering two vegetation types (strong and weak vegetation), two clearance methods (full clearance and partial clearance), and two maximum discharge. The full clearance scenario stood for the removal of above-ground and underground biomass simultaneously, and the partial clearance scenario stood for the removal of above-ground biomass. Braided rivers had developed for both no vegetation and river with weak vegetation cover. The bedform affected by strong vegetation coverage consisted of a main channel and small channels on the floodplain, which was consistent with previous experimental results. The distinctive morphology of developed bed form depended on the dominant factor in the vegetation-geomorphology interaction: vegetation dominant or physical process dominant. River morphology responded differently to the vegetation control measure based on the dominating factor. For the vegetation dominated river, the developed main channel tended to be braiding after the vegetation removal, and the river morphology change was sensitive to the vegetation clearance method. By contrast, river morphology changed insignificantly by vegetation colonization and after vegetation removal if the river physical process was dominant. We also found that the small channels on the floodplain promoted sediment transport from the floodplain to the main channel after the vegetation clearance. Thus, the morphological response to the vegetation clearance method was also affected by the reduction of maximum discharge because the connectivity between floodplain and channel was reduced. To improve vegetation clearance effectiveness, we recommend increasing the connectivity between the floodplain and the main channel, such as excavating small channels on the floodplain.

How to cite: Zhu, R. and Tsubaki, R.: Impact of vegetation control measures on the bedform of braided gravel-bed river, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3753, https://doi.org/10.5194/egusphere-egu21-3753, 2021.

The retrospective simulation of the Pyoza river (Arkhangelsk region, Russia) meander cut-off in 2003-2008 has been undertaken. As a result of the river bend straightening two large villages were cut off from the road network of the region.

The initial data for modeling were obtained by analyses of archive satellite images for the period from 1997 together with the runoff data, as well as by the field survey of September 2019. The simulation was performed by the latest version of the STREAM_2D CUDA software, using a new method for the numerical solution of two-dimensional Saint-Venant equations [1]. It was adapted for the complicated bottom topography typical for a wide floodplain with a meandering channel flooded in high water stage.

The mass-exchange equations for three layers of sediment over the unerodible bed were solved together with the hydrodynamic equations. When calculating channel deformations, the gravitational effect due to bottom slope and the influence of secondary currents on the sediment shift were taken into account [2].

The Pyoza river is the lowest large tributary of the Mezen’ river flowing into the White sea. It is distinguished by a typical alluvial channel, meandering along wide floodplain composed by sands and sandy loams. The maximum runoff usually corresponds to spring snow-melting and can reach 1500-2000 m³/s.

To schematize the computational domain of the Pyoza river section of 13 km long, a hybrid grid of irregular structure was constructed, consisting of 37 329 cells of a quadrangular shape for the channel and a triangular one for the floodplain.

The simulation started at the year 1997 when where was no any rill across the meander neck. The time step of calculation was taken to be one day.

Modeling made it possible to simulate realistically the essential steps and mechanisms of the meander cut-off: the development of a pioneer straightening rill, its widening and deepening, as well as blocking of the old channel by a point bar in its upper reaches, as well as its further silting and aggradation.

1. Aleksyuk A.I., Belikov V.V. (2017): Simulation of shallow water flows with shoaling areas and bottom discontinuities. Computational Mathematics and Mathematical Physics, Volume 57, issue 2, pp. 318–339. https://doi.org/10.1134/S0965542517020026

2. Aleksyuk А. I., Belikov V. V., Borisova N. M., Fedorova T. A. (2018): Numerical modeling of non-uniform sediment transport in river channels. Water Resources, Volume 45, Special Issue S1, pp. 11–17. http://dx.doi.org/10.1134/S0097807818050275

How to cite: Fedorova, T., Belikov, V., and Alabyan, A.: Simulation of the meander cut-off by 2D hydrodynamic model for erodible bed, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6016, https://doi.org/10.5194/egusphere-egu21-6016, 2021.

The importance of water reservoirs has been increasing with population growth and the need of water supply. Understanding the environmental processes in these water bodies is essential for the correct management performed by stakeholders. In this context, numerical models appear as a great tool for simulating hydrodynamics and sediment related processes and presenting them in way easy to understand. However, a wide range of data is required as input for a good performance of these models and its quality have a direct influence on the simulations results. Long term high resolution input data is the ideal case for simulations, but in developing countries this is usually not the case due to the absence of measured or simulated data to be used as input. The main objective of this research is to understand how the reduction of input data resolution and/or use of wrong datasets may influence final results of the processes taking place in reservoirs regarding the hydrodynamics, water temperature and sedimentation. Passaúna reservoir located in Curitiba, Brazil, is used as a study case with high resolution measured and simulated datasets which will be implemented as input data for numerical simulations using Delft3D. For the simulation of the horizontal velocities and the water temperature, parameters related to heat flux calculations showed the strongest influences on the results. A specific important parameter was the secchi depth, which is a single value used as input data and shows great differences for the reproduction of periods with mixed or stratified water column. On the other hand, sediment simulations showed sensitivity to the main river flow discharge temporal resolution and its corresponding sediment concentrations. Reanalysis data used as heat flux parameters and wind presented great differences from the use of measured datasets, but in cases where measured data is not available, this source may be the best choice for users.

How to cite: Gonzalez, W., Mees Delfes Varela, D., and Seidel, F.: Study on the complexity reduction of the input data for 3D numerical modeling of the hydrodynamics and sediment transport in a Brazilian reservoir , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8900, https://doi.org/10.5194/egusphere-egu21-8900, 2021.

The large-scale turbulent structures that develop at confluences fall into three main categories: vertically orientated (Kelvin-Helmholtz) vortices, large-scale secondary flow helical cells and smaller strongly coherent streamwise orientated vortices. The causal mechanisms of each class, how they interact with one another and their respective contributions to mixing is still unclear. Our investigation emphasises the role played by the instantaneous flow field in mixing at a mesoscale confluence (Mitis-Neigette, Quebec, Canada) by complementing aerial drone observations of turbulent suspended sediment mixing processes with results from a high-resolution eddy-resolved numerical simulation. The high velocity near-surface flow of the main channel (Mitis) separates at the crest of the scour hole before downwelling upon collision with the slower tributary (Neigette). Fed by incursions of lateral momentum of the Mitis, shear generated Kelvin-Helmholtz instabilities expand as they advect along the mixing-interface. As the instabilities shed, water from the deeper Neigette passes underneath the fast, over-topping Mitis, causing a large portion of the Neigette’s discharge to cross under the mixing-interface in a short distance. The remaining flow of the tributary crosses over inside large-scale lateral incursions farther downstream. The downwelling Mitis, upwelling Neigette and recirculatory cell interact to generate coherent streamwise vortical structures which assist in rapidly mixing the waters of the two rivers in the vicinity of the mixing-interface. Evidence of large-scale helical cells were not observed in the flow field. Results suggest that flow interaction with bathymetry, and both vertical and streamwise orientated coherent turbulent structures play important roles in mixing at confluences. Our findings strongly suggest that investigating mixing at confluences cannot be based solely on mean flow field variables as this approach can be misleading. Visualization of a confluence’s mixing processes as revealed by suspended sediment gradients captured in aerial drone imagery complemented with eddy-resolved numerical modelling of the underlying flow is a promising means to gain insights on the role of large-scale turbulent structures on mixing at confluences.

How to cite: Duguay, J., Biron, P., and Buffin-Bélanger, T.: Study of turbulent mixing processes at a mesoscale confluence through aerial drone imagery and eddy-resolved modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9676, https://doi.org/10.5194/egusphere-egu21-9676, 2021.

Floating units/booms are used to trap or guide floating debris in watercourses. In a relatively shallow depth, these floats could affect the velocity distribution, sediment transport and channel bed deformation.  A three-dimensional non-hydrostatic numerical modelling was performed in a 180 degree channel bend with floats to see the effects in flow distribution and bed deformation as a conceptual study. Different configurations of the floats were simulated. The results showed that the flow velocity increased and deposition decreased at the inner bank of the bend. Use of floating units could be studied to alter sediment deposition pattern and sediment transport phenomenon in watercourses.

How to cite: Maskey, D. L. and Ruther, N.: Numerical modelling of sediment transport in a channel bend with floating units, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16114, https://doi.org/10.5194/egusphere-egu21-16114, 2021.

The presented work is part of the optimization of the sediment management at the hydroelectric powerplants in Reutte/Höfen in Austria. The weirs of the two powerplants are situated at the alpine river Lech, located about 3 km upstream of the Lechaschau gauge (A=1012.2 km²). Totally five sluice gates and a fixed overflow weir are controlling the upstream reservoir, being subjected to high rates of coarse bed load material. In frame of a coupled approach of physical and numerical modelling, different options to (i) avoid/minimize sediment deposition and (ii) allow improved sediment flushing were tested and optimized. Besides a lowering of energy losses (reduced spilling times) the reduction of depositions downstream close to the turbine outlet were considered.

The physical model covers the hydropower and weir system of both power plants within a stretch of 400m / 150m using a model scale of 1:25. Investigated situations covered periods of reservoir sedimentation, flushing of the reservoir and typical flood flow situations (e.g. HQ1 and an unsteady HQ5 event). For model parametrization, sediment samples to quantify size distribution were taken in the field. Sediment inputs to the model were realized dynamically and were required (due to scaling effects) to exclude cohesive fractions having a minimum particle size of 0.5 mm. The full-area surface measurement of the river bed was made by means of airborne laser bathymetry and echo sounding.

As part of an optimization of the overall sediment management strategy, the focus of the presented research is on the western located runoff power plant Höfen. Via a lateral water intake, a maximum design flow of 15 m³/s is withdrawn causing that the intake structure is subjected to sediment depositions. Within the described scale model (1:25) and a partial scale model (1:15) covering the western side, several management options and configurations of sediment guiding walls were tested. Erosion and deposition as well as the transported material are assessed by 3D laser scanning and permanent monitoring of transported sediment load entering and leaving the scale model.

Complementary, a 2D hydro numerical model using a layer based multi fraction approach for sediment transport is set up for an extended area to simulate the morpho-dynamic behavior. The numerical model covers the whole weir system and 750 m of the upstream part of the Lech. The simulations made were realized at nature scale and allowed to mimic the erosion and deposition pattern obtained within the physical modelling for different tested options. Regardless of a chosen guiding wall setup, the results showed that each one is compromise between sediment defense and the effectiveness of the subsequent flushing processes.

How to cite: Siedersleben, J., Schuster, M., Ties, D., Zwick, B., Aufleger, M., and Achleitner, S.: Physical and numerical modelling of sediment guiding walls in an alpine river, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14680, https://doi.org/10.5194/egusphere-egu21-14680, 2021.

Nowadays, the aquatic biodiversity is highly under pressure due to anthropogenic changes of the rivers such hydraulic structures changing the diversity of flow and aquatic fauna as well as sediment continuity. This can have severe consequences on the fish population in the river reach. Fish are strongly depending on a certain substrate composition throughout all their life stages. Juveniles for example are depending on a certain availability of shelter in the substrate in order to survive this stage.

Therefore, we investigate the effects of changes in the sediment composition at a hydropower plant in Switzerland on the availability of potential shelter for juvenile fish. By utilizing the observed correlation between parameters describing the fine tail of a riverbed’s grain size distribution and shelter abundance for juvenile Atlantic salmon, we predict the available shelter in a river reach by using a 3D hydrodynamic numerical model directly coupled to a morphodynamic model. The initial substrate composition was assumed to be spatially uniform, its parameters based on a grain size distribution curve derived from collected sediment samples.

This model can now be used for habitat improvement scenario modeling. Based on the assumption that a specific mixture of sediment coming from upstream travelling through the river reach will positively influence the potential shelter availability, different scenarios can be investigated. The baseline for comparison was the simulation of the bed changes without any sediment supply from upstream. The baseline discharge was set to 100 m³ /s and was applied for 24 hours. The resulting bed changes create a map of the potential shelter availability of this grain size mixture. Then, two scenarios with sediment inflow from the upstream boundary were simulated. One coarse and one fine mixture of sediment were chosen as inputs, with the goal of investigating their impact on shelter abundance. The former designed to have a positive effect while the latter expected to reduce interstitial voids in the substrate and have a negative effect on available shelter.

The investigation is conducted as part of the EU Horizon 2020 funded project FIThydro (funded under 727830)

How to cite: Pritsis, S., Ruther, N., Schwarzwälder, K., and Stamou, A.: Modeling shelter abundance for juvenile Atlantic salmon in a residual flow reach of a hydro power plant using SSIIM 2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13319, https://doi.org/10.5194/egusphere-egu21-13319, 2021.

Prediction of the dispersion of sediment plumes induced by potential mining activities is still very limited due to operational limitations on in-situ observations required for a thorough validation and calibration of numerical models. Here we report on a plume dispersion experiment carried out in the German License Area for the exploration of polymetallic nodules in the northeastern tropical Pacific Ocean. The dispersion of a sediment plume induced by a dredging experiment in April 2019 was investigated by employing a hydrodynamic high-resolution regional ocean model coupled to a sediment transport module.

Various aspects including sediment characteristics and ocean hydrodynamics are examined to obtain the best statistical agreement between observation and model results. Results show that the model is capable to reproduce suspended sediment concentration and re-deposition patterns observed in the dredging experiment. Due to a strong southward current during the experiment, the model predicts no sediment deposition and plume dispersion north of the dredging tracks. The sediment re-deposition thickness reaches up to 9 mm at the dredging tracks and 0.01 mm at far-field at a distance of about 500 m from the dredging tracks.

The model results suggest that seabed topography and variable sediment release heights above the seafloor cause significant changes especially for the low sedimentation pattern in the far-field region due to different current regimes. The termination of seawater stratification can rise sediment plume above the seafloor and spread it in a larger vertical distances up to 10 m from the seafloor.

How to cite: Purkiani, K., Gillard, B., Paul, A., Haeckel, M., Haalboom, S., Greinert, J., de Stigter, H., Hollstein, M., Baeye, M., Vink, A., Thomsen, L., and Schulz, M.: Numerical simulation of the dispersion of a sediment plume induced by seabed dredging in the northeastern tropical Pacific Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10446, https://doi.org/10.5194/egusphere-egu21-10446, 2021.

Numerical hydro-morphodynamic models can simulate the impact of future changes in climate and land cover on river channel dynamics. Accurate predictions of the hydro-morphological changes within river channels require a realistic representation of controlling factors and boundary conditions (BC), such as the sediment load. This is, in particular, true where simulations are run over longer timescales and when sparse data on sediment load is available. Using sediment rating curves to reconstruct the missing sediment load data can lead to poor estimates of temporal variations in sediment load, and hence, erroneous predictions of channel morphodynamics. Furthermore, when simulating channel morphological changes at longer timescales, this comes at a high computational cost making it impossible to run various scenarios of changing boundary conditions to long river reaches with sufficient spatial detail.  Here, we apply different methods (morphological factors (MFs) and wavelet transform (WT)) to overcome these problems and to arrive at faster and more accurate predictions of long-term morphodynamic simulations.

We modelled river channel bed level changes of the River Dijle (central Belgium) from 1969 to 1999. Detailed cross-sectional surveys every 20 to 25 m along the river axis were collected in 1969, 1999 and 2018. Since 1969, the river has been incised by about 2 m most probably as a response to land-use/land-cover changes and subsequent changes in discharge and sediment load.  Daily discharge and water level measurements are available for the entire period; however, daily suspended sediment load was only collected between 1998 and 2000. Therefore, WTs were coupled with artificial neural networks (WT-ANN) to calculate long-term sediment load BCs (1969-1999) from the short-term collected suspended sediment concentration samples. Sediment load predictions with sediment rating curves only obtain an R² of 0.115, whereas WT-ANN predictions of suspended sediment load data show an R² of 0.902.

Using MFs the reference hydrograph was condensed with a factor of 10 and 20. WT is a mathematical tool that can convert time-domain signals into time-frequency domain signals by passing through low and high-level filters. Passing sediment load time series through these filters create another synthetic BCs containing the frequential and spatial information with half the original signal's temporal length. Thus we also compare the modelling performance using WT generated synthetic BCs with MFs. Similarly, 36x1 to 36x10 processors of an HPC was used to simulate 16 km river reach containing 3,33,305 mesh nodes (with 1.5 m mesh resolution).  Interestingly, with a significant reduction in computational cost, there was a mild difference (R²=0.802 using MFs 10 and R²=0.763 using MFs 20) in model performance without using MFs during initial trials. Surprisingly, generating a synthetic time series using WT did not perform well. Therefore, hydrograph compression using MFs is found the best option to reduce the computational cost, significantly. Although the computational time reduced from 30 days to only 3 days using MFs and more precise BCs calibrated model with R²=0.70, WT poor performance needs to be still investigated.

How to cite: Ateeq-Ur-Rehman, S., Broothaerts, N., Swinnen, W., and Verstraeten, G.: Numerical modelling of hydro-morphodynamic channel processes at decadal timescales using optimization methods: an application to the Dijle River, Belgium, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1329, https://doi.org/10.5194/egusphere-egu21-1329, 2021.