Search

The study of the characteristics of fine-grained and cohesive sediments due to its filling characteristics in harbors and ports is an important subject in coastal studies. Fine-grained cohesive sediments have special complicated characteristics as compared to other sediments regarding their behavior. Hence, numerous research works have been carried out to establish well validated physical and mathematical descriptions of the behavior and outcome of concentrated near-bed cohesive sediment suspensions and their interaction with the water column and the bed as well as the turbulence characteristics of sediment laden flow. A two-dimensional model is developed in this study that includes: a cohesive sediment simulator module, processes such as advection and diffusion of cohesive sediment, flocculation and its effect on the settling velocity of cohesive sediment particles, consolidation of bed layers and sediment transport between layers, substrate shear stress variations affecting the simultaneous presence of wave and flow, bed morphology, deposition and erosion. In all these processes, the depth is considered according to the actual topography of the bed. For verification of model performance the model results have been compared with the MIKE 21 model results against the field data reported during the construction phase and at the simulation stage of Ho Bay, a case study presented by DHI (MIKE, 15). The comparisons indicate a favorable accuracy of the present model performance in simulation of cohesive sediment transport. [ABSTRACT FROM AUTHOR]

Floods are among the most common natural disasters responsible for severe damages and human losses. Combining numerical modelling with user-friendly tools for geographically referenced data has been adopted to increase preparedness and reduce vulnerabilities. This paper describes the numerical dataset of hydrometric variables that characterize a flood event occurred in February 2016 in the Portuguese Águeda river, shortly defined as Agueda.2016Flood. The dataset was numerically produced and managed through the RiverCure Portal, a collaborative web platform connected to a validated shallow-water model featuring modelled dynamic bed geometries and sediment transport. The dataset Agueda.2016Flood can be used as a starting point to design other experiments and tools, and to learn and apply the proposed approach by directly using the RiverCure Portal. This dataset includes modelled hydrodynamic data (output data) and the topographic, geometrical, land-use and hydrologic data (input data) necessary to carry out the numerical simulation of the flood event. [ABSTRACT FROM AUTHOR]

This article aims to provide a comprehensive appraisal on the current status of three-dimensional (3D) numerical modeling of local scour around instream hydraulic structures. Many computer models have been developed or are in development to simulate riverbed morphology. Local scour, however, is a special category that requires in general a 3D model based on some averaged/filtered form of the Navier-Stokes (NS) equations and a 3D sediment transport model. Only limited number of 3D scour codes are available for practical usage; there is also a lack of literature on the limitations of the models in capturing the scour-producing flow and the sediment transport solvers embedded within. For 3D local scour modeling, the main intrinsic uncertainty lies primarily in the sediment part. In this article, the basic theory and the state of the art of current 3D local scour models are reviewed, focusing primarily on the sediment solver, and future research needs are discussed. [ABSTRACT FROM AUTHOR]

• An improved Eulerian method with the movable IBM is employed to study the deformation of the river bed. • A Simultaneous Bed Adjustment Method is proposed to consider the sand sliding on the riverbed in the DNS. • A Segment Plane Method is proposed to calculate the geometrical parameter of the complex riverbed. • The improved Eulerian method can capture both the development of the local scour and the flow structure in detail. An improved Eulerian method with a novel morphologic model is developed based on the direct numerical simulation (DNS) approach to study the local scour. A movable immersed boundary method is proposed to prescribe the geometry of the riverbed. Two techniques, namely the Simultaneous Bed Adjustment Method (SBAM) and (Segment Plane Method) SPM, are proposed to model the complex time changing geometry of the riverbed. The SBAM technique considers sand sliding in the model and the SPM is an algorithm calculating the geometrical parameter of the complex riverbed. A wind tunnel experiment obeying Reynolds similarity and measuring the pressure in a fixed scour hole is carried out to compare with the numerical results. The results show that the movable immersed boundary method performs well in capturing the local scour and qualitatively agrees with the experimental study in how the scour hole develops. The numerical results also show that the DNS can capture the formation of the local scour in details at the beginning and find that the Reynolds stress u'v' plays an important role in an erosion at the leading edge of the cylinder. Therefore, the newly improved Eulerian method is a valuable tool for DNS studies of local scour with the implementation of MPI. [ABSTRACT FROM AUTHOR]

This paper presents the numerical simulations of flow and dam body sediment transport over a movable bed due to an earthfill dam break. The RANS equations, together with the k-ω SST turbulent model, are employed. The phase characteristic parameter is used as the phases of air, water, sediment, and bulk of dam body. The system of equations is solved numerically using the PISO algorithm. The numerical model is first verified using the dam break experimental data from the literature. The model successfully captures the temporal changes in the measured flow depths, pressures, wave fronts, and arrival times. The ve rified mod el is the n app lied to simulate the flow and sediment transport as a result of an artificial earthfill dam break having an obstacle at its downstream section. The simulations show that there is a noticeable decrease in the shock pressures at all points around the obstacle and there is an increase in the water levels. The bulk dam body sediment moves together with the water flow while spreading. It takes longer time for the sediment laden flow to reach the obstacle. The investigation of dam body formed by different soils shows that the soil type has minor effect while the transport of sediment can raise the water levels and change the morphology of the downstream section. [ABSTRACT FROM AUTHOR]

Spyrou, D. and Karambas, T., 2021. Experimental and numerical simulation of cross-shore morphological processes in a nourished beach. Journal of Coastal Research, 37(5), 1012–1024. Coconut Creek (Florida), ISSN 0749-0208. This paper presents the results of an experimental and numerical research carried out to reproduce the cross-shore evolution of nourished sandy beaches. An extensive two-dimensional laboratory investigation was performed to study the cross-shore profile evolution after the application of a beach nourishment method. A 1:20 scale physical model was carried out in the Laboratory of the Department of Hydraulics and Environmental Engineering, Aristotle University of Thessaloniki. The analysis considered only cross-shore sediment transport. Long-shore sediment transport is considered negligible. Consequently, the obtained results are related to the short-term response of the cross-shore profile. Ten different wave combinations were generated in an attempt to reproduce erosive conditions. Experimental results are given in terms of wave parameters, bed slope, and berm height of the nourished beach. An advanced nonlinear wave model was developed to simulate cross-shore morphodynamical processes. The comparison between the experimental and numerical beach profile indicated good agreement. Also, comparisons between Dean's analytical method and the experimental results were made. [ABSTRACT FROM AUTHOR]

The flow field and the sediment motion generated over a rippled bed by a uniform and oscillating pressure gradient are evaluated by means of a direct numerical simulation of Navier–Stokes and continuity equations and assuming that small spherical particles, free to move, mimic the sediment grains. The numerical approach determines the flow around each sediment particle, along with the force and torque that the fluid exerts on it. Then, the particle trajectories are computed by means of Newton-Euler's laws. The parameters of the problem are the same as those of one of the laboratory experiments described by Blondeaux and Vittori (1991a) and the initial ripple profile is similar to that observed during the experiment. The numerical results are qualitatively compared with the experimental observations and support the reliability of the numerical simulation. Moreover, the results provide further valuable information on sediment dynamics under surface waves which propagate over a rippled bed. In particular, the obtained results allow a detailed analysis of the dynamics of the vortices generated by flow separation at the ripple crest and of the mechanism through which the sediments are lifted up from the bottom and carried into suspension. [Display omitted] • Large vortices shed from the ripple crests strongly affect the sediment transport mechanics. • Sediment pick-up and deposition rates were found lagged by a constant interval throughout the wave-cycle. • Indications for the modelling of suspended sediment concentration over a rippled bed are provided. [ABSTRACT FROM AUTHOR]

Current-induced scour around a submarine pipeline during onset and tunnel erosion are investigated in both clear-water and live-bed regimes using an Eulerian–Eulerian two-phase model for sediment transport. The Hybrid Fictitious Domain-Immersed Boundary (HFD-IB) method is adopted to simulate the pipeline structure. The model is validated against two published experimental data: (1) flow past a pipeline near a rigid bed and (2) scour around a pipeline over a sand-bed, showing favorable results for the flow and scour patterns around the pipe. The driving mechanisms of the onset and tunnel erosion are investigated by examining the detailed fluid–sediment interactions around the pipe in both regimes. The driving mechanisms for the onset and tunnel erosion are found similar in both regimes, except their time and magnitude scales. The onset is triggered by the piping mechanism induced by the upstream–downstream pressure gradient beneath the pipe. Meanwhile, the jet flow is the driving mechanism for the tunnel erosion. The simulated live-bed condition shows a higher upstream–downstream pressure gradient beneath the pipe, faster piping mechanism, more intense jet flow velocity and sediment flux compared to the clear-water condition. The model well capture these underlying mechanisms, suggesting its promising capability as a predictive tool for scouring. • Onset of scour and tunnel erosion of a submarine pipeline are simulated by using a two-phase model in clear-water and live-bed regimes. • Hybrid Fictitious Domain-Immersed Boundary (HFD-IB) method is adopted to simulate the pipeline structure. • Simulated fluid-flow velocity and scour profile around the pipe are in good agreement with published experimental results. • Piping mechanism is the dominant driving mechanism for the onset of scour, while jet flow is dominant during the tunnel erosion. • Comparison of the driving mechanisms of scour in clear-water and live-bed regimes are discussed. [ABSTRACT FROM AUTHOR]

The dynamics of suspended sediment transport in horizontal open channel flow is analysed using point-particle one-way coupling Direct Numerical Simulations (DNS), with a virtual wall as a simple particle resuspension model. In sediment transport, the bed-load is dominated by the inter-particle interactions, but the suspended sediments are transported essentially in a one-way coupling situation. The validity of one-way coupling DNS with point-particle approach for the transport of suspended sediment is analysed, by comparing the simulations with existing well-designed experiments performed under very similar conditions. The range of the relevant non-dimensional parameters of the simulations is roughly the same as in actual sediment transport, except for the flow Reynolds number. Good agreement is observed between the simulations and the experiments; furthermore, the simulation results of the motion of the suspended sediment are insensitive to the position of the virtual wall, provided this wall is placed in a region where the fluid velocity fluctuation in the wall-normal direction is comparable to the particle settling velocity. Using the simulation results, the interplay between the different fluid–particle interaction forces is analysed, with and without gravity. In the absence of gravity, the dynamics is dominated by the balance between the stress-gradient force and the turbophoretic effects; as the particle-to-fluid density ratio for the sediment particles is on the order of one, the situation is quite different when compared to the dynamics with a density ratio on the order of 1000. When gravity is included, the dynamics is dominated by the interplay between the drag and gravitational forces, and they balance each other. Both with and without gravity, the lift and added-mass forces have only secondary effects and do not play an important role. • Point-particle one-way coupled DNS with virtual wall for solid–liquid flow is done. • The stress-gradient and turbophoretic effects dominate in the absence of gravity. • The results of suspended sediment motion agree with well-designed experiments. • The motion of suspended sediment is insensitive to the virtual wall position. • The drag and gravity dominate the motion of suspended sediment. [ABSTRACT FROM AUTHOR]

• An integrated 1D morphodynamic model was proposed covering three different modules. • Bank erosion and accretion processes in the braided reach of the LYR were simulated. • Large-scale bank deformation played an important role in the channel evolution. The braided reach of the Lower Yellow River (LYR) often experienced dramatic channel evolution during flood seasons, with both bed and bank deformations occurring simultaneously, which led to several negative effects such as land loss, channel migration and overbank flooding. A coupled one-dimensional model was proposed to simulate both bed evolution and bank deformation, including three modules of flow-sediment transport, bed deformation, and bank erosion and accretion. The proposed model was applied respectively to simulate the longitudinal and lateral channel deformations in the braided reach in the years of 2018 and 2006, in order to conduct detailed processes of model calibration and verification. The results indicate that: the proposed model can accurately simulate the flow and sediment transport processes, and the majority of the Nash-Sutcliffe efficiency coefficients (NSEs) were larger than 0.60 between the calculated and measured hydrographs of water discharge, water level and sediment concentration at different sections, with the maximum value up to 0.96; the calculated erosion and accretion widths of the riverbanks closely agreed with the measured data, and the absolute errors between the calculated and measured post-flood reach-scale bankfull widths were 18 m in 2018 and 14 m in 2006, with the relative errors of 1.7% and 1.3%; the effects of riverbank deformation on sediment discharge, flow velocity and channel deformation were also analyzed, which indicate that the module of riverbank erosion and accretion can play a significant role in predicting hydrodynamic and morphodynamic processes in the braided reach of the LYR. [ABSTRACT FROM AUTHOR]

Direct numerical simulation is employed with the immersed boundary method to study how the ISWs create local scour around the cylinder. Numerical results show that the geometrical morphology of the local scour depends on the relative location of the cylinder to the breaking point of the ISWs. It is concluded from the results that the erosion mainly occurs when the ISWs shoal on a slope. In addition, when the ISWs begin to break, the shear stress induced by the ISWs reduces and tends to fail to erode the bed, so in all cases the local scours are backfilled to some extent after the ISWs breaking. If the cylinder is near the breaking point, a vertical upward flow is induced near the cylinder that can bring the suspended sediment to the pycnocline when the ISWs shoal. As a result, the suspended sediment concentration at the bottom is small. On the other hand, if the cylinder is away from the breaking point without the vertical upward flow, the backfill is in a great quantity due to the fact that deposition will occur on the condition that the suspended sediment concentration is high at the bottom. • 3D-DNS with improved IBM is employed. • The erosion mainly occurs when the ISWs shoal. • The local scour may backfill. [ABSTRACT FROM AUTHOR]