A High-Order Conservative Semi-Lagrangian Solver for 3D Free Surface Flows with Sediment Transport on Voronoi Meshes

In this paper,we present a conservative semi-Lagrangian scheme designed for the numeri-cal solution of 3D hydrostatic free surface flows involving sediment transport on unstruc-tured Voronoi meshes.A high-order reconstruction procedure is employed for obtaining a piecewise polynomial representation of the velocity field and sediment concentration within each control volume.This is subsequently exploited for the numerical integration of the Lagrangian trajectories needed for the discretization of the nonlinear convective and viscous terms.The presented method is fully conservative by construction,since the transported quantity or the vector field is integrated for each cell over the deformed vol-ume obtained at the foot of the characteristics that arises from all the vertexes defining the computational element.The semi-Lagrangian approach allows the numerical scheme to be unconditionally stable for what concerns the advection part of the governing equations.Furthermore,a semi-implicit discretization permits to relax the time step restriction due to the acoustic impedance,hence yielding a stability condition which depends only on the explicit discretization of the viscous terms.A decoupled approach is then employed for the hydrostatic fluid solver and the transport of suspended sediment,which is assumed to be passive.The accuracy and the robustness of the resulting conservative semi-Lagrangian scheme are assessed through a suite of test cases and compared against the analytical solu-tion whenever is known.The new numerical scheme can reach up to fourth order of accu-racy on general orthogonal meshes composed by Voronoi polygons.

Numerical simulations of flow and sediment transport within the Ning-Meng reach of the Yellow River,northern China

Effective management of a river reach requires a sound understanding of flow and sediment transport generated by varying natural and artificial runoff conditions. Flow and sediment transport within the Ning-Meng reach of the Yellow River（NMRYR）, northern China are controlled by a complex set of factors/processes, mainly including four sets of factors：（1） aeolian sediments from deserts bordering the main stream;（2） inflow of water and sediment from numerous tributaries;（3） impoundment of water by reservoir/hydro-junction; and（4） complex diversion and return of irrigation water. In this study, the 1-D flow ＆ sediment transport model developed by the Yellow River Institute of Hydraulic Research was used to simulate the flow and sediment transport within the NMRYR from 2001 to 2012. All four sets of factors that primarily control the flow and sediment transport mentioned above were considered in this model. Compared to the measured data collected from the hydrological stations along the NMRYR, the simulated flow and sediment transport values were generally acceptable, with relative mean deviation between measured and simulated values of 〈15%. However, simulated sediment concentration and siltation values within two sub-reaches（i.e., Qingtongxia Reservoir to Bayan Gol Hydrological Station and Bayan Gol Hydrological Station to Toudaoguai Hydrological Station） for some periods exhibited relatively large errors（the relative mean deviations between measured and simulated values of 18% and 25%, respectively）. These errors are presumably related to the inability to accurately determine the quantity of aeolian sediment influx to the river reach and the inflow of water from the ten ephemeral tributaries. This study may provide some valuable insights into the numerical simulations of flow and sediment transport in large watersheds and also provide a useful model for the effective management of the NMRYR.

Experimental investigation of the effect of flow turbulence and sediment transport patterns on the adsorption of cadmium ions onto sediment particles

The mechanism of flow turbulence, sediment supply conditions, and sediment transport patterns that affect the adsorption of cadmium ions onto sediment particles in natural waters are experimentally simulated and studied both in batch reactors and in a turbulence simulation tank. By changing the agitation conditions, the sediment transport in batch reactors can be categorized into bottom sediment-dominated sediment and suspended sediment-dominated sediment. It is found that the adsorption rate of bottom sediment is much less than that of suspended sediment, but the sediment transport pattern does not affect the final （equilibrium） concentration of dissolved cadmium. This result indicates that the parameters of an adsorption isotherm are the same regardless of the sediment transport pattern. In the turbulence simulation tank, the turbulence is generated by harmonic grid-stirred motions, and the turbulence intensity is quantified in terms of eddy diffusivity, which is equal to 9.84F （F is the harmonic vibration frequency） and is comparable to natural surface water conditions. When the turbulence intensity of flow is low and sediment particles stay as bottom sediment, the adsorption rate is significantly low, and the adsorption quantity compared with that of suspended sediment is negligible in the 6 h duration of the experiment. This result greatly favors the simplification of the numerical modeling of heavy metal pollutant transformation in natural rivers. When the turbulence intensity is high but bottom sediment persists, the rate and extent of descent of the dissolved cadmium concentration in the tank noticeably increase, and the time that is required to reach adsorption equilibrium also increases considerably due to the continuous exchange that occurs between the suspended sediment and the bottom sediment. A comparison of the results of the experiments in the batch reactor and those in the turbulence simulation tank reveals that the adsorption ability of the sediment, and in particular the adsorption rate, is greatly over-estimated in the batch reactor.

A Higher-Efficient Non-Hydrostatic Finite Volume Model for Strong Three-Dimensional Free Surface Flows and Sediment Transport

In order to accurately simulate strong three-dimensional （3-D） free surface flows and sediment transport, the fully 3- D non-hydrostatic pressure models are developed based on the incompressible Navier-Stokes equations and convection-diffusion equation of sediment concentration with the mixing triangle and quadrilateral grids. The governing equations are discretized with the unstructured finite volume method in order to provide conservation properties of mass and momentum, and flexibility with practical application. It is shown that it is first-order accurate on nonuniform plane two-dimensional （2-D） grids and second-order accurate on uniform plane grids. A third-order approximation of the vertical velocity at the top-layer is applied. In such a way, free surface zero stress boundary condition is satisfied maturely, and very few vertical layers are needed to give an accurate solution even for complex discontinuous flow and short wave simulation. The model is applied to four examples to simulate strong 3-D free surface flows and sediment transport where non-hydrostatic pressures have a considerable effect on the velocity field. The newly developed model is verified against analytical solutions with an excellent agreement.

Numerical Modeling of Sediment Transport and Its Effect on Algal Biomass Distribution in Lake Pontchartrain Due to Flood Release from Bonnet CarréSpillway

In order to protect the city of New Orleans from the Mississippi River flooding, the Bonnet Carré Spillway (BCS) was constructed from 1929 to 1936 to divert flood water from the river into Lake Pontchartrain and then into the Gulf of Mexico. During the BCS opening for flood release, large amounts of freshwater, nutrients, sediment, etc. were discharged into Lake Pontchartrain, and caused a lot of environmental problems. To evaluate the environmental impacts of the flood water on lake ecosystems, a two-dimensional numerical model was developed based on CCHE2D and applied to simulate the flow circulation, sediment transport and algal biomass distribution in Lake Pontchartrain. The effect of sediment concentration on the growth of algae was considered in the model. The numerical model was calibrated using field measured data provided by USGS, and then it was validated by the BCS Opening Event in 1997. The simulated results were generally in good agreement with filed data and satellite imagery. The field observation and numerical model show that during the spillway opening for flood release, the sediment concentration is very high, which greatly restricts the growth of algae, so there is no algal bloom observed in the lake. After the closure of BCS, the sediment concentration in the lake reduces gradually, and the nutrient concentration of the lake is still high. Under these conditions, numerical results and satellite imagery showed that the chlorophyll concentration was high and algal bloom might occur.

Combined effects of massive reclamation and dredging on the variations in hydrodynamic and sediment transport in Lingdingyang Estuary,China

Anthropogenic disturbances associated with the rapid development of coastal cities have drastically influenced the hydrodynamics and sediment transport processes in many large estuaries globally.Lingdingyang Estuary(LE),located in the central and southern part of the Pearl River Delta,southern China with a long history of high-intensity anthropogenic disturbances,was studied to explore the contribution rate and mechanism underlying the alteration in hydrodynamics and sediment transport under each phase of human activity.A state-of-the-art modeling tool(TELEMAC-2D),was used to study the variations in the hydrodynamics and sediment transport,accounting for reclamation-induced shoreline and dredging-induced topography changes.The results indicated that:i)under the influence of successive land reclamation,the general distribution of the Confluence Hydrodynamic Zone(CHZ)in LE varied from scattered to concentrated,and these zones moved 3–5 km seaward.ii)Large-scale channel dredging weakened the residual flow in LE,decreasing the residual flow in the Inner-Lingding Estuary(ILE)by 62.45%.This was initiated by the enhancement of tidal dynamics through changes in the bottom friction caused by dredging in the ILE.In contrast,massive reclamation decreased the residual flow in the ILE by 17.55%and increased that in the Outer-Lingding Estuary(OLE).iii)Despite disturbances related to land reclamation and dredging,the estuarine jet flow in LE remained a turbulent jet system,and the estuarine jet flow became more asymmetrical.In addition,the position of the estuarine jet source moved 6–13 km seaward.iv)Both reclamation and dredging decreased the SSC in the ILE and increased the SSC in the OLE.Reclamation weakened the SSC in the ILE by 62.19%,whereas dredging enhanced the SSC in the OLE by 49%.Spatially,reclamation resulted in an increase in the SSC near the outlets and a decrease in the SSC in the northern portion of the Western Channel.Dredging mainly increased the SSC in the northern part of the OLE.v)The increase in the barotropic pressure gradient was the main factor driving the enhancement of the residual flow and SSC near the outlets.Moreover,the southward location of the“artificial outlets”favored the transport of suspended sediments to the OLE,which was one of the primary reasons for the increase in the SSC in the OLE.Finally,the tidal dynamics of the ILE intensified due to massive reclamation and dredging.The findings of this study indicate that hydrodynamics and sediment transport in LE have greatly changed over the last decades,with reclamation and dredging being the crucial drivers.The insights obtained from this study can serve as a reference for the comprehensive management of the Pearl River Estuary and other large estuaries experiencing similar anthropogenic forcing.

Interactions between vegetation, water flow and sediment transport: A review

The vegetation, as one of the most important components, plays a key role in the aquatic environment. This paper reviews recent progress on the complex interaction between the vegetation and the water flow. Meanwhile, the relationships between the vegetation and the sediment transport are discussed. The vegetation characteristics, such as the shape, the flexibility and the height, have significant effects on the flow structures. The density and the arrangement of the vegetation influence the flow velocity in varying degrees and the flow resistance increases with the increase of the plant density. In turns, the growth of aquatic plants is influenced by the water flow via the direct effect （stretching, breakage, uprooting, etc.） and the indirect effect （changes in gas exchange, bed material distribution, sediment resuspension etc.）. Numerical models were developed and widely used for the flow through vegetated waterways, and the results could be applied to solve engineering problems in practice. The sediment is essential for the survival of most vegetation. The existence of the vegetation helps to resist the deformation and the erosion of the bed sediment, to maintain the bed stability and to improve the water quality by removing suspended particles. Additionally, the effects of the sediment transport on the growth of the vegetation mainly consist of the reduction of their photosynthetic capacity by decreasing the water transparency and hindering the exchange of gas and nutrients between plants and water by attaching particles to plant leaves. Therefore, the interaction between the vegetation and the sediment transport is great and complicated. In order to establish a healthy aquatic ecosystem, it is important to study the relationships between the vegetation, the water flow and the sediment transport.

FLOW STRUCTURE AND SEDIMENT TRANSPORT WITH IMPACTS OF AQUATIC VEGETATION

Aquatic vegetation plays an important role in the flow structure of open channels and thus changes the fate and the transport of sediment. This article proposes a three-dimensional turbulence model by introducing vegetation density and drag force into the control equations of water flow in the presence of vegetation. The model was used to calculate the impacts of submerged vegetation on the vertical profiles of longitudinal flow velocities, the changes of the depth-averaged flow velocities in a compound channel with emergent vegetation in the floodplain, the removal of suspended sediment from the channels by emergent vegetation, and the bed changes around and in a vegetated island. Numerical investigations show that aquatic vegetation retards flow in the vegetation zone, reduces the sediment transport capacity, and contributes to erosion on both sides of the vegetated island. Calculated results agree well with experimental results.

Flow dynamics and sediment transport in vegetated rivers:A review

The significance of riparian vegetation on river flow and material transport is not in dispute.Conveyance laws,sediment erosion and deposition,and element cycling must all be adjusted from their canonical rough-wall boundary layer to accommodate the presence of aquatic plants.In turn,the growth and colonization of riparian vegetation are affected by fluvial processes and river morphology on longer time scales.These interactions and feedbacks at multiple time scales are now drawing significant attention within the research community given their relevance to river restoration.For this reason,a review summarizing methods,general laws,qualitative cognition,and quantitative models regarding the interplay between aquatic plants,flow dynamics,and sediment transport in vegetated rivers is in order.Shortcomings,pitfalls,knowledge gaps,and daunting challenges to the current state of knowledge are also covered.As a multidisciplinary research topic,a future research agenda and opportunities pertinent to river management and enhancement of ecosystem services are also highlighted.

Coupled Model of Two-phase Debris Flow,Sediment Transport and Morphological Evolution

The volume fraction of the solid and liquid phase of debris flows, which evolves simultaneously across terrains, largely determines the dynamic property of debris flows. The entrainment process significantly influences the amplitude of the volume fraction. In this paper, we present a depth-averaged two-phase debris-flow model describing the simultaneous evolution of the phase velocity and depth, the solid and fluid volume fractions and the bed morphological evolution. The model employs the Mohr–Coulomb plasticity for the solid stress, and the fluid stress is modeled as a Newtonian viscous stress. The interfacial momentum transfer includes viscous drag and buoyancy. A new extended entrainment rate formula that satisfies the boundary momentum jump condition （Iverson and Ouyang, 2015） is presented. In this formula, the basal traction stress is a function of the solid volume fraction and can take advantage of both the Coulomb and velocity-dependent friction models. A finite volume method using Roe’s Riemann approximation is suggested to solve the equations. Three computational cases are conducted and compared with experiments or previous results. The results show that the current computational model and framework are robust and suitable for capturing the characteristics of debris flows.

An approach to estimating sediment transport capacity of overland flow

Estimating sediment transport capacity of overland flow is essential to the development of physically based soil erosion models.Correlation analysis indicates that stream power is a dominant factor for sediment transport in overland flows and a new sediment transport capacity equation is proposed based on dimensional analysis.The coefficients of the new equation are calibrated using the published laboratory data,and rainfall impact is taken into consideration by adding an empirical factor on the dimensionless critical stream power.The new sediment transport capacity equation is a function of stream power,rainfall impacted critical stream power and slope.The new equation is applied in a one-dimensional soil erosion model to simulate field data of a runoff plot and the simulation results are reliable.

Numerical modeling of sediment transport based on unsteady and steady flows by incompressible smoothed particle hydrodynamics method

The purpose of the present paper is to introduce a simple two-part multi-phase model for the sediment transport problems based on the incompressible smoothed particle hydrodynamics（ISPH） method. The proposed model simulates the movement of sediment particles in two parts. The sediment particles are classified into three categories, including the motionless particles, moving particles behave like a rigid body, and moving particles with a pseudo fluid behavior. The criterion for the classification of sediment particles is the Bingham rheological model. Verification of the present model is performed by simulation of the dam break waves on movable beds with different conditions and the bed scouring under steady flow condition. Comparison of the present model results, the experimental data and available numerical results show that it has good ability to simulate flow pattern and sediment transport.

1-D coupled non-equilibrium sediment transport modeling for unsteady flows in the discontinuous Galerkin framework

A high-resolution, 1-D numerical model has been developed in the discontinuous Galerkin framework to simulate 1-D flow behavior, sediment transport, and morphological evaluation under unsteady flow conditions. The flow and sediment concentration variables are computed based on the one-dimensional shallow water flow equations, while empirical equations are used for entrainment and deposition processes. The sediment transport model includes the bed load and suspended load components. New formulations for Harten-Lax-van Leer （HLL） and Harten-Lax-van Contact （HLLC） are presented for shallow water flow equations that include the bed load and suspended load fluxes. The computational results for the flow and morphological changes after two dam break events are compared with the physical model tests. Results show that the modified HLL and HLLC formulations are robust and can accurately predict morphological changes in highly unsteady flows.

Sedimentation motion of sand particles in moving water(Ⅰ):The resistance on a small sphere moving in non-uniform flow

In hydraulics,when we deal with the problem of sand particles moving relative to the surrounding water,Stokes'formula of resistance has usually been used to render the velocity of sedimentation of the particles.But such an approach has not been proved rigorously,and its accuracy must be carefully considered.In this paper,we discuss the problem of a sphere moving in a non-uniform flow field,on the basis of the fundamental theory of hydrodynamics.We introduce two assumptions:i)the diameter of the sphere is much smaller than the linear dimension of the flow field,and ii)the velocity of the sphere relative to the surrounding water is very small.Using these two assumptions,we solve the linearized Navier-Stokes equations and equations of continuity by the method of Laplace transform,and finally we obtain a formula for the resistance acting on a sphere moving in a non-uniform flow field.

Sediment Transport in Rivers with Overbank Flow

Some concepts related to sediment transport in rivers with overbank flow are described.Following a description of the physical processes that are involved when a river inundates its floodplains,some simple com- putational methods are presented which permit the depth-averaged velocity and boundary shear stress to be pre- dicted within a cross section of variable,but prismatic shape.The methoda account for the strong transverse shear in velocity that occurs when the stage is just above bankfull,as well as ...

A numerical study for boundary layer current and sheet flow transport induced by a skewed asymmetric wave

An analytical model with essential parameters given by a two-phase numerical model is utilized to study the net boundary layer current and sediment transport under skewed asymmetric oscillatory sheet flows. The analytical model is the first instantaneous type model that can consider phase-lag and asymmetric boundary layer development. The two-phase model supplies the essential phase-lead, instantaneous erosion depth and boundary layer development for the analytical model to enhance the understanding of velocity skewness and acceleration skewness in sediment flux and transport rate. The sediment transport difference between onshore and offshore stages caused by velocity skewness or acceleration skewness is shown to illustrate the determination of net sediment transport by the analytical model. In previous studies about sediment transport in skewed asymmetric sheet flows, the generation of net sediment transport is mainly concluded to the phase-lag effect.However, the phase-lag effect is shown important but not enough for the net sediment transport, while the skewed asymmetric boundary layer development generated net boundary layer current and mobile bed effect are key important in the transport process.

Coupled flood and sediment transport modelling with adaptive mesh refinement

Coupled flood and sediment transport modelling in large-scale domains has for long been hindered by the high computational cost.Adaptive mesh refinement is one of the viable ways to solving this problem without degrading the accuracy.This goal can be accomplished through mesh adaptation,e.g.,mesh coarsening and refining based on the dynamic regime of the flow and sediment transport along with bed evolution.However,previous studies in this regard have been limited to cases either without involving sediment transport or featuring flow-sediment-bed decoupling and the assumption of sediment transport capacity,which are not generally justified.Here,a coupled hydrodynamic and non-capacity sediment transport model is developed on adaptive non-uniform rectangular mesh.The proposed model is validated against experimental tests and numerical results based on the fixed meshes.It is demonstrated that the proposed model can properly capture shock waves,resolve the wetting/drying transition and reproduce morphological evolution.Compared with models based on the fixed meshes,the proposed model features great advantage in computational efficiency and holds promise for wide applications.

Shear velocity criterion for incipient motion of sediment

The prediction of incipient motion has had great importance to the theory of sediment transport. The most commonly used methods are based on the concept of critical shear stress and employ an approach similar, or identical, to the Shields diagram. An alternative method that uses the movability number, defined as the ratio of the shear velocity to the particle＇s settling velocity, was employed in this study. A large amount of experimental data were used to develop an empirical incipient motion criterion based on the movability number. It is shown that this approach can provide a simple and accurate method of computing the threshold condition for sediment motion.

CRITICAL UNIT STREAM POWER FOR SEDIMENT TRANSPORT

Yang's (1996) sediment transport theory based on unit stream power is one ofthe most accurate theories, but in his equations the use of product of slope and critical velocityinstead for critical unit stream power is not suitable. Dimensionless critical unit stream powerrequired at incipient motion can be derived from the principle of conservation of power as afunction of dimensionless particle diameter and relative roughness. Based on a lot of data sets,this new criterion was developed. By use of this new criteria, Yang's (1973) sand transport formulaand his 1984 gravel transport formula could be improved when sediment concentration is less thanabout 100 ppm by weight.

Experimental Study of Sediment Incipience Under Complex Flows

Sediment incipience under flows passing a backward-facing step was studied. A series of experiments were conducted to measure scouring depth, probability of sediment incipience, and instantaneous flow velocity field downstream of a backward-facing step. Instantaneous flow velocity fields were measured by using Particle Image Velocimetry (PIV), and an image processing method for determining probability of sediment incipience was employed to analyze the experimental data. The experimental results showed that the probability of sediment incipience was the highest near the reattachment point, even though the near-wall instantaneous flow velocity and the Reynolds stress were both much higher further downstream of the backward-facing step. The possible me- chanisms are discussed for the sediment incipience near the reattachment point.