Empirical model for estimating vertical concentration profiles of re-suspended,sediment-associated contaminants

Vertical distribution processes of sediment con-taminants in water were studied by flume experiments. Experimental results show that settling velocity of sedi-ment particles and turbulence characteristics are the major hydrodynamic factors impacting distribution of pollutants, especially near the bottom where particle diameter is similar in size to vortex structure.Sediment distribution was uniform along the distance, while contaminant distribution slightly lagged behind the sediment.The smaller the initial sediment concentration was,the more time it took to achieve a uniform concentration distribution for suspended sediment. A con-taminants transportation equation was established depending on mass conservation equations.Two mathematical estima-tion models of pollutant distribution in the overlying water considering adsorption and desorption were devised based on vertical distribution of suspended sediment:equilibrium par-tition model and dynamic micro-diffusion model.The ratio of time scale between the sediment movement and sorption can be used as the index of the models.When this ratio was large,the equilibrium assumption was reasonable,but when it was small,it might require dynamic micro-diffusion model.     

细颗粒泥沙净冲刷和输移的大涡模拟研究

在传统水沙输移数值模拟研究中一般采用雷诺时均模拟技术(Reynolds-averaged simulation,RANS).与RANS相比,大涡模拟技术(large eddy simulation,LES)能够更加精确反映细部流动结构,计算机的发展使得采用LES探讨水流和泥沙运动规律成为可能.本文尝试给出净冲刷条件下悬沙计算的边界条件,采用动态亚格子模式对循环槽道和长槽道中的水流运动和泥沙输移进行了三维大涡模拟研究.利用直接数值模拟(direct numerical simulation,DNS)结果对LES模型进行了率定,计算结果符合良好,在此基础上初步探讨了泥沙浓度、湍动强度和湍动通量等的分布特征.结果表明,净冲刷条件下输沙平衡时泥沙浓度符合Rouse公式分布,单向流动中泥沙浓度沿着流向逐渐增大.泥沙浓度湍动强度和湍动通量都在近底部达到最大值,沿着垂向迅速减小.湍动黏性系数和扩散系数基本上在水深中间处达到最大.湍动Schmidt数沿着水深方向不是常数,在近底部和自由水面附近较大,在水深中间处较小.     

Transport of dissolved oxygen at the sediment-water interface in the spanwise oscillating flow

The distribution and concentration of dissolved oxygen(DO)play important roles in aerobic heterotroph activities and some slow chemical reactions,and can affect the water quality,biological communities,and ecosystem functions of rivers and lakes.In this work,the transport of high Schmidt number DO at the sediment-water interface of spanwise oscillating flow is investigated.The volume-averaged Navier-Stokes(VANS)equations and Monod equation are used to describe the flow in the sediment layer and the sediment oxygen demand of microorganisms.The phase-averaged velocities and concentrations of different amplitudes and periods are studied.The dependence of DO transfer on the amplitude and period is analyzed by means of phase-average statistical quantities.It is shown that the concentration in the sediment layer is positively correlated with the turbulence intensity,and the DO concentration and penetration depth in the sediment layer increases when the period and amplitude of the oscillating flow increase.Moreover,in the presence of oscillating flow,a specific scaling relationship exists between the Sherwood number/oxygen consumption of aerobic heterotrophs and the Reynolds number.     

河口混合与泥沙输运

根据振荡边界层理论和波流分解方法,导出了河口往复水流的流速垂向分布廓线,据此建立了河口垂向准二维水流、盐度、泥沙运动模型。对泥沙输运,完整地考虑了其对流、扩散、起动和沉降的动力学过程。模拟结果与实测资料进行了对比。应用该模型研究河口泥沙输运,分析了河口混合对泥沙输运的影响及最大浑浊带的时空变化规律。     

Mobile bed thickness in skewed asymmetric oscillatory sheet flows

A new instantaneous mobile bed thickness model is presented for sediment transport in skewed asymmetric oscillatory sheet flows. The proposed model includes a basic bed load part and a suspended load part related to the Shields parameter, and takes into account the effects of mass conservation, phase-lag, and asymmetric boundary layer development, which are important in skewed asymmetric flows but usually absent in classical models. The proposed model is validated by erosion depth and sheet flow layer thickness data in both steady and unsteady flows, and applied to a new instantaneous sediment transport rate formula. With higher accuracy than classical empirical models in steady flows, the new formula can also be used for instantaneous sediment transport rate prediction in skewed asymmetric oscillatory sheet flows.     

A finite volume shock‐capturing solver of the fully coupled shallow water‐sediment equations

This paper describes a numerical solver of well‐balanced, 2D depth‐averaged shallow water‐sediment equations. The equations permit variable horizontal fluid density and are designed to model water‐sediment flow over a mobile bed. A Godunov‐type, Harten–Lax–van Leer contact (HLLC) finite volume scheme is used to solve the fully coupled system of hyperbolic conservation laws that describe flow hydrodynamics, suspended sediment transport, bedload transport and bed morphological change. Dependent variables are specially selected to handle the presence of the variable density property in the mathematical formulation. The model is verified against analytical and semi‐analytical solutions for bedload transport and suspended sediment transport, respectively. The well‐balanced property of the equations is verified for a variable‐density dam break flow over discontinuous bathymetry. Simulations of an idealised dam‐break flow over an erodible bed are in excellent agreement with previously published results, validating the ability of the model to capture the complex interaction between rapidly varying flow and an erodible bed and validating the eigenstructure of the system of variable‐density governing equations. Flow hydrodynamics and final bed topography of a laboratory‐based 2D partial dam breach over a mobile bed are satisfactorily reproduced by the numerical model. Comparison of the final bed topographies, computed for two distinct sediment transport methods, highlights the sensitivity of shallow water‐sediment models to the choice of closure relationships. Copyright © 2016 John Wiley & Sons, Ltd.     

Generalized Aifantis strain gradient plasticity model with internal length scale dependence on grain size,sample size and strain

Sediment-water interfaces are important interfaces for lakes, which are related to most environmental and ecological problems. Wind-induced waves cause secondary pollution via sediment resuspension. Since the coupling mechanism of water, resuspended sediments, and phosphorus affects the release of phosphorus (P) near the interface, a coupled model was explored for two sediment types with different adsorption-desorption capabilities to examine sediment resuspension and P release. The relationships among wind speed, wave characteristics, sediment distribution and P concentration were obtained. For different sediments, the unit sediment desorption release is negatively correlated with wind speed. When sediments are resuspended under low or moderate wind speed, the P concentration in the overlying water increases abruptly, hampering diffusion. P release exhibits the characteristics of concentrated release in a small region and changes the water environment rapidly.

     

Numerical modeling of cohesive sediments dynamics in estuaries: Part I—Description of the model and simulations in the Po River Estuary

The present work contributes to the numerical modeling of complex turbulent multiphasic fluid flows occurring in estuarine channels. This research finds its motivation in the increasing need for efficient management of estuaries by taking into account the complex turbulent stratified flows encountered in estuaries and costal zones. A time‐dependent, 3D finite element model of suspended sediment transport taking into account the effects of cohesiveness between sediments is presented. The model estuary is the forced time‐dependent winds, time elevation at open boundaries and river discharge. To cope with the stiffness problems a decoupling method is employed to solve the shallow‐water equations of mass conservation, momentum and suspended sediment transport with the conventional hydrostatic pressure. The decoupling method partitions a time step into three subcycles according to the physical phenomena. In the first sub‐cycle the pure hydrodynamics including the k–ε turbulence model is solved, followed by the advection–diffusion equations for pollutants (salinity, temperature, suspended sediment concentration, (SSC)), and finally the bed evolution is solved. The model uses a mass‐preserving method based on the so‐called Raviart–Thomas finite element on the unstructured mesh in the horizontal plane, while the multi‐layers system is adopted in vertical with the conventional conforming finite element method, with the advantage that the lowermost and uppermost layers of variable height allow a faithful representation of the time‐varying bed and free surface, respectively. The model has been applied to investigate the SSC and seabed evolution in Po River Estuary (PRE) in Italy. The computed results mimic the field data well. Copyright © 2007 John Wiley & Sons, Ltd.     

Multiple time scales of fluvial processes — theory and applications

Fluvial processes comprise water flow, sediment transport and bed evolution, which normally feature distinct time scales. The time scales of sediment transport and bed deformation relative to the flow essentially measure how fast sediment transport adapts to capacity region in line with local flow scenario and the bed deforms in comparison with the flow, which literally dictates if a capacity based and/or decoupled model is justified. This paper synthesizes the recently developed multiscale theory for sediment-laden flows over erodible bed, with bed load and suspended load transport, respectively. It is unravelled that bed load transport can adapt to capacity sufficiently rapidly even under highly unsteady flows and thus a capacity model is mostly applicable, whereas a non-capacity model is critical for suspended sediment because of the lower rate of adaptation to capacity. Physically coupled modelling is critical for fluvial processes characterized by rapid bed variation. Applications are outlined on very active bed load sediment transported by flash floods and landslide dam break floods.     

Erosion and sedimentation of a bump in fluvial flow

The 2D laminar quasistationary interacting boundary layer flow with mass transport of a suspended sediment is solved over an erodable bump (or dune) in the case of a fluvial régime. It is assumed that if the skin friction goes over a threshold value, the bump is eroded, then, the concentration of sediment in suspension is convected but falls at a constant settling velocity. This changes the shape of the dune, exemples of displacement toward final equilibrium states are presented.     

1‐D numerical modelling of shallow flows with variable horizontal density

A1‐D numerical model is presented for vertically homogeneous shallow flows with variable horizontal density. The governing equations represent depth‐averaged mass and momentum conservation of a liquid–species mixture, and mass conservation of the species in the horizontal direction. Here, the term ‘species’ refers to material transported with the liquid flow. For example, when the species is taken to be suspended sediment, the model provides an idealized simulation of hyper‐concentrated sediment‐laden flows. The volumetric species concentration acts as an active scalar, allowing the species dynamics to modify the flow structure. A Godunov‐type finite volume scheme is implemented to solve the conservation laws written in a deviatoric, hyperbolic form. The model is verified for variable‐density flows, where analytical steady‐state solutions are derived. The agreement between the numerical predictions and benchmark test solutions illustrates the ability of the model to capture rapidly varying flow features over uniform and non‐uniform bed topography. A parameter study examines the effects of varying the initial density and depth in different regions. Copyright © 2009 John Wiley & Sons, Ltd.     

再悬浮底泥中非吸附性污染物释放的数值模拟

环境水动力学中, 湖库底泥中污染物释放是人们研究的主要问题之一. 在水动力学条件作用下, 污染底泥再悬浮使大量污染物被重新释放出来, 造成水体的二次污染. 本文基于水槽实验研究提供的大量实测数据, 建立上覆水体-底泥-污染物的耦合力学模型. 在上覆水体不同流速条件下, 数值模拟底泥起动再悬浮过程以及污染物释放过程. 分析流场特性和污染物浓度之间的关系, 得到速度、颗粒体积分数、污染物浓度、湍动能以及时间等参数之间的定量关系. 研究表明, 底泥再悬浮污染物释放过程, 是由上覆水体-底泥-污染物构成的耦合过程. 迅速进入上覆水体的底泥颗粒, 影响了上覆水体流动特性, 进而影响到污染物的释放. 对于非吸附性污染物, 底泥起动后复杂的流场特性是底泥再悬浮污染物释放的主要影响因素. 当流场特性(如雷诺数)改变时, 对流和湍流扩散作用在污染物输运过程贡献不同. 建立上覆水体-底泥-污染物的耦合模型, 研究水动力学条件与底泥污染物释放规律的定量化关系, 可为构建湖库区域水污染模型提供支撑.      

A dynamic coupling numerical model for pollutant transport under wave condition

In order to study the diffusion, migration, and distribution of pollutants among overlying water-body and porous seabed under wave conditions, a dynamic coupling numerical model is proposed. In this model, the coupling between wave field of overlying water-body and seepage of porous bed, the capture and release of pollutants in porous media, and the transport process between the two different regions are taken into account. We use the unified equations for pressure correction and pollutant concentration to solve the numerical model, which avoids repeated iteration on the interface boundary. The model is verified by several case studies. Afterwards, the processes involving release of pollutant from porous seabed and transportation to overlying water-body under different wave conditions are investigated. The results show that the water depth, wave height,and wave period have great influences on the release, capture, and transport processes for phosphorus pollutant.     

振荡潜流带沉积层?水界面污染物输运的研究

潜流带中污染物质交换与输运特性是影响水资源环境的重要问题之一. 本文对底部为高渗透沉积层的三维槽道振荡流高Schmidt数传质问题进行了大涡模拟研究. 采用动力学亚格子模型来封闭滤波后的三维不可压缩Navier-Stokes方程以及污染物输运方程, 同时采用修正的Darcy-Brinkman-Forcheimer模型来描述沉淀有锌离子污染溶质的可渗透沉积层. 通过对沉积层内外流场和浓度场的统计特性以及瞬态结构的分析, 探究了上覆水体中振荡流驱动作用对污染物输运的动力学影响以及扩散率随振荡周期和振荡角的变化规律. 研究结果表明, 浓度通量中的湍流浓度分量在垂向物质交换中起主导作用, 流向、展向速度, 湍流强度和污染物浓度的波动跟随振荡驱动力呈现准周期变化, 同时发现沉积层?水交界面处的湍流浓度通量与法向湍流强度两者之间的变化具有明确的相关性. 并且在较大振荡角和低频振荡的情况下, 沉积层?水交界面处的有效扩散率增大, 这主要是来自于沉积层?水交界面处流体的猝发行为有效促进了湍流混合和物质交换, 进而增强了污染物的垂向输运.      

明渠中悬移质的弥散-对流方程及悬浮机理

悬移质泥沙通常构成冲积河流总输沙量的主体, 研究悬移质的悬浮机理具有重要的意义. 以双流体模型为基础, 通过引入弥散速度的概念, 建立了悬移质泥沙的输沙方程以及泥沙扩散系数的本构关系. 应用该方程分析了二维明渠均匀流中悬移质泥沙浓度垂向分布规律, 并与Einstein 和Chien 的泥沙浓度实验资料及经典扩散理论进行了对比. 以此为基础, 分析了紊动扩散、颗粒自身的紊动、颗粒碰撞应力对泥沙悬浮的影响在垂向上的变化, 以及浓度、粒径等对这些因素的影响. 结果表明, 泥沙颗粒在明渠紊流中的扩散是浑水的紊动扩散、颗粒自身的紊动、颗粒碰撞应力3 部分不同机制共同作用的结果, 把泥沙颗粒的悬浮简单归因于水流的紊动是不全面的.     

Influence of Internal Structure and Medium Length on Transport and Deposition of Suspended Particles: A Laboratory Study

A laboratory study was undertaken on the transport and the deposition of suspended particles (silt of modal diametre 6 μm) in three columns of different length, filled with glass beads or gravel. Tracer tests were carried out at various flow velocities by short pulses of a mixture of suspended particles/dissolved tracer. The breakthrough curves were competently described with the analytical solution of a convection dispersion equation with a first-order deposition rate and the hydro-dispersive parameters were deduced. For the same experimental conditions, the results showed a difference in the behaviour of the suspended particles transport and deposition rates within the two porous media tested. The internal structure of both media governs the particle-grain collision frequency as well as the particles trapping. The scale effect was highlighted and affects the dispersivity, the size exclusion effect, the recovery rates and the deposition rates. Longitudinal dispersion increases with mean pore velocity and is described with a nonlinear relationship. The dispersivity increases with the column length. The size exclusion effect is more important in the short column. The recovery rate increases with flow velocity and decreases while increasing column length. The deposition rates increases until a critical flow velocity then decreases. This critical velocity is also sensitive to the scale effect, and increases with the column length.     

The experimental investigation of heat transport and water infiltration in granular packed bed due to supplied hot water from the top: Influence of supplied water flux, particle sizes and supplied water temperature

In the present study an experimental investigation of heat transport and water infiltration in granular packed bed (unsaturated porous media) due to supplied water flux is carried out. The study is focus on the one-dimensional flow in a vertical granular packed bed column assuming local thermal equilibrium between water and particles at any specific space. This experimental study described the dynamics of heat transport and water infiltration in various testing condition. Experimentally, the influences of particle sizes, supplied water flux and supplied water temperature on heat transport and water infiltration during unsaturated flow are clarified in details. The results showed that the granular packed bed with larger particle size results in faster infiltration rate and form a wider infiltration depth. Furthermore, the increase of the supplied water flux and supplied water temperature corresponds to faster infiltration rate, but the results not linearly related to the interference between the heat transport and hydrodynamics characteristics in granular packed bed.     

Electrical measurement of sediment layer thickness under suspension flows

 This paper reports a new technique to measure the thickness of a layer of deposited sediment as a function of time, independent of the flow conditions or presence of suspended sediment above the layer. Small electrodes on the bottom and a reference electrode in the fluid above were used to measure the resistance of the layer with a small AC current and a bridge circuit. Using a multiplexer and an Analog-to-Digital converter the growth of the layer can be accurately monitored at many locations on the tank bottom. In a trial experiment the sedimentation under a stagnant column of a monodisperse suspension was examined. The results show that changes in the sediment layer thickness of less than 0.3% can be measured for layers up to 0.2 g/cm². Received: 8 February 1998/Accepted: 19 July 1998     

An unstructured finite‐volume method for coupled models of suspended sediment and bed load transport in shallow‐water flows

The aim of this work is to develop a well‐balanced finite‐volume method for the accurate numerical solution of the equations governing suspended sediment and bed load transport in two‐dimensional shallow‐water flows. The modelling system consists of three coupled model components: (i) the shallow‐water equations for the hydrodynamical model; (ii) a transport equation for the dispersion of suspended sediments; and (iii) an Exner equation for the morphodynamics. These coupled models form a hyperbolic system of conservation laws with source terms. The proposed finite‐volume method consists of a predictor stage for the discretization of gradient terms and a corrector stage for the treatment of source terms. The gradient fluxes are discretized using a modified Roe's scheme using the sign of the Jacobian matrix in the coupled system. A well‐balanced discretization is used for the treatment of source terms. In this paper, we also employ an adaptive procedure in the finite‐volume method by monitoring the concentration of suspended sediments in the computational domain during its transport process. The method uses unstructured meshes and incorporates upwinded numerical fluxes and slope limiters to provide sharp resolution of steep sediment concentrations and bed load gradients that may form in the approximate solutions. Details are given on the implementation of the method, and numerical results are presented for two idealized test cases, which demonstrate the accuracy and robustness of the method and its applicability in predicting dam‐break flows over erodible sediment beds. The method is also applied to a sediment transport problem in the Nador lagoon.Copyright © 2013 John Wiley & Sons, Ltd.