Two dimensional river flow and sediment transport model

Frequency and intensity of extreme rainfall events is expected to increase. Extreme rainstorms cause redistribution of contaminated sediments which is a significant problem due to the presence of many contaminated sites in heavily populated urban coastal areas. This study evaluates contaminated sediment transport and redistribution in rivers due to extreme rainfall. To do so, a conceptual model that couples hydraulic, sediment and contaminant transport is formulated to predict erosion and deposition of sediments and the transport of adsorbed heavy metals in a river after extreme rainfall.One dimensional energy equation and Manning's equation were used to predict water surface elevations, the sediment continuity equation is used for sediment transport predictions and calculate sediment erosion and deposition, hydrodynamics and sediment transport predictions were linked to predict contaminated sediment transport assuming equilibrium and linear soil-contaminant partitioning. The hydraulic model was calibrated using water surface elevations from USGS. The sediment transport model was calibrated using USGS sediment loads data. Metal concentrations reported by USGS were also used to calibrate contaminant transport model and simulate contaminant-sediment sorption. Lead is used as a model contaminant due to its high sorption on sediment particles. A sensitivity analysis was first carried out to assess the impacts of key parameters on contaminated sediment transport. The approach was then implemented to evaluate the effect of Hurricane Harvey, which hit Texas and Louisiana in 2017, on contaminated sediment transport at the Neches River watershed, one of the most contaminated waterways in Texas, as a case study. The study shows that extreme rainfall events cause substantial transport and redistribution of contaminated sediments in rivers. Redistribution of heavy metals contamination is controlled by transport of fine particles due to their high sorption capacity. This causes significant spreading of contamination after such event as fine particles constitute the majority of transported sediments. Further, the increase in Lead concentrations in surface water and sediments from upstream to downstream the river lasts for long periods after the rain event.

A 3D investigation of sediment transport processes across long, straight channels exposed to an oblique incoming current is presented. For this purpose a detailed mathematical flow and sediment transport model is applied. The governing flow and sediment transport equations are solved in a curvilinear coordinate system. The sediment transport model is composed of bed and suspended load, applying a convection-diffusion equation for the latter. As the flow crosses a channel obliquely, it is refracted toward the channel alignment. The effect of the current refraction on the sediment transport is studied using both 2D (depth-averaged) and 3D models. Shortcomings of models based on depth-averaged descriptions are underlined through comparison with the 3D model. A reliable description of the backfilling process requires a flow and sediment transport model capable of capturing the following aspects: (1) The direction and magnitude of the bed-shear stress; and (2) the adaptation of suspended sediment to local sediment transport capacities. Calculations of the morphological development of small-scale excavations (e.g., pipeline trenches) exposed to an oblique incoming current are presented.

Understanding processes controlling sediment transports at the mouth of a highly energetic inlet system (San Francisco Bay, CA)

An integrated multi-scale simulation method of flow and sediment transport in rivers is required for river channel designs and maintenance managements. This paper highlights the need of a new depth integrated model without the assumption of the shallow water flow for integrated simulations of 2D and 3D phenomena in rivers, indicating the applicable ranges and limitations of previous depth integrated models. The general Bottom Velocity Computation method without the shallow water assumption is proposed and its applicability for flows with 3D structures and bed variations is discussed in this paper. The unified calculation method for integrated multi-scale simulations is presented to elucidate flow and sediment transport in laboratory channel and rivers during flood. Finally, we discuss issues to be solved and the future works toward integrated multi-scale simulations of flow and sediment transport in rivers.

The mathematical simulation of sediment and Kepone (a highly chlorinated pesticide) transport in the James River estuary was conducted by applying the sediment and contaminant transport model, FETRA, to an 85-km river reach between Bailey and Burwell Bays. The FETRA code is an unsteady, two-dimensional, finite model, utilizing the Galerkin weighted residual method. The model consists of three submodels coupled together to take into account sediment-contaminant interaction. the submodels are: (1) sediment transport model, (2) dissolved contaminant transport model, and (3) particulate contaminant (contaminants adsorbed by sediment) transport model. Transport of sediment and particulate contaminants is simulated for each sediment type or size fraction. The FETRA code was applied to simulate the migration of sediment and Kepone for three river discharges. Tidally influenced depth and velocity distributions in the study area were obtained by the unsteady, one-dimensional code, EXPLORE. These results were used by the two-dimensional code, FETRA, to obtain longitudinal distributions of sediment and Kepone. Hence, the results presented here are cross-sectionally averaged values changing with tidal flow. Comparison of computed results and field data for both sediment and Kepone concentrations indicates very good agreement, confirming the validity of the model. Mathematical simulation of Kepone transport under most probable flow conditions yields an estimate of 89.1 kg/yr of Kepone transported seaward from Burwell Bay. Of this 89.1 kg of total Kepone, 25.3% is carried by sediment, while 74.7% is in a dissolved phase.

A depth-averaged 2-D model has been developed to simulate unsteady flow and nonuniform sediment transport in coastal waters. The current motion is computed by solving the phase-averaged 2-D shallow water flow equations reformulated in terms of total-flux velocity, accounting for the effects of wave radiation stresses and general diffusion or mixing induced by current, waves, and wave breaking. The cross-shore boundary conditions are specified by assuming fully developed longshore current and wave setup that are determined using the reduced 1-D momentum equations. A 2-D wave spectral transformation model is used to calculate the wave height, period, direction, and radiation stresses, and a surface wave roller model is adopted to consider the effects of surface roller on the nearshore currents. The nonequilibrium transport of nonuniform total-load sediment is simulated, considering sediment entrainment by current and waves, the lag of sediment transport relative to the flow, and the hiding and exposure effect of nonuniform bed material. The flow and sediment transport equations are solved using an implicit finite volume method on a variety of meshes including nonuniform rectangular, telescoping (quadtree) rectangular, and hybrid triangular/quadrilateral meshes. The flow and wave models are integrated through a carefully designed steering process. The model has been tested in three field cases, showing generally good performance.

An integrated modeling system has been developed to simulate sediment and contaminated sediment transport in the Upper Columbia Slough System during the Flood of 1996. The modeling system consists of a hydrodynamic model, a sediment transport model, and a contaminant transport model. The hydrodynamic model predicts the Slough flow dynamics and sediment transporting power; the sediment transport model predicts flow-induced sediment transport; and the contaminant transport model predicts the migration of contaminated sediments in the Slough system. The Upper Columbia Slough modeling system is characterized by its recognition of the complex interplay of Slough hydrology, hydrodynamics, sediment transport, contaminant transport. The modeling system is also characterized by its ability to simulate both cohesive and noncohesive sediments and the associated contaminant transport. The hydrodynamic model was calibrated using water level obtained over a two-month dry period in 1993. The sediment transport model was not calibrated. The sediment model utilized the typical values from extensive literature review and from the calibration results of the Lower Columbia Slough sediment transport model to specify the parameters governing deposition and resuspension processes. The model results indicate that the hydraulic power in Upper Slough is weak under normal conditions, and existing sediment in the Slough moves very little unless there is a major stonn. Winter large storm events dominate Upper Slough dynamics and dictates sediment transport. The model predicted significant sediment resuspension but minimal bedload transport further downstream from the Mid-Dike and upstream of MCDD#4 on the Slough main channel during the 1996 flood. The model predicted little or no sediment transport throughout the southern arm system The study also concluded that none of the contaminated sediment priority sites would have significant sediment transport during the major storm events. The study points out that future model improvement should be focused on calibrating the key model parameters that determine sedimentation rates, such as critical shear stress for erosion and deposition.

Hummel, Ryan, Jennifer G. Duan, and Shiyan Zhang, 2012. Comparison of Unsteady and Quasi‐Unsteady Flow Models in Simulating Sediment Transport in an Ephemeral Arizona Stream. Journal of the American Water Resources Association (JAWRA) 48(5): 987‐998. DOI: 10.1111/j.1752‐1688.2012.00663.xAbstract: Hydrodynamic and sediment transport models are useful engineering tools for predicting unsteady flood flow and sediment transport. Many models such as HEC‐RAS, HEC‐6, and IALLUVIAL apply quasi‐unsteady flow model, whereas others apply the unsteady flow model. It remains unknown if a quasi‐unsteady flow model is sufficiently accurate for simulating sediment transport in rapidly varied unsteady flood events, especially in ephemeral rivers in arid and semiarid regions. This study compared the quasi‐unsteady HEC‐RAS 4.1 model with one‐dimensional (1D) Finite Volume Method (FVM) based model in simulating flood flow and sediment transport in the Pantano Wash, a dryland river in the state of Arizona. The objective is to determine which sediment transport method is appropriate in predicting bed elevation changes in an ephemeral stream, Pantano Wash, and if an unsteady model is more accurate than a quasi‐unsteady flow model in predicting sediment transport. Results showed that the quasi‐unsteady HEC‐RAS model and the 1D FVM yielded similar results of bed degradation and aggradation for this dryland stream, although the FVM model predicted better flood hydrographs. Among the seven sediment transport formulas embedded in HEC‐RAS, Yang’s and Engelund‐Hansen’s equations gave the best matches with the field measurements for this particular case study.

The paper presents recommendations for using the results obtained in sediment transport simulation and modeling of channel deformations in rivers. This work relates to the issues of empirical modeling of the water flow characteristics in natural riverbeds with a movable bottom (alluvial channels) which are extremely complex. The study shows that in the simulation of sediment transport and calculation of channel deformations in the rivers, it is expedient to use the calculation dependences of Chézy’s coefficient for assessing the roughness of the bottom sediment mixture, or the dependences of the form based on the field investigation data. Three models are most commonly used and based on the original formulas of Meyer-Peter and Müller (1948), Einstein (1950) and van Rijn (1984). This work deals with assessing the hydraulic resistance of the channel and improving the river sediment transport model in a simulation of riverbed transformation on the basis of previous research to verify it based on 296 field measurements on the Central-East European lowland rivers. The performed test calculations show that the modified van Rijn formula gives the best results from all the considered variants.

It is a challenge to apply coupled hydrodynamic, sediment process, and contaminant fate and transport models to the studies of surface water systems. So far, there are few published modeling studies on sediment and metal transport in rivers that simulate storm events on an hourly basis and use comprehensive data sets for model input and model calibration. The United States Environmental Protection Agency (USEPA) in 1997 emphasized the need for credible modeling tools that can be used to quantitatively evaluate the impacts of point sources, nonpoint sources, and internal transport processes in 1D/2D/3D environments. A 1D and time-dependent hydrodynamic, sediment, and toxic model, within the framework of the 3D Environmental Fluid Dynamics Code (EFDC), has been developed and applied to Blackstone River, Mass. The Blackstone River Initiative (USEPA) in 1996, a multiyear and multimillion-dollar project, provided the most comprehensive surveys on water quality, sediment, and heavy metals in the river, and served as the primary data set for this study. The model simulates three storm events successfully. The river flow rates are well calculated both in amplitude and in phase. The sediment transport and resuspension processes are depicted satisfactorily. The concentrations of sediment and five metals (cadmium, chromium, copper, nickel, and lead) during the three storm events are also simulated very well. Numerical analyses are conducted to clarify the impacts of contaminant sources and sediment resuspension processes on the river. While point sources are important to sediment contamination in the river, other sources, including nonpoint sources from watershed and bed resuspension, were found to contribute significantly to the sediment and metals in the river. Point sources alone cannot account for the total metals in the river. The model presented in this paper can be a useful tool for studying sediment and metals transport in shallow rivers and for water resource management.

Rahman, S., Mano, A. and Udo, K., 2013. Quasi-2D Sediment Transport Model Combined with Bagnold-type Bed Load Transport. Suspended load and bed load sediment transport are important component for the sediment transport in surf zone. The purpose of this study is to obtain a good model for sediment transport in surf zone by combining a quasi-2D sediment transport model and the Bagnold-type sediment transport model. The quasi-2D sediment transport model is used to simulate the suspended load transport, while the Bagnold-type for the bedload transport. A quasi-2D numerical wave model called Funwave was expanded to accommodate the sediment transport model. The model is validated by the published data for sediment transport in a wave flume. Two mode of morphological change is compared to evaluate the influence of wave-current (mode A) and instantaneous bottom velocity (mode B) in the third and forth velocity moment of Bagnold-type sediment transport model. Four sets of bed load transport parameters are...

Flash‐flood events in mountain basins often involve large volumes of sediment transported by the rivers, with significant changes in the bed morphology even within a single, short‐term event. By contrast, flood scenarios are most frequently devised under the assumption of a fixed‐bed condition, that is, neglecting the possible change of the river bed elevation during the reference flood. In this work, feasibility and positive outcome of incorporating sediment transport modelling into the evaluation of flood hazard are assessed with reference to the case of the Mallero River in northern Italy. A past flood event has been modelled by means of a fully coupled model of flow and sediment transport. Particular attention has been focused onto the town of Sondrio, located at the downstream end of the river. Results show that interpretation of the event dynamics and proper quantification of flood hazard for the town cannot be obtained without considering the morphologic evolution of the river bed due to sediment transport. In addition, reliability of the results of hydro‐morphologic modelling has been proved by extensive sensitivity analysis, showing a weak dependence of the findings on external forcing. Some arguments are thus provided towards the incorporation of morphologic processes into hazard assessment, landscape protection, scenario modelling and emergency management.

The use of multiple-level non-uniform rectangular mesh in coupled flow and sediment transport modeling is preferred to achieve high accuracy in important region without increasing computational cost greatly. Here, a robust coupled hydrodynamic and non-equilibrium sediment transport model is developed on non-uniform rectangular mesh to simulate dam break flow over movable beds. The enhanced shallow water and sediment transport equations are adopted to consider the mass and momentum exchange between the flow phase and sediment phase. The flux at the interface is calculated by the positivity preserving central upwind scheme, which belongs to Godunov-type Riemann-problem-solver-free central schemes and is less expensive than other popular Riemann solvers while still capable of tracking wet/dry fronts accurately. The nonnegative water depth reconstruction method is used to achieve second-order accuracy in space. The model was first verified against two laboratory experiments of dam break flow over irregular fixed bed. Then the quantitative performance of the model was further investigated by comparing the computational results with measurement data of dam break flow over movable bed. The good agreements between the measurements and the numerical simulations are found for the flow depth, velocity and bed changes.

A depth-averaged two-dimensional (2D) numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered, curvilinear grid. The 2D shallow water equations are solved by the SIMPLE(C) algorithms with the Rhie and Chow’s momentum interpolation technique. The proposed sediment transport model adopts a nonequilibrium approach for nonuniform total-load sediment transport. The bed load and suspended load are calculated separately or jointly according to sediment transport mode. The sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes. An empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed-load movement direction in channels with steep slopes. Flow and sediment transport are simulated in a decoupled manner, but the sediment module adopts a coupling procedure for the computations of sediment transport, bed change, and bed material sorting. The model has been tested against several experimental and field cases, showing good agreement between the simulated results and measured data.

Purpose In this paper, a novel modeling approach is applied to assess the unique transport characteristics of hydrophobic (bitumen containing) cohesive sediment for the Ells River, AB, Canada. The modeling offers a new way of treating the transport and fate of fine sediment in rivers and points to the importance of including a sediment entrapment process in the modeling of the Ells River sediment dynamics.