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Development of a reach scale two-dimensional finite element model for floodplain sediment deposition
Abstract
This paper discusses the development of a two-dimensional finite element coupled hydraulic-sediment transport scheme for application to lowland floodplain environments. Recent developments in numerical algorithms have led to the development of two- and three-dimensional hydraulic models which are capable of simulating open channel and floodplain hydraulics at river reach scales of 1-60 km. However, these hydraulics schemes have not been linked with suitable sediment dynamics schemes. This paper describes the numerical developments necessary to extend an existing two-dimensional finite element solution of coupled hydraulic and sediment transport to fluvial floodplains. These include correct choice of numerical solvers to prevent artificial build-up of tracer mass in dry areas, control of artificial diffusion and implementation of an equation base appropriate to moving-boundary problems. Four experiments of increasing complexity were undertaken in order to ascertain the predictive ability of the model to produce realistic simulations of floodplain sediment deposition. Results showed that a two-dimensional-depth averaged flow field representation captures much of the gross behaviour of suspended sediment transport for reach scale fluvial applications.
... The use of flow modelling in fluvial geomorphology and hydrology, as well as fish habitat research, is a rapidly growing area. Hydraulic modelling of rivers for practical purposes has generally used one-dimensional (1D; i.e. width-and depth-averaged) simplifications of the fundamental equations (Horritt, 2002), but 2D (depth-averaged) models are increasingly used (Bates and Anderson, 1993;Hardy et al., 1999Hardy et al., , 2000 and there has been much recent research on the application of 3D computational fluid dynamics (CFD) codes to natural rivers (Hodskinson and Ferguson, 1998;Booker et al., 2001;Lane et al., 2002Lane et al., , 2004. Lane and Ferguson (2005) review this whole subject area, including the coupling of sediment transport models to flow dynamics models. ...
Large-eddy simulation (LES) is a method for resolving the time-dependent structure of high Reynolds number, turbulent flows. With LES it is possible to model and track the behaviour of coherent turbulent structures and study their effect on the flow field. Hence, LES is potentially an important research tool in the fluvial sciences where flow mixing, sediment entrainment and sediment transport are all affected by the presence of coherent vortices and their interactions with channel boundaries and other flow structures. This paper introduces the LES methodology, discusses a variety of ways for representing small-scale processes within LES (the subgrid-scale modelling problem), and provides some examples of early work into the use of LES in a fluvial context. A number of advances in computational power and numerical methods are required before LES can be effectively applied at the river reach scale. This paper considers some recent developments and their potential for providing validated large-eddy simulations of river flow at the channel scale.
This chapter reviews some aspects of sediment transport modeling for a range of environments. It discusses that sediment transport modeling is far from a complete science due to our current processes understanding and the complexity introduced from the heterogeneity of the natural environment. The chapter concludes by suggesting that the most promising development route for an improved understanding of the processes appears to be discrete particle modeling. Such processes understanding will enable an improved parameterization or adapt the methodology of lower dimensionality codes.
The bed change analysis near the opening gate of a dam or weir to release deposited sediments have been conducted mostly using the numerical models. However, the use of unverified input parameters in the numerical model is able to produce the different results with natural and real conditions. Also, the bed changes near the opening gate of a dam or weir calculated with a numerical model could be varied depending on the geometry extent included the downstream area with supercritical flow in the model. In addition, the different time steps could provide different results in the bed change calculation, even though other conditions such as input parameters, geometries, and total simulation time were same. Therefore, in this study, hydraulic experiments were performed to validate the eddy viscosity coefficient which is the one of important input parameters in the RMA2 model and relevant to variation of simulation results. The bed changes were calculated using the SED2D model based on flow results calculated in the RMA2 model with the verified and selected eddy viscosity coefficient and also compared with experimental results. The bed changes near the opening gate were underestimated in the numerical model comparing with experimental results except only the numerical case without the modeling section of sediment release pipe and downstream area where the supercritical flow was produced. For the simulation of minimum time steps, different shapes of scour hole were produced in numerical and physical modeling.
In the framework of a river regulation design of the Po River Delta (Northern Italy), a study on a large physical model of the bifurcation Po di Goro-Po di Venezia was conducted with the main objective of determining the discharge subdivision rate at the river node, in order to assess the inflow conditions in the Po di Goro River for flood risk analysis. In this context, a two-dimensional depth averaged numerical model was tested against measured values, with reference to the prototype. In this paper a comprehensive analysis and discussion of the results is reported in order to highlight the applicability of numerical models in comparison with physical ones in river engineering applications.
IntroductionFluid dynamics of moving boundary problems in environmental hydraulicsDeforming grid methodsFixed grid methodsTreating moving boundaries in advection–diffusion modelsFuture research needsAcknowledgementsReferences
The potential for using high-resolution numerical modelling to understand flow over complex rough heterogeneous surfaces has yet to be fully realized, largely due to problems of designing numerically stable meshes for use with complex bed geometries. Representation of such geometries has tended to rely upon sub-grid scale treatments involving roughness parameterization within numerical schemes. This paper develops, verifies and validates a technique for dealing with the representation of complex bed geometries within a basic numerical scheme. The method is based upon a numerical porosity treatment, which uses a structured grid but specifies cell porosities to block out bottom topography ( =1 for cells that are all water, P =0 for cells that are all bed and 0
A finite element model for simulating tidal flooding and dewatering of shallow estuaries is described and applications to hypothetical embayments and to the Great Bay, New Hampshire estuary system, are presented. The model incorporates two-dimensional kinematic wave physics, with a porous medium beneath the open channel to incorporate the realistic drainage of dry elements on a fixed, high resolution mesh. The Galerkin method is used on simple linear finite elements and solved implicitly with iteration in time.Simulations of idealized channels conserve mass, display physically correct behaviour, and agree with applicable one-dimensional results. Solutions for Great Bay further illustrate the physics of tidal flat hydrodynamics, characteristic distributions of bottom shear stress and the influence of topography on the overall circulation in the region. Sediment transport implications are discussed.
Describes upper and middle reaches characterised by meandering and lateral sedimentation and lower reaches displaying channel stability, low sinuosity and overbank sedimentation. Radiocarbon dates show different accumulation rates in different floodplain environments with maximum rates around 2-3000 BP probably due to deforestation and arable agriculture. The Severn is typical of many low-energy systems where low slope, bank resistance and relatively high suspended sediment load can produce floodplain fills dominated by vertical sedimentation associated with semi-stable straight, meandering or anastomosing channels -from Author
Within a 13 km reach of the lower course of the River Culm, Devon, UK, the river has a well-developed floodplain which is frequently inundated at times of high flow. Detailed records of suspended sediment load obtained from upstream and downstream gauging stations indicate that approximately 28% of the upstream load may be deposited within this reach. Direct measurements of floodplain sedimentation using 'plastic grass' sediment traps evidence both macro- and microscale spatial variations in deposition which can be related to the duration of inundation, the topography of the floodplain surface and the flow velocity of the inundating water. Rates of deposition have been estimated from conveyance losses and these have been compared with data obtained from the sediment traps. Average rates of approximately 490 g m-2 (0.49 mm year-1) are indicated. A comparison of the characteristics of storm and floodplain sediment indicated that conveyance losses are associated with a preferential loss of the coarser fractions, although appreciable deposition of the clay fraction also occurs.
This paper describes waveform sampling lidar systems for airborne remote sensing applications. Measurements over complex terrains and open ocean using these lidar systems are described. The lidar detected terrain features during these experiments included forested lands, farm lands, cleared lands, farm roads and lake water. Lidar waveform characteristics as a function of these terrain features are described. Comparisons of lidar waveform over lake water and ocean water are also described.
This paper describes a new numerical method used within the SUBIEF software to compute the transport of diluted or suspended tracers in a two-dimensional depth-averaged free surface flow for domains where the flood extent varies with time. The transport equations and the numerical choices for time and space discretization in finite elements are described prior to developing two alternative formulations for dealing with partially dry elements. Finally, we depict two typical applications of the software: the simulation of sedimentation in the intake of a power plant qualitatively validated by comparing numerical results obtained from SUBIEF with those from a physical model, and a simulation of heavy metal adsorption/desorption on to sediment particles in a fluvial flow which are subsequently deposited and resuspended. © 1998 John Wiley & Sons, Ltd.
A two-dimensional horizontal finite element numerical model (RMA-2) was applied to a 24 km river channel-floodplain reach in West Germany. Initial results indicate that finite element schemes may successfully estimate inundation in large-scale floodplain applications. Potentially, the resulting detailed velocity vector distributions and identification of inundation zones throughout storm events could provide an insight into the present day sedimentary environment on the floodplain.
A prototype two-dimensional finite element flow model for depth-averaged free surface flows was developed for floodplain environments. Limited refinement of the model's physical representation was undertaken and the enhanced scheme applied to an 11 km river channel/floodplain reach in the U.K. Preliminary model results indicate that this modelling approach can be used to identify dynamic variations in the flow field parameters over length scales of the order of 10-100 m. Potentially, such data have the ability to permit detailed analysis of short-term floodplain sedimentary dynamics.
Samples of sediment collected from the Severn floodplain between Worcester and Gloucester following the severe flooding in January and February 1990, were analysed for their grain size distribution. The results show that most sand was deposited within 20 m of the channel bank, but that fine sand may contribute to flood sediment across the width of the floodplain. James' (1985) numerical model of overbank sedimentation attempts to predict the transfer of sediment to the floodplain during flooding. Geometrical and hydraulic data relating to the Severn flood are used as input for a computer program of James' (1985) model. The pattern of sediment concentrations predicted by the model was compared with that obtained from statistical analysis of the flood sediment. The patterns were found to be similar, so James' (1985) model was considered to predict in a relative sense the distribution of flood sediment.
A new finite element formulation for convection dominated flows is developed. The basis of the formulation is the streamline upwind concept, which provides an accurate multidimensional generalization of optimal one-dimensional upwind schemes. When implemented as a consistent Petrov-Galerkin weighted residual method, it is shown that the new formulation is not subject to the artificial diffusion criticisms associated with many classical upwind methods.The accuracy of the streamline upwind/Petrov-Galerkin formulation for the linear advection diffusion equation is demonstrated on several numerical examples. The formulation is extended to the incompressible Navier-Stokes equations. An efficient implicit pressure/explicit velocity transient algorithm is developed which accomodates several treatments of the incompressibility constraint and allows for multiple iterations within a time step. The effectiveness of the algorithm is demonstrated on the problem of vortex shedding from a circular cylinder at a Reynolds number of 100.
A two dimensional finite element model, RMA-2, is applied to a 25 km floodplain reach in West Germany. This scheme is shown to be capable of providing a good predictive capability over the comparatively long reach length. In addition, when the scheme is driven by an input hydrograph generated by an ungauged flood forecasting scheme (HYMO3), the results remain encouraging in terms of both outflow hydrograph and downstream stage prediction.These initial results are supportive of the possibility of large scale applications of finite element schemes providing base (or ‘ground truth’) discharge data for ungauged catchment model validation, where no downstream gauged flow records are available. Secondly, such schemes can themselves be driven by ungauged semi-lumped forecasting models to provide an improved resolution of inundation and flow characteristics, (including velocity vectors) in those reaches where such data are required.
A two-dimensional finite difference numerical model, capable of predicting depth-averaged tidal flow fields in coastal and estuarine waters, has been extended to include tide-induced non-cohesive sediment transport processes. The partial differential equations governing the conservation of mass, momentum and suspended sediment in an incompressible turbulent flow are included in a depth-integrated form in the model. For the representation of the processes of erosion and deposition of sediment from the bed an empirically based source-sink term was refined, based on the results of three mobile bed flume studies. The model has been tested by simulating tidal flows and suspended sediment fluxes in two estuaries, with particular application to the Humber estuary in the U.K. The model was calibrated and found to produce an encouraging degree of agreement between the numerical predictions and corresponding field measurements for this estuary. Furthermore, the predicted gross deposition and erosion features of the estuary were found to be in close agreement with interpretations from Eulerian tidal residual predictions.
A prototype two dimensional finite element model for the simulation of flood flows is presented. This is shown to have significant advantages over other methods of flow simulation for compound meandering channels, such as the ability to predict dynamic inundation zones. In particular, this paper demonstrates that reasonable predictions may be obtained on the basis of extremely sparse data sets, such as topographic maps and available flow monitoring records. Finally, a more rigourous method of determining the most suitable modelling approach in a given situation is called for.