River computations: artificial backwater from the momentum advection scheme (original) (raw)
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A new approach to river bank retreat and advance in 2D numerical models of fluvial morphodynamics
2011
River bank retreat and advance are modes of morphological evolution in addition to bed level changes and changes in bed sediment composition. They produce planform changes such as width adjustment and meander bend migration. However, their reproduction in a 2D numerical model still remains a challenge. Defining bank-lines along the nearest grid lines of a rectangular computational grid leads to staircase lines that impede any reasonable determination of the hydraulic loads on the banks. An adaptive curvilinear boundary-fitted grid may seem to solve this problem, but arbitrary bank retreat and advance appear to deform such a grid prohibitively within a few bank-line update steps. We therefore present a new approach in which shifting bank-lines are followed as separate moving objects on a fixed grid, using local immersed-boundary techniques to solve the flow and sediment transport in the vicinity of the bank-lines. This means that the grid itself remains stationary but the flow domain is adapted each (morphological) time step. The use of separate moving objects also gives the opportunity to track bank-lines that are not on the border of the computational domain, but somewhere inside this domain, e.g. the main river channel between floodplains or the channels in an estuary.
Advances in Water Resources, 2010
This work deals with the formulation and numerical implementation of a two-dimensional mathematical and numerical model describing open channel hydrodynamics, sediment and/or scalar transport and riverbed evolution in curved channels. It is shown that a well balanced 2D model can predict flow features, sediment and scalar concentration, and bed elevation with an accuracy that is suitable for practical river engineering. The term ''balanced" implies that important physical processes are modeled with a similar degree of complexity and exhaustiveness. The starting point of the model formulation is the assumption of self-similarity of vertical velocity profiles (horizontal velocities in the longitudinal and transverse directions), that are scaled by shear velocity and streamline curvature, both resolved by the model. The former is scaled by a bed-resistance coefficient that must be estimated or calibrated -as usualon a application-specific basis, and the latter is computed by a new, grid-based but grid orientation independent, scheme that acts on the discrete solution. All processes, including bottom shear, momentum dispersion, scalar dispersion, turbulent diffusion, bed load, and suspended load, are modeled using physically based, averaged values of empirical or semi-empirical constants, and consistently with the assumed velocity profiles (and bed-generated turbulence). Bed deformation modeling can be implemented with either an equilibrium or non-equilibrium formulation of the Exner equation, depending on the adaptation length scale, which must be taken under consideration if significantly larger than the length scale of the spatial discretization. The governing equations are solved by high-resolution, unstructured-grid Godunov method, which is elsewhere tested and shown to be reliable and second-order accurate. Application of the model to laboratory test cases, using standard parameter values and previously reported bed-resistance coefficients, gives results comparable to many 2D and 3D models previously applied to the same cases, most part of which benefit from case-specific parameter tuning. There are obviously intrinsic limits to the descriptive ability of 2D models in river modeling, but the results of this study point to the utility and cost-effectiveness of a well-designed 2D model.
Treatment of natural geometry in finite volume river flow computations
Journal of Hydraulic …, 2003
A method is proposed for the treatment of irregular bathymetry in one-dimensional finite volume computations of openchannel flow. The strategy adopted is based on a reformulation of the Saint-Venant equations. In contrast with the usual treatment of topography effects as source terms, the method accounts for slope and nonprismaticity by modifying the momentum flux. This makes it possible to precisely balance the hydrostatic pressure contributions associated with variations in valley geometry. The characteristic method is applied to the revised equations, yielding topographic corrections to the numerical fluxes of an upwind scheme. Further adaptations endow the scheme with an ability to capture transcritical sections and wetting fronts in channels of abrupt topography. To test the approach, the scheme is first applied to idealized benchmark problems. The method is then used to route a severe flood through a complex river system: the Tanshui in Northern Taiwan. Computational results compare favorably with gauge records. Discrepancies in water stage represent no more than a fraction of the magnitude of typical bathymetry variations.
Computational fluid dynamics and the physical modelling of an upland urban river
Geomorphology, 2002
This paper describes the application of a commercially available, three-dimensional computational fluid dynamic (CFD) model to simulate the flow structure in an upland river that is prone to flooding. Simulations use a rectangular channel geometry, smooth sidewalls and a bed topography obtained from the field site that contains a subdued pool-riffle sequence. The CFD model uses the RNG je turbulence closure scheme of Yakhot and Orszag (J. Sci. Comput. 1 (1986) 1), as implemented in FLUENT 4.4.4, with a free surface. Results are shown for numerical runs simulating a 1:100 year return interval flood. Output from the numerical model is compared to a physical model experiment that uses a 1:35 scale fibreglass mould of the field study reach and measures velocity using ultrasonic Doppler velocity profiling (UDVP). Results are presented from the numerical and flume models for the water surface and streamwise velocity pattern and for the secondary flows simulated in the numerical model. A good agreement is achieved between the CFD model output and the physical model results for the downstream velocities. Results suggest that the streamwise velocity is the main influence on the flow structure at the discharge and channel configuration studied. Secondary flows are, in general, very weak being below the resolution of measurement in the physical model and less than 10% of the streamwise velocity in the numerical model. Consequently, there is no evidence for a 'velocity dip'. It is suggested that the subdued topography or inlet morphology may inhibit the development of secondary flows that have been recorded in previous flat-bed, rectangular open channel flows. A significant corollary of these results is that the morphological evolution of the pool-riffle sequence at high discharges may be controlled primarily by the downstream distribution of velocity and sediment transport with little role for lateral sorting and sediment routing by secondary flows. This paper also raises a number of issues that may be of use in future CFD modelling of three-dimensional flow in open channels within the geomorphological community.
Validation of a Numerical Model for Flow and Bedform Dynamics
PROCEEDINGS OF HYDRAULIC ENGINEERING, 2007
An explorative study has been carried out within the scope of this paper in order to validate a morphodynamic numerical model with the assessment of model sensitivity to some factors and parameters. The flow model component is a vertical two-dimensional with non-hydrostatic, unsteady free surface flow condition. The flow model has been coupled with sediment transport models. Two different sediment transport approaches have been used, namely Ashida & Michiue's bedload transport formula and a stochastic pick up deposition formulation for non-equilibrium sediment transport proposed by Nakagawa & Tsujimoto. The model performance has been tasted for different turbulence closures, namely a zero-equation, a standard k-ε and a non-linear k-ε models, in the context of morphodynamic simulation. Model performance has been evaluated for the prediction of temporal variation of flow-depth and boundary shear stress induced by bedform evolution based on experimental measurements.
Two-dimensional finite element river morphology model
Canadian Journal of Civil Engineering, 2007
We report the development and application of a river morphology model based on the two-dimensional depth-averaged hydrodynamic model River2D. This new movable bed version of River2D was applied to simulate the bed elevation changes in four experiments: bed aggradation due to sediment overload, bed degradation by sediment supply shut-off, knickpoint migration, and bar formation in a variable-width channel. Some conditions in these experiments involved quick changes in the upstream boundary conditions, rapidly varied flow, supercritical flow, hydraulic jumps, and secondary flows. The results of the model agreed well with measured data. Notable features of the model are the use of a flexible unstructured mesh based on triangular finite elements to provide higher spatial resolution in areas of interest and transcritical flow capabilities to simulate supercritical flow and hydraulic jumps over movable beds. Key words: numerical modeling, rivers, scour, sedimentation, two-dimensional, fin...
Earth Surface Processes and Landforms, 2019
Results from computational morphodynamics modeling of coupled flow-bed-sediment systems are described for ten applications as a review of recent advances in the field. Each of these applications is drawn from solvers included in the public-domain International River Interface Cooperative (iRIC) software package. For mesoscale river features such as bars, predictions of alternate and higher mode river bars are shown for flows with equilibrium sediment supply and for a single case of oversupplied sediment. For microscale bed features such as bedforms, computational results are shown for the development and evolution of two-dimensional bedforms using a simple closure-based two-dimensional model, for two-and three-dimensional ripples and dunes using a three-dimensional large-eddy simulation flow model coupled to a physics-based particle transport model, and for the development of bed streaks using a threedimensional unsteady Reynolds-averaged Navier-Stokes solver with a simple sedimenttransport treatment. Finally, macroscale or channel evolution treatments are used to examine the temporal development of meandering channels, a failure model for cantilevered banks, the effect of bank vegetation on channel width, the development of channel networks in tidal systems, and the evolution of bedrock channels. In all examples, computational morphodynamics results from iRIC solvers compare well to observations of natural bed morphology. For each of the three scales investigated here, brief suggestions for future work and potential research directions are offered.
Computational fluid dynamics (CFD) in river engineering : a general overview
Computational Fluid Dynamics is a branch of fluid mechanics with the purpose of discretizing the Navier-Stokes equations mainly by using algorithms, numerical methods and computers. CFD is becoming more and more popular among river engineers. Nevertheless the "computational costs" of some of these techniques are still expensive for engineering purposes. This paper presents a general overview of the most common computational methods that can be applied in river engineering.
Numerical modelling of river morphodynamics: latest developments and remaining challenges
Numerical morphodynamic models provide scientific frameworks for advancing our understanding of river systems. The research on involved topics is an important and socially relevant undertaking regarding our environment. Nowadays numerical models are used for different purposes, from answering questions about basic morphodynamic research to managing complex river engineering problems. Due to increasing computer power and the development of advanced numerical techniques, morphodynamic models are now more and more used to predict the bed patterns evolution to a broad spectrum of spatial and temporal scales. The development and the success of application of such models is based upon a wide range of disciplines from applied mathematics for the numerical solution of the equations to geomorphology for the physical interpretation of the results. In this light we organized this special issue (SI) soliciting multidisciplinary contributions which encompass any aspect needed for the development and applications of such models. Most of the papers in the SI stem from contributions to session HS9.5/GM7.11 on numerical modelling and experiments in river morphodynamics at the European Geosciences Union (EGU) General Assembly held in Vienna, April 27th to May 2nd 2014.