Physical complexity to model morphological changes at a natural channel bend

This study developed a two‐dimensional (2‐D) depth‐averaged model for morphological changes at natural bends by including a secondary flow correction. The model was tested in two laboratory‐scale events. A field study was further adopted to demonstrate the capability of the model in predicting bed deformation at natural bends. Further, a series of scenarios with different setups of sediment‐related parameters were tested to explore the possibility of a 2‐D model to simulate morphological changes at a natural bend, and to investigate how much physical complexity is needed for reliable modeling. The results suggest that a 2‐D depth‐averaged model can reconstruct the hydrodynamic and morphological features at a bend reasonably provided that the model addresses a secondary flow correction, and reasonably parameterize grain‐sizes within a channel in a pragmatic way. The factors, such as sediment transport formula and roughness height, have relatively less significance on the bed change pattern at a bend. The study reveals that the secondary flow effect and grain‐size parameterization should be given a first priority among other parameters when modeling bed deformation at a natural bend using a 2‐D model

A 3D numerical model of graded sediment transport in nonequilibrium condition

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732The computer code FAST3D has been developed to calculate flow and sediment transport in open channels. In the code the flow field is calculated by solving the full Reynolds-averaged Navier-Stokes equations with k- ε turbulence model; the bed-load transport is simulated with a non-equilibrium model containing an important parameter the so-called non-equilibrium adaptation length, which characterizes the distance for sediment to adjust from a non-equilibrium state to an equilibrium state; the bed deformation is obtained from an overall mass-balance equation for sediment transport. The governing equations are solved numerically with a finitevolume method on an adaptive, non-staggered grid. The former model assumed uniform bed material. In order to take into account the influence of grain size distribution of the bed-surface on the evolution of the bed topography and consequently also on the flow field, a sediment transport module has been presently developed at the Institute of Hydraulic Engineering, University of Innsbruck, Austria, for fractional sediment transport using a multiple layer model. This paper presents the numerical results for sediment sorting and the bed deformation in a curved alluvial channel under unsteady flow conditions according to Yen and Lee (1995). The calculations were compared with data from laboratory measurements. Further, the sensitivity of the simulated results to the non-equilibrium adaptation length is investigated

Modelling the Eddy Pattern in the harbour of Zeebrugge

Hydrodynamic

Implementation of a new Layer-Subroutine for fractional sediment transport in SISYPHE

Water Qualit

A gravel-sand bifurcation:a simple model and the stability of the equilibrium states

A river bifurcation, can be found in, for instance, a river delta, in braided or anabranching reaches, and in manmade side channels in restored river reaches. Depending on the partitioning of water and sediment over the bifurcating branches, the bifurcation develops toward (a) a stable state with two downstream branches or (b) a state in which the water discharge in one of the branches continues to increase at the expense of the other branch (Wang et al., 1995). This may lead to excessive deposition in the latter branch that eventually silts up. For navigation, flood safety, and river restoration purposes, it is important to assess and develop tools to predict such long-term behavior of the bifurcation. A first and highly schematized one-dimensional model describing (the development towards) the equilibrium states of two bifurcating branches was developed by Wang et al (1995). The use of a one-dimensional model implies the need for a nodal point relation that describes the partitioning of sediment over the bifurcating branches. Wang et al (1995) introduce a nodal point relation as a function of the partitioning of the water discharge. They simplify their nodal point relation to the following form: s*=q*k , where s* denotes the ratio of the sediment discharges per unit width in the bifurcating branches, q* denotes the ratio of the water discharges per unit width in the bifurcating branches, and k is a constant. The Wang et al. (1995) model is limited to conditions with unisize sediment and application of the Engelund & Hansen (1967) sediment transport relation. They assume the same constant base level for the two bifurcating branches, and constant water and sediment discharges in the upstream channel. A mathematical stability analysis is conducted to predict the stability of the equilibrium states. Depending on the exponent k they find a stable equilibrium state with two downstream branches or a stable state with one branch only (i.e. the other branch has silted up). Here we extend the Wang et al. (1995) model to conditions with gravel and sand and study the stability of the equilibrium states

Modelling of sediment transport and bed deformation in rivers with continuous bends

Peer reviewedPostprin

A review on recent development of numerical modelling of local scour around hydraulic and marine structures

This paper reviews the recent development of numerical modelling of local scour around hydraulic and marine structures. The numerical models for simulating local scour are classified into five categories: sediment transport rate models, two-phase models, CFD-DEM models, equilibrium scour models and depth-averaged models. The sediment transport rate models are the most popularly used models because of their high calculation speed and availability of empirical formulae for predicting sediment transport rates. Two-phase models were developed to simulate sediment transport in the format of sheet flow under strong current velocity or strong turbulence. The CFDDEM model simulates the motion of every individual sediment particle. Its speed is the slowest, but it provides the opportunity to understand fundamental mechanisms of flow–particle interaction and particle–particle interaction using small-scale simulations. Equilibrium scour models predict the final scour profile at the equilibrium stage but cannot predict scour history. The depth-averaged models that were developed early are not recommended for local scour problems because they are not able to predict three-dimensional features around structures. Although many numerical models have been developed and many studies have been conducted to investigate local scour, some challenging problems remain to be solved, for example, the effects from scaling and sediment gradation. In addition, people’s understanding of local scour of cohesive sand is still very shallow, and more experimental and numerical research in this area is needed

Assessment of a numerical model to reproduce event-scale erosion and deposition distributions in a braided river

Becky Goodsell and Eric Scott are thanked for field assistance. Antony Smith assisted with figure production. The field campaign wa funded by NERC Grant NE/G005427/1 and NERC Geophysical Equipment Facility Loan 892. Richard Williams was funded by NERC Grant NE/G005427/1during fieldwork and by a British Hydrological Society Travel Grant whilst visiting NIW