Sediment transport and dunes in pipe flow

In most 1D or 2D depth-averaged sediment transport models, the sediment in suspension is assumed to be advected by the depth-averaged velocity. This contribution highlights the fact that the depth-averaged velocity must be weighted by the concentration profile to take into account the fact that the largest part of the sediment is transported near the bed. For this reason, a correction factor is introduced and an analytical formulation of this factor is provided. Through comparison with 3D computations, the efficiency of this correction factor is evaluated on a test case representing a gentle dune (with different dune steepnesses) propagating downstream under the action of a steady flow. For small dune steepness, the correction factor enables results from 2D computation to be closer to 3D simulation.

MARID IV Marine and River Dune Dynamics 2013, 2013

Often river bed form modelling is done with an equilibrium bed load transport formula like that of Meyer-Peter & Müller (1948). However, a physically more correct way would be to model it with separate models for the sediment pickup and deposition processes as described by Nakagawa & Tsujimoto (1980). Besides the physics of the sediment transport itself, using such a method allows for the modelling of higher-order processes as well like spatial lag in bed load transport. As shown by Shimizu et al. (2009) applying the aforementioned pickup and deposition model in a dune evolution model, makes is possible to model dunes well. Specifically it made it possible to determine a transition to upper stage plane beds, as well as capturing hysteresis well. In this paper we will explore the effect of using different kinds of bed load models in a relatively simple dune evolution model. The Nakagawa & Tsujimoto (1980) bed load model, will be implemented in the dune evolution model of Paarlberg et al. (2009). Results of this model version will be compared with the original version (using the Meyer-Peter & Müller formula) and a later version that directly models spatial lag with a relaxation equation.

Sedimentology, 1981

Theoretical and empirical analyses of flow structure, sediment transport, and sediment size characteristics at the crest of dune-like bedforms indicate that it is possible to describe, at least semi-quantitatively, the diffusion and deposition of sediment on the leeside of such structures. A numerical program based on this analysis simulates the grain-size distribution and deposition rate on the leeside of dunes for specified flow conditions and bed material. Evaluation of flow and sediment variables through the numerical simulation program shows that flow velocity, flow depth and sediment size have a strong influence on the deposition rate and texture of leeside sediment before avalanching. Sorting of the bed material, in particular, appears to exert a strong control on both the grain-size and the deposition-rate gradients.

Sediment pickup rate is useful for deriving bedload function and specifying boundary conditions for numerical modeling of sediment transport. Previous pickup functions were developed based only on the data measured under the plane bed condition, and they may not be applicable for dune-covered beds. The present study compares pickup rates for three bed conditions, i.e. plane bed, fixed dunes and mobile dunes. All pickup functions are formulated by involving the densimetric Froude number instead of the Shields number. The results show that the sediment pickup rates measured over fixed dunes can be described using the pickup function developed for the plane bed. However, the presence of mobile dunes causes a considerable reduction in the pickup rate when the densimetric Froude number is greater than 5, which is largely due to flow-particle and particle-particle interaction. This study also shows that under mobile dune conditions, the particle step length for dune bed conditions scales with the dune length rather than the particle diameter.

Coastal Engineering Proceedings, 1982

An experimental study is described in which the rate of transport of fine sand over a flat bed was measured as a function of the shear stress exerted on the bed by both laminar and turbulent flows, for a range of water temperatures. The results confirm the great sensitivity of the transport rate to changes in both shear stress and temperature over the test range. The hypothesis is propounded that in certain specified circumstances, the net rate of local bed load transport generated by an arbitrary unsteady flow, can be estimated by integration, using the measured steady laminar flow transport relationships in conjunction with the time history or probability distribution of the fluctuating bed shear stress. The concept has been tested using the measured data with encouraging results. It is suggested that this method of linking the characteristics of the near bed flow field to the local induced rates of sediment transport could be productively applied in a range of studies including t...

Journal of Geophysical Research, 2005

1] Dunes are present in nearly all fluvial channels and are vital in predicting flow resistance, sediment transport, and deposition within many rivers. Progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field, laboratory, and numerical investigations. Progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune. Additionally, the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood, with the feedback of dune three-dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital. Such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features. This review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes. Five areas are then highlighted and discussed as a possible focus for future research: (1) the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field, (2) the influence of three-dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress, (3) flow field modification resulting from bed form superimposition and amalgamation, (4) the scale and topology of dune-related turbulence and its interactions with sediment transport and the flow surface, and (5) the influence of suspended sediment on the dune flow field and dune morphology. Citation: Best, J. (2005), The fluid dynamics of river dunes: A review and some future research directions,

Ocean Engineering, 2009

Sand transport beneath large waves occurs in the plane-bed, sheet-flow regime. Comparisons between two sediment transport models and oscillatory sheet-flow experiments conducted in a flow tunnel are presented here. The experiments represent field-scale asymmetric (velocity-skewed) wave conditions over fine, medium and coarse sands, with median grain diameters of 0.13, 0.27 and 0.46 mm, respectively. The two numerical models used in the study are a two-phase flow model and a simpler two-layer, turbulence-closure model, both of which are one-dimensional vertical (1DV). The two-phase model takes account of the complete fluid–particle interactions, and the two-layer model uses an empirical description of the processes within the sheet-flow layer. The measured and predicted time-varying velocity, concentration and flux profiles, as well as the erosion depth and the net transport rates are compared and analysed. Overall, the predictions of both models are shown to be in good agreement with the measurements. The models predict the changing characteristics of the sheet-flow layer with grain size including the increasing importance of phase-lag effects for finer sands and the change in the net transport rate direction from onshore for coarse and medium sands to offshore for fine sands, with important implications for sediment sorting in the nearshore zone.

E3S Web of Conferences

There is disagreement in the literature as to whether a shear stress-based approach can be used to accurately predict sediment transport over dunes. This study aims to address this disagreement. To this end, use is made of an experiment involving the study of naturally formed, fully developed dunes produced in a laboratory flume. The bed shear stress is estimated through a combination of velocity, Reynolds stress measurements, and results of a CFD RANS rough wall model. The validity of using Bagnold’s equation to predict the bed-load rate is subsequently analyzed. In contrast to what has been previously suggested by some authors, it is found from the present experiment that the bed-load rate correlates well with the bed shear stress, and that Bagnold’s equation yields realistic values of the bed-load rate over the stoss side of the dune downstream of the reattachment point. This work also highlights the difficulties in reliably estimating the bed shear stress in complex flows. Such ...

Water Resources Research, 2013

1] The paper describes a numerical model for simulating sediment transport with eddyresolving 3-D models. This sediment model consists of four submodels: pickup, transport over the bed, transport in the water column and deposition, all based on a turbulent flow model using large-eddy simulation. The sediment is considered as uniform rigid spherical particles. This is usually a valid assumption for sand-bed rivers where underwater dune formation is most prominent. Under certain shear stress conditions, these particles are picked up from the bed due to an imbalance of gravity and flow forces. They either roll and slide on the bed in a sheet of sediment or separate from the bed and get suspended in the flow. Sooner or later, the suspended particles settle on the bed again. Each of these steps is modeled separately, yielding a physics-based process model for sediment transport, suitable for the simulation of bed morphodynamics. The sediment model is validated with theoretical findings such as the Rouse profile as well as with empirical relations of sediment bed load and suspended load transport. The current model shows good agreement with these theoretical and empirical relations. Moreover, the saltation mechanism is simulated, and the average saltation length, height, and velocity are found to be in good agreement with experimental results.