Modelling sediment pick-up and deposition in a dune model

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.

Fluvial Sedimentology VII, 2005

The importance and role of sorting processes in dune-phase bedload transport and deposition is demonstrated with flume experiments, vibracores from the River Rhine (The Netherlands) and with sediment transport and dune data from the River Rhine. The entrainment and deposition depth of the sediment depend on dune trough levels below the average bed level and therefore on the dune height. As bedload sediment transport depends partly on grain size, it will be dependent upon the relict vertical sorting left in the bed by former discharge waves. The vertical sorting is created by sorting in grain flows on the lee side of dunes and by gravel lag formation in the dune troughs. Based on these principles, a simple reach-representative process model is developed for the prediction of bed sediment reworking, vertical sorting and deposition by dunes. The model is applied to two successive discharge waves of different magnitude, and predicts qualitatively the same vertical sorting characteristics as observed in the vibracores from the River Rhine (The Netherlands) after two successive discharge waves. The effect of the sorting on the sediment transport, and how to include this feedback in future models, is discussed.

River dunes influence the hydraulic roughness and water levels in rivers. Especially during floods the interplay between dunes and the flow field could become significant. Therefore, it is desired to forecast the dimensions of dunes during such extreme and unsteady flood events. At present it is not yet feasible to predict the dimensions of dunes by computing the flow and sediment transport in full detail for operational spatial and temporal scales. In this paper, we present a morphodynamic model that simulates dune development, without computing all details in the flow field. The flow model is based on the 2D-vertical hydrostatic equations, with a constant eddy viscosity over depth. Flow separation and the sediment transport behaviour in the flow separation zone are included in a parameterized way. Results of the morphodynamic model using steady flow conditions, are in general agreement with flume experiments. A next step in the research will be to use forecasted dune dimensions for hydraulic roughness predictions.

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.

2009

This work presents the sand dune study using the morphodynamic numerical model for nonuniform sediment. We assess the influence of mean step length, an important parameter used in sediment transport formula proposed by Tsujimoto and Motohashi, on sand dune dynamics. The equilibrium bed load transport formula is also employed in order to compare the results using different sediment transport formulae. From the results of the nonequilibrium sediment transport model, we found that the dune geometry significantly depends on the mean step length, and the appropriate value of mean step length for the simulation of nonuniform sediment is found to be 20-30 times sediment diameter. The results using the nonequilibrium sediment transport model provide better agreement with the experiments than that using the equilibrium sediment transport model. Moreover, the model sensitivity assessment, i.e. the initial perturbation and the domain length is conducted.

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.

2007

River bed configuration, as a roughness element, exerts flow resistance and influences sediment transport process. In this study, a movable bed experiment has been carried out to evaluate the influence of form drag, exerted by microscale bedforms, on sediment transport rate. The experimental results have been compared with the bedload transport rate calculated by using Ashida & Michiue's and Meyer-Peter & Muller's formulae. The bedload transport rate, measured directly in the present experiments, appears to be in good agreement with the bedload transport estimated by both relationships. Furthermore, experimental result has been compared with the result of bedload transport calculated by using a numerical model proposed by Giri & Shimizu. This numerical model incorporates a stochastic pick up-deposition model for non-equilibrium sediment transport with a distribution function of mean step-length proposed by Nakagawa & Tsujimoto. It is revealed that the mean step-length has an...

Journal of Geophysical Research: Earth Surface, 2015

It is observed, during flood events, that bed forms initially grow in height and make the riverbed rougher. But later, under high discharge, the bed forms grow longer with the opposite effect of making the riverbed smoother. After the discharge drops to a lower value, new bed forms regenerate on top of the elongated bed forms. This mechanism leads to a significant variation in the bed roughness and the water stage and hence determines the behavior of floods and the risk of flood disasters. This work presents detailed modeling of bed forms under discharge hydrographs and simulates the conditions under which the bed is flattened out in the upper plane bed regime. The flow was simulated by large-eddy simulation, and the sediments were considered as rigid spheres and modeled in a Lagrangian framework. The bed morphodynamics were the result of entrainment and deposition of sediment particles. We examined several discharge hydrographs. In the first case, we increased the discharge linearly and then kept it constant after reaching the upper plane bed condition. The dunes were generated and grew during the rising stage of discharge. When the flow conditions reached the upper plane bed regime, high-frequency ripples were generated and helped to flatten the bed. The results also showed that in contrast with mechanisms in the dune regime, the flattening of the bed was associated with a distinct pattern of sediment transport which deposited sediment mainly in the lee side of the dunes and led to flattening of the bed. After flattening, the sediments were mainly transported in suspension mode. As long as flow conditions stayed in the upper plane bed regime, the bed remained flat with small high-frequency ripples. We also examined two other scenarios: one with an immediate falling stage of discharge after the rising stage and the other with a period of constant discharge between the rising and falling stages. Dunes were regenerated during the falling stage of discharge for both cases. In the first case, the dunes were regenerated before the bed was flattened because of a time lag between the flow and the bed deformation. Bed flattening was followed by dune regeneration in the second case. Moreover, a significant reduction in the form drag coefficient was observed after the flattening in the second case. The reduction in form drag led to a reduction in the flood intensity. The model showed its ability to reproduce the key phenomena of the development of subaqueous dunes under conditions of a passing flood wave.

Journal of Geophysical Research, 2009

1] This paper presents an idealized morphodynamic model to predict river dune evolution. The flow field is solved in a vertical plane assuming hydrostatic pressure conditions. The sediment transport is computed using a Meyer-Peter-Müller type of equation, including gravitational bed slope effects and a critical bed shear stress. To avoid the necessity of modeling the complex flow inside the flow separation zone, we follow an approach similar to one used earlier to simulate the dynamics of wind-blown desert dunes. In case of flow separation, the separation streamline acts as an artificial bed and sediment avalanches down the leeside distributing evenly on the leeside at the angle of repose. Model results show that bed slope effects play a crucial role in the determination of the fastest-growing wavelength from a linear analysis. Flow separation is shown to be crucial to take into account if the dune lee exceeds a certain threshold slope. If flow separation is not included, dune shapes are incorrectly predicted and the dune height saturates at an early stage of bed form evolution, yielding an underprediction of dune height and time to equilibrium. The local bed slope at the dune crest plays a critical role for obtaining an equilibrium dune height. The simulation model is able to predict the main characteristics of dune evolution, such as dune asymmetry, dune growth, and saturation at a certain dune height. Dune dimensions, migration rates, and times to equilibrium compare reasonably well to various data sets. Citation: Paarlberg, A. J., C. M. Dohmen-Janssen, S. J. M. H. Hulscher, and P. Termes (2009), Modeling river dune evolution using a parameterization of flow separation,

Water Resources Research, 2006

This paper presents a morphodynamic model for free surface flows over bed forms. The model reproduces bed form evolution and migration in a physically based manner. The flow model component of the coupled morphodynamic model, which is twodimensional in the streamwise and vertical directions, explicitly treats unsteadiness and nonhydrostatic effects. Comparison with measurements shows that the flow model accurately simulates both mean flow and turbulence structure induced by bed forms under free surface flow. The flow model performance is enhanced by the use of a high-order Godunov scheme known as the cubic interpolated pseudoparticle (CIP) technique to compute the advection phase of the Navier-Stokes equation. In addition, an enhanced k-e turbulence closure with nonlinear terms is incorporated in the present model in order to allow better predictions of Reynolds stress anisotropy in regions with flow separation. Performance of the nonlinear k-e model is compared with standard k-e closure in the context of the flow as well as morphodynamic simulation. Nonequilibrium bed load sediment transport is treated using a Eulerian stochastic formulation of the sediment exchange process in terms of pickup and deposition functions. The proposed approach for sediment transport explicitly considers the local flow variability during morphodynamic computation. Model results show that the vertical grid distribution pattern affects the simulation result, particularly for bed form morphodymics. The model successfully simulates some important physical features of bed form evolution, such as amalgamation and asymmetric geometry. The simulation results are in agreement with laboratory experiments as well as some commonly used prediction methods for geometric characteristics of bed forms.