Formulating Fine to Medium Sand Erosion for Suspended Sediment Transport Models

François Dufois,Pierre Hir

doi:10.3390/jmse3030906

Abstract

The capacity of an advection/diffusion model to predict sand transport under varying wave and current conditions is evaluated. The horizontal sand transport rate is computed by vertical integration of the suspended sediment flux. A correction procedure for the near-bed concentration is proposed so that model results are independent of the vertical resolution. The method can thus be implemented in regional models with operational applications. Simulating equilibrium sand transport rates, when erosion and deposition are balanced, requires a new empirical erosion law that involves the non-dimensional excess shear stress and a parameter that depends on the size of the sand grain. Comparison with several datasets and sediment transport formulae demonstrated the model’s capacity to simulate sand transport rates for a large range of current and wave conditions and sand diameters in the range 100–500 μm. Measured transport rates were predicted within a factor two in 67% of cases with current only and in 35% of cases with both waves and current. In comparison with the results obtained by Camenen and Larroudé (2003), who provided the same indicators for several practical transport rate formulations (whose means are respectively 72% and 37%), the proposed approach gives reasonable results. Before fitting a new erosion law to our model, classical erosion rate formulations were tested but led to poor comparisons with expected sediment transport rates. We suggest that classical erosion laws should be used with care in advection/diffusion models similar to ours, and that at least a full validation procedure for transport rates involving a range of sand diameters and hydrodynamic conditions should be carried out.

Highlights

Modelling strategies are different for sand and mud transport
Transporting sand in suspension is necessary in sediment transport models dealing with cohesive sediment, in order to allow the management of mixtures of sand and mud
It has been shown that using classical erosion rate formulations from the literature in our model did not lead to consistent results

Summary

Introduction

Modelling strategies are different for sand and mud transport. This is due to the fact that, whereas mud is advected with fluid velocity, sands are not, and this is all the more true the larger their diameter [1]. Transport rates are computed as the vertical integration of the product of fluid velocity and the concentration of suspended sediments (e.g., [2]). Because equilibrium is reached very rapidly, for instance in flume tests, sand transport dynamics have been generally formulated as vertically-integrated horizontal transport rates, using so-called parametric models empirically deduced from experimental data (e.g., [3,4,5])

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Discussion

Conclusion