Observed and predicted bed forms and their effect on suspended sand concentrations

Abstract

In this paper, we study the effect of bed forms on suspended sand concentrations and we compare three existing bed form predictors to field and laboratory measurements over a wide range of conditions and propose a new predictor that better collapses the measured bed form dimensions. We apply the different bed form predictors to estimate the suspended sediment concentration distributions based on the 1D advection diffusion equation and the bed-form-enhanced shear stress, which includes the flow contraction effect according to Nielsen [Coast. Eng. 10 (1986) 23]. Predictions of suspended sand concentration improve when including a bed form predictor. Interestingly, it does not make much difference which bed form predictor is implemented but more whether a bed form predictor is implemented. However, the choice of a particular bed form predictor is more important when the bed form effect in the sand transport model is based a roughness length scale k s and thus on η 2/ λ instead of η/ λ, where k s is Nikuradse's equivalent roughness length scale, η is the bed form height and λ is the bed form length. This is the case in the Van Rijn [Van Rijn, L.C., 2000. General view on sand transport by currents and waves: data analysis and engineering modelling for uniform and graded sand (TRANSPOR 2000 and CROSMOR 2000 models). Z2899.20 Z2099.30 Z 2824.30, WL∣Delft Hydraulics, Delft, The Netherlands] sand transport model, for example. We discuss the effect of so-called long wave ripples on the suspended sand concentrations.