New bed shear stress estimator for net sand transport rate predictions under non-linear waves

Abstract

The estimation of the bed shear stress is a crucial step in many sediment transport formulae since, when bedload is dominant, sediment transport can be parameterized in terms of the bed shear stress. In this work, the performance of a new bed shear stress estimator for nonlinear waves is analysed. The effects of velocity and acceleration skewness are incorporated in the time-varying bed shear stress using two parameters: the index of skewness or nonlinearity, r, and the waveform parameter, φ. The results are compared with two different data sets obtained in the Aberdeen Oscillatory Flow Tunnel, under accelerated-skewed oscillatory flows. When applied to a typical bedload formula, the new expression considerably improves the prediction of the net sand transport rates. A sensitivity analysis for the phase-lead between the bed shear stress and the outer flow indicates that the new expression provides the best trends between predicted and measured transport rates when compared to other parameterizations. This work provides further insights in the correct prediction of sediment transport modelling, accounting for the effect of nonlinear wave shapes.