Modelling coastal boundary layer flows over typical bed-forms

Abstract

This paper presents details of hydrodynamics, turbulence characteristics, sediment concentrations and transport rates computed by a new three-dimensional numerical boundary layer model above typical coastal bed-forms. Comparisons are made against a large number of available laboratory and field measurements covering situations of current alone, wave alone, and combined wave and current, which demonstrate the model's ability to simulate the complex structure of flow interactions as well as the associated sediment transport processes with reasonably good accuracy. The model results are also parameterised by averaging over both bed-form length and wave period in an attempt to produce improved relations of the overall bed-form-induced roughness height and equivalent near-bed reference concentration for engineering morphological modelling. Verifications of these averaged results against some simple engineering predictors show realistic agreements along with some noticeable differences. Where differences are found, modifications are also suggested to the existing engineering formulae in order to improve their predictions for complex flows. Further model testing is also suggested to refine the constants used for the proposed modifications.