Bed frictional characteristics in a turbulent flow driven by nonlinear waves

Waves may have highly nonlinear features that will affect the near-bed flow, particularly in shallow water regions. Here we examine such effects in both the wave analysis and the calculation of the turbulent periodic boundary layer over rough surfaces. Attention is focused upon the peak bed shear stress and the rate of energy dissipation due to bed friction. The boundary layer computation is based upon a k– ε turbulence model which is of established reliability in relation to linear wave boundary layers characterized by moderate-to-high ratios of freestream particle amplitude to surface roughness. The computations reported for 30≤ a/ k s≤1000 apply to natural conditions of relative roughness from the lower end of the range to values in the region of 100 and above. It is found that energy dissipation rates are reduced sharply under nonlinear conditions, a result that is considered in relation to wave power. A number of questions concerning modelling strategy and numerical procedures are addressed in the course of the study.