Discussion of 'Apparent Roughness in Wave-Current Flow: Implication for Coastal Studies' by Alexander Perlin and Eliezer Kit

摘要

The authors have carried out a numerical study on the dependence of the apparent bed roughness k_w/k_d on four dimensionless parameters (k_d/h,U_(fc)/u_b,A/k_d,φ) using a one-dimensional k-l turbulence closure model. It was found that k_w/k_d was strongly dependent on U_(fc)/u_b, A/k_d, and φ but weakly on k_d/h. A fitting function, Eq. (23) for k_w/k_d, was then suggested based on their numerical results and was used in the three-dimensional Coastal and Marine Engineering Research Institute (CAMERI) flow model to calculate the current velocities in water depths of up to 8 m. The mean current velocities calculated by the flow model with Eq. (23) were shown to be always much smaller than those without Eq. (23) (see Figs. 13 and 14) especially outside the surf zone. The combination of Eq. (23) with the CAMERI flow model is of practical importance in eliminating the unnecessary computing time in coastal flow simulations. However, the authors did not make a direct comparison of Eq. (23) with the experimental data to test the goodness of Eq. (23). In this discussion, the original expression Eq. (21) is used to compare directly with the available experimental data to show how well it performs. The effects of the physical bed roughness k_d and the current friction velocity U_(fc) on k_w/k_d are also discussed.