Laboratory measurement of seabed shear stress and the slip factor over a porous seabed

This paper provides a simple relationship to theoretically estimate the wave friction factor in various flow regimes in porous media based on the slip factor formula. The theoretical formula shows that the wave friction factor varies inversely with the relative bed roughness, A=ks, over a rough bed and that it can be conveniently determined if wave conditions and sediment parameters are known without using a specific regression formula deduced from experiments.Alaboratory experiment that directly measures the wave-driven bed shear stress dominant in the turbulent regime with a permeable bed is used to examine the newly derived relationship. In the laminar regime, the comparison demonstrates that the theoretical results determined by the proposed formula are in good agreement with existing measurements. In the turbulent-rough regime, the influence of eddy viscosity is considered in the slip factor formula and the zero-equation model is used in estimating the average eddy viscosity. The theoretical wave friction factor is reasonably close to the experimental measurement, and considerably better than that obtained by other existing regressions. It is also found that the wave friction factor in the small A=ks zone can be described by the present model, with comparisons showing that the slip factor theory can be extended to estimate the wave friction factor in the turbulent-rough regime. Additionally, the proposed relationship is demonstrated to be effectively used in an alternate rough bed. Experimental results further indicate that the wave friction factor in a porous medium is affected by the permeability of the sediment.