Acceleration and Skewness Effects on the Instantaneous Bed-Shear Stresses in Shoaling Waves

Instantaneous bed-shear stresses in shoaling waves are studied. The shear stress, or in nondimensional form the Shields parameter, is a classical basic forcing parameter in coastal engineering to calculate sediment transport due to wave motion. Based on our recent work on incipient motion of particles under shoaling waves, we became interested in the effects on the shear stress if $u_{\infty }$ is determined by a more arbitrary wave shape, such as when waves are shoaling. We therefore present a derivation of the Shields parameter ${\theta }_{wf}$ as a function of the waveform parameter $\Phi$ , which is a further generalization of Nielsen’s studies. The results indicate that shear stress distributions are modified depending on the waveform parameter. Based on laboratory observations, we are able to validate some of our findings. The use and quantification of ${\theta }_{wf}$ should be helpful in the formulation of sediment transport due to shoaling waves confirming the role of wave skewness and asymmetry.