On estimation of coastal wave parameters and wave-induced shear stresses

Wave parameters, e.g., wave height, near-bed wave orbital velocity, and wave-induced shear stresses, are important hydrodynamic parameters for sediment processes in coastal oceans. Wave orbital velocity is particularly critical in sediment resuspension. Several algorithms to calculate wave orbital velocity have been proposed, including linear wave theory, spectrum, and Joint North Sea Wave Project methods, but the validity of these algorithms in relatively shallow waters is not well understood. In this study, we compared the wave parameters obtained by different instruments and algorithms at four sites, one within the intertidal zone with a mean depth of 1 m and the remainder three in deeper offshore water with mean depths of 15–30 m. We found a high consistency of the estimated wave height, peak wave period, and wave orbital velocity among different datasets and different algorithms at the offshore sites, while there were significant discrepancies at the shoreline site. Using Ursell number, our study suggests that it is reliable to apply any of the three algorithms and different instruments (acoustic Doppler velocimeter and buoy) in deeper water. However, for very shallow water, it is recommended to use the measured high-frequency velocity and spectrum method to calculate wave orbital velocity, and use wave gauge instrument or zero-crossing algorithm to obtain wave height and period information. Finally, the effect of turbulence and bedform morphology on wave-induced shear stress is discussed: without removing the turbulence or taking onto account bedforms (e.g., ripples), the orbital velocity will be remarkably over-estimated or under-estimated.