Variation of frictional resistance under long waves in cross-shore direction

The importance of an existing criterion, to distinguish between ordinary and quasi-steady wave boundary layers, has been elucidated by considering two typical field examples. It has been shown that only longer period of oscillation is not sufficient to induce quasi-steady properties, the ratio of boundary layer thickness to the distance from the bottom to free-stream must be used instead, in order to distinguish a quasi-steady wave boundary layer from an ordinary one.