Observations of wave shear stress on a steep beach

Observations are presented of the wave shear stress 〈u∼w∼〉 on a steeply sloping beach. Above the wave boundary layer (WBL), positive values of 〈u∼w∼〉 were observed and are attributed to a combination of both wave shoaling due to the large-scale bed slope, and dissipation due to wave breaking, in agreement with the wave theory of Zou et al. (2003). Within the WBL, observed vertical profiles of 〈u∼w∼〉 were also in good agreement with theory, in cases where the wave height was small. As wave heights increased, however, the WBL profile of 〈u∼w∼〉 generally did not agree with theory. Near-simultaneous rotary sonar observations of the bed suggest the disagreement with theory was due to the presence of orbital-scale ripples, which the present theory does not accommodate.