Wind/wave interactions in the surf zone
Wind/wave interactions in the surf zone are studied using a wave tank and environmental wind tunnel. The wind simulation is achieved over a relatively short fetch using accelerated growth techniques at a scale of roughly 1:100. Waves are scaled at approximately 1:50, and consequently there is some scaling mis-match between the wind and wave simulations. Results show that wind has a significant effect on the breaking of the waves. Both breaker location and breaker type are shown to be affected by the wind. Results are in agreement with those of Douglass (1989 & 1990), who used a wind/wave flume to simulate the prototype conditions, but made no attempt to correctly simulate the turbulence in the air flow. The main findings, are that onshore winds promote spilling waves and increase the surf zone width, whereas offshore winds promote plunging waves, decreasing surf zone width. Hot-film measurements of the air flow over the waves show that there exists significant differences between the air flow structure of offshore and onshore winds over the surf zone. Under offshore winds, the surf zone exerts a large drag on the air flow, dramatically increasing turbulence intensities aerodynamic roughness z0, and friction velocity, u*, near the point of wave breaking. Under onshore winds the air flow is less affected and at the point of wave breaking, z0 for onshore winds is an order of magnitude lower than the value under offshore winds. Phase-averaging techniques indicate large wave-induced perturbations to the mean velocity over the waves, and these are present to heights of up to 5 or 6 times the breaker height over the point of wave breaking. Spectra indicate that for onshore winds large wave-frequency fluctuations are present at the shore. Additionally, studies of particle motion offshore of the surf zone indicate wind effects on the drift velocities of suspended particles, although the precise nature of the wind effect was not clear.