Solute Mixing in the Surf Zone

An experimental study has been undertaken within the U.K. Coastal Research Facility (CRF), located at HR Wallingford Ltd. Detailed measurements were made to examine the on-off shore spreading of a solute, mostly inside the surf zone, for shore normal waves combined with a longshore current. The results show that the on-off shore mixing is significantly increased by almost 70 times, when compared to current-only conditions. A theoretical advection-dispersion mixing mechanism has been postulated. Using suitable estimates for the turbulent diffusion and on-off shore wave-induced velocity, a theoretical approximation to the overall mixing within the surf zone can be obtained. This theoretical analysis suggests that the on-off shore mixing in the surf zone is proportional to H-b(3/2), where H-b is the breaker wave height. It is demonstrated that within the surf zone, the on-off shore mixing is dominated by the effects of the on-off shore velocity. Some previous experimental studies to determine the magnitude of the mixing within the surf zone have been performed. However, the mechanisms responsible for generating the mixing were neither fully investigated nor reported. The previous studies, together with the results obtained from the CRF, are compared to estimates for the theoretical description of mixing developed within this present study. A good agreement exists.