Field experiments on vertical mixing of sand in the surf zone

Abstract The depth to which sand in the surf zone is vertically mixed by waves was determined from large numbers of cores taken in situ in eight tracer experiments performed on high-energy beaches. The most common tracer concentration profile showed a maximum at the bed surface and a monotonic decrease with depth; significant numbers of irregular profiles were also found and classified. Several methods of estimating the mixing depth were compared. In the method finally selected, the mixing depth was defined as that depth which includes 80% of the tracer in a core. Reliability of the results was reinforced by a consistency check made possible by the use of different colored tracers. Local mixing depths exhibited considerable variation. However, the average value for the surf zone was found to increase only slightly over time intervals of 2 to 5 hr. The most common on-offshore distribution of the mixing depth was bimodal, with maxima near the breaker line and near the shoreline. The average mixing depth alongshore was almost constant for distances of up to 200 m. The experiment-average mixing depth, Z, was well correlated with the significant breaking-wave height, H b , and the relation Z = 0.027 H b was obtained.