SUSPENDED SEDIMENT LOAD DURING AN ASYMMETRIC WAVE CYCLE OVER A PLANE BED

Suspended sediment measurements made by Ribberink and Al-Salem (1992) between 5 mm and 46 mm from a plane bed exhibited apparent changes to the boundary layer thickness and eddy diffusivity during the wave cycle. Bed ripples were absent but the data still showed entrainment near the time of flow reversal. These laboratory measurements were examined using a numerical model to study sediment suspension at short time scales. To assess relevance, the data were considered in conjunction with concentration time series of similar character from a natural beach. Each of three distinct concentration peaks during one wave cycle evident in the laboratory measurements had to be treated differently. However, all peaks exhibited a high correlation to steady-state logarithmic concentration profiles with uniform, but unequal, vertical eddy diffusivities. The first peak under the wave crest could be predicted with standard empirical relationships derived from long-term averaged data. The remaining peaks could be simulated only when unexpected entrainment was permitted for a very short time (0.1 s) near flow reversal and separately during the accelerating phase of the wave trough. The paper provides the basis for further work but, at present, the results cannot be used for predictive purposes.