Grain size effects on suspended sediment concentration profiles under breaking waves

Presently, conceptual and numerical sediment concentration modelling is largely based on the diffusion approach. However, several works have shown that this approach is, frequently, unsuitable to model the profile distribution of different grain size particles in the same flow. In order to understand the grain size effects on suspended sediment concentration profiles under breaking wave conditions, a new data set was collected at three west coast Portuguese beaches (Almagreira, Caparica and Comporta beaches). As expected, the relative concentration for the entire grain-size spectrum shows a systematic decrease with distance to the bed, though with different profile shapes (exponential and power law). The analysis of the relative concentration profiles for each sediment fraction show similar profile patterns; however, as expected, the decrease in the relative concentration with height strongly depends on particle size and assumes a different behaviour at each site. In this case, diffusion approach was unable to explain all the observed profile patterns, namely the overlapping of coarse sediment profiles and the different profile shapes in the same flow. These deficiencies were overcome by the use of the convection-diffusion approach.