The response of an erodable bed to the shear stresses of wave and current motions takes place on a grain-by-grain basis. However, it is the integrated quantities that are of interest in the prediction of beach response. Thus, models of sediment transport vary from those focusing on small detailed descriptions to those dealing only in bulk quantities. Ideally, the large-scale models are simply the integral of the small-scale behavior. The truth of this assumption is not well tested.
Again, a great deal of work has done in both the theoretical and
laboratory contexts, most originally derived from models based on
unidirectional flows. Many excellent summaries are available, [for
example, Freds
and Deigaard, 1992; Nielsen, 1992] The
following discussion will focus on results from field data, with the
discussion split into small scale (less than an incident band
wavelength) and large scale processes.