Energetics prediction of frequency-dependent suspended sand transport rates on a macrotidal beach

The on–offshore (cross-shore) transport of sand on beaches is highly time-variable, which has made it difficult to model or predict. In this paper, simple energetics modelling is used to compare velocity moment predictions with field observations of suspended sand transport rates. Separate consideration is given to transport associated with the three main frequency-dependent cross-shore transport processes: that associated with the short (incident) waves, that due to the long (infragravity) waves, and transport associated with the mean flow. Direct comparison between the depth-averaged model predictions, and the in-situ point measurements was facilitated by making the first order assumption that the time-averaged suspension profile is exponential and the wave velocity profile is vertically uniform. An appropriate rippled bed roughness was used to provide the drag coefficient in the energetics model and the vertical length scale of the exponential suspension profile. Despite these simple assumptions, comparison of the velocity moment predictions with the field observations of suspended sand fluxes reveals that this approach has the capacity to predict transport magnitudes due to short wave, long wave, and mean flow components to within about one order of magnitude. However, owing to the limitations of the model, the transport direction of the short wave component could not, on occasion, be correctly determined, probably due to ‘reverse’ transport over ripples. © 1998 John Wiley & Sons, Ltd