Observations of cross-shore sediment transport and formulation of the undertow

Graduation date: 2010A new set of data from a large-scale sand bar migration experiment is presented. During this experiment, two sandbars were generated. One of the bar was generated by the action of undertow, and sediment moved offshore. The other bar was generated by the shoreward movement of sediment coming from the first bar. The principal mechanism responsible for shoreward movement is associated with effects of velocity asymmetry. Analysis of bathymetric surveys and suspended sediment concentration data reveals that suspended load contributed to a large extent to the formation of the first bar. Bed load was also important, and was moving in the same direction as suspended load. For the second bar, shoreward sediment movement occurred as bedload. Suspended load was moving in the opposite direction as bed load. This difference in sediment movement is explained by the predominance of the undertow in the suspended sediment flux. Two models were tested to reproduce the observed sediment transport, a wave-averaged (energetics) model and a wave-resolved model. After proper calibration, both models yielded satisfactory results. Calibration efforts highlighted the need for robust models of sediment pickup functions and sediment eddy diffusivity. They also showed the need for a deterministic undertow models. Formulations of the undertow are presented. These formulations are valid for all relative water depths, and include mean current advective terms. These formulations show that the forcing of the undertow is depth-uniform, assuming linear water wave theory. The model is tested against four datasets to evaluate the possibility of a deterministic model. Although a constant eddy viscosity would yield such a model, no universal parametrization could be determined