Undertow over a barred beach

The spatial distribution of the mean cross-shore flow (undertow) over a barred beach is examined with field data obtained on three energetic wave days during the Duck94 experiment. The vertical structure of the undertow is modeled using a turbulent eddy viscosity closure and includes the important effects of wave breaking (described using the roller concept) and convective acceleration of the current. Other than a more realistic description of observed turbulence variations, a depth-dependent eddy viscosity (compared with a constant) does not improve the agreement between predicted and observed undertow profiles. The effect of using different boundary conditions is investigated by extending the formulations of Stive and Wind [1986] and Svendsen et al. [1987] to include random waves by ensemble averaging over the wave height distribution. The contribution of breaking wave rollers to the surface mass flux can be of the same order or greater than the contribution associated with the organized wave motion. The largest discrepancies between model predictions and observations occur over the sandbar, where the mass transport of the breaking waves appears to be underestimated.