Cross-shore suspended sediment transport in the surf zone: A field-based parameterization

Existing cross-shore sediment transport models for two-dimensional surf zone bathymetries almost invariably predict offshore-directed sand transports and bar migrations during storm conditions. However, onshore-directed suspended sediment fluxes and associated nearshore bar migration were observed during recent field experiments on a gently sloping beach on the Danish North Sea coast. Field measurements of suspended sediment flux obtained during three experiments on two different beaches are used to parameterize the observed fluxes. This parameterization predicts suspended sediment transport due to incident waves and undertow across bars in two-dimensional surf zones. First, a non-dimensional sediment flux index is formulated which describes the tendency towards net onshore or offshore transport and the strength of that tendency. The non-dimensional formulation circumvents the problem of measurement inconsistencies due to varying elevations of sediment concentration sensors relative to the bed. The index is found to depend upon the undertow velocity, the incident wave skewness and the cross-correlation between orbital velocity and sediment concentration. However, some of these parameters are difficult to predict, particularly in barred surf zones and therefore, the independent variables are recast in terms of a set of more easily obtainable parameters. The sediment flux index depends on a combination of the following: non-dimensional bed shear stress (the Shields parameter), relative water depth, wave orbital velocity, relative wave height and bed slope. Finally, a formulation of suspended sediment transport across bars is obtained by linking the flux index with a parameterization of the sediment concentration/distribution in the water column. These concentrations are found to depend on non-dimensional bed shear stress, relative wave height and water depth. The formulation predicts a tendency for onshore-directed sediment transport due to incident waves on gently sloping beaches and/or with large bed shear stresses. On steeply sloping beaches and/or in the inner part of the surf zone there is a tendency towards offshore sediment transports due to the undertow.