Field investigation of sediment transport in the swash zone

Concurrent measurements of swash flow velocity and sediment transport were obtained from a natural beach to develop a means for predicting sediment transport in the swash zone. Twenty-seven swash events were monitored and the sediment load of each event was correlated to the flow velocity for both the wave uprush and backwash. The sediment load displayed a strong relationship with the time-averaged velocity cubed, consistent with equations for both bedload transport and total load transport under sheet flow conditions. Validation of the energetics-based model of Bagnold (1963, 1966) revealed different constants of proportionality for wave uprush and backwash, with the uprush value being approximately twice that obtained for the backwash. Similarly, validation of a modified Shields parameter (to account for the effect of a sloping bed) also indicated different coefficients. The physical explanation for the different constants of proportionality is that neither sediment transport model adequately accounts for the disparate nature of the two phases of the swash cycle.