A numerical and field study on inner-surf and swash sediment transport

Field observations and numerical model simulations are used to investigate mechanisms of sediment transport in the inner-surf and swash zones. Both the detailed two-phase and (dilute) turbulent suspension model results suggest that sediment transport is in phase with the bottom stress and can be parameterized by a Meyer–Peter-type power law for typical sandy-beach grain sizes (0.2⩽0.2⩽ diameter ⩽0.5mm) and wave conditions (wave period ⩾5s). However, comparison of bottom stress (and the resulting sediment transport) predicted from observed flows by the detailed models with that predicted by a quasi-steady model suggests that the phase lag between the bottom stress and the fluid forcing may be important under strongly pitched-forward, saw-tooth-shaped swell and sea waves. Bottom stress predicted by a boundary-layer model that accounts for flow turbulence, but not particle interactions, is similar to that from the two-phase model if a large roughness is used to compensate for neglected intergranular and fluid–sediment interactions. Preliminary analysis of field observations in the swash zone suggests that breaking-wave (surface) generated turbulence affects the near-bed flow during passage of the breaking wave (bore) front and may have significant effects on sediment transport.