MAGNITUDE AND CROSS-SHORE DISTRIBUTION OF BED RETURN-FLOW MEASURED ONNATURAL BEACHES

Field measurements of cross-shore currents 0.25 m from the bed were ma de on two natural beaches under a range of incident wave conditions. T he results indicated the presence of a relatively strong, offshore-dir ected mean current, both within and seaward of the surf zone. Typical velocities within the surf zone were of the order of 0.2-0.3 m/s. This bed return flow, or ''undertow'', represents a mass conservation resp onse, returning water seaward that was initially transported onshore i n the upper water column, primarily above the trough of the incident w aves. The measurements demonstrated that the bed return flow velocity increases with the incident wave height. In addition, the cross-shore distribution of the bed return flow is characterised by a mid-surf zon e maximum, which exhibits a strong decrease in velocity towards the sh oreline and a more gradual decay in the offshore direction. Several be d return how models based on mass continuity were formulated to predic t the cross-shore distribution df the bed return flow under an irregul ar wave field and were compared with the field data. Best agreement wa s obtained using shallow water linear wave theory, after including the mass transport associated with unbroken waves. The contribution of th e unbroken waves enables net offshore-directed bottom currents to pers ist outside the region of breaking waves, providing a mechanism, other than rip currents, to transport sediment offshore beyond the surf zon e.