G. Masselink et Kp. Black, MAGNITUDE AND CROSS-SHORE DISTRIBUTION OF BED RETURN-FLOW MEASURED ONNATURAL BEACHES, Coastal engineering, 25(3-4), 1995, pp. 165-190
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.