This suggests that surf zone flow patterns depend on a balance between
cross-shore and longshore flows. Cross-shore flows and rip currents
are activated by wave breaking on the bar and thus depend on the
wave height and the water level (e.g. MacMahan et al., 2006) whilst
longshore flows are driven by either the tide or obliquely incident
waves (e.g. Ruessink et al., 2001). Thus, the tide may influence the flow
pattern in two ways: the vertical tide by modulating the water level
and the wave breaking pattern, and the horizontal tide that is an alongshore
current.During the experiment, drifter exits and strong rip currents
were only observed during medium and low water levels when waves
were breaking on the bar. As the water level increased during flood,
wave breaking shifted shoreward, waves ultimately broke at the shoreline
and rip currents were no longer generated. Tidal longshore currents
were shown to deflect the rip current significantly outside the surf zone.
An example from the field observations shows that small changes in
water level and/or tidal current occurring within 45 min can cause a
shift from flow pattern 2 to flow pattern 3 (compare Figs. 6a and 7a).
In Section 3 a numerical approach is used to further investigate these processes
because it enables the effects of tide and wave-induced longshore
currents and water level to be considered separately.