Water-level controls on macro-tidal rip currents

Field measurements and numerical modelling have been used to investigate the water-level control of rip current dynamics on a macro-tidal beach. Field data collected over 32 complete tidal cycles, spanning a range of wave and tide conditions, demonstrate that rip current strength and behaviour is modulated at the semi-diurnal frequency by tide-induced changes in the water-level over bar/rip morphology. Peak flow speeds in the rip neck 〈 uv 〉 correspond to the time of maximum wave breaking 1.5 h before and after low water. hore-directed water surface gradients ∂ η / ∂ y were measured along the feeder channel and around the ends of the inter-tidal bar, with head differences O(0.1 m). The numerical model reproduced ∂ η / ∂ y with a good level of skill and showed that ∂ η / ∂ y and 〈 uv 〉 increase with the proportion of breaking waves Qb over the inter-tidal bar; but 〈 uv 〉 was maximised during peak Qb, maximum ∂ η / ∂ y occurred when wave breaking moved offshore to the sub-tidal bar and Qb was reduced. Around low water, the forcing of the rip current by the alongshore pressure-driven feeder current was reduced by the decrease in Qb over the bar and feeder regions, but an offshore flow through the rip channel was maintained by a localised intensification of ∂ η / ∂ y around the ends of the inter-tidal bar.