Rip currents

A laboratory study of a rip current in the presence of a submerged reef

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Detailed laboratory measurements of the wave and current field induced by bi-chromatic waves incident on an artificial surfing reef are presented. Strong wave focussing occurs on the surfing reef. Both PTV and in-situ measurements are reported that show the presence of a strong rip current flowing oblique off-shore. Observed water level set-up and set-down indicate the locations of feeder currents, driving the rip current. Wave-current interaction between the incoming bi-chromatic waves and the outgoing rip-current are apparent in the measurements. The stochastic character of meandering of the rip current is shown by the variance of the low pass filtered time-averaged local velocities. The vertical velocity distribution could not be determined reliably due to limitations in the test set-up and measurement devices. The dispersion relationship is utilized to connect the PTV surface measurement with the underlying velocity field obtained by in-situ measurements. The total dataset gives a detailed synoptic view of the wave-current field that can be used for model verification and validation.