Bedform Dynamics in a Rip Current

Abstract

A Sand Ripple Profiler (SRP) was deployed in a rip channel on a dissipative sandy beach to measure bedform height (Δ), length (λ) and migration rate (Mr) throughout a macro-tidal cycle. Data were collected in significant wave heights ranging from 0.5 m to 2.1 m and water depths of between 0.9 m and 7.4 m. Periods when the rip current was active were identified by comparing measured cross-shore velocity (U) with predicted cross-shore velocity from a bed return flow model. Rip flow was found to commence when offshore velocities reached approximately 0.1 m/s, maximum rip velocity as a 5 minute mean was 0.74 m/s. At lower tidal elevations (h < 2.5 m), in the strong offshore flow of the rip current megaripples were found to migrate offshore (Mr = 0-4 cm/min). When migration occurred in a rip current migration rates were correlated with U with the highest migration rates found in the strongest flows. In non-rip current conditions megaripples exhibited two types of migration behaviour; (1) migrating onshore at a maximum rate of 2.1 cm/min when h was in the range of 2.5 m to 5 m and wave skewness was high and (2) megaripples were stable when mean flows and skewness were low, this occurred at higher tidal elevations when h > 5m.