Rip currents

Time exposure video images provide a means of observing rip currents under all incident wave conditions for an extended period of time. Time sequences of images illustrate the development and decay of rip currents in response to varying incident wave conditions over periods of days to months. The variability in the morphology as revealed by time exposures has been quantified using a fractal approach. High wave events typically form a shore parallel offshore bar with a low fractal dimension while the formation of the rip channels occurs during the subsequent period of lower waves. Rip formation appears to be associated with a gradual onshore migration of the offshore bar and a corresponding increase in fractal dimension. Once formed the rip channels remain relatively stable until the next high wave event. A relatively short period (1-2 days) of high waves can remove the rip channels and restore the initial shore parallel bar and the process repeats, though the location and length scales of the rip cells may be quite different. Converging longshore currents in the feeder channels either side of a rip current are revealed using video time stacks with estimated speeds of 0.5-0.6 m/s.