Morphodynamics of a large-scale rip current system at Muriwai Beach, New Zealand

Field measurements of rip spacing, nearshore morphology, water surface elevation and eulerian and lagrangian Rows were made in order to investigate the morphodynamics of a large-scale rip current system at Muriwai Beach, New Zealand. Muriwai is a high-energy meso-tidal beach characterised by modal breaker wave heights of 2.5 m and incident wave periods of 10-15 s. The monitored rip system was characterised by a 400 m long, 75 m wide longshore feeder channel and a 150 m wide rip-neck channel oriented obliquely to the shore and extending over a distance of almost 400 m. During the experiment, the beach evolved from a longshore bar-trough and rip state to a transverse bar and rip configuration. Mean eulerian how velocities obtained from ducted flowmeters deployed on the margin of the feeder channel and rip-neck were on the order of 1 m s(-1) and instantaneous Rows were commonly in excess of 2 m s(-1). Mean lagrangian surface flow velocities extending from the base of the feeder through to the rip-head were obtained by tracking rip Boaters and were on the order of 0.7 m s(-1), with maximums in the rip-neck region of 1.4 m s(-1). A distinct tidal modulation of rip current flow existed with maximum velocities occurring at low tide and minimum velocities at high tide. Comparison with other rip studies suggests that although the magnitude of the morphodynamic and hydrodynamic processes occurring within large-scale rip systems is extreme, the behaviour of these rip systems is very similar to low-energy rips with much smaller spatial scales. There is evidence to suggest that distinct morphodynamic scaling relationships exist between these environments. (C) 2000 Elsevier Science B.V. All rights reserved.