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

Field measurements of rip spacing, nearshore morphology, water surface elev ation and eulerian and lagrangian Rows were made in order to investigate th e morphodynamics of a large-scale rip current system at Muriwai Beach, New Zealand. Muriwai is a high-energy meso-tidal beach characterised by modal b reaker wave heights of 2.5 m and incident wave periods of 10-15 s. The moni tored rip system was characterised by a 400 m long, 75 m wide longshore fee der channel and a 150 m wide rip-neck channel oriented obliquely to the sho re and extending over a distance of almost 400 m. During the experiment, th e beach evolved from a longshore bar-trough and rip state to a transverse b ar 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 Boater s and were on the order of 0.7 m s(-1), with maximums in the rip-neck regio n of 1.4 m s(-1). A distinct tidal modulation of rip current flow existed w ith maximum velocities occurring at low tide and minimum velocities at high tide. Comparison with other rip studies suggests that although the magnitu de of the morphodynamic and hydrodynamic processes occurring within large-s cale rip systems is extreme, the behaviour of these rip systems is very sim ilar 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.