Rip current dynamics on an embayed beach

Abstract

Rip currents are fast, narrow currents which traverse the surf-zone in the seaward direction. The most important effect of rip currents is that they can pose a deadly hazard to beach-users. Rip currents and their interaction with waves and underwater morphology are still poorly understood. This is often attributed to a lack of high quality long-term datasets. This shortcoming is due to the difficulty of sampling in the turbulent surf-zone. Past attempts to compare rip current behaviour (e.g. alongshore spacing) to waves have failed to show that they interact.

In this thesis, an improved technique of locating rip channels in video imagery is presented. Previous studies to create computer algorithms to locate rips in video imagery have only looked at one alongshore transect which is averaged in the cross-shore direction, and there have been issues making the algorithms work in complicated cases. The method created in this thesis uses computer algorithms to locate light intensity minima across the entire expanse of the surf-zone in video imagery. This was applied to a dataset from Tairua Beach. The light intensity minima are sorted into distinct rip channels to create a dataset spanning 3.3 years from 1999 until April 2002. Using the high quality rip data output from the algorithms, rip channel morphological reconfiguration events were defined using a measure of change. Wave climate was compared to the timing of these reconfiguration events. It was found that mean wave energy averaged over ten days and wave event duration showed a better relationship to the reconfiguration events than immediate, instantaneous measures of significant wave height.

Rip channel spatial scale (i.e. cross-shore extent) was found to be critical in determining how rip channels behave during high wave events. At Tairua Beach, it was not uncommon for the surf-zone to be wide on one half of the beach and narrow on the other. This 'dual' surf-zone can be attributed to wave shadowing by offshore islands under certain wave directions. Smaller rip channels on the narrow half of the beach changed rapidly whereas larger rips were stable during the same period. This situation shows the importance of both hydrodynamic-control and topographic-control of rips, where rips may respond directly to changes in the wave conditions or be stabilised by the pre-existing morphology respectively. There was also a tendency for rips to form and persist at the headlands of the beach.

A conceptual model was created to demonstrate how rip channels of different spatial scales respond to changes in the wave conditions. Small rips relative to the wave energy are more likely than larger rips to evolve, and vice versa.