Citation

Bogle JA, Bryan KR, Black KP, Hume TM, Healy TR. J. Coast. Res. 2001; (ICS 2000): 117-127.

Copyright

(Copyright © 2001, The Coastal Education and Research Foundation)

DOI

10.2307/25736280

PMID

unavailable

Abstract

To observe and quantify the changes in rip channels and sand bars, a computer-controlled video camera was deployed on Paku Hill at Tairua Beach, on the East Coast of Coromandel Peninsula, New Zealand. Over the last 11 months, 600 images were captured every hour by the camera for 15.66 minutes and then averaged. Rip characteristics (such as rip spacing, shape, migration direction and rates) were identified by the decreased wave breaking in the rip channel leading to reduced light intensity in the image. Seven out of twelve storm events over the 11 months were chosen for detailed study. During these storms, the spatial separation between the rips channels did not vary significantly over the length of the beach or during a storm event. Rip channel spacing altered between storm events so that the average spacing for the main channels was either approximately 130 m, 169 m or 205 m, with the exception of a single storm in which rip spacing was approximately 100 m. A "transverse rip and bar" beach state occurred with the smaller rip spacing and a "longshore bar-trough" beach state occurred with the wider rip spacing. Migration of the main rip channel was measured for each of the eight rips typically present by calculating the distance moved between two consecutive images. Migration rate, averaged over all storms and rips, was 0.6 m/hr. However migration direction was generally not consistent over time, so that over all the storms there was no net migration (the mean net migration over an hour over all storms was -0.04 m/hr +/- a standard deviation of 0.4 m/hr). At any one time, the migration rate and direction for all rips during a particular storm was similar, so that rip spacing was preserved during a storm. Sequences of rip current genesis were also observed in the averaged images. The most common type occurred when a shore-parallel segment of the longshore bar evolved into a crescentic bar, in which case the rip channel formed between two horns of the crescentic bar. Rip currents also evolved within a circulation cell by a transverse bar extending either outward from the shoreline or inward from the crescentic bar. The most unusual pattern that was observed was a "rip extension" where a new rip formed at the one end of the alongshore extension of a pre-existing rip.

Keywords: Drowning; Drowning Prevention; Water Safety