Video observations of rip formation and evolution

Abstract

To observe and quantify the changes in 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 spacing, shape, migration direction and rates) were identified by the decreased wave breaking in the 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 spacing was approximately 100 m. A and beach state occurred with the smaller spacing and a bar-trough beach state occurred with the wider spacing. Migration of the main 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 spacing was preserved during a storm. Sequences of 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 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 where a new formed at the one end of the alongshore extension of a pre-existing rip.