Video-imaging of transient rip currents on the Gold Coast open beaches

Transient rips vary in location and occurrence making them hard to predict, and unlike topographically controlled, fixed or semi-permanent rips, they are believed to be controlled by surf zone hydrodynamics rather than beach and surf zone morphology. This paper reports on the first video analysis of transient rip behaviour in the Southern Hemisphere. Transient rips were identified from a high camera located approximately 100 m above mean sea level at Narrowneck, Gold Coast, Australia. Transient rip occurrence, duration and flow behaviour were identified from the video and occurrence was correlated with environmental factors including beach state, wave, tide and wind characteristics. Transient rips were found to display a mushroom cloud rip head and a narrow trailing rip neck consistent with characteristics often displayed by topographically controlled rips. Transient rip events were characterised by short life-spans (30 - 236 s) and low temporal frequency, but high temporal variance, occurring for only 0.52% of the time in the video. Around 66% of transient rip events identified occurred when the beach was generally alongshore uniform in a shore-attached terrace state. Transient rip occurrence was also found to be significantly greater on the ebb tide as opposed to the flood tide and when wave breaking was absent on the outer bar. There was a lack of a clear correlation between averaged wave and wind conditions and transient rip formation. Current and future study is focussing on the effects of wave groupiness and sea state on transient rip occurrence, spacing, duration and intensity.