February 2020 (ROTM#134) North Cronulla Beach, Sydney, NSW Australia

North Cronulla Beach is a popular beach in Sydney's Sutherland Shire that is the southern part of a much longer beach that extends several kilometres to the north and changes its' name a few times (Wanda, Elouera, Greenhills). Most of the beach is typically characterised by semi-regularly spaced channel rip currents about 150-200 m apart. 

 I took this picture on January 23, after giving one of my community Science of the Surf talks to the public thanks to Sutherland Shire Council. It was a 40 degree Celsius day and the beach was busy, mostly with teenagers, and the Lifeguards had their hands full keeping people between the flags (which is where most of the people in this picture are). They were particularly concerned about the rip current that dominates this photo as evident by the Lifeguard 4WD situated right at the shoreline of the rip embayment. Lifeguards on jet skis were also making sure that people remained in the flags. 

 This is a classic example of a channelised rip current that is common under normal, and even gentle, wave conditions. Channel rip currents can persist in the same location for days and weeks and tend to carve out their own embayment into the beach. This rip actually had distinct alongshore feeder channels transporting water into a narrow rip neck that is only about 5-10 metres wide. Flow in the rip current gradually increases from the feeders to the rip neck, where it becomes strongest and eventually slows down seaward of the line of breaking waves (i.e. the seaward limit of the surf zone) and decelerates into an expanding rip head. Most of that water is eventually returned landward across the sand bars by breaking waves.

Vigilant lifeguards in front of the rip at North Cronulla