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Abstract
As a major coastal process and hazard, rip currents are a topic of considerable interest from both a scientific and safety perspective. Collaborations between these two areas are a recent development, yet a scientific basis for safety information is crucial to better understanding how to avoid and mitigate the hazard presented by rip currents. One such area is the field of swimmer escape strategies. Contemporary safety advice is divided on the relative merits of a ‘Stay Afloat’ versus ‘Swim Parallel’ strategy, yet conceptual understanding of both these strategies is largely based on an idealised model of rip current morphology and flow dynamics where channels are incised in shore-connected bars.
Two field studies of swimmer escape methods were conducted in NSW, Australia, making use of Lagrangian flow measurements and GPS-equipped swimmers to determine the viability of escape actions in rip current systems differing from this idealised model. At North Cronulla Beach two rip current systems were observed, with a rhythmic detached bar system compared to a current incised in shore-welded transverse bars. The detached bar system was found to produce long duration floats that were unsuccessful as an escape mechanism, and presented a distance-based hazard to those seeking to swim out of the system. At Bulli Beach, a topographic rip current in the lee of a headland was measured, with a Stay Afloat strategy, aided by strongly recirculating flow, producing rapid escapes and a high success rate. Of note in both studies, a newly tested Swim Onshore strategy was also found to be highly successful, with potential implications for future study and safety campaigns.
The results of these studies are synthesised with the existing literature to produce a conceptual model of escape viability linked to morphological beach state, finding a decreasing gradient of escape viability with increased wave energy, and hypothesised negative outcomes for swimmers of limited ability in most scenarios, with implications for future safety education and research.