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Evaluation of swimmer-based rip current escape strategies

Abstract

Rip currents are the primary hazard on surf beaches, and early studies described them as fast, shore-normal flows that extended seaward of the surf zone. Based on this traditional view, commonly promoted safety advice was to escape a rip current by swimming parallel to the beach. However, recent studies have shown dominant rip current re-circulation within the surf zone and have endorsed floating as an appropriate escape strategy. Here, a first quantitative assessment of the efficacy of various rip current escape strategies, with a focus on the underlying physical processes, is presented. A field study was conducted at Shelly Beach, NSW, Australia, measuring three rip currents (two open beaches, one topographic) over 3 days in varying wave conditions. Floating was found to be a longer duration, more variable escape strategy (\( \overline{t} \) = 3.8 min, σ = 2.4 min), than swimming parallel (\( \overline{t} \) = 2.2 min, σ = 1.0 min). Neither of the scenarios is 100 % foolproof, and both fail in some scenarios, making simplified safety recommendations difficult. Swim parallel failures are related to swimming against the alongshore current of the rip circulation. Float failures related to surf zone exits, with the highest exit rate occurring in the topographic rip. Float failures also occurred due to multiple re-circulations without the person attaining safe footing on the bar. The variable spatial and temporal behaviour of rip currents suggests that a single escape strategy safety message is inappropriate. Instead, a combined approach and scenario-specific safety advice should be considered by beach safety practitioners to promote to the public.

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Acknowledgments

This project was funded by Australian Research Council (ARC) Linkage Project LP110200134 and by Surf Life Saving Australia (SLSA). MacMahan and Brown were also supported by NSF OCE-0926750. Thank you to Patrick Rynne for his video documentary of the experiment (http://waterlust.org/RIP.html). Special thanks to the many volunteers who helped with the fieldwork, particularly the swimmers and floaters.

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Correspondence to R. Jak McCarroll.

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McCarroll, R.J., Brander, R.W., MacMahan, J.H. et al. Evaluation of swimmer-based rip current escape strategies. Nat Hazards 71, 1821–1846 (2014). https://doi.org/10.1007/s11069-013-0979-1

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Keywords

  • Beach safety
  • Lagrangian drifters
  • Swimming hazards
  • Drowning prevention