Beach Safety in Atypical Rip Current Systems: Testing Traditional Beach Safety Messages in Non-Traditional Settings
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Beach safety in atypical rip current systems: testing traditional beach safety messages in non-traditional settings Benjamin Robert Van Leeuwen A thesis in fulfilment of the requirements for the degree of Master of Science School of Biological, Earth and Environmental Science (BEES) Faculty of Science Supervisors: Associate Professor Robert Brander, School of Biological, Earth and Environmental Sciences, UNSW Australia, Sydney, NSW, 2052, Australia Professor Ian Turner, Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Australia, Manly Vale, NSW, 2093, Australia July 2015 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Van Leeuwen First name: Benjamin Other name/s: Robert Abbreviation for degree as given in the University calendar: MSc School: School of Biological, Earth and Environmental Sciences Faculty: Science Title: Beach safety in atypical rip current systems: testing traditional beach safety messages in non-traditional settings Abstract 350 words maximum: (PLEASE TYPE) 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. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). …………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS ORIGINALITY STATEMENT ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed ................................................................. Date ................................................................ Preliminary Items Preliminary Items ................................................................................................................. i Table of Contents ................................................................................................................. i Abstract ............................................................................................................................... iii Acknowledgements .............................................................................................................. v List of Figures ...................................................................................................................... vi List of Tables ........................................................................................................................ x List of Symbols .................................................................................................................... xi Table of Contents Chapter 1. Introduction ................................................................................................ 1 1.1 Rip current flow circulation and implications for bathers .............................. 2 1.2 Rip current forcing mechanisms, morphology and types ............................... 5 1.2.1 Open beach rip current systems ................................................................. 5 1.2.2 Structurally controlled rip currents ............................................................. 9 1.2.3 Embayed beaches and rip currents ........................................................... 10 1.3 Beach morphology and the rip current hazard ............................................. 12 1.4 Thesis aims .................................................................................................... 15 1.4.1 Thesis structure ......................................................................................... 15 Chapter 2. Methodology ............................................................................................. 16 2.1 Study locations .............................................................................................. 16 2.2 Topographic surveys ..................................................................................... 17 2.3 Environmental observations ......................................................................... 19 2.4 Lagrangian flow measurements .................................................................... 20 2.5 Swimmer measurements .............................................................................. 21 2.5.1 Swimmer action outcomes ....................................................................... 25 2.5.2 Swimmer analysis ...................................................................................... 25 2.6 Using Lagrangian drifters as proxies for human floaters .............................. 26 2.7 Flow hazard rating......................................................................................... 28 Chapter 3. Rip current escape strategies in a detached Rhythmic Bar System .......... 31 3.1 Introduction .................................................................................................. 31 3.2 Study location ............................................................................................... 32 3.3 Results ........................................................................................................... 33 i 3.3.1 Rip current flow characteristics ................................................................ 34 3.3.2 Swimmer escape outcomes ...................................................................... 35 3.3.3 Simultaneous actions ................................................................................ 40 3.4 Discussion ...................................................................................................... 42 3.4.1 Morphological controls on swimmer escape outcomes ........................... 43 3.4.2 Implications for beach safety .................................................................... 45 3.5 Conclusions ................................................................................................... 48 Chapter 4. Swimmer escape