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Bed Shear Stress Under Wave Runup on Steep Slopes

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Bed Shear Stress Under Wave Runup on Steep Slopes BED SHEAR STRESS UNDER WAVE RUNUPONSTEEPSLOPES DANIELHOWE a thesis in fulfilment of the requirements for the degree of DOCTOROFPHILOSOPHY School of Civil and Environmental Engineering Faculty of Engineering UNSW Australia March 2016 Thesis/Dissertation Sheet Surname/Family Name Howe : Given Name/s Daniel : Abbreviation for degree as give in the University calendar : PhD Faculty Engineering : School School of Civil and Environmental Engineering : Thesis Title Bed shear stress under wave runup on steep : slopes Abstract 350 words maximum: Extensive measurements of swash hydrodynamics and bed shear stress were obtained for surging, collapsing, plunging and spilling swashes, in both medium and prototype scale fixed-bed laboratory wave flumes with steep slopes (1:3 and 1:6, respectively). The hydrodynamics of swash flows were found to adhere surprisingly well to principles from classical fluid mechanics, considering they are far from the uniform, steady conditions for which the principles were originally intended. The quadratic stress law was observed to provide accurate estimates of bed shear stress, based on depth-averaged velocity. This was evidenced by back-calculated friction factors, which remained constant during uprush and backwash (except around flow reversal). Friction factors remained consistent for many different wave cases, and appeared to be only affected by bed roughness. These results show much more consistency than previous studies, because of the quality of the prototype scale measurements. Furthermore, the Colebrook-White equation was shown to provide reasonable estimates of friction factors, given a representative bed roughness and Reynolds number. The leading edge of the shoreline during wave runup was found to follow a parabolic trajectory, but only complete collapse of the incident wave. This suggests that the Shen and Meyer (1963) model is valid for all swash types, and not just for well-developed bores. Collapsing and surging swashes tended to collapse slowly on the beachface, with the instantaneous shoreline accelerating in the landward direction during most of the uprush cycle. This extended period of landward acceleration for collapsing and surging waves is not described well in existing swash zone definitions, which assume the bore collapse process is almost instantaneous. A new generalised definition of the swash zone is proposed to clarify this situation, and allow direct comparison between different swash types. Presently the most robust estimates of swash zone bed shear stress can only be obtained from direct measurements, where the active face of the instrument is mounted flush with the bed and fully exposed to the moving fluid, so that no assumptions about boundary layer structure are required. Existing instruments with this capability are hot film anemometers and mechanical shear plates. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents a non-exclusive licence to archive and to make available (including to members of the public) my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known. I acknowledge that I retain all intellectual property rights which subsist in my thesis or dissertation, such as copyright and patent rights, subject to applicable law. I also retain the right to use all or part of my thesis or dissertation in future works (such as articles or books). …………………………………………………………… ……….……………………...…….… Signature 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 can be made when submitting the final copies of your thesis to the UNSW Library. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. 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 …………………………………………….............. COPYRIGHT STATEMENT ‘I hereby grant the University of New South Wales or its agents a non-exclusive licence to archive and to make available (including to members of the public) my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known. I acknowledge that I retain all intellectual property rights which subsist in my thesis or dissertation, such as copyright and patent rights, subject to applicable law. I also retain the right to use all or part of my thesis or dissertation in future works (such as articles or books).’ ‘For any substantial portions of copyright material used in this thesis, written permission for use has been obtained, or the copyright material is removed from the final public version of the thesis.’ Signed ……………………………………………........................... Date …………………………………………….............................. AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis.’ Signed ……………………………………………........................... Date …………………………………………….............................. ACKNOWLEDGEMENTS I have been blessed to work under the supervision of Dr Chris Blen- kinsopp and Professor Ian Turner, both of whom are completely un- afraid to get their feet wet (and more) in the name of coastal research, whether on the French Atlantic coast in near-zero conditions, or in the 39 ◦C heat of a shed in the middle of Germany. To Chris, thank you for your constant enthusiasm, and creative ideas. To Ian, thank you for sharing your vast experimental knowledge, and for helping me focus on the bigger picture. I am also grateful to Associate Professor William Peirson, for as- sisting with the derivation of the momentum equation in Chapter 6. Bill, thank you for taking such a keen interest in my research, and encouraging me to pursue ideas that I believed were dead ends. I was fortunate to work with Dr Michael Allis and Assistant Pro- fessor Jack Puleo during the gwk experiments. Mike, thank you for your eagerness in everything you do, and for your organisation skills in compiling the masses of data we collected. Jack, thank you for making experimental work such fun, all day long. Thank you to the technical staff at gwk, for helping us make the most of our time in the flume. In particular thank you to Stefan Schim- mels, for making us feel so welcome, and for going beyond the call of duty to help us complete our experiments. Thank you to Rob Jenkins, Larry Paice, and Mark Whelan, for your assistance with all things mechanical and electrical, for inspiring me with your elegant craftsmanship, and for always making me feel wel- come in the workshop. Thank you to my parents Peter and Janet, for always showing in- terest in my work. To my wife Vicky, thank you for your love and support, and for waiting patiently for me whenever I stand on the beach to watch the waves. Lastly, I thank the Lord for giving me an eagerness to solve prob- lems and a curiosity to learn more about the processes of the natural world. It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience. Albert Einstein (1879–1955) CONTENTS 1 introduction1 1.1 Wave runup 1 1.2 Sediment transport 4 1.3 Motivations 7 2 review9 2.1 Bed level and hydrodynamics 9 2.2 Bed shear stress 14 2.3 Conclusion 20 3 methodology 23 3.1 Instrumentation 23 3.2 Experimental design 28 3.3 Conclusion 34 4 data processing 35 4.1 Definitions 35 4.2 Depth measurements 35 4.3 Leading edge trajectories 40 4.4 Depth-averaged velocity 43 4.5 Shear plate processing 46 4.6 Conclusion 53 5 anatomy of wave runup 55 5.1 Classical models for wave runup 55 5.2 Different modes of runup 57 5.3 Swash zone definitions 58 5.4 Experimental observations 61 5.5 General definition of swash 67 5.6 Discussion 68 5.7 Conclusion 70 6 bed shear stress and fluid momentum 71 6.1 Motivations for indirect measurement 71 6.2 Derivation of momentum equation 72 6.3 Application of momentum equation 74 6.4 Discussion of results 78 6.5 Alternative methods 83 6.6 Conclusion 86 7 comprehensive bed shear measurements 89 7.1 Variation with hydrodynamic conditions 89 7.2 Quadratic stress law and friction factors 95 7.3 Friction factors for numerical modelling 101 7.4 Estimating f from runup trajectories 106 7.5 Conclusion 109 8 conclusion 111 bibliography 115 xiii LISTOFFIGURES Figure 1.1 Nearshore wave transformation 1 Figure 1.2 Different phases of swash cycle 4 Figure 1.3 Modes of sediment transport 5 Figure 2.1 Typical bed shear stress time series 15 Figure 2.2 Turbulent boundary layer structure
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