TIDAL BORES, AEGIR, EAGRE, MASCARET, POROROCA: THEORY AND OBSERVATIONS

Transcription

1

2 TIDAL BORES, AEGIR, EAGRE, MASCARET, POROROCA: THEORY AND OBSERVATIONS by Hubert CHANSON Professor, School of Civil Engineering, School of Engineering, The University of Queensland, Brisbane QLD 4072, Australia Ph.: (61 7) , Fax: (61 7) , Url: December 2009 Tidal bores of the Garonne River (Top left), Dordogne River (Top right), Sélune River (Bottom left) and Sée River (Bottom right) in 2008

3 Abstract A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising. It forms during spring tide conditions when the tidal range exceeds 4 to 6 m and the flood tide is confined to a narrow funnelled estuary. The existence is based upon a fragile hydrodynamic balance between the tidal amplitude, the freshwater river flow conditions and the river channel bathymetry, and it is shown that this balance may be easily disturbed by changes in boundary conditions and freshwater inflow. This book demystifies the physics of a tidal bore and it documents thoroughly the tidal bores on our Planet with reliable and accurate informations. It aims to share a passion for a beautiful, but fragile geophysical process and it is supported by over 190 illustrations and photographs. Keywords: Tidal bores, Mascarets, Aegir, Pororoca, Observations, Theory, Turbulence, Mixing, Environmental impact. ii

4 DEDICACE To all the people who share my passion for tidal bores, in particular late Professor Howell Peregrine. To Ya Hui, pour Bernard, Nicole et André. iii

5 TABLE OF CONTENTS Page Abstract Keywords Dédicace Table of contents List of Symbols Acknowledgments Préface 1. Introduction 1.1 Presentation 1.2 Related processes 1.3 Structure of the book 2. Basic theory 3. Observations 3.1 Presentation 3.2 Tidal bores in Europe 3.3 Tidal bores in Asia 3.4 Tidal bores in the Americas 3.5 Tidal bores in Australia 3.6 Tidal bores in Africa 4. The rumble noise of tidal bores 4.1 Presentation 4.2 Field observations 4.2 Physical processes 5. Turbulence and mixing in tidal bores 5.1 Turbulence in tidal bores 5.2 Mixing in tidal bores 6. Interactions between the tidal bore, environment and mankind iv

6 6.2 Impact of tidal bores on the estuarine processes 6.3 Fragility of tidal bores 6.4 Surfing a tidal bore 7. Conclusion Appendix A - Incomplete list of tidal bore affected rivers and estuaries Appendix B - Undular wave theory Appendix C - Tales of a surfer Appendix D - Glossary and tidal bore vocabulary REFERENCES Bibliography Internet references Audiovisual references Index of Authors Index v

7 List of symbols The following symbols are used in this report: A a w flow cross-section area (m 2 ) measured normal to the velocity; wave amplitude (m); B free-surface width (m); C celerity (m/s) of a small disturbance: C = g d ; D H hydraulic diameter (m) or equivalent pipe diameter: D H = 4 A/P w ; d d c d max d 1 d 2 water depth (m) measured normal to the invert; critical flow depth (m); first wave crest elevation (m) above the river bed; water depth (m) before the tidal bore arrival; water depth (m) immediately after the tidal bore passage; E specific energy (m); Fr Froude number defined as: Fr = ( V U) / g d ; Fr 1 bore Froude number defined as: Fr 1 = ( V1 U) / g d1 ; f g H b L L w frequency (Hz); gravity acceleration (m/s 2 ): g = 9.81 m/s 2 in Europe; breaker height (m); bubble cloud length (m); wave length (m); M momentum function (m 2 ); P P w q R S f S o T t pressure (Pa); wetter perimeter (m); discharge per unit width (m); bubble radius (m); friction slope defined as the slope of the total head line; bed slope: S o = sin ; tidal period (s); time (s); Note: all given times are local times, with daylight saving between the last week-end of March and the last week-end of October in Europe typically; U V V s V x V y V z V 1 V 2 tidal bore celerity (m/s) for an observer standing on the bank, positive upstream; velocity (m/s) positive downstream; surfer speed (m/s); longitudinal velocity component (m/s) positive downstream; vertical velocity component (m/s) positive upwards; horizontal transverse velocity component (m/s); flow velocity (m/s) before the tidal bore arrival; flow velocity (m/s) immediately after the tidal bore passage; vi

8 V depth-averaged velocity (m/s); x longitudinal co-ordinate (m); x s location (m) of the tidal bore front; void fraction: 0 1 with = 0 in pure water and = 1 in air; momentum correction coefficient, also called Boussinesq coefficient, defined as: 2 Vx da A 2 V A polytropic index; angle between the river bed and the horizontal; water density (kg/m 3 ); stream function (m 2 /s); Subscript o initial flow conditions; s tidal bore location; x longitudinal direction positive downstream; y vertical direction positive upwards; z horizontal transverse direction positive towards the right bank; 1 initial flow conditions, before the tidal bore arrival; 2 new flow conditions, immediately after the tidal bore passage; Abbreviations D/S downstream; U/S upstream. vii

9 Acknowledgements The author wants to thank especially Dr Eric JONES, Proudman Oceanographic Laboratory, and late Professor Howell PEREGRINE, for their enthusiastic support and helpful assistance all along. He thanks also Dr Pierre LUBIN, University of Bordeaux for his positive feedback and advice on the book material. He expresses his gratitude to the following people who provided photographs and illustrations of interest: Mr and Mrs J. CHANSON (France); Bernard, Nicole and André CHANSON (Australia); Dr Shenliang CHEN (China); Ya-Hui CHOU (Australia); Jean-Yves COCAIGN (France); Antony COLAS (France); Jean-Michel Cousteau Ocean Adventures (USA); Michel DEYRICH (France); Angela EGOLD; Lou EVANS; Nathanaelle EUDES (France); Francis FRUCHARD (France); Google Earth; Mark HUMPAGE (UK); Lim Hiok HWA and Department of Irrigation & Drainage, Sarawak (Malaysia); Dr Eric JONES (UK); Dr Pierre-Yves LAGREE (France); Loïck LE LOUARGANT (France); Dr Cheng LIU (China); Dr Pierre LUBIN (France); J.J. MALANDAIN (France); National Oceanic and Atmospheric Administration NOAA / Department of Commerce; NASA Earth Observatory; late Professor D. Howell PEREGRINE (UK); Petitcodiac Riverkeeper (Canada); Sonya PREIN; Sequana-Normandie; G. Warren SWIRE & John Swire & Sons (UK); Walter TAPE (USA); Dr Bernadette TESSIER (France); viii

10 Carrie VONDERHAAR (USA); Dr Pierre WEILL (France); Sherman WILLIAMS (Canada); Dr Eric WOLANSKI (Australia); Dr YU Dajin (China); At last, but not the least, the author thanks all the people (incl. friends, relatives, colleagues, former students, students, professionals) who gave him information, feedback and comments on this material. In particular, he acknowledges : Professor Shin-ichi AOKI (Toyohashi, Japan); Robert BICKERS (UK); Dave BUTTERTON (UK); Mr and Mrs Jacques CHANSON (Paris, France); Bernard, Nicole and André CHANSON (Brisbane, Australia); Dr Shenliang CHEN (China); Ms Y.H. CHOU (Brisbane, Australia); Antony COLAS (France); Fabrice COLAS (Saint Pardon, France); Dr Jean CUNGE (France); Frédéric DANEY (France); Mrs Jacqueline DUPEYRAT (Saint Martial, France); Dr Richard W. FASS (USA); Dr David HUNTLEY (UK); Dr Eric JONES (UK); Mrs Nathalie LEMIERE (France); Karen HICKOX (Australia); Daniel LEBLANC (Canada); Dr Pierre LUBIN (Bordeaux, France); Mr J.J. MALANDAIN (Rouen, France); Roger MARCEL (France); Alain MARHIC (France); Jean-Paul PARISOT (France); late Professor Howell PEREGRINE (Bristol, UK); David PETERSON (Australia); Professor Roger RULIFSON (USA); Prof. Hubert SAVENIJE (The Netherlands); John Swire & Sons (UK); Dr Carl TAPE (USA); Dr Bernadette TESSIER (France); Don THIEDERMAN (USA); Dr Eric WOLANSKI (Australia);... ix

11 Préface Tidal Bores, Aegir, Eagre, Mascaret, Pororoca: what's all about it? A tidal bore is a surge of waters propagating upstream as the tidal flow turns to rising and the flood tide rushes into a funnel shaped river mouth (Fig. aa). The bore forms during the spring tides when the tidal range exceeds 4 to 6 m and the rising tide waters are confined to the narrow funnelled estuary. It is estimated worldwide that over 400 estuaries are affected by a tidal bore, on all continents but Antarctica. A bore is a discontinuity of the water depth and it represents a hydrodynamic shock. The tidal bores have a significant impact on the environmental system and the ecology of the river mouth. Recent studies demonstrated in particular the significant impact of small tidal bores and of non-breaking undular surges on natural channels. Surprisingly, the tidal bore remains a challenging research topic to theoreticians, and many hydrodynamic features remain unexplained. The existence of a tidal bore is based upon a fragile hydrodynamic balance between the tidal flow range, the freshwater river flow conditions and the channel bathymetry. Some simple theoretical considerations show that this balance may be too easily disturbed by changes in boundary conditions and freshwater inflow. For examples, a number of tidal bores disappeared because of river training, dredging and damming. Man-made interventions led to the loss of several bores with often adverse impacts onto the eco-system: e.g., the mascaret of the Seine River (France) no longer exists after extensive training works and dredging; the Colorado River bore (Mexico) is drastically smaller after dredging. Although the fluvial traffic gained in safety in some case, the ecology of estuarine zones were adversely affected. The tidal bores of the Colorado (Mexico), Couesnon (France) and Petitcodiac (Canada) Rivers almost disappeared after construction of upstream barrage(s). At Petitcodiac, this yielded the elimination of several native fish species. The proposed construction of the Severn Barrage in UK is a major threat to one of the best documented tidal bores: the Severn River bore. Herein the author aims to share his enthusiasm and passion for the tidal bores by documenting numerous tidal bores on our Planet. More than 160 photographs are presented and documented. After a brief introduction, some basic theoretical considerations are developed (section 2). Then the major tidal bores in our Planet's continents are introduced (section 3). The rumble noise of tidal bores is discussed (section 4). The turbulence and turbulent mixing induced by a tidal bore are documented (section 5) before the interactions between tidal bores and mankind are discussed (section 6). The book is supported by several appendices including a reliable reference list of tidal bores, with all information on each tidal bore doublechecked by independent, reliable reports (App. A), a technical appendix on undular tidal bores (App. B), some comments of tidal bore surfers (App. C) and a glossary of technical terms (App. D), followed by a list of bibliographic, Internet and audio-visual references. The book aims to bridge the gap between the general knowledge and scientific expertise on tidal bores. The general readership would be interested to read first the sections 1, 3, 4 and 6, while the sections 2 and 5 would provide these readers with more scientific data. The scientific readership would start first with the sections 1, 2, 4, and 5, and these readers would find some relevant illustrations the section 3 and some discussion in section 6. x

12 (A) Tidal bore of Sée/Sélune River in the Baie du Mont Saint Michel (France) downstream of Pointe du Grouin du Sud on 19 October 2008 at 09:31:09 - Bore propagation from right to left with Mont Saint Michel in the background (B) Incoming tidal bore of River Parrett at Bridgewater (UK) - Note the spectators on the bridge Fig. aa - Photographs of tidal bores xi