Modelling the Tidal and Sediment Dynamics in Darwin Harbour, Northern Territory, Australia
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Modelling the Tidal and Sediment Dynamics in Darwin Harbour, Northern Territory, Australia Li Li School of Physical, Environmental and Mathematical Sciences The University of New South Wales Canberra, ACT, 2600, Australia A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy August 2013 ii ABSTRACT The suspended-sediment dynamics in Darwin Harbour, Northern Territory Australia, were investigated using a combination of field measurements and numerical modelling. After analysing the harbour’s geophysical characteristics from the field data and an extensive literature review, a hydrodynamic model for the harbour was built using the Finite Volume Coastal Ocean Model (FVCOM), a model suited to coastal ocean simulation. This model was then coupled in this study to the estuarine suspended- sediment model (ESSed) of Wang (2002) to produce the FVCOM-ESSed model. Both the hydrodynamic and sediment-transport components were calibrated using the field data on sea-surface level, current velocity and suspended-sediment concentration. The sediment-transport model focuses on suspended sediments, with improvements that allow wetting-drying processes, different bathymetry types and a variable thickness of the fine-sediment layer on the harbour bed to be included. The combined hydrodynamic and sediment model provides a reasonable simulation of the tidal and suspended-sediment dynamics in the harbour. Numerical experiments using this model were then designed to determine the effect of the mangrove areas and tidal flats in Darwin Harbour on the tides and tidal asymmetry, and subsequently, on the suspended-sediment dynamics. This study shows that the hydrodynamics of Darwin Harbour are driven mainly by tides, with the effects of wind and rivers small. The M2 tidal component is dominant, with amplitude near Darwin City about 1.9m and phase 249 degrees. Current flow is dominated by the tides. The current, which reaches a maximum speed of about 2.0 ms-1 at the surface of Middle Arm and decreases gradually from the surface to the bottom, is -1 dominated by the M2 tidal current, with an average vertical speed of about 0.4 ms near East Arm Wharf. The water-flow patterns are in accordance with the shoreline, and rectilinear in all three arms. The energetic hydrodynamic regime, together with the availability of erodible sediment on the seabed, determines the variation in suspended- sediment concentration (SSC) in the harbour. Stronger currents induced by the larger tidal ranges during spring tides generate higher bottom SSC values than those during neap tides, as observed in the harbour in November 2012. A turbid zone appears in the iii outer harbor, with bottom SSC values about 0.1kgm-3 during the spring tides and 0.07kgm-3 during neap tides. The water is less turbid during neap tides than during spring tides; vertical gradients of SSC are formed in the channel during neap tides due to weaker currents. Net sediment transport is seaward in the channel and landward at the entrance to East Arm, dominated by Eulerian advection. Mangrove areas and tidal flats play key roles in modulating the tides and water- flow dynamics of an estuary. Suspended sediments are redistributed as a result of these modulated water dynamics. Removal of the mangrove areas and tidal flats from Darwin Harbour, a possibility due to planned harbour development, would dampen the M2 amplitude because of decreased shoaling effects in the inner harbour, but would generate a greater M4 amplitude. Removal of the mangroves and tidal flats would also lessen the tidal choking effect, thereby, reducing the current speed; as a result, the water would be less turbid in the harbour, for example, the bottom SSC values in East Arm would be reduced by about 70% during spring tides. Mangrove areas and tidal flats affect tidal asymmetry via their influence on the amplitudes and phases of the tides, and therefore affect net sediment transport. In Darwin Harbour, these areas significantly reduce tidal asymmetry: for example, the tidal elevation skewness would increase by about 120% in Middle Arm if the mangrove areas and tidal flats were removed. Due to the increased flood dominance, the tidal pumping effect would overtake the Eulerian residual to dominate sediment transport both in the channel and at the entrance to East Arm. This would reverse the transport of suspended sediment in the channel to landward. The landward net sediment flux would be decreased at the entrance to East Arm as a result of reduced currents in the arms because of the reduction in the tidal choking effect, if the mangrove areas and tidal flats were removed. iv ORIGINALITY STATEMENT Li Li August 2013 v vi COPYRIGHT STATEMENT AUTHENTICITY STATEMENT vii viii ACKNOWLEDGEMENTS This study was carried out as a PhD student of Oceanography in the School of Physical, Environmental and Mathematical Sciences (PEMS), the University of New South Wales (UNSW) at Canberra. There are many people I would like to thank who helped me at various stages throughout the task of researching and writing this thesis. My supervisors deserve special thanks and deepest gratitude. Associate Professors Xiaohua Wang and Harvinder Sidhu, PEMS, UNSW at Canberra, always made time in their busy schedule to help me with various problems during my study. Their scientific ideas, many discussions, proof reading, support and encouragement over the years have been greatly appreciated. Thank you for introducing me to Oceanography and for your encouragement along the way. Thank you for keeping me on track throughout the hard grind of writing a thesis. Dr David Williams, Australian Institute of Marine Science (AIMS) provided this research with field assistance and field data. He has been enormously helpful, tirelessly answering many questions regarding data analysis and the use of field equipment. Professor Peter Ridd and the Marine Geophysics Laboratory at the James Cook University are highly appreciated for the generous loans of nephelometers and their efforts during field work. Dr Fernando Andutta assisted me with his useful physical background and constructive discussions. In addition, I would like to thank Donghui Jiang, Younjong Sun and Dehai Song for their generous help in answering my questions and giving me the benefit of their physical and numerical experience during this study. Conducting a PhD study can be expensive, and both the Chinese Scholarship Council and the University of New South Wales at Canberra must be acknowledged for their scholarship support. The University of New South Wales funded travel expenses incurred during fieldwork, and PEMS provided equipment, charts and library resources. In particular, I would like to thank Australian Research Council (ARC) and INPEX for their generous financial support all through this study via an ARC Linkage Project (LP110100652), and Dr David Williams for his financial support during field trips and data observation. Without this financial support, this work would never have been possible. ix Heartfelt thanks go to PEMS staff: Deborah Bator and Dianne Ferguson for their patient help with all kinds of travel; and Tessa Hodson, Nadia Seselja, Annabelle Boag, Steve James and Peter Scott for their warm introduction and assistance, when I first arrived and during my study. Particular thanks go to Colin Symons and Paul Mckie for answering many questions regarding the use of field equipment. Thanks to Dr Peter McIntyre for his editorial efforts and help with proof reading and constructive criticism. Sincere thanks also go to Julie Kesby, Research Officer, PEMS, for her fantastic help in providing advice on the PhD confirmation during my early research phase, for her patient assistance with my academic writing and chart reading, and for help with reference advice and proof reading. Dr Kate Wilson and Geoff Millar from the Academic Language and Learning Unit, UNSW Canberra, helped me to improve my grammar in English writing. Sincere thanks to colleagues from the library for helping me with book reservations, database search methods and online literature retrieval. Many thanks go to Information Communication and Technology Service for patiently helping me with computer and internet problems. As any researcher and writer can attest, no such work would be possible without the support of family and friends. In particular I would like to pay special tribute to the incredible support of my family, my father, mother, brother, and my aunt and her family. Heartfelt thanks and sincere appreciation go to Ms J. Xu and her family for bringing me the warmest feeling of home in Australia. Thank you for giving me so much love and encouragement. I also would like to thank all my friends for their warm care, lovely company and sincere friendship during this study. x TABLE OF CONTENTS ABSTRACT ....................................................................................................... III ORIGINALITY STATEMENT ........................................................................... V COPYRIGHT STATEMENT ........................................................................... VII AUTHENTICITY STATEMENT ..................................................................... VII ACKNOWLEDGEMENTS ............................................................................... IX TABLE OF CONTENTS .................................................................................... XI LIST OF FIGURES ..........................................................................................