Seafloor gravity currents: flow dynamics in overspilling and sinuous channels Robert William Kelly The University of Leeds School of Computing EPSRC Centre for Doctoral Training in Fluid Dynamics Submitted in accordance with the requirements for the degree of Doctor of Philosophy. October 2018 This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. The candidate confirms that the work submitted is his/her/their own, except where work which has formed part of jointly authored publications has been included. The contribution of the candidate and the other authors to this work has been explicitly indicated below. The candidate confirms that appropriate credit has been given within the thesis where reference has been made to the work of others. A version of Chapter 5 has been accepted for publication in Journal of Geophysical Research: Oceans: “The structure and entrainment characteristics of partially-confined gravity currents”. Robert Kelly, Robert Dorrell, Alan Burns and William McCaffrey. Journal of Geophysical Research: Oceans. Under review. In this manuscript the work is the candidate’s own, with the other authors having acted in a supervisory role, providing feedback and suggestions. i “In every outthrust headland, in every curving beach, in every grain of sand there is the story of the earth.” – Rachel Carson ii Acknowledgements It feels a bit bizarre to finally be writing this, as it means my time as a PhD student must be drawing to a close. Firstly, I would like to thank my team of supervisors, Bill McCaffrey, Robert Dorrell and Alan Burns, who gave me their utmost support throughout this project. Thank you to Bill, for imparting a smidge of his vast geological knowledge upon this physicist and whose keen insights and ideas were, and continue to be, invaluable. Thank you to Rob, for being my first port of call for any technical query and whose unique and unparalleled skillset were a vital resource to tap into. And thank you to Alan, a total authority on CFD, who taught me almost everything I know on the subject and whose expertise undoubtedly prevented many a futile endeavour on my part. Next, I would like to thank the CDT management, for providing me with the opportunity to study for a PhD in the first place. I am especially grateful to Peter Jimack, who always made time, and to Claire Savy, who bore the brunt of my unwillingness to deal with paperwork and bureaucracy. Thank you to my fellow students, who worked, laughed, travelled and drank with me. My CDT cohort is a group of immensely talented individuals and they have made the past 4 years unforgettable. A special thanks to Sam Williams, who took me climbing and, in doing so, introduced me to a whole new world. I would also like to thank the Sorby team of Gareth Keevil, Robert Thomas and Helena Brown. Gareth in particular is thanked for his time, effort, dry wit and for still allowing me access to the lab after the minor flooding incident of 2018. I am forever indebted to Mum and Dad, both for their constant support and for allowing me the total freedom to make my own choices and mistakes. It doesn’t seem to have gone too badly so far. Finally, I would like to thank Dev. This could very well be the only page of the thesis she will ever read. Somehow, you have been completely supportive while simultaneously showing a total indifference to fluid dynamics or turbidity currents. Frankly, this has been a welcome relief and perhaps ensured I am still sane? I’ll let you decide on that one. iii Abstract Turbidity currents are the largest agent of global sediment transport and their deposits, submarine fans, are the largest sedimentary structures on Earth. Submarine fans consist of networks of seafloor channels, which are vital pathways for sediment and nutrient transport to the deep ocean. This work focusses on flow dynamics within these channels, with the aim of understanding the role of the channel form on flow development and identifying implications for the development of channels and, ultimately, for submarine fans. Laboratory experiments have been conducted of continuous saline gravity currents traversing fixed-form channel models with a range of planform geometries. Both velocity and density data were gathered to investigate the effect of a channel on the flow field. Numerical simulations have also been conducted, using a Reynolds-averaged Navier- Stokes model and a shear stress transport turbulence closure. These allow an extension of the laboratory analysis, both in terms of physical domain size, data resolution and measured variables. Velocity data reveal how partial confinement exerts a first order control on the vertical variation in flow structure. The channel half-depth acts to limit the height of the velocity maximum, resulting in the development of a confined, high-velocity flow core. The channel form also constrains the lateral and three-dimensional flow structure. Secondary flow rotation, characterised by a local reversal in the radial pressure gradient, is shown here to be inhibited by low channel sinuosity and large levels of overbank fluid losses. A change in cross-sectional channel profile is capable of switching the dominant cross- stream basal flow direction of these structures. Furthermore, channels are shown to cause flow tuning, whereby flows of differing magnitudes entering a channel reach are rapidly modified to show a much restricted magnitude range, that remains quasi-stable thereafter. For the cases studied, this quasi-equilibrium state is characterised by a symmetrical cross- channel basal stress profile. The existence of such a state could explain how seafloor channels can achieve a degree of planform stability. iv Contents Acknowledgments ........................................................................................ ii Abstract ....................................................................................................... iii Contents ....................................................................................................... iv List of figures ............................................................................................... x List of tables ............................................................................................. xvii 1 Introduction .............................................................................................. 1 1.1 Research background and thesis aims ..................................................................... 1 1.2 Methodological approaches ..................................................................................... 3 1.3 Thesis Outline .......................................................................................................... 5 2 Gravity current dynamics and submarine channel morphology ........ 6 2.1 Gravity and turbidity current dynamics ................................................................... 6 2.1.1 Definitions of flow classifications .................................................................... 6 2.1.2 Mean and bulk flow properties ......................................................................... 9 2.1.3 Anatomy of a gravity current .......................................................................... 11 2.1.4 The head of the current ................................................................................... 12 2.1.5 The body of the current .................................................................................. 13 2.1.6 Velocity and density structure ........................................................................ 15 2.1.7 Turbulence structure ....................................................................................... 18 2.1.8 Field observations ........................................................................................... 19 v 2.1.9 Turbidity currents: initiation and deposition .................................................. 21 2.2 Submarine fans and channels ................................................................................ 22 2.2.1 Classifications ................................................................................................. 23 2.2.2 Morphology and geometry ............................................................................. 24 2.3 Experimental modelling of gravity currents .......................................................... 28 2.3.1 Technologies and techniques .......................................................................... 29 2.3.2 Straight channel studies .................................................................................. 30 2.3.3 Sinuous channel studies .................................................................................. 31 2.4 Numerical modelling of gravity currents .............................................................. 33 2.4.1 Eddy viscosity models .................................................................................... 34 2.4.2 Reynolds stress models ................................................................................... 36 2.4.3 Large eddy simulation and direct numerical simulation................................. 37 2.5 Theoretical modelling of gravity currents ............................................................. 40 2.6 Summary ................................................................................................................ 43 3 Laboratory methodology and design ..................................................