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The Influence of Coriolis Forces on Flow Structures of Channelized Large-Scale Turbidity Currents and Their Depositional Patterns

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The Influence of Coriolis Forces on Flow Structures of Channelized Large-Scale Turbidity Currents and Their Depositional Patterns The Influence of Coriolis Forces on Flow Structures of Channelized Large-Scale Turbidity Currents and their Depositional Patterns by Remo Cossu A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Geology University of Toronto © by Remo Cossu 2011 The Influence of Coriolis Forces on Flow Structures of Channelized Large-Scale Turbidity Currents and their Depositional Patterns Remo Cossu Doctor of Philosophy Department of Geology, University of Toronto 2011 Abstract Physical experiments are used to investigate the influence of the Coriolis forces on flow structures in channelized turbidity currents, and their implication for the evolution of straight and sinuous submarine channels. Initial tests were used to determine whether or not saline density currents are a good surrogate for particle-laden currents. Results imply that this assumption is valid when turbidity currents are weakly-depositional and have similar velocity and turbulence structures to saline density currents. Second, the controls of Coriolis forces on flow structures in straight channel sections are compared with two mathematical models: Ekman boundary layer dynamics and the theory of Komar [1969]. Ekman boundary layer dynamics prove to be a more suitable description of flow structures in rotating turbidity currents and should be used to derive flow parameters from submarine channels systems that are subjected to Coriolis forces. The significance of Coriolis forces for submarine channel systems were determined by evaluating the dimensionless Rossby number Ro W. The Rossby number is defined as the ratio of the flow velocity, U, of a turbidity current to the channel width, W, and the rotation rate of the Earth represented by the Coriolis parameter, f. Coriolis forces are very significant for channel systems II with Ro W ≤ O(1) . Third, the effect of Coriolis forces on the internal flow structure in sinuous submarine channels is considered. Since previous studies have only considered pressure gradient and centrifugal forces, the Coriolis force provides a crucial contribution to the lateral momentum balance in channel bends. In a curved channel, both the Rossby number Ro W and the ratio of the channel curvature radius R to the channel width W, determine whether Coriolis forces affect the internal flow structure. The results demonstrate that Coriolis forces can cause a significant shift of the density interface and the downstream velocity core of channelized turbidity currents. The sediment transport regime in high-latitude channel systems, which have Ro W << R/W , is therefore strongly influenced by Coriolis forces. Finally, these findings are incorporated into a conceptual model describing the evolution of submarine channels at different latitudes. For instance, the Northern Hemisphere channels have a distinctly higher right levee system and migrate predominantly to the left side and generally exhibit a low sinuosity. In contrast, low latitude channel systems have Ro W >> R/W so that centrifugal forces are more dominant. This results in more sinuous submarine channel systems with varying levee asymmetries in subsequent channel bends. In conclusion, Coriolis forces are negligible around the equator but should be considered in high latitude systems, particularly when Ro W ~ O(1) and Ro W << R/W . III Acknowledgements There are many people I would like to thank for their help and support over the last four years. Without them I could not call my experience in Canada and the completion of my Phd an exciting one. • First, and foremost, my supervisor Mathew Wells for giving me the opportunity to come to Canada and to go on this adventurous journey. I am especially grateful for his truly unconditional support regarding all aspects of the scientific world. I enjoyed working with Mathew from the first to the very last minute of my PhD. With his guidance, teaching and understanding he was the most pleasant supervisor I could have asked for. Mathew, thank you for everything! • Members of my PhD Committee: Brian Greenwood, Nick Eyles, Joe Desloges and Julian Lowman for contributing timely and healthy criticism and inspiration. Special thanks go out to Brian for his help with the ADV, to Nick for extending my knowledge of glacial sedimentology/geology and Julian for his patient, nonchalant help with mathematics in the late stages of this thesis. • Bruce Sutherland for his helpful comments and suggestions for the thesis. Special thanks for his readiness and willingness to travel in from Edmonton to attend the thesis defense. • Jeff Peakall and Gareth Keevil for loaning me the the UDVP which I used for a major part of this thesis. Thank you Jeff, for all of your support, advice and input for my experiments and publications. • Anna Wåhlin for her mathematical modelling skills and contribution to this thesis. • Gary Parker and Carlos Pirmez for their intreguing discussions at the AGU Chapman conference in Oxnard. • I would also like to thank a few key influencers and mentors from back home who had guided me professionally in academia and industry: Birgit Brinkmann, Karl-Friedrich Daemrich and Joerg Osterwald. I would not have undertaken the step of going to Canada without your support, advice and help. • Ulrich Langer for proofreading gazillions of documents in preparation for my acceptance into the PhD program at UofT. IV • The province of Ontario for its financial support of this thesis project. • All of the administrative staff in the Geology Department, as well as the Department of Physical & Environmental Sciences. • Bob Ush and Joan Mills for being so supportive during those long nights of writing and editing. • C.C. for countless free coffees on my days out at UTSC. • Joerg Bollmann for being a great and generous friend and counsellor in academia and the weird ways of life. • Christoph Schrank for being a great schoolmate during the first years in Toronto. • The wild bunch of grad students who started this journey with me in 2007. You made my first year probably the most memorable. Cheers mates, the next international dinner is on me! • My various officemates I had over the years, especially Bronwyn and Taronish for enduring the occasional odor of gym gear with only minor complaints. I also want to thank Taronish for becoming a companion, who I could always rely on for down time and being such an awesome workout partner. • Many longtime and most loyal friends back home. Although separated by an ocean from you, I did not get lost being so far from home because I knew you were still there for me. You know who you are! “Wir werden immer laut durchs Leben ziehen, jeden Tag in jedem Jahr…” • The honorary final position is reserved for those that I owe the most: Kristina, for standing beside me along this journey. You helped me grow through things and have kept me in balance (not only with the rotating table!). Thank you for who you are! I finally want to thank my family and relatives: especially my magnificent sister and my wonderful, loving parents. Leaving home is never easy…You all rule! V Table of Contents Abstract II-III Acknowledgements IV-V Statement of authorship XII Chapter 1: Introduction 1.1 The significance of turbidity currents 1-8 1.2 The importance of the Coriolis force 9-13 1.3.1 Physical models 13-16 1.3.2 Velocity structure 16-17 1.3.3 Secondary flow cells due to centrifugal forces 17-19 1.3.4 Secondary flow cells due to Coriolis forces 20-22 1.4. Motivation and thesis overview 22-24 References 24-30 Chapter 2: A comparison of the shear stress distribution in the bottom boundary layer of experimental density and turbidity currents Abstract 31-32 2.1 Introduction 32-35 2.2 Shear stresses in the BBL 36-38 2.3 Method 38-42 2.4.1 General flow properties 42-44 2.4.2 Velocity fluctuations 45-46 2.4.3 Reynolds stresses 47-51 2.4.4 Turbulent kinetic energy profiles 51-52 2.4.5 Drag coefficients 53-54 2.5. Discussion 55-60 2.6. Summary and Conclusions 60-61 Acknowledgements 62 References 62-65 VI Chapter 3: Influence of the Coriolis force on the velocity structure of gravity currents in straight submarine channel systems Abstract 66-67 3.1 Introduction 67-71 3.2.Theory 72-80 3.3 Method 80-83 3.4.1 General observations 83-85 3.4.2 Observations of downstream velocity U 85-89 3.4.3 Slope of the interface with changing f 89-91 3.4.4 Secondary flow cells and across-stream velocities 91-95 3.5 Discussion 95-99 3.6 Conclusions 100-101 Acknowledgements 101 References 101-105 Chapter 4: Coriolis forces influence the secondary circulation of gravity currents flowing in large-scale sinuous submarine channel systems Abstract 106 4.1 Introduction 106-109 4.2 Theory 109-110 4.3 Experiments 110-112 4.4 Results and Discussion 112-117 4.5 Conclusions 118 Acknowledgements 118 References 119-120 VII Chapter 5: Flow structures and sedimentation processes in submarine channels under the influence of Coriolis forces: experimental observations in rotating gravity currents Abstract 121 5.1 Introduction 122-124 5.2 Method 124-125 5.3 Results 125-128 5.4 Discussion 129-133 5.5 Summary and conclusions 133 Acknowledgements 134 References 134-135 Chapter 6: Final remarks 6.1 Summary and implication 136 6.2 1 Density currents as an analogue for turbidity currents 136-137 6.2.2 Coriolis forces in straight submarine channels 137-138 6.2.3 Coriolis forces in sinuous submarine channels and their implication for the evolution of channel systems 138-140 6.2.4 Future work 140-141 References 141-142 VIII List of Tables Chapter 2 Table 2.1: Summary of parameters for experimental gravity currents 44 Chapter 4 Table 4.1: Experimental conditions 112 IX List of Figures Chapter 1 Fig.
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