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Exploring the Interaction Between Rivers and Sand Dunes – Implications for Fluvial-Aeolian Geomorphology

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Exploring the Interaction Between Rivers and Sand Dunes – Implications for Fluvial-Aeolian Geomorphology THE UNIVERSITY OF HULL Exploring the interaction between rivers and sand dunes – implications for fluvial-aeolian geomorphology being a Thesis submitted for the Degree of Doctor of Philosophy in the University of Hull by Baoli Liu MSc from Xiamen University, China September, 2014 For my mum Chunrong (杨春荣) and young brother Zehua (刘泽华) And in memory of my father Kefu (刘克富) Abstract The interaction between fluvial and aeolian processes can significantly influence landforms. When rivers and sand dunes meet, the interaction of sediment transport between the two systems can lead to change in either one or both systems. However, these two systems are usually studied independently which leaves many questions unresolved in terms of how they interact. This study investigated interactions between fluvial and aeolian processes, focusing on the triggers that switch the dominance between one process and the other, and the consequent changes in geomorphology that may occur. Firstly, a global inventory of fluvial-aeolian interactions at 230 globally distributed locations was collected using satellite imagery. At each site, the following attributes were measured: net sand transport direction, fluvial-aeolian meeting angle, dune type and river channel pattern. From these data, six different types of interaction were classified that reflect a shift in dominance between the fluvial and aeolian systems. Results from this classification confirm that only certain types of interaction were significant: the meeting angle and dune type, the meeting angle and interaction type and finally the channel pattern and interaction type. These results show the importance of fluvial – aeolian interactions, but also reveal the difficulties of understanding dynamic geomorphic systems from images taken at a single moment in time. A highly novel cellular fluvial and aeolian/dune model was then developed to simulate the process interactions over longer periods of time. Results from the global inventory were used to set up the computational domain and different flow regimes (perennial and ephemeral) were simulated interacting with different magnitudes of aeolian sand transport. The model results demonstrated the same six types of interaction that were identified in the global survey, and also provided a better understanding of the dynamics of landform change which cannot be interpreted from single ‘snapshot’ images. In perennial fluvial systems, when the ratio between fluvial discharge (Rs) and aeolian sand transport rate (Dc) was in a range of critical values, the landscape exhibited cyclic behaviour with abrupt large-scale changes in the absence of external forcing. River channels would avulse around dunes significantly altering the river/dune i configuration and affecting sediment output. The interaction types observed in the modelled scenarios also exhibited transient characteristic with gradual or rapid shifts between various interaction types even under stable conditions. In contrast, landform evolution in ephemeral fluvial systems was very different from perennial environments. Dunes crossed the river channel more easily in ephemeral environments and channel development was more irregular with wetland areas developing and frequent changes in channel pattern. The results suggest that the duration of the wet season is more important than the timing of the wet season on the sediment output rates. This study has demonstrated the importance of fluvial-aeolian interactions for understanding landform development in certain fluvial-aeolian environments and has shown that these systems may show complex responses even with constant input conditions. ii Acknowledgements I cannot express enough thanks to my principle supervisor Professor Tom Coulthard for his continued support and encouragement. The inspiration, thoughtful guidance and critical comments from every meeting and discussion with him not only helped me to get through my PhD study and finish writing my thesis but also altered my perception about research. The considerable help of my second supervisor, Dr Stuart McLelland, has also been invaluable, for which I am very grateful. The encouragement, comprehensive advice and feedback from Dr Andreas Baas (King’s College London, UK), Professor Martin Williams (University of Adelaide, AU), Professor Jonathan Phillips (University of Kentucky, USA) and all the other researchers I have met have enriched my research - it means so much to a junior researcher like me. I would like to extend my grateful thanks to all of the people from the department of Geography, Environment and Earth Sciences at University of Hull. Sincerely thanks to Dr Malcolm Lillie, for equipment, advice, mentoring, and help during the final stage of corrections. Thanks also to Dr Jane Reed, Mike Dennett, Dr Sonja Boehmer- Christiansen and Dr Jane Bunting, for their good advice, support and friendship all of which have been so valuable on both academic and personal levels. The understanding and help from other PhD students, especially Tim Horsfield with whom I shared an office and faced the challenge of writing together to the end, have been very important and made my PhD a bit easier. Also, I would like to take this opportunity to thank the joint scholarship from the University of Hull and China Scholarship Council, and the support from Professor Wenzhi Cao (Xiamen University, China). Without this support, I would not have been able to come to study in England, which has been one of the most important experiences in my life. Heartfelt thanks also go to all the friends and people I have met during my PhD life, for their help and good wishes for the successful completion of this study. Finally, I am forever indebted to my mum and my young brother for their understanding, endless support and love. To all of the above and any I have forgotten to mention, my sincere thanks. iii Contents Abstract ..................................................................................................................... i Acknowledgements .......................................................................................................... iii List of figures ................................................................................................................. viii List of tables ................................................................................................................... xi List of symbols ................................................................................................................. xii Chapter 1 Introduction ................................................................................................ 1 1.1 Fluvial and aeolian systems versus geomorphologys....................................... 1 1.2 Aims and contributions .................................................................................... 1 1.2.1 Aims ............................................................................................................. 1 1.2.2 Contributions .............................................................................................. 2 1.3 Approach .......................................................................................................... 2 1.4 Summary ........................................................................................................... 2 Chapter 2 Literature review ........................................................................................ 3 2.1 Introduction ...................................................................................................... 3 2.2 Aeolian geomorphology ................................................................................... 3 2.2.1 Dune type classification .............................................................................. 4 2.2.2 Dune morphology ....................................................................................... 5 2.2.3 Dune field metrics ..................................................................................... 11 2.2.4 Dune dynamics and migration .................................................................. 15 2.3 River geomorphology ..................................................................................... 19 2.3.1 River and channel pattern ......................................................................... 19 2.3.2 Channel dynamics ..................................................................................... 20 2.4 Fluvial-aeolian interaction .............................................................................. 23 2.4.1 Fluvial-aeolian interaction examples ........................................................ 25 2.4.2 Current approaches to the study of fluvial-aeolian interactions .............. 31 iv 2.5 Numerical modelling of landscape development .......................................... 33 2.5.1 Computational simulation of dune field development ............................ 34 2.5.2 Computational simulation of fluvial landscapes ....................................... 36 2.6 Synthesis ......................................................................................................... 38 Chapter 3 Mapping fluvial-aeolian interaction ........................................................ 41 3.1 Introduction .................................................................................................... 41 3.2 Methodology .................................................................................................. 41 3.2.1 Field identification
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