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Morphological Bedload Transport in Gravel-Bed Braided Rivers

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Morphological Bedload Transport in Gravel-Bed Braided Rivers Western University Scholarship@Western Electronic Thesis and Dissertation Repository 6-16-2017 12:00 AM Morphological Bedload Transport in Gravel-Bed Braided Rivers Sarah E. K. Peirce The University of Western Ontario Supervisor Dr. Peter Ashmore The University of Western Ontario Graduate Program in Geography A thesis submitted in partial fulfillment of the equirr ements for the degree in Doctor of Philosophy © Sarah E. K. Peirce 2017 Follow this and additional works at: https://ir.lib.uwo.ca/etd Part of the Physical and Environmental Geography Commons Recommended Citation Peirce, Sarah E. K., "Morphological Bedload Transport in Gravel-Bed Braided Rivers" (2017). Electronic Thesis and Dissertation Repository. 4595. https://ir.lib.uwo.ca/etd/4595 This Dissertation/Thesis is brought to you for free and open access by Scholarship@Western. It has been accepted for inclusion in Electronic Thesis and Dissertation Repository by an authorized administrator of Scholarship@Western. For more information, please contact [email protected]. Abstract Gravel-bed braided rivers, defined by their multi-thread planform and dynamic morphology, are commonly found in proglacial mountainous areas. With little cohesive sediment and a lack of stabilizing vegetation, the dynamic morphology of these rivers is the result of bedload transport processes. Yet, our understanding of the fundamental relationships between channel form and bedload processes in these rivers remains incomplete. For example, the area of the bed actively transporting bedload, known as the active width, is strongly linked to bedload transport rates but these relationships have not been investigated systematically in braided rivers. This research builds on previous research to investigate the relationships between morphology, bedload transport rates, and bed-material mobility using physical models of braided rivers over a range of constant channel-forming discharges and event hydrographs. Morphology changes were estimated using the morphological method, which infers information from changes in channel topography over time, from an extensive dataset of digital elevation models (DEMs) generated using digital photogrammetry and ‘Structure-from-Motion’ principles. Results suggest that the active width is highly variable even at constant discharge but increases with stream power and is positively related to bedload transport rates, bulk change (i.e., total volume of erosion and deposition), and active braiding intensity. Morphologically- derived sediment budgets provided reasonable estimates of bedload transport rates that were similar to independent measurements of bedload transport rates from sediment baskets. In addition, grain size distributions and bed mobility evolved from a state of partial mobility towards equal mobility with increasing discharge. This is rare in most gravel-bed rivers, but in braided rivers the high levels of sediment supply and lack of armouring allow for greater mobility of the channel bed and subsurface. Finally, the lower detection threshold for the morphological active width, bedload transport, and transition to selective mobility all coincided with a dimensionless stream power of ~0.08. Overall, these results suggest that while braided rivers are dynamic, they may be restricted in ways like their single-threaded counterparts so that measures of morphology (i.e., the active width) can be used as general predictors of bedload transport rates and the morphological stability of the river. This knowledge contributes to our overall understanding of braided river i morphodynamics while also building on theory for use in applied geomorphology and engineering practices for the management, conservation, and restoration of complex braided rivers systems. Keywords braided, gravel-bed rivers, morphological method, active width, digital photogrammetry, bed mobility ii Acknowledgements I would first like to acknowledge and thank my thesis advisor, Dr. Peter Ashmore. I am grateful your guidance, patience, wisdom, and general love of braided rivers. Along with my advisor, I would like to thank all of those who had a direct hand in making this Ph.D. possible. Dr. Pauline Leduc, whose brilliant ideas on physical modelling and coding made it possible collect (and process) the rather ridiculous amount of data that was measured in the flume. Furthermore, your guidance and friendship was truly invaluable throughout my Ph.D. experience. I also want to thank Lara Middleton, who spent endless hours in the flume, in the dark, collecting data with me. Thank you for your help, friendship, and of course, for your enthusiasm for braided rivers. Thank you Danie Barr for everything, but especially for all of your support throughout the last two years. I also want to extend my gratitude to Dr. Matidle Welber for sharing her wisdom about statistics, flumes, and cooking with me. Thank you to all of those who came to the flume to help out including Erika Hill, Sarah McFadden, Cliff Davidson, Clayton Cook, Chris Vandelaar, and in particular Cody Ruthman, who often came at a moment’s notice to save the day! I also want to thank Dr. Darren Sjogren, Dr. Joe Wheaton, Wally Macfarlane, and Dr. Chris Hugenholtz for their help and experience in the field. I would like to thank Dr. Andy Binns, Dr. Marco Van De Wiel, Dr. Adam Yates, Dr. Katrina Moser, and Dr. Cheryl McKenna-Neuman for taking the time to challenge me as part of my thesis and examination committees. A special thank you to Dr. Desmond Moser, my second reader, for your time and feedback on my manuscript. Thank you to all of the wonderful staff at the university including Lori Johnson, Joe Smrekar, Karen Vankerkoerle, Kathy Tang, Angelica Lucaci, and Rita Mendis-Mogenson for all of your help. Last but not least, I want to thank my family and friends for their continued love and support throughout these last few years. Like so many other graduate students I can honestly say that my Ph.D. experience did not go exactly as planned, but I am so lucky to iii have people encourage me to just keeping going. Thank you to my mom and brother, Michael, I could not have done this without you. I also want to thank my Dad, whom I miss every day, for bestowing on me a love of learning and teaching that helped make this journey possible. Finally, I want to thank my husband, Aaron. I am not sure I can even find the words to express my gratitude to you but we both know I would not be submitting this thesis if it wasn’t for your unwavering love and support. Thank you for everything. iv Table of Contents Abstract ................................................................................................................................ i Keywords ............................................................................................................................ ii Acknowledgements ............................................................................................................ iii Table of Contents .................................................................................................................v List of Tables ..................................................................................................................... ix List of Figures ......................................................................................................................x List of Notations .............................................................................................................. xiv 1 Introduction .................................................................................................................1 1.1 Thesis Format and Research Objectives ............................................................. 1 1.1.1 Thesis Rationale .......................................................................................... 1 1.1.2 Purpose and Methodological Background .................................................. 1 1.1.3 Thesis Format and Research Objectives ..................................................... 2 2 Background ..................................................................................................................4 2.1 Conditions for Braiding in Gravel-Bed Rivers ................................................... 4 2.2 Morphological Characteristics of Braided Channels .......................................... 6 2.2.1 Bar Unit ....................................................................................................... 7 2.2.2 Confluences and Bifurcations ................................................................... 10 2.2.3 Braiding Intensity...................................................................................... 11 2.2.4 Channel Geometry .................................................................................... 13 2.3 Bedload Transport Processes ............................................................................ 15 2.3.1 Active Layers and Phase Flow .................................................................. 15 2.3.2 Bedload Pulses .......................................................................................... 15 2.3.3 Bed Mobility ............................................................................................. 18 2.4 The Morphological Active Width ..................................................................... 20 2.5 Morphological Methods for Estimating Bedload Transport Rates ................... 22 2.6 Measuring Morphological Change ..................................................................
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