Doctor of Philosophy
Dr. Peter Ashmore
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 fundamentally the result of bedload transport processes but our understanding of the fundamental relationships between channel form and bedload processes 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 morphological 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 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.
Peirce, Sarah E. K., "Morphological Bedload Transport in Gravel-Bed Braided Rivers" (2017). Electronic Thesis and Dissertation Repository. 4595.
Available for download on Sunday, December 31, 2017