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UBC Theses and Dissertations

Gravel-bed river behaviour : modelling and analysis of process feedbacks over spatio-temporal scales Adams, David Lawson


Alluvial rivers arise through process interactions between water and a deformable boundary. These processes are poorly understood in gravel-bed rivers as they are difficult to measure, and are often described using simple conceptual models. Perhaps the simplest is the 'fluvial trinity' which describes the feedbacks between sediment transport, morphology, and hydraulics. There are two aspects of channel dynamics not explicated by this model: (1) the spatial distribution of channel feedbacks, and (2) interactions between processes across spatial-temporal scales. This thesis examined the effect of these two aspects using a recirculating stream table, from which conceptual frameworks and analytical tools were developed. First, I modelled the response of a channel to successive increases in discharge and identified stage-dependent feedbacks that controlled the adjustment of the system towards a steady-state. Notably, at low flows, transport capacity was modulated by the spatial concentration of shear stress and channel deformation was controlled by larger-than-average grains. The following chapters focused on two components of the fluvial trinity. I combined recent advances in fluid dynamics, statistics, and remote sensing to estimate how roughness length varied across different physical scales. The novel techniqe provided an intuitive representation of how morphology and hydraulics vary across different scales, and affirmed the importance of bar-scale roughness in pool-riffle settings. I evaluated the performance of 1D and 2D bedload transport functions under high relative shear stress conditions. The results clarified that although the spatial distribution of shear stress may be required to predict transport under low stresses, 1D simplifications may be effective under high stresses. Despite being similarly accurate, the 1D and 2D approaches provided substantially different estimates of critical dimensionless shear stress, suggesting that the estimate and interpretation of critical shear stress may depend on whether it is averaged or considered a distribution. Finally, I reviewed hierarchical conceptual frameworks based on ideas of self-organisation or 'emergence' and their potential application to explain the relationship between processes occurring across different spatio-temporal scales within fluvial systems. These conceptual frameworks provide testable hypotheses about the relationship between process scales, and may have implications for understanding sediment transport and approaching physical modelling of rivers.

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