Morphodynamic processes in semi-alluvial rivers

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An alluvial river has bed and banks composed of sediment transported by the flow. Consequently, through the morphodynamic processes of erosion and deposition, an alluvial river is free to adjust its morphology to equilibrium dimensions that can convey the flow and incoming sediment. The majority of rivers in Canada, however, are semi-alluvial because they flow over, and cut into, glacial deposits (e.g. glaciomarine clays, glacial tills), and/or are controlled by bedrock outcrops. Relations between flow, sediment transport, and channel form are poorly understood in semi-alluvial rivers, thus models are not available to predict channel geometry (e.g., width, depth, slope) as a function of the flow and sediment regimes. Canadian river engineers and scientists require these models as design tools for river restoration to improve aquatic habitat, and to predict morphological and flood response to changes in flow or sediment regime (e.g., climate change adaptation), river realignment (e.g., highways, navigation), or implementation of instream infrastructure such as hydroelectric dams, bridges, or pipelines. This proposal outlines a research plan to study semi-alluvial river morphodynamics, with the ultimate objective of developing river channel design tools suitable for semi-alluvial rivers, including clay-bed channels and rivers influenced by bedrock. Deeper understanding of semi-alluvial rivers in Canada will be gained by means of systematic field measurement of morphodynamic processes in a set of case study reaches that encompass the range of semi-alluvial river types. For each case study reach, hydraulic geometry, channel topography, habitat features, and spatiotemporal distributions of bed material, suspended sediment, bed material transport, and three-dimensional flow will be surveyed and mapped at high resolution over several years. Most of these measurements will be conducted with an acoustic Doppler current profiler (aDcp) using methodologies developed by the applicant. Observed sediment transport distributions will elucidate sediment transport pathways between zones of erosion and deposition, which are themselves dictated by interactions between the channel morphology and three-dimensional spatiotemporal turbulent flow distribution. Thus, the morphodynamic processes that determine channel morphology will be measured directly. Special emphasis will be placed on assessing differences between more and less alluvial reaches. In particular, relations between extreme flows, channel morphology, available alluvium, and associated habitat will be assessed. The field data will be used to develop and validate numerical predictive modelling tools for semi-alluvial river morphology and available habitat. A three-dimensional morphodynamic river modelling tool specifically formulated for semi-alluvial channels will be developed for the first time. This research will build upon advances in morphodynamic modelling including adaptive grid generation, turbulence closure, free-surface simulation, bank stability, and bed material transport routines. In order to model semi-alluvial channels, the bank stability and sediment transport routines will consider the erodibility and transport of both the parent material and alluvial sediments received from upstream, which will be a novel contribution to the field of morphodynamic river modelling. The model output will be the equilibrium channel morphology for a given flow. Importantly, each study-reach morphodynamic model will be calibrated and validated using the observed empirical field observations, which is a crucial step too often ignored.