Multifrequency Acoustic Doppler Profilers

点击翻译按钮获取中文摘要

River channels are conduits for water and sediment through landscapes. Their morphology and dynamics result from the interaction of turbulent flow with sediment and a boundary that may be labile, in the case of alluvial rivers, or relatively resistant to erosion, as for bedrock rivers. Prediction of river channel behavior has improved dramatically over the past few decades, in large part because of our ability to test theory and numerical models of river behaviour against high-resolution observations of sediment dynamics, river bed configuration, and turbulent flow. Yet our understanding of the largest rivers on Earth remains poor, largely due to a lack of high-resolution observations of flow and bed topography. It remains difficult to model the behaviour of large river systems and predict their evolution without such observations. The requested instrumentation will support research programs exploring the morphodynamics of large rivers. The work centers around three major outstanding morphodynamic problems in river science: 1) How a labile sand bed resists flow and mediates flooding, 2) How complex sediment-laden flows carve bedrock canyons, and 3) How rivers are impacted by ice formation, breakup and discharge. The research requires high-resolution observations of flow, using oceanographic hydroacoustic instruments adapted for use in rivers. We request two dual frequency acoustic Doppler current profilers (ADCPs) and accompanying RTK-GNSS equipment designed for marine environments. Each ADCP has two sets of four transducers with different frequencies (300/600 kHz and 600/1200 kHz) and different profiling ranges from a few to a hundred meters in clear water. The lower frequency ranges are required simply to penetrate the water column in rivers powerful enough to suspend gravel-sized particles. The higher frequency ranges provide greater resolution in shallow flows (<5m) where the 300/600 kHz ADCP would provide just one or two points per profile. The combination of three frequencies allows for interleaved measurements of velocity and acoustic backscatter, which can be used in acoustic inversions to measure suspended sediment concentration, grain size and grain size specific sediment flux. The new knowledge of river flow dynamics and flood processes that will result from this research is valuable to all Canadians, particularly decision makers who manage, conserve and preserve river landscapes. There has never been a more pressing time to understand how rivers flood. In economically rich countries like Canada, predicting when rivers flood is worth billions of dollars in infrastructure protection costs. In economically poorer countries, where flood defenses are non-existent, accurate flood water level predictions are a matter of life and death when included in survival, rescue, and relocation plans.