Acoustically derived suspended sediment concentrations and flux in the Fraser River, Canada

In Canada, there is no existing systematic sediment data collection program for river systems and limited resources are available to mount manual measurement programs. Yet, there is a pressing need to understand and predict sediment fluxes because the quantity and caliber of transported sediment controls river channel stability, influences river ecology and should be considered in river management. In the Lower Fraser River, British Columbia, Canada conventional methods for estimating sediment flux are based on historical data that are no longer reliable due to the river's ongoing adjustment to land use practices, climate change, sea level variation and dredging. This research establishes methods to monitor suspended sediment delivery to the Fraser Delta using hydroacoustic signals as a surrogate of suspended sediment concentration (SSC) and grain size. Both single- and multi-frequency sediment detection approaches are evaluated. Acoustic signals from an array of three horizontally-mounted acoustic Doppler current profilers (ADCPs) are coupled with physical bottle samples within the acoustically ensonified volume. Bottle samples are analyzed for SSC and for grain size distribution. Discharge, channel-average SSC and flux are measured. Twenty-five sampling campaigns were carried-out in the Fraser River at Mission between 2012 and 2014. I develop data processing methods for acoustic signals near the ADCP noise-floor and establish threshold concentrations below which attenuation measured in-situ is unreliable. A single-frequency, two-stage acoustic inversion is developed for application in large rivers where the ADCPs cannot penetrate the full channel width. The method involves calibration of ADCPs and a correlation between ADCP SSC and the channel-averaged SSC. Strong calibrations for total SSC, sand SSC and silt/clay SSC are obtained. Good correlations between acoustically derived SSC and channel-average SSC allow for continuous SSC and flux estimates. Annual flux fell within the same order of magnitude as historical flux from the same location, computed with traditional methods, supporting the robustness of the method. Explicit and implicit multi-frequency inversions are explored. Comparisons between the inversion results and sample data show that the implicit method tends to perform best for estimating concentration at all flows. Realistic estimates of particle size are obtained for high flows only using this method.