Bedload Hysteresis and Bedform Deformation Rates Investigated with Physical Samples, Multibeam Sonar, and Seismic Monitoring

Abstract:

Field and laboratory observations indicate that bedload transport frequently exhibits clockwise hysteresis. Several grain-scale mechanisms could account for this, including increases in the median surface particle size, development of stable particle arrangements on the bed surface, or reductions in the sediment supply. Alternatively, bedload hysteresis at steady flow could occur if the bed configuration stabilizes as bedforms approach a configuration that is in quasi-equilibrium with prevailing hydraulic conditions. The roles of bedform development and changes in coarse sediment availability as potential causes of hysteresis are investigated with a unique dataset obtained during a high flow release in the Trinity River, a regulated gravel-bed river in California. Physical bedload samples were obtained over the release hydrograph along with continuous seismic monitoring with 3-component broadband seismometers at four locations along the river. At one location, changes in bed topography during the peak of the release were monitored with repeated multibeam sonar surveys over a channel length of about 1 km. In addition, a network of 80 vertical-component seismometers was deployed adjacent to the channel to support development of a time series of maps showing local variations in seismic energy production on the stream bed. Finally, a gravel augmentation operation was being implemented at the upstream end of the reach during the release peak, permitting evaluation of how changes in sediment supply affect downstream transport rates. Sampled bedload transport rates were found to increase briefly during gravel augmentation operations, but return to pre-augmentation levels within a few hours after augmentation activities stop and generally decline over a period of peak flow lasting about 3 days. The sonar data indicate that most of the topographic change observed during the peak flow period occurred in the first several hours of the period, supporting the hypothesis that decreasing bedload transport rates in the latter part of the hydrograph peak correspond to the development of relatively stable bedforms. Seismic measurements were found to correspond well to temporal and spatial variation in bedload transport and bed deformation rates, as determined by the physical samples and sonar surveys.