HR: 1340h
AN: H43A-0356    [Abstracts]
TI: Implications of Acoustic Bed Velocity Measurements for Sediment Transport and Bedform Dynamics in Large Sand-bed Rivers
AU: * Gaeuman, D
EM: dgaeuman@usgs.gov
AF: National Research Council Associate, USGS-CERC 4200 New Haven Rd, Columbia, MO 65201 United States
AU: Robert, J B
EM: rjacobson@usgs.gov
AF: US Geological Survey, Columbia Environmental Reseach Center 4200 New Haven Rd, Columbia, MO 65201 United States
AU: Johnson, H E
EM: hejohnson@usgs.gov
AF: US Geological Survey, Columbia Environmental Reseach Center 4200 New Haven Rd, Columbia, MO 65201 United States
AB: Field investigations into the relationship between acoustic Doppler current profiler (ADCP) bed-velocity measurements and bedload transport rates have served to narrow the range of the possible near-bed phenomena that influence ADCP bottom-track response, and provide a glimpse into the nature of sediment dynamics in the near-bed region of a large sand-bed river. We collected about 80 bedload samples from the lower Missouri River while concurrently recording ADCP, real-time kinematic GPS, and echo-sounder data. We calculated apparent bed velocities using an approach introduced by Rennie et al. (2002), who suggested that the difference between GPS positions and ADCP bottom-track positions represents the velocity of particles moving on or near the stream bed. We found the relationship between ADCP bed velocities and measured bedload transport rates to be non-linear, with a decreasing rate of increase in the bedload transport rate for higher bed velocities. This nonlinearity appears to be generated primarily by steepening of the near-bed velocity gradient at higher transport stages, and is linked to changes in bedload particle size and bedform morphology. An analysis of near-bed acoustic backscatter profiles indicates that water bias, i.e., acoustic backscatter from faster-moving suspended sediments traveling higher in the water column, is a relatively minor contributor to the non-linearity. The acoustical returns recorded by our instrument appear to be influenced by particles traveling at a relatively consistent height above the bed, despite differences in sream flow conditions. Data collection spanned a period of approximately five-months, during which several heavy precipitation events in the region produced highly variable discharge and periodic pulses of tributary sediment inputs. We were therefore able to evaluate the effects of simultaneous changes in both sediment characteristics and hydraulic conditions on ADCP bed velocity measurements.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting