Laboratory Measurements of Coarse Sediment Bedload Transport Velocity Using a Prototype Wideband Coherent Doppler Profiler (MFDop)

A prototype wideband coherent Doppler profiler (MFDop) was tested for measuring bedload velocity of different gravel and coarse-sand-sized fractions (d = 1-32 mm) in the laboratory. The sediment was spread out on a smooth-surface tray, and motion was initiated by tilting the tray at angles of alpha = 20 degrees-39 degrees from the horizontal. Particle velocities downslope (u), cross slope (upsilon), and vertical to the tray (w) were determined for different MFDop parameter settings, such as monostatic/bistatic configuration, acoustic beam angle, and pulse length. Video observations of bed particle velocity were made for comparison to the acoustic measurements. Velocities estimated using the MFDop equal to, on average, 71%-74% of the velocities determined using the video observations. Standard deviations ranged from 21% to 35%, including observed irregular motion. Three stages of sediment motion were observed: (i) single particles moving with u <= 5 cm s(-1), (ii) varying motion of particles and particle groups with predominantly 5 cm s(-1) <= u <= 20 cm s(-1), and (iii) fast sheetlike motion with u >= 20 cm s(-1). The cross-slope velocity upsilon and the vertical velocity w were significantly smaller than u, hinting at slipping as the major particle motion rather than rolling or saltation. Comparisons between MFDop and video-determined velocities showed good agreement. Standard deviations for the MFDop velocity estimates ranged from 22% to 35%. The trials with different gravelly sediments and coarse sand revealed a significant influence of grain size, as well as grain shape impacting the initiation of sediment transport and transport velocities.