Remote acoustic measurement of the velocity within water-immersed gravity-driven granular flows

Measuring bedload transport at high spatial and temporal resolution in energetic aqueous environments is challenging. Acoustic remote-sensing technologies are attractive because the measurement can be made without disturbing the mobile bed or the near-bed flow. Of particular interest is the development of broadband MHz-frequency acoustic systems capable of simultaneous measurements of backscatter amplitude and phase at mm-scale range resolution and 100 Hz sampling frequencies. Using such an instrument, we study granular flow in a water-submerged rectangular chute. By releasing sediments in the upstream portion of the chute, a O(1)cm-thick layer of avalanching sediment is produced. Trials were carried out for both erodible and fixed roughness beds. Natural sand and glass beads with median grain sizes ranging from 0.22 to 0.4 mm were used. The thickness of, and velocity profile within, the moving layer were measured using a wide bandwidth coherent Doppler profiler operating at 1.2 MHz. The velocity profiles are compared to estimates made with video imagery through the chute sidewall. The velocities at the sediment-water interface are compared to estimates made with a commercially available Doppler profiler (Vectrino) operating at 10 MHz and with imagery from a submerged video camera.