Measurement of Near-Bed sediment load, particle size, settling velocity and turbulence from a multi-frequency acoustic backscatter instrument

Observations of near-bed sediment suspension processes typically rely on a combination of acoustic and other ancillary sensors to monitor the dynamic suspended sediment environment. In addition to the suspended load, it is also of interest to quantify the settling and resuspension processes. Multi-frequency acoustic backscatter is an established technique for estimating vertical profile time series of mean particle size and concentration. Acoustic Doppler instruments are also used to measure water velocity intrusively at a single point, or in profiles of 3-dimensional velocity. Siting multiple acoustic instruments alongside one another can cause interference, leading to measurements having to be taken with either spatial or temporal separation. In this paper, we describe an approach for monitoring not only suspended load, but also parameters relating to settling velocity and turbulence in a single acoustic instrument with coincident beams. By exploiting the instrument?s capability to acquire a complex backscatter signal, profiles of the velocity of particles resolved along the acoustic beam can be calculated. When coupled with knowledge of particle size, this could be used to assess the density of the suspended particles and identify bubbles and flocs. Further statistical analysis of the velocity profiles has the potential to yield information about the turbulent kinetic energy dissipation rate over various length scales, which in turn can be used to assess the likelihood of floc formation. This has applications in science and industry, including observation of fundamental oceanographic processes, monitoring of coastal and civil engineering operations, and analysis of industrial processes.