The Simulation of Bedload Transport Measurement by Coherent Doppler Backscatter

The movement of sediment within the ocean bottom boundary layer is a significant component of total sediment transport. Measurement of this bedload transport is challenging, as mechanical systems inevitably interfere and alter the flow and transport in the boundary, while optical systems have short operating ranges due to the high concentration of sediment in boundary layer, and therefore must be placed intrusively close to the flow. However, an acoustic system has the capability of measuring the transport profile of the bedload from a noninvasive distance on the order of a meter from the bottom. A computer simulation of sediment transport measurement from the acoustic return generated by a bistatic pulse-to-pulse coherent Doppler profiler in the presence of a dynamic bottom boundary layer is used to investigate the effectiveness of an Multi-Frequency Doppler Instrument (MFDop) for field measurements. The performance of the MFDop is investigated by simulating results for various configurations of the instrument's geometry, pulse length, pulse frequencies and bandwidth. Simulation results are compared over a range of sediment transport profiles, where the suspended sediment velocity profile, bedload velocity profile, bedload layer thickness and concentration are independently varied. The limitations of an inversion algorithm for measuring boundary layer sediment properties are investigated by applying it to the simulated results for a variety of transport profiles. Results from lab experiments and simulations are compared in order to asses the accuracy of the simulation.