Remote determination of suspended sediment size and concentration by multi-frequency acoustic backscatter

Sheng, Jinyu (1991) Remote determination of suspended sediment size and concentration by multi-frequency acoustic backscatter. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (12MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Suspended sediment concentration and size are obtained simultaneously from multi-frequency acoustic profiling data collected in the nearshore zone with the newly developed Remote Acoustic Sediment TRANsport (RASTRAN) System 1. It appears to be the first time that both suspended sediment size and concentration have been obtained in the ocean from acoustic profiling data. The inversion method for data analysis is based on a semi-empirical expression for the scattering cross section of sand. Straight stainless steel wires are used as standard targets to determine the acoustic sounder sensitivities. Laboratory measurements, with a suspended sediment jet, and four periods of field measurements from Stanhope Beach, Prince Edward Island are analysed. It is demonstrated that in the laboratory measurements the systematic error is about +12% in concentration, and about -19% in size. The RASTRAN concentration data from the field experiments are in reasonably good agreement with Optical Backscattering Sensor (OBS) measurements at the same height above bottom. The time-mean profiles of particle concentration for the four selected field runs follow a power law in the nearbed region, above which the concentration profile can be expressed mainly by an exponential function. Suspended particle sizes are more uniform than concentration in the vertical. Both size and concentration profiles vary significantly with the wave energy level. The vertical profile of sediment eddy diffusivity Ks is calculated from the time-mean profiles of particle concentration and size by assuming a balance on average between vertical diffusion and settling. It is shown that the sediment eddy diffusivity is a linear function of height in the nearbed region, then decreases slightly with height. It is demonstrated that the horizontal wave flux affects the total suspended sediment transport, while the vertical wave flux has important effects on the vertical profiles of sediment eddy diffusivity. By eliminating the effects of vertical wave flux from Ks, it is found that the vertical profile of eddy diffusivity due to turbulence is analogous to the profile of eddy viscosity proposed by Smith and McLean.

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