Field deployment and evaluation of a prototype autonomous two dimensional acoustic backscatter instrument: The Bedform And Suspended Sediment Imager (BASSI)

The processes of sediment entrainment, transport and deposition over bedforms are highly dynamic and temporally and spatially variable. However, most measurements of these processes tend to be collected in a vertical line at one spatial location above the bed; one dimensional in the vertical, 1D-V. Such measurements capture the temporal signature, and, to a lesser degree, the spatial variability when bedforms migrate, and they have contributed greatly to our understanding of sediment transport processes. It is generally acknowledged, however, that such 1D-V systems provide a limited description of the spatially three dimensional processes occurring at the turbulent and intra-wave time scales. It would undoubtedly facilitate the interpretation of fundamental sediment processes above bedforms if other spatial dimensions could be simultaneously interrogated. To this end a multi-frequency acoustic array has been developed to measure suspended sediments and bedforms over a horizontal transect of the bed in the vertical, providing two dimensional observations in the vertical and horizontal, 2D-HV. This new acoustic instrument, the BASSI; Bedform And Suspended Sediment Imager, has been deployed in the River Dee tidal estuary in the UK. The design of the BASSI and results from the deployment are presented and its performance assessed against more conventional instrumentation. Measurements and images of 2D-HV suspended sediments and bedforms are provided to illustrate the capability and future use of the BASSI for the investigation of sediment transport processes.