Comparison between ADCP and transmissometer measurements of suspended sediment concentration

For more than a decade, acoustic Doppler current profilers, ADCPs, have been in common use measuring current profiles. It has been recognised over this period that the backscattered ADCP signal could be used to not only evaluate the Doppler shift, but also offered the possibility to extract information on the scatterers. The present work reports on an analysis of opportunistic backscatter measurements collected using a 1 MHz ADCP system, to assess the potential of ADCPs to measure suspended sediment concentration quantitatively. The data were gathered during a water monitoring campaign which deployed ADCPs, near-bed and profiling transmissometers, and in situ bottle samplers. Although the original study was not specifically designed to test the capability of ADCPs to evaluate suspended sediment concentration, sufficient data were collected to examine the use of ADCPs for such measurements. The backscattered amplitude from one ADCP beam was recorded for quality control to assess the accuracy of velocity measurements. However, in this study these data have also been used to examine the potential of ADCPs for suspended sediment measurements. To investigate ADCPs in this role, the backscattered signals from one range cell has been calibrated against in situ bottle samples of the suspended material. Using this calibration, the backscattered signals have been inverted to give time series profiles of suspended particulate matter. To assess these profiles, comparisons have been made with in-situ calibrated profiling and moored transmissometers. The outcome from the present study shows ADCP results which are comparable with the transmissometer observations, and clearly demonstrate the potential of ADCPs for directly measuring suspended sediment profiles.