Using a Down‐Looking Multifrequency ABS for Measuring Suspended Sediments in Rivers

In the last decade, much progress has been made in continuously measuring suspended sediment concentration (SSC) in rivers using horizontal side‐looking Acoustic Doppler Current Profilers. However, these techniques do not provide information on the spatial variability of the suspension. In this study, we explore some new possibilities offered by the down‐looking deployment of a multifrequency acoustic backscatter system (ABS) in order to obtain information on the suspension throughout an entire river cross section. Two sites, with low and high levels of SSC, were investigated. The acoustic signal was processed using multifrequency inversion methods. Both water sample calibration data and modeling were used for retrieving the acoustic properties of the suspended particles. At the low SSC site ( ∼30 mg/L), we successfully inverted the acoustic signal, except in some areas of high acoustic backscatter close to the surface that were probably generated by air microbubbles. At the high SSC site ( ∼10 g/L), estimates of both fine and sand SSCs throughout the river cross section were successfully obtained, except in some areas close to the bottom where the acoustic signal was totally attenuated due to distance and high concentration of fine sediments. This work confirms the capacity of hydroacoustic technology for providing spatial information on river suspensions. Key Points Multi‐frequency inversion methods were applied to down‐looking multi‐frequency ABS Inversion of bimodal river suspension was successful for high fine and sand concentrations Acoustic response was not always dominated by solid suspended particles for low concentration