Estimation of Mud and Sand Fractions and Total Concentration From Coupled Optical‐Acoustic Sensors.

Optical turbidity and acoustic sensors have been widely used in laboratory experiments and field studies to investigate suspended particulate matter concentration over the last four decades. Both methods face a serious challenge as laboratory and in‐situ calibrations are usually required. Furthermore, in coastal and estuarine environments, the coexistence of mud and sand often results in multimodal particle size distributions, amplifying erroneous measurements. This paper proposes a new approach of combining a pair of optical turbidity‐acoustic sensors to estimate the total concentration and sediment composition of a mud/sand mixture in an efficient way without an extensive calibration. More specifically, we first carried out a set of 54 bimodal size regime experiments to derive empirical functions of optical‐acoustic signals, concentrations, and mud/sand fractions. The functionalities of these relationships were then tested and validated using more complex multimodal size regime experiments over 30 optical‐acoustic pairs of 5 wavelengths (420, 532, 620, 700, 852 nm) and six frequencies (0.5, 1, 2, 4, 6, 8 MHz). In the range of our data, without prior knowledge of particle size distribution, combinations between optical wavelengths 620–700 nm and acoustic frequencies 4–6 MHz predict mud/sand fraction and total concentration with the variation <10% for the former and <15% for the later. The results also suggest that acoustic‐acoustic signals could be combined to produce meaningful information regarding concentration and mud/sand fraction, while no useful knowledge could be extracted from a combination of optical‐optical pairs. This approach therefore enables the robust estimation of suspended sediment concentration and composition, which is particularly practical in cases where calibration data is insufficient. Plain Language Summary: Crucial decisions to govern the development of an estuary, delta, or coastal zone often rely heavily on the knowledge of where sediment accumulates. Such knowledge primarily comes from long‐term, high‐frequency monitoring of the transport of mud and sand particles in the water column. Optical or acoustic sensors are usually used for this task. Optical/acoustic sensors emit a light/sound beam to the particles and then measure the strength of the reflecting signals to estimate the concentration of the suspension. Since particles with different shape, size, and density respond differently to the light/sound signals, intensive calibrations are required whenever there is a significant change in the water column, for example, during a tidal cycle, seasonal variations between summer and winter. To avoid these tedious calibrations processes, we experimentally show that combination of optical and acoustic sensors in one measurement will help to derive empirical functions which in turn allow us to estimate the ratio of mud/sand and total concentration. Key Points: A new approach of coupling uncalibrated optical and acoustic signals to predict mud/sand fraction and total concentration was proposedWe experimentally show that without knowledge of the suspension estimations of mud/sand ratio and concentration can be as accurate as 10%–15%The new approach is particularly useful in cases where calibration data is insufficient or impractical [ABSTRACT FROM AUTHOR]