Optical and acoustic sensors have been widely used in laboratory experiments and field studies to investigate suspended particulate matter concentration and particle size 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-acoustic signals 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 6 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. This approach therefore enables the robust estimation of suspended sediment concentration and composition, which is particularly useful in cases where calibration data is insufficient.

Sediment fluxes at the estuary-sea interface modulate estuary morphologies and impact particle matter exchanges between marine and continental sources along the land-sea continuum. However, meteorological forcing (e.g., extreme events) and human activities (e.g., estuary deepening) drive pressures on estuary physical functioning, hence threatening estuarine habitats and their ecosystem services. There is an increasing societal need to better predict the potential trajectories of estuarine sediment fluxes resulting from natural and anthropogenic pressures. Nevertheless, it is difficult to derive generalizations from site-specific studies; thus, multi-site approaches appear necessary to move toward a global conceptualization of estuarine sediment transfers. This study explores 10-year numerical hindcasts of three contrasted macrotidal estuaries (Gironde, Loire, and Seine estuaries; France) to disentangle the relative contributions of hydrometeorological and morphological forcing on net sediment fluxes between estuaries and coastal seas. Our results highlight that intense wave events induce fine sediment (≤100 μm) export to the sea but coarser sediment (≥210 μm) import within the estuary. Remarkably, moderate to large river flows support mud import within the estuary. In addition, the Seine Estuary morphological changes due to human activities (i.e., estuary deepening and narrowing) increase fine sediment import within the estuary, shifting the estuary from an exporting to importing system. We propose a conceptualization of mud flux response to river flow and wave forcing, as well as anthropogenic pressures. It provides valuable insights into particle transfers along the land-sea continuum, contributing to a better understanding of estuarine ecosystem trajectories under global changes.

Quantifying and characterizing suspended sediment is essential to successful monitoring and management of estuaries and coastal environments. To quantify suspended sediment, optical and acoustic backscatter instruments are often used. Optical backscatter systems are more sensitive to fine particles ($<63 \mu m$) and flocs, whereas acoustic backscatter systems are more responsive to larger sand grains ($>63 \mu m$). It is thus challenging to estimate the relative proportion of sand or mud in environments where both types of sediment are present. The suspended sediment concentration measured by these devices depends on the composition of that sediment, so it is also difficult to measure concentration with a single instrument when the composition varies. The objective of this paper is to develop a methodology for characterizing the relative proportions of sand and mud in mixed sediment suspensions by comparing the response of simultaneous optical and acoustic measurements. We derive a sediment composition index (SCI) that can be used to directly predict the relative fraction of sand in suspension. Here we verify the theoretical response of these optical and acoustic instruments in laboratory experiments, and successfully apply this approach to field measurements on the ebb-tidal delta of Ameland Inlet in the Netherlands. Increasing sand content decreases SCI, which was verified in laboratory experiments. A reduction in SCI is seen under more energetic conditions when sand resuspension is expected. Conversely, the SCI increases in calmer conditions when sand settles out, leaving behind finer sediment. This approach provides crucial knowledge of suspended sediment composition in mixed sediment environments.