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.

Coral reefs represent an efficient natural mechanical coastal defense against ocean waves. The focus of this study is the La Saline coral reef, located in the West of La Reunion Island in the Indian Ocean. The area is microtidal and frequently exposed to Southern Ocean swell as well as cyclonic events. The objective of this paper is to provide a better understanding of the coastal defense characteristics of the reef system for a range of Southern Ocean swell events and tides. Pressure sensors were placed across the reef to measure water level fluctuations and to determine gravity wave and infragravity wave components and their transformation across the coral reef. A numerical model (XBeach surf beat), validated using field observations, was used to deepen understanding of wave transformation, wave setup and runup. Field measurements and model outputs show that as gravity waves break over the reef, the reef acts as a low-pass filter. Study results also suggest frequency-dependent dissipation of infragravity waves by bottom-friction. The resulting wave-induced setup is found to be the dominant hydrodynamic component. The setup and runup are each 95% and 71% driven by the significant wave height (HS) with which they increase, and transfer functions relating incident wave characteristics to reef system hydrodynamics are proposed. At a semi-diurnal tidal timescale, the setup and runup are in anti-phase, as the runup is highest in conditions of reduced wave dissipation on the reef flat, corresponding with high tides. These conditions also result in a lower wave setup.

The morphodynamic response of the Dutch Wadden Islands to the effects of climate change (e.g. sea level rise) or human interventions (e.g. nourishments) is closely tied to the evolution of the ebb-tidal deltas between them. To understand the fate of these ebb-tidal deltas, we must quantify the behaviour and transport patterns of sediment as it moves across them. In September 2017, 2000 kg of dual signature (fluorescent and ferrimagnetic) sediment tracer was deployed on the seabed at Ameland ebb-tidal delta in the Netherlands. The tracer’s physical characteristics (d50= 285 μm, ρ = 2628 kg/m3) closely matched those of the native sediment to ensure that it was eroded, transported and deposited in a similar manner. The tracer study was complemented by simultaneous measurements of hydrodynamics and suspended sediment at four locations across the ebb-tidal delta. Over the subsequent 41 days, the tracer’s dispersal was monitored via the collection of seabed grab samples and determination of tracer content and particle size within each sample. In addition, high-field magnets mounted on mooring lines 1, 2, and 5 m above the seabed at strategic locations around the deployment site were used to sample tracer particles travelling in suspension. Tracer particles were recovered from over 60 of approximately 200 samples, despite the occurrence of two significant storm events (Hs > 4 m). Although hydrodynamic measurements suggest an eastward tidal residual flow, the spatial pattern of the recovered tracer indicates that transport is highly dispersive, likely due to the storms. Furthermore, the samples recovered from the suspended magnets show an upward fining trend in grain size through the water column. The active sediment tracing approach provides useful insight into sediment transport patterns and sorting processes in energetic coastal environments. The study also demonstrated the potential of dual signature sediment tracers to monitor sand nourishment effectiveness. In particular, the use of magnets proved highly effective at sampling tracer travelling in suspension, enabling both bed load and suspended load transport processes to be investigated. The data obtained through this study will serve as a basis for future numerical model calibration and validation.

The curious undular bore// Propagates onward to shore// The energy flies// From low freqs to high// Until the wavefront is no more

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.

Connectivity provides a framework for analyzing coastal sediment transport pathways, building on conceptual advances in graph theory from other scientific disciplines. Connectivity schematizes sediment pathways as a directed graph (i.e., a set of nodes and links). Existing techniques in graph theory and network analysis provide a low barrier to entry for using connectivity to quantify complex coastal systems, exemplified here using Ameland Inlet in the Netherlands. We divide the study site into geomorphic cells (i.e., nodes), and then quantify sediment transport between these cells (i.e., links) using a numerical model. The system of cells and fluxes between them are then schematized in a network described by an adjacency matrix. Network metrics like link density, asymmetry, and modularity quantify system-wide connectivity. The degree, strength, and centrality of individual nodes identify key locations and pathways through the system. These metrics allow us to address fundamental questions about sediment bypassing of Ameland Inlet and the optimal placement of sand nourishments. Connectivity thus provides a novel and valuable technique for predicting the response of our coasts to climate change and the human adaptations it provokes.