Sediment dynamics in coastal waters has relevance to a broad spectrum of marine science and engineering. Through the processes of accretion, erosion and transport, sediments define our coastlines, marine habitats, and nearshore recreation zones. Seabed sediments critically affect the stability of marine structures, shoreline protection and development, and navigation in coastal waters. Sediments act as reservoirs for nutrients and contaminants and as regulators of light transmission, thereby impacting significantly on water chemistry and primary production. Thus, predicting sediment transport, erosion and deposition is of great practical importance on a global scale, but is also very challenging. The challenge is associated in part with the complex and often non-linear interactions among combined forcing by waves and currents, the dynamic response of the bed, and sediment size and type, as well as the fundamental role of turbulence. Part of the challenge has also been the absence, historically, of suitable technologies for making the comprehensive measurements needed to adequately test or validate the models.
To measure and predict sediment movement requires a detailed understanding of the fundamental physical processes that lead to the transport of sediments. The processes can be thought of as dynamic interactions between: (i) the seabed morphology; (ii) the sediment field; and (iii) the hydrodynamics. These three components relate to each other in complex ways, being mutually interactive and interdependent with feedback; this interaction has been coined the ‘Sediment triad’. To advance our understanding of sediment transport, we need therefore to obtain simultaneous co-located measurements of these three components, in differing sedimentary and hydrodynamic environments and under a range of forcing conditions.
The vision two to three decades ago was that acoustics would offer distinct advantages over optical methods, particularly in the high turbidity conditions encountered during active transport conditions in coastal and continental shelf environments and that it may provide simultaneous measurements of all three components of the sediment triad. Acoustics was considered to have the potential to measure non-intrusively, with high temporal–spatial resolution, co-located profiles of suspended sediment particle size, concentration, the three orthogonal components of flow and bedforms, at intra-wave, intra-ripple and turbulent scales. Much of this potential has been realised.
Initially, single frequency acoustic backscatter systems (ABS), were deployed; these required independent estimates of particle size, to obtain suspended sediment concentration. These were followed by multi-frequency ABS, which coupled with developing theoretical works on scattering and inversion methodologies, provided profiles of concentration and particle size. To assess bedforms, high spatial resolution sector scanners were developed; these provided images of the bed features. In more recent years, these have been superseded by two-dimensional and three-dimensional profilers, which provide a direct measure of the local bed topography. Coupled with these developments have been advances in the utilisation of coherent Doppler technology, leading to systems capable of near-bed profiling of turbulent and intra-wave hydrodynamic processes. Integrated together, these complementary acoustic technologies provide direct access to the fundamental processes composing the sediment triad.
In this Special Issue of Continental Shelf Research, the aim has been to bring together papers which reflect progress in the application of acoustics to sediment transport dynamics. The papers cover turbulent, intra-wave and wave group processes, network data-driven approaches, reformulations of inversions, sediment scattering properties, discrimination of sand and clays and horizontal suspension profiling. By no means does this Special Issue cover all applications of acoustics to sediment transport. However, it does provide an overview of the contemporary use of acoustic devices, by coastal and estuarine scientists, for the study of the dynamic processes of sediment transport.
Finally, looking towards the future, there is still much work to be undertaken. Bedload sediment transport is still difficult to measure, particularly in wave-dominated environments, and acoustics will almost certainly have a role to play. The use of acoustics for estimating sediment concentration in flocculating suspensions is still problematic and requires fundamental studies on the interaction of sound with aggregated fine-grained particles, before quantitative inversions can be formulated. The requirement to examine boundary layer processes in even more detail necessitates further developments in the temporal–spatial resolution of acoustic instrumentation. In most circumstances, bedforms, hydrodynamic and sediment processes are three dimensional. Therefore, the one-dimensional vertical profiles generated using present acoustic systems are limited and the next generation of acoustic technology should expand capabilities into the other spatial dimensions. We look forward to these developments over the coming years.