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Abstract: Measurements are presented from a multi-frequency acoustic backscatter study of aqueous suspensions of sand sized particles having different and mixed mineralogical compositions. Seven different mineral sands are examined: quartz, crushed shell fragments, aragonite, muscovite mica, olivine, zircon, and magnetite. For each of these sands, measurements of the normalised total scattering cross section and backscatter form function were obtained. These measurements show some significant differences in the acoustic scattering properties between the different mineral sands. If the composition is erroneously assumed to be quartz when inverting the acoustic backscatter data collected from these different mineral sands, relatively poor estimates of mean size and suspended sediment concentration are obtained. To overcome this problem, the scattering measurements are used to construct a generic scattering description by normalising the scattering properties by grain density and averaging over the different minerals examined. This generic scattering description is evaluated by testing its application to the inversion of the acoustic backscatter data obtained from the seven different mineral sands, as well as two different mixtures each containing multiple mineral sands spanning size distributions typical of those encountered in the marine environment. These inversion results show close agreement to independent estimates of suspended sediment concentration and mean size. [Copyright &y& Elsevier]

Abstract: The utilisation of sound backscattered from sediments in suspension, to measure profiles of near-bed particle size and concentration, has been gaining increasing acceptance and usage by sedimentologists and coastal engineers over the past two decades. To obtain the sediment parameters from the backscattered signal requires an inversion to be conducted on the signal and this necessitates a system calibration. The calibration can be carried out by detailed acoustic and electronic measurements, or alternatively by measuring the backscattering from suspensions with known scattering characteristics. Here, we explore the latter approach and describe in some detail the calibration of a triple frequency acoustic backscatter system. The aim is to provide coastal scientists involved in using acoustics as a tool for sediment transport research, with a clear exposition of the calibration process. Suspensions of glass spheres of varying particle size were used as the calibration scatterers. To interpret the signal backscatter from the suspension of glass spheres a simple model for sphere scattering is presented. The results show that consistent calibration results can be obtained in a relatively simple and robust manner. [Copyright &y& Elsevier]

Abstract: Multi-frequency acoustics backscattering has been used for over a decade, to quantitatively measure profiles of suspended sediment particle size and concentration, in the bottom 1–2m above the seabed. Central to obtaining the sediment parameters from the backscattered signal is a description of the scattering characteristics of the particles in suspension. Therefore, formulations are required for the attenuation and backscattering properties of the suspended particles with size and acoustic frequency. There is no single formulation for these scattering properties and different researchers have used somewhat different expressions. However, these expressions are all based on a variation of sphere scattering, modified to fit available scattering data. Here we bring together all the published data on acoustic backscattering and attenuation by suspensions of sandy sediments. The aim is to provide coastal scientists, who use acoustics for sediment transport measurements, with simple expressions which best represent the observed scattering properties of sandy sediments. [Copyright &y& Elsevier]

As part of the U.S. Office of Naval Research (ONR, Arlington, VA) mine burial program, an experiment was conducted off the pier at Santa Cruz, CA, to measure the near-bed suspended sediment reference concentration under waves and currents. Two tripods were deployed to carry out the measurements; one consisting mainly of acoustical instrumentation and the other solely of optical instruments. The tripods were located within 15 m of one another on a sandy bed and measurements of the suspended sediment were made using acoustics and optics. Although the experiment was not primarily designed to conduct an intercomparison of acoustical and optical measurements, it was considered interesting to take advantage of the situation and to examine if these two techniques gave comparable results. In particular, measurements of particle size and concentration, obtained using a triple frequency acoustic backscatter system (ABS) have been compared with the commercially available laser miniature scattering and transmissometry instrument (MSCAT). It was found that the mean grain size estimated by the two methods was consistent; however, in contrast, the concentration time series showed differences, both in magnitude and form. [ABSTRACT FROM AUTHOR]

The simultaneous measurement of current flow and suspended sediment concentration in the marine environment is central to the study of sediment transport processes. In view of this, two acoustic approaches for measuring flow were tested in a tidal estuary to assess their capabilities in this environment. A coherent Doppler velocity profiler and a cross-correlation velocity profiler were assessed using conventional current meters and a commercially available acoustic Doppler velocimeter. Mean velocity profiles were obtained up to a range of 1.47 m in 0.046-m range bins over a number of flood tides. The measurements compared well with the reference instruments and regression analysis produced gradients close to unity. Turbulent velocities measured with the coherent Doppler profiler were comparable with turbulent fluctuations measured with the acoustic Doppler velocimeter. The cross-correlation velocity profiler was shown to be unable to measure turbulent velocities. The backscattered signals received on the cross-correlation transducers were also used to compute the sediment concentration profiles using an explicit solution to the acoustic backscatter equation. Combining the concentration and flow measurements enabled sediment flux profiles to be obtained, the results of which compared favorably with flux measurements obtained from the conventional current meters and pumped sampling. [ABSTRACT FROM AUTHOR]

Over the past two decades the application of acoustics to the measurement of small-scale sediment processes has been gaining increasing acceptance within the sedimentological community. This has arisen because acoustics has the potential to measure non-intrusively, with high temporal and spatial resolution, profiles of suspended sediment size and concentration, profiles of flow, and the bedform morphology. In the present article we review the capability of acoustics to deliver on its potentiality to make a valuable and unique contribution to the measurement of small-scale sediment processes. The article introduces the reasons for using acoustics, the physics underlying the approach, a series of examples illustrating collected data, a discussion on some of the difficulties encountered when applying acoustics and finally a look to the future and possible new developments. [Copyright &y& Elsevier]

The majority of reported field studies, using acoustic backscattering, for the measurement of nearbed suspended sediment processes, have been focussed on field sites with sand size fractions and unimodal size distributions. However, in many sedimentary environments, and particularly for estuaries and rivers, sands and muds coexist in the bed sediment substrate, forming a size regime that is often bimodal in nature. To examine the interaction of sound in these more complex sedimentary environments a numerical study is presented based on observations of sediment size distributions measured in the Dee estuary, UK. The work explores the interpretation of the backscatter signal from a mixed sediment composition in suspension, with mud-sand fractions varying with height above the bed. Consideration is given to the acoustical scattering properties and the inversion of the backscatter signal to extract information on the suspension. In common with most field deployments, the scenarios presented here use local bed sediments for the acoustic inversion of the backscattered signal. The results indicate that in general it is expected that particle size and concentration will diverge from what is actually in suspension, with the former being overestimated and the latter underestimated. • A numerical study explores acoustic backscatter from a suspension of a mud-sand mixture varying with height above the bed. • Changing mud-sand composition above the bed may lead to errors in the acoustic estimates of particle size and concentration. • Using bed samples, for acoustic inversions can leadto overestimates of suspended size and underestimates of concentration. [ABSTRACT FROM AUTHOR]

Improved understanding and modelling of interactions between biological and physical processes has required progress in the understanding of sediment processes and their relationship to bedforms, hydrodynamics and benthic biology. Recently, simultaneous, co-located in situ measurements of suspended sediments, near-bed velocities and bed morphology have been made possible using new acoustic instruments, including a triple frequency acoustic backscatter system, (ABS); a uniaxial, and triple axis, coherent Doppler velocity profiler, (CDVP); a sand ripple imager (SRI); and a sand ripple profiler (SRP). Together these instruments measure the relevant hydrodynamic and sediment parameters necessary to investigate sediment processes in the bottom metre above the bed, with centimetric resolution. The range of data from such instruments is required to fully understand the related processes occurring near the seabed. The outcome of such measurements should improve our understanding of benthic processes in general. [Copyright &y& Elsevier]

The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments. [ABSTRACT FROM AUTHOR]

The RIPCOM (RIPples on COMPLEX) experiments are part of the experimental program developed in the COMPLEX (Cross disciplinary Observations of Morphodynamics and Protective structures, Linked to Ecology and eXtreme events) Joint Research Activity within the EU Hydralab+ project. The main objective of the experiments was to study ripple growth and dynamics in a large wave flume under waves above fine unimodal and mixed sands. The experiments presented were conducted in the CIEM large scale wave flume of Barcelona, having dimensions of 100 m in length, 3 m in width and 4.5 m in depth.The experimental plan was divided into three steps:1. Finding the optimum wave conditions that ensured appropriate scaled ripples (based on measured velocities and previous literature studies) in the study area. Test the targeted waves with unimodal fine sediment (d50=0.250 mm) and measure the ripples obtained under these tested conditions. From the measurements, the waves to be used on the next two steps are selected in order to produce the "best" ripples within the experimental constrains.2. The upper 13 cm of the fine sediment were removed and replaced by a coarser sediment (d50=0.545 mm). The previously selected waves were run while measuring wave height, velocity, suspended sediment concentration and collecting bathymetric information.3. The fine and coarser sediments previously employed were mixed homogeneously along the study area. The selected wave conditions were repeated while hydro-morphodynamic information were collected under the new mixed sediment conditions.The waves tested were selected in order to produce ripples in accordance with different formulation (Nielsen 1981, Mogridge et al. 1994, or Faraci and Foti 2001), but also trying to control the reflection coefficient within the wave flume. Finally wave periods of 5, 7 and 9 seconds with wave heights of 0.25 and 0.3 m were chosen for the three different sediment size distributions.The water surface elevation was measured by means of Resistive Wave Gauges (RWG), Acoustic Wave Gauges (AWG) and Pore Pressure Transducers (PPT). Acoustic Doppler Velocimeters (ADV) and Optical Bacskatter Sensors (OBS) were used to collect information of velocity and suspended sediment concentration. In this presentation the main results will tackle ripple growth and development which were obtained using an Acoustic Ripple Profiler (ARP) and four Echosounders mounted on the flume mechanical profiler.The ripples were formed faster than on previous experiments (Pedocchi and Garcia, 2009), since within the first two hours of waves the ripples were already developed. The measured ripples ranged between small 3D ripples which had a ripple height of 0.05 m and ripple length of 0.5 m, up to 2D ripples with a ripple height of 0.13 m and a ripple length of 0.9 m. The presentation will end with some conclusions and ideas for continuing research on this topic. [ABSTRACT FROM AUTHOR]