Peter D. Thorne

A review of acoustic measurement of small-scale sediment processes

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.

Analysis of acoustic measurements of suspended sediments

Estuarine measurements of suspended sediments over sand waves have been taken with the objective of clarifying the role of turbulence in suspension transport, sand waves development, and migration. To discern the mechanisms of transport, observations of the turbulent flow were taken and records of the response of the suspended sediment concentration were obtained using acoustic backscattering (ABS). As an intermediate goal of the overall objective an analysis of the ABS data has been conducted with the aim of assessing and quantifying the acoustic technique. Recent theoretical descriptions of the interaction of sound with suspensions are presented, and predictions are compared with a laboratory study on suspensions of spheres and noncohesive sediments. The laboratory data show the theoretical approach to be broadly correct, and it has been implemented to interpret the estuarine observations. Acoustic estimates of the suspended sediment concentration have been compared with pumped sample data and shown to be similar. Comparison of the laboratory and estuarine estimates for the form function and total scattering cross section for the estuarine sediments is presented, and some unresolved differences are considered. The effects of the sediment attenuation and the random amplitude of the backscattered signal are also highlighted, and their implications regarding acoustic estimates of the suspended sediment concentration are addressed. Finally, after assessing the accuracy of the acoustic approach, high-resolution estimates of the suspended concentration are presented with some speculation on the origins of the concentration variability observed.

Backscattering from a suspension in the near field of a piston transducer

The problem of acoustic backscattering from suspended particulates in the near field of a piston source is examined. A monostatic configuration is used, with the transducer acting as a transceiver. Predictions for the range dependence of the backscatter signal are calculated, and formulated into a nondimensional general form. To compare predictions with observations, a series of laboratory experiments, using a number of transducers insonifying varying homogeneous suspensions, has been conducted. To first order the outcome shows good agreement between prediction and observation.

Acoustic measurements of suspended sediments in turbulent currents and comparison with in-situ samples

Increasingly in recent years the application of acoustic backscattering to the quantitative measurement of suspended sediment particle size and concentration at sea has gained acceptance. A number of works describing the interaction of sound with suspensions have been published, and the scattering properties of suspended sediments formulated. However, there have been relatively few experiments conducted in the marine environment, which have attempted to assess the accuracy of the acoustic measurements by direct comparison with in-situ samples, taken simultaneously with the acoustic observations. The purpose of the present work is to report on such an experiment, and to evaluate the accuracy of the acoustic technique. To this end multifrequency acoustic measurements of suspended sediment profiles were collected in an estuarine environment, subject to strong turbulent tidal currents, which generated high concentrations of suspended sediments. To obtain the sediment parameters from the acoustic data an inver...

Backscattering by a suspension of spheres

Predictions of backscattering from a suspension of nominally spherical particles are compared with a series of experimental observations. Initially measurements were taken on single particles to establish the form function for the glass spheres employed and comparisons are made with calculations based on resonance scattering theory. The form function description of sphere scattering has been employed to describe the interaction of sound with a suspension of spheres, with first‐order multiple scattering being included by accounting for the attenuation introduced by the presence of the suspension itself. The study is in part stimulated by the application of acoustic backscattering for measuring suspended sediment concentrations in the marine environment and some discussion is presented on this topic.

Laboratory and marine measurements on the acoustic detection of sediment transport

This paper describes a series of measurements taken both in the laboratory and at sea, to examine the feasibility of monitoring marine gravel transport, by measuring the acoustic self‐generated noise (SGN) produced by the interparticle collisions of bedload material. This study is a continuation of work previously reported on the SGN of agitated artificial sediments. Laboratory measurements are presented on the relationship between the SGN spectrum and particle size for naturally occurring gravels undergoing interparticle collisions. The dependence of the acoustic intensity level upon the mass of agitated material is also studied. Attempts are made to utilize the acoustic spectrum to obtain information on the particle size distribution for poorly sorted gravel samples. Using the laboratory results, a marine system was developed consisting of two hydrophones to monitor the SGN, an underwater TV camera to independently assess bedload transport, and electromagnetic current meters to obtain information on the flow condition. The instrumentation package was deployed on a seabed composed of mobile gravels. The results of an intercomparison between the acoustic and visual data are presented. Also, some discussion on the relationship between flow conditions and gravel movement is pursued. Attempts are also made to obtain the size distribution of the mobile material by an examination of the SGN spectrum taken at sea.

The pervasive role of biological cohesion in bedform development

Sediment fluxes in aquatic environments are crucially dependent on bedform dynamics. However, sediment-flux predictions rely almost completely on clean-sand studies, despite most environments being composed of mixtures of non-cohesive sands, physically cohesive muds and biologically cohesive extracellular polymeric substances (EPS) generated by microorganisms. EPS associated with surficial biofilms are known to stabilize sediment and increase erosion thresholds. Here we present experimental data showing that the pervasive distribution of low levels of EPS throughout the sediment, rather than the high surficial levels of EPS in biofilms, is the key control on bedform dynamics. The development time for bedforms increases by up to two orders of magnitude for extremely small quantities of pervasively distributed EPS. This effect is far stronger than for physical cohesion, because EPS inhibit sand grains from moving independently. The results highlight that present bedform predictors are overly simplistic, and the associated sediment transport processes require re-assessment for the influence of EPS.

Field observations and predictions of bed shear stressesand vertical suspended sediment concentration profiles in wave-current conditions

During a study of sediment dynamics at an offshore field site adjacent to Middlekerke Bank, Belgium, high-frequency measurements of turbulence and vertical profiles of the time-averaged suspended sediment concentration, C, were obtained in the bottom 1.2 m of the water column above a rippled bed in a water depth of approximately 20 m using the autonomous multisensor instrument STABLE. During the experiment, a combination of large waves and strong currents resulted in the resuspension and transport of bottom sediments. Values for the physical roughness of the sea bed, ks, have been derived. Estimates of the bed shear stress attributable to currents in the presence of waves, τc(tke), and the peak wave-only bed shear stress, τw, have been obtained using the turbulent kinetic energy (tke) method and linear wave theory, respectively, and have been combined to obtain peak, τwc, and time-averaged, τwc, wave-current (w–c) bed shear stress values for gain- and ripple-scale roughness using existing models. A new semi-empirical expression giving accurate prediction of measured vertical C profiles for a wide range of w–c conditions has been derived. Using τw, τwc and ks values as input parameters to the expression, estimates of the dynamic in situ grain settling velocity, wave mixing coefficient and total diffusive bed shear stress that agree well with previous measurements and with theory have been obtained. Results indicate it may now be possible to predict vertical C profiles, and hence suspended sediment transport rates, with knowledge of flow turbulence, wave orbital motion and C measured accurately at only one location near the bed.

The measurement of acoustic noise generated by moving artificial sediments

Laboratory measurements relating to the acoustic detection of marine sediment transport are reported. A series of underwater measurements have been conducted on agitated artificial sediments, and the acoustic noise generated by interparticle collisions monitored. Measurements of the total acoustic pressure and spectral pressure levels were made on a wide range of particle diameters for different values of the mass of agitated material, the collision velocities, and the swept bandwidth of the spectrum analyzer. The objective was to relate the total pressure level to the mass of mobile material and ascertain if there was an acoustic spectral signature unique to particle size. This information could then be used to guide the interpretation of acoustic measurements of bedload transport taken in the marine environment and provide specifications for the development of an acoustic system to monitor remotely the material moving along the seabed.

Observations and analysis of sediment diffusivity profiles over sandy rippled beds under waves

Acoustic measurements of near-bed sediment diffusivity profiles are reported. The observations were made over two sandy rippled beds, classified as ‘medium’ and ‘fine’ in terms of sand grain size, under slightly asymmetric regular waves. For the medium sand, the ripples that formed had relatively steep slopes, while for the fine sand, the slopes were roughly half that of the medium sand. In the medium sand case, the form of the sediment diffusivity profiles was found to be constant with height above the bed, to a height equal approximately to the equivalent roughness of the bed, ks, while above this the sediment diffusivity increased linearly with height. For the case of the fine sand there was no constant region; the sediment diffusivity simply increased linearly with height from the bed. To understand the difference between the respective diffusivity profiles, advantage has been taken of the high temporal-spatial resolution available with acoustic systems. Using intra-wave ensemble averaging, detailed images have been built up of the variation in concentration with both the phase of the wave and also height above the bed. These intra-wave observations, combined with measurements of the bed forms and concepts of convective and diffusive entrainment, have been used to elucidate the mixing mechanisms that underlie the form of the diffusivity profiles observed over the two rippled beds. These mechanisms centre on coherent vortex shedding in the case of steeply rippled beds and random turbulent processes above ripples of lower steepness.