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Biologically active, fine-grained sediment forms abundant sedimentary deposits on Earth's surface, and mixed mud-sand dominates many coasts, deltas, and estuaries. Our predictions of sediment transport and bed roughness in these environments presently rely on empirically based bed form predictors that are based exclusively on biologically inactive cohesionless silt, sand, and gravel. This approach underpins many paleoenvironmental reconstructions of sedimentary successions, which rely on analysis of cross-stratification and bounding surfaces produced by migrating bed forms. Here we present controlled laboratory experiments that identify and quantify the influence of physical and biological cohesion on equilibrium bed form morphology. The results show the profound influence of biological cohesion on bed form size and identify how cohesive bonding mechanisms in different sediment mixtures govern the relationships. The findings highlight that existing bed form predictors require reformulation for combined biophysical cohesive effects in order to improve morphodynamic model predictions and to enhance the interpretations of these environments in the geological record. Subaqueous bed form dimensions are controlled more by biological than physical cohesionExisting predictors require reformulation for combined biophysical cohesive effectsSmall amounts of substrate biological cohesion act to reduce bed roughness 2 orders of magnitude

Estuarine salt intrusion greatly threatens freshwater supply in surrounding lands. Physical barriers, which reduce the estuary length (L), are widely constructed to control salt intrusion. Yet, the role of Lin salt intrusion remains unknown. Using a process‐based, idealized, semi‐analytical three‐dimensional model, we systematically investigate for the first time this unknown for tide‐dominated, periodically weakly stratified estuaries. Results show decreasing Lsignificantly reduces salinities for short estuaries (L< Lw/4, with Lwthe dominant tidal wavelength), but not for long estuaries. Tidal pumping remains a key salt importer in most estuaries, regardless of L. However, substantial decreases in Lrelative to Lw/4 can change the dominant landward salt importer from tidal pumping to horizontal diffusion. The latter, together with gravitational circulation, weakens responses of salt intrusion to changes in tidal and river forcing in short estuaries. This study highlights the importance of considering Lto understanding and mitigating salt intrusion. Salty seawater can enter rivers through estuaries, where rivers meet the sea, thus affecting the surrounding freshwater supply and the healthy functioning of estuarine ecosystems. To mitigate the adverse impact of salt intrusion under aggravating climate change and human activities, expensive salinity barriers are commonly built around the world. These barriers result in an abrupt change in the estuary length and can substantially affect estuarine salt intrusion. This study uses a simplified computer model to understand the dependency of salt intrusion on estuary length in estuaries with small top‐to‐bottom salinity differences. Our results show that estuary length substantially influences the salinity distribution and estuaries of different lengths can respond to changes in tides and river flow contrastingly. Our study highlights the importance of considering the impact of estuary length to effectively mitigate estuarine salt intrusion. Estuary length strongly affects tidal propagation, residual circulation, and salt intrusion in periodically weakly stratified estuariesThe dominant salt balance may change when substantially reducing estuary lengthSteady lateral shear dispersion can reduce responses of salt intrusion to forcing changes in short estuaries with weak stratification Estuary length strongly affects tidal propagation, residual circulation, and salt intrusion in periodically weakly stratified estuaries The dominant salt balance may change when substantially reducing estuary length Steady lateral shear dispersion can reduce responses of salt intrusion to forcing changes in short estuaries with weak stratification

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.

In recent years the application of resonance scattering theory, RST, to the interaction of underwater sound with elastic bodies, has yielded a description of the scattering process that has provided valuable insight into the underlying mechanisms of scattering. Comparisons of this approach with observations have usually been carried out in the frequency domain, using the form function notation. In the present study results are reported of an examination of the backscattered echo from a selection of spheres conducted both in the time domain and the frequency domain. The structure of the background and resonance components of the backscattered signal are investigated, and examined in both domains. To obtain the measurements a linear frequency‐modulated signal or chirp was employed. Using a Fourier analysis approach, with pulse compression, measurements have been taken in the time domain, which readily show the components of the backscattered echo. Using these data, sections of the record have been separately transformed to obtain in the frequency domain the background and resonance form functions. Further, time‐frequency plots have been generated to examine the association between the time domain echo and the form function structure. To compare the observations with predictions, resonance scattering theory has been applied both in the time domain and the frequency domain.

The application of acoustic backscattering to the measurement of suspended sediment transport is being increasingly employed. This is due to the unique property of acoustics being able to measure profiles using a single instrument. Also the profiles can readily be obtained with a high spatial and temporal resolution, and this is beginning to provide detailed images of suspended sediments which have the potential to significantly advance our understanding of suspension processes. However, at periods of high concentration, when sound attenuation due to sediment scattering becomes significant, the inversion technique used to obtain the suspended sediment profiles, can, due to relatively small errors ∼5%–10%, generate unbounded concentration profiles which diverge to infinity or zero. Therefore at the periods of greatest scientific interest, the acoustic technique can encounter difficulties. In the present work the bed echo is utilized to identify the input errors, and constrain the calculated concentration profiles, and therefore enable acoustics to play a useful role at high concentration levels. © 1995 Acoustical Society of America.

Although many scatterers in nature are irregular in shape, the data on the scattering of sound by asymmetrical bodies is relatively limited. Our understanding of the scattering of underwater sound by nonregular shaped targets is therefore significantly less developed than that due to scattering by scatterers of symmetry. In the present work, measurements are reported on the backscattering from irregularly shaped solid elastic particles in both the time and frequency domains. Backscatter data have been collected for a variety of irregularly shaped particles at varying orientations. These observations show a number of characteristics which are comparable with scattering by targets of symmetry, particularly the property of the scattered echo being significantly longer in duration than the insonifying pulse. This pulse elongation for symmetrical targets is normally interpreted using resonance scattering theory, RST, as arising from radiation due to surface eigen waves on the body of the scatterer. In the present work the interpretation of the scattering by irregular shaped targets has been guided by resonance scattering studies, and an understanding of the response formulated within this framework.

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.

Although the seabed has not generally been noted as an active originator of underwater sound, it can in some circumstances behave as an acoustic source, and contribute to marine ambient noise conditions. This arises when near‐bed current flow, induced by tides and waves, exerts sufficient shear stress at the bottom boundary to cause the bedload transport of sediments. Interparticle collisions of this mobile material occur and sound is generated by particle impacts. To understand this addition to ambient noise, an examination of this source of sound generation has been conducted in the laboratory and at sea. The laboratory observations were taken using colliding sediments covering size fractions from medium sand to large cobbles. The marine records of ambient noise conditions were obtained over a mobile shingle bed. Comparison of predictions based on rigid body radiation theory with the data has been carried out and similar results obtained.

Measurements of the acoustic backscattered amplitude, attenuation, and suspended concentration have been recorded for a number of suspensions consisting of spheres with a known particle size distribution. These data have been employed to obtain the form function and total scattering cross section for suspensions of spheres. Calculations for these parameters are compared with the observations and the effect of having a size distribution is examined.

The application of acoustics for measuring suspended sediment transport processes potentially offers the opportunity for examining suspensions with a spatial and temporal resolution sufficient to probe turbulent and within wave sediment mechanisms. This should provide a significant advance in the understanding of the coupling between the fluid dynamics and the sediment response. Although acoustic backscattering devices have been employed by sedimentologists for over a decade to investigate marine suspensions, it is only relatively recently that a theoretical framework for interpreting the interaction of sound with marine suspensions has become available. In the present work a comparison is made between laboratory measurements of the backscattering and attenuation from a suspension and recent developments based on incoherent first‐order multiple scattering. This analysis is also applied to acoustic data collected in an estuarine environment over sandwaves under turbulent tidal conditions. Acoustic estimates of the suspended concentration have been calculated and compared with independent tranditional measurements of the suspended concentration. The two observations of suspended sediments are shown to be in good agreement and this has provided confidence for calculating high spatial and temporal resolution estimates of suspended load. The latter display a highly variable concentration field and the relationship between this variability and turbulent structures is speculated upon. [Work supported by ARL:UT, NOARL, and MAFF UK.] a)On leave from the Proudman Oceanographic Laboratory, Merseyside L43 7RA, UK.

The present study focuses on the fine‐scale flow and sand transport processes above onshore migrating ripples below skewed surface gravity waves in the shoaling zone. A set of acoustic instruments was deployed in the shoaling region of the large‐scale wave channel at Canal d'Investigacío i Experimatacío Marítima, Universitat Poltiècnica de Catalunya, Barcelona, Spain, in order to provide high‐resolution velocity and sediment concentration profiles with an acoustic concentration and velocity profiler (ACVP). Measurements are analyzed relative to the positions of the measured nonmoving sand bed and the interface separating the suspension from the near‐bed load layer. This interface is detected here by the application of a novel acoustic bed echo detection method. Furthermore, the use of the dual‐frequency inversion proposed in the work of Hurther et al. (2011) allows for the calculation of the sediment concentration profile across both the suspension and near‐bed load layers. The sand bed was covered by quasi‐two‐dimensional suborbital ripples migrating onshore. As proposed by O'Donoghue et al. (2006), the occurrence of quasi‐two‐dimensional ripples is attributed to the fine‐size sand of D50= 250 μm used in the present study under full‐scale forcing conditions. In order to determine the effect of shoaled wave skewness on the ripple vortex entrainment and sediment transport, the instantaneous and mean measurements of the flow, sediment concentration, and sediment flux along the ripple profile are discussed in terms of (1) the occurrence of ripple vortex entrainment on either side of the ripple crest; (2) the wave velocity phase lagging driven by the ripple vortex entrainment process and the turbulent bed friction effects in the wave boundary layer; (3) phase lagging between velocity and maximum concentration and sediment flux events; (4) the structure of bed friction and ripple‐driven turbulence across the suspension and the near‐bed load layers; and (5) the streaming components. The results on these aspects strongly support that the wave velocity skewness effect under shoaling waves is fairly similar to the one obtained in skewed oscillatory water tunnel flows. Furthermore, it is found that the onshore‐oriented net bed load sediment transport is at the origin of the onshore ripple migration. This flux is roughly twice as much as the opposite offshore‐oriented net suspension flux dominated by the ripple vortex entrainment processes. Wave skewness effect in the shoaling on ripple vortex‐dominated sand transportNovel acoustic flow measuring system combining ADVP and ABS technologies

In general, descriptions of suspended sediment transport beneath surface waves are based on the turbulent diffusion concept. However, it is recognised that this approach is questionable for the suspension of sediment when the seabed is rippled. In this case, at least if the ripples are sufficiently steep, the entrainment process is likely to be well organised, and associated with vortex formation and shedding from the ripples. To investigate the entrainment process above ripples, a study was carried out in a large‐scale wave flume facility. Utilising acoustic techniques, visualisations of the intra‐wave sediment entrainment above vortex ripples have been generated. The observations provide a detailed description of entrainment, which is interpreted here in relation to the process of vortex formation and shedding. It is anticipated that such measurements will contribute to the development of improved physical process models of sediment transport in the rippled bed regime.

Scattering by spheres is a classical problem in underwater acoustics, and developments over the past two decades have established a detailed description of the scattering process in terms of the form function structure. More recently, interest has focused on sound scattering by spheroids and a number of studies have successfully applied the T ‐matrix formulation to solving the scattering problem. Currently the scattering of sound by irregular bodies is receiving increasing attention because of the application of acoustics in studying suspended sediment processes, the effect of irregular bodies on sea bottom scattering, and the use of acoustics in medical physics. In the present study a series of measurements have been taken on a number of solid irregular bodies over a broad frequency range to examine their scattering behavior. A statistical approach has been adopted to measure the ensemble average form function for the irregular bodies and these observations are compared with predictions based on a semiheuristic approach to solving the scattering problem.