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The vertical structure of surface gravity wave‐induced pore pressure is investigated within the intertidal zone of a natural, steeply sloping, megatidal, mixed sand‐gravel‐cobble beach. Results from a coherent vertical array of buried pore pressure sensors are presented in terms of signal phase lag and attenuation as functions of oscillatory forcing frequency and burial depth. Comparison of the observations with the predictions of a theoretical poro‐elastic bed response model indicates that the large observed phase lags and attenuation are attributable to interstitial trapped air. In addition to the dependence on entrapped air volume, the pore pressure phase and attenuation are shown to be sensitive to the hydraulic conductivity of the sediment, to the changing mean water depth during the tidal cycle, and to the redistribution/rearrangement of beach face material by energetic wave action during storm events. The latter result indicates that the effects on pore pressure of sediment column disturbance during instrument burial can persist for days to weeks, depending upon wave forcing conditions. Taken together, these results raise serious questions as to the practicality of using pore pressure measurements to estimate the kinematic properties of surface gravity waves on steep, mixed sand‐gravel beaches. First‐time measurements of wave‐induced pore pressure on a steep, mixed sand and gravel beach compare well with model predictionsPore pressure phase lags are proportional to mean water depth, and persist through the spring‐neap cyclePhase lag is due to entrapped air, and is sensitive to wave‐forced changes in hydraulic conductivity

Analytical results are obtained for the amplitude of a spherical wave backscattered from a solid cylinder immersed in water. The effects of transducer directivity and transmitted pulse duration are included. Backscatter measurements for comparison with theory are made using straight stainless steel wires ranging from 0.13 to 0.26 mm diameter, and acoustic transceivers operating at 1, 2.25, and 5 MHz. The results are used to evaluate the use of thin wires as standard targets.

A method using a broadband transducer to measure the acoustic backscatter cross section of suspended sand particles is investigated. The frequencies used range from 1.3 to 2.8 MHz, and the sand sizes from 100‐ to 350‐μm diameter. The measurements are made in the transducer near field. The measured form factor is compared with the theoretical result for the movable rigid sphere model, and with previous narrow‐band measurements.

A closed‐form expression for the pressure field produced by a circular piston radiator with simply supported boundary condition and continuous‐wave excitation is derived. The method of Hasegawa etal. [J. Acoust. Soc. Am. 74, 1044–1047 (1983)], developed for a uniformly vibrating piston, is extended to include the nonuniform case. The resulting expression is valid for all field points and for large ratio of source diameter to wavelength. The average pressure on the surface of a circular receiver as a function of receiver size and distance from the transducer is also calculated.

An inversion algorithm for extracting suspended sand size and concentration from simultaneous backscattered acoustic pressure amplitude at three operating frequencies is presented. The algorithm is based on the differences in signal amplitude between different frequency pairs, and is tested using laboratory measurements of multifrequency backscatter from a turbulent sediment‐carrying jet. Concentration and size profiles inverted from field and laboratory data are compared with results from a previously developed algorithm based on signal ratios. The difference inversion scheme is less sensitive to errors arising from low signal levels, allowing the size/concentration measurement range to be extended to regions of lower concentration. The concentrations from the field data agree well with independent optically determined estimates. The results demonstrate sensitivity to the backscatter form factor.

Multifrequency acoustic scattering experiments were carried out in the laboratory using a free, turbulent water jet carrying solid particles in suspension. A crossed‐beam geometry was used, in which the axis of the jet was perpendicular, or nearly so, to the direction of the incident sound. The suspended particles were either natural sand grains or lead‐glass beads, 100–500 μm in diameter. The results, based on measurements made simultaneously at 1, 2.25, and 5 MHz, include backscatter and attenuation as a function of suspended sediment concentration, measurements of the total scattering and backscattering cross sections of natural sand grains and glass beads as a function of frequency and particle size, and two‐point correlation measurements of the fluctuations in apparent suspended sediment concentration. The lead‐glass bead cross sections exhibit both the resonance and diffraction extrema expected of solid spherical scatterers; the sand cross sections do not and, for acoustic wavelengths comparable to or less than the particle circumference, are larger than spherical scatterer theory would predict. The two‐point correlation estimates of jet velocity and measurements of the time‐averaged jet width as a function of particle size are related to the mean and turbulent structure of two‐phase jets and illustrate the potential of acoustic methods for noninvasive investigations of two‐phase turbulent flow.

Prior studies demonstrate that bottom velocity measurements from Doppler sonar systems are proportional to bedload transport rates. These observations suggest that acoustically based systems offer a capability for rapid sampling of bedload transport processes. Before these measurements can be fully utilized, validation and understanding of the sampling mechanism are essential. We explore the measurement mechanism through a series of laboratory trials with a field instrument, the multi‐frequency coherent Doppler profiler (MFDop). The MFDop system is a multi‐frequency (1.2–2.2 MHz), bistatic Doppler sonar that provides three‐component ensemble‐averaged velocity profiles over a ∼30 cm depth interval with up to 1 mm resolution at a rate of 50 profiles/sec. Tests of the MFDop system were carried out in the main flume in field‐scale conditions at the St. Anthony Falls Laboratory (SAFL) using 1 ms−1mean flows over a mobile bed of sand with median grain size d50= 0.4 mm. We find agreement between MFDop transport measurements and measurements based on bedform migration rates, and sediment traps built into the SAFL flume. Predictions using the Meyer‐Peter and Müller (1948) empirical equation closely match our observations while in contrast, predictions using the Nielsen (1992) equation are a factor of two higher. Field‐scale laboratory trials demonstrate that bedload transport can be measured by bistatic coherent Doppler sonarAcoustic, bedform migration, sediment trap, and Meyer‐Peter & Muller estimates of bedload all agree Field‐scale laboratory trials demonstrate that bedload transport can be measured by bistatic coherent Doppler sonar Acoustic, bedform migration, sediment trap, and Meyer‐Peter & Muller estimates of bedload all agree

Sediment transport in the oceanic bottom boundary layer involves threefold coupling among: (1) the motion of sediment grains; (2) the response of the bed; and (3) the fluid forcing. The appeal of acoustic remote sensing methods is that in principle all three components of the problem can be addressed with minimal disturbance to the system. This paper reviews the progress that has been made to date toward developing acoustic instrumentation systems for sediment transport measurements in the nearshore zone, where sand is the principal component of the bottom sediments. Instrument developments are discussed in relation to several types of measurement requiring multiple acoustic beams: (1) range‐gated multifrequency backscatter to obtain profiles of suspended sediment concentration and size, and the associated inverse problem; (2) bedform imaging with fan‐beam sonars; (3) suspended sediment fluctuations under waves. The results provide revealing insights into the sediment transport process, and thus demonstrate the effectiveness of the multibeam approach. The results also raise a number of fundamental questions. Some of these questions can be addressed with the instrumentation currently at hand. Others will require a new generation of instruments.

Field observations and model simulations are presented of flow generated by waves breaking over a shoal at the entrance to a shallow bay. The shoal is composed of a series of steep and narrow bedrock ridges with depths of 2–8 m at the ridge crests. Observations from instruments on the shoal indicated peak significant wave heights during a storm event were 2.4–4.0 m across the observation sites; this spatial variability is due to wave breaking over the ridges. The 2‐D depth‐averaged hydrodynamic model Delft3D, coupled to the wave model SWAN, was used to simulate the waves and wave‐driven flow over the shoal and throughout the entire bay with a nested fine grid (5 m resolution) to resolve the shoal bathymetry. The model predicts a well‐defined jet behind the shoal, with mean axial speeds of 0.4–0.7 m/s. The observations indicate maximum speeds behind the shoal of 0.3–0.4 m/s, exceeding the maximum tidal current speed by more than a factor of 4, that are consistent in timing and direction with the model predictions. The model overpredicts wave breaking over the steep slopes and as a result the current speeds are overestimated.

Results are presented from 70+ days of nearly continuous in situ acoustic imagery of the nearshore sandy seabed in ∼3‐m mean water depth, at two locations separated by 40‐m cross‐shore distance. The bottom sediments were 150 μm median diameter sand, with nearly identical size distributions at the two locations. Five principal bed states were observed: irregular ripples, cross ripples, linear transition ripples, lunate megaripples, and flat bed. The linear transition and flat bed states were the most frequent, together accounting for 68% of the total time. Bed state occurrence was a strong function of incident wave energy, each bed state occurring within a relatively narrow range of sea‐and‐swell energies. During the 12 major storm events spanned by the record, the bed response was characterized by a repeatable bed state storm cycle, involving four of the five principal states (lunate megaripples did not appear repeatedly, and thus may be a special case), with no obvious dependence of bed state occurrence on prior bed state, or on third‐moment measures of wave nonlinearity. Radial spectra from the rotary acoustic images indicate pronounced differences in the anisotropy of spatial scales for the different bed states, and exhibit onshore‐offshore differences which are likely related to ripple migration.

The field of sediment transport measurement using acoustic remote sensing technique is reviewed, summarizing recent advances and possible directions for the future. The focus is on elucidating sediment transport mechanisms in the bottom boundary layer, and particularly on estimating terms in the sediment mass transport equation. A key problem is the inversion of acoustic backscatter profiles to obtain suspended sediment size and concentration. Several fundamental issues are involved: the scattering cross sections of irregular particles; estimating scatterer concentration fluctuations from fluctuations in signal amplitude; the related problem of inversion stability in the presence of fluctuations; the spatial coherence scales of the concentration field; and sediment mass flux estimation. These issues are illustrated with results from measurements made both in the laboratory and in the field. While several of the above issues remain unsolved, the results are nevertheless quite promising, and indicate a bright future for the application of acoustics to the sediment transport measurement problem.