Pulse-coherent Acoustic Doppler Research Articles

Study on the Methods to Estimate the Bed Shear Stresses in Yangtze Estuary

High spatial and temporal current resolution velocities were measured by use of an acoustic Doppler velocimeter (ADV) in the tidal bottom layer of North Passage Deep water Navigational Channel (DNC) of Yangtze Estuary in China. And the vertical profiles of mean velocity were measured by using a pulse-coherent acoustic Doppler profiler (PC-ADP). The measured data show the vertical profiles of mean velocity are almost logarithmic. For discussing, the turbulent kinetic energy (TKE) method based on the velocity for ADVs-based is chosen and a new logarithmic profile (LP) method ignoring the influence of complex variation of the parameter Z 0 is suggested to estimate the bed shear stresses respectively. The calculated results show that the bed shear stresses estimated by LP method are larger than the ones estimated by TKE method mostly in Yangtze estuary. At the moment of ebb peak, the maximum error can be about twice occurring and the error of ebb tide is almost greater than the one of flood tide. These phenomena are explained in this study by the deduction of formula for steady and unsteady flow. It is concluded that the vertical logarithmic profile of velocities will be influenced by unsteady flow and there is not the same bed shear stress even if there is the same logarithmic velocity profile in vertical. Generally, this has led to a necessary adjustment of the drag coefficient Cd of steady flow in flow model to avoid overestimating the bed stress for unsteady flow in estuary based on the observed data.

Observing Finescale Oceanic Velocity Structure with an Autonomous Nortek Acoustic Doppler Current Profiler.

This paper describes the instrumentation and techniques for long-term targeted observation of the centimeter-scale velocity structure within the oceanic surface boundary layer, made possible by the recent developments in capabilities of autonomous platforms and self-contained pulse-coherent acoustic Doppler current profilers (ADCPs). Particular attention is paid to the algorithms of ambiguity resolution ("unwrapping") of pulse-coherent Doppler velocity measurements. The techniques are demonstrated using the new Nortek Signature1000 ADCP mounted on a Lagrangian float, a combination shown to be capable of observing ocean turbulence in a number of recent studies. Statistical uncertainty of the measured velocities in relation to the ADCP setup is also evaluated. Described techniques and analyses should be broadly applicable to other autonomous and towed applications of pulse-coherent ADCPs.

Suspended sediment flux statistics in unidirectional flow, using a pulse-coherent acoustic Doppler profiler

An experiment was conducted in the St. Anthony Falls Lab main channel flume, involving sand sediment (d50 = 0.4 mm) in unidirectional flow. The flow conditions were 1 m/s flow rate and 1 m depth, and the bed state consisted of quasi-linear sand dunes with ~1 m wavelength and 10-20 cm height. A pulse-coherent acoustic Doppler instrument (MFDop) measured high-resolution near-bed vertical profiles of velocity and backscatter amplitude at various positions spanning the dune profile. These measurements are used to obtain probability distributions of instantaneous suspended sediment concentration and flux, which are compared to predictions from two stochastic theories. While the stochastic theories were previously developed and validated for low transport rates over a flat bed, the acoustic measurements enabled measurements at a much higher transport rate; despite this, the new measurements remain in remarkably good agreement with the theories.

Turbulent particle flux measurement with pulse-coherent Doppler sonar

The accuracy of velocity estimates from pulse-coherent acoustic Doppler systems is related to the magnitude, R, of the ensemble-averaged complex pulse-pair correlation. The closer R is to unity, the more accurate the estimate. The accuracy of particle concentration estimates is also related to the value of R but, in contrast, high accuracy requires R to be less than unity, i.e., the amplitudes from individual pulses in an ensemble should be independent, in order to beat down Rayleigh statistics associated with configuration noise. Consequently, when estimating the turbulent component of the particle flux, i.e., the product of the particle concentration and velocity fluctuations, a tradeoff arises between these conflicting requirements for velocity and concentration accuracy. This tradeoff is investigated through a statistical model of sound scattering from particles embedded in idealized turbulence, and laboratory experiments in which particle velocities in turbulent flow are measured both acoustically, via pulse-coherent sonar, and optically, using particle imaging velocimetry.

Estimation of bed shear stresses in the pearl river estuary

Mean and fluctuating velocities were measured by use of a pulse coherent acoustic Doppler profiler (PC-ADP) and an acoustic Doppler velocimeter in the tidal bottom boundary layer of the Pearl River Estuary. The bed shear stresses were estimated by four different methods: log profile (LP), eddy correlation (EC), turbulent kinetic energy (TKE), and inertial dissipation (ID). The results show that (a) all four methods for estimating bed stresses have advantages and disadvantages, and they should be applied simultaneously to obtain reliable frictional velocity and to identify potential sources of errors; (b) the LP method was found to be the most suitable to estimate the bed stresses in non-stratified, quasi-steady, and homogeneous flows; and (c) in the estuary where the semi-diurnal tidal current is dominant, bed shear stresses exhibit a strong quarter-diurnal variation.

Winter storm-induced hydrodynamics and morphological response of a shallow transgressive shoal complex: Northern Gulf of Mexico

Using extended deployments during seasons of low and high discharge from the Atchafalaya River, meteorological, hydrodynamic and bottom boundary layer parameters were monitored from Tiger and Trinity Shoal complex, off Louisiana coast, USA. During winter storms, the surface current speed measured at both shoals exceeded 0.5 m/s and the entire water column followed the prevailing wind direction. The current speed close to the bottom exceeded 0.3 m/s during high energy northerly winds. The mean water level in the shoal complex increased during southerly winds and decreased during northerly winds, such that the difference between wind set-up and set-down exceeded 0.7 m in Tiger Shoal and 0.6 m in Trinity Shoal during high energy frontal passages. The swell height was inversely correlated with mean water level, and increased during pre-frontal phase and decreased during post-frontal phase of winter storms. The sea (short waves) height responded quickly to wind direction and speed; and within a few hours after the wind shifted and blowing from the north, the sea height increased during both deployments. Bimodal wave frequency spectrum was observed during wind veering from southerly to northerly, when both sea and swell intensities were significant. The Tiger Shoal bed sediment texture transformed drastically, from mud to shell and shell hash assemblage, within a period of two weeks during the December 2008 deployment. Backscatter signal intensity from a Pulse Coherent Acoustic Doppler Profiler (PCADP) and its velocity estimates were used to determine the vertical extend and timing of mud resuspension and their eventual flushing out from the shoal environment, when exposed to high energy winter storm passages. The computed time frame for a total transformation of bottom sediment texture (from muddy bottom to shell and shell hash assemblage) was supported by the combined wave and bottom current induced shear stress at shoal bed. The bed samples collected from Tiger Shoal before and after the deployment in spring 2009 consisted of more than 80% shell and shell hash, which again confirmed a stable bottom as predicted from the PCADP data. However, the fine sand and mud dominated bed at Trinity Shoal was highly dynamic and experienced a few cm of ephemeral sediment deposition during the passage of each cold front, as revealed from the analysis of acoustic backscatter data from the PCADP. Suspended sediment concentration estimated from Optical Backscatterance Sensors (OBS) and PCADP were in good agreement during low river discharge events in December 2008; but significantly diverged during the spring 2009 deployment, when a high suspended sediment load was discharged into the shelf from the Atchafalaya River, and subsequently pushed farther offshore into the deployment sites by wind-induced strong currents during the passage of cold fronts.

Flow Separation over a Prograding Beach Step at Pensacola Beach, Florida

ABSTRACT Houser, C., 2013. Flow separation over a prograding beach step at Pensacola Beach, Florida. Beach steps are a common feature of poorly sorted intermediate and reflective beaches that can develop as an accretionary wedge from foreshore sediment moved seaward by the backwash. Although there have been several studies describing the development and evolution of steps in response to wave and tidal forcing, characterization of the separated flow and development of the backwash vortex has been limited to a small number of laboratory experiments. This study presents the first field data in which flow separation and the backwash vortex are quantified using a pulse-coherent acoustic Doppler profiler over a 0.10-m prograding beach step. As the step migrated beneath the instrument station under a quasi-steady wave forcing it maintained its height and length, allowing for a space-for-time substitution. Flow separation developed under subcritical flows, when the backwash exceeded ∼0.50 m s−1 and extended over ...

Comparison between optical and acoustical estimation of suspended sediment concentration: Field study from a muddy coast

The aim of this study is to evaluate the ability of a 1.5MHz Pulse-coherent Acoustic Doppler Profiler (PCADP) to measure suspended sediment concentration (SSC) in a muddy environment where SSC varies over several orders of magnitude. Two seasonal deployments were conducted south of the Atchafalaya–Vermilion Bay system along the Louisiana coast, USA. During a low discharge period of the Atchafalaya River, acoustical estimates of SSC were in good agreement with OBSs deployed. The second deployment was conducted during a high discharge period of the Atchafalaya River. The passage of a cold front across the coast resulted in a rapid advection of the river plume to the study area. High sensitivity of the OBS to fine-grained suspended sediments led to high values from the OBS readings. On the other hand, the PCADP barely sensed the transportation of fine-grained sediments and significantly responded to the combined wave and current shear stress close to sea bed. To reduce the influence of temporal variation of grain size distribution, acoustical backscatter measurements were calibrated over a time span of a few hours. The resulting SSC estimates from the time-adaptive calibration of the PCADPs were in good agreement with data from OBS sensors.

Near and far field flow disturbances induced by model hydrokinetic turbine: ADV and ADP comparison

Wake flows downstream of hydrokinetic turbines are characterized by hub and tip vortices, a velocity deficit and an increase in turbulence intensity. Velocity and turbulence recovery in the wakes of individual turbines constrains the density of turbines in an array and limits the amount of energy that can be produced by a turbine farm. However, few hydrokinetic turbine flow recovery studies have been conducted, especially on the far-field flow characteristics. Nor have studies evaluated the accuracy of acoustic Doppler profiler measurements in the wakes of turbines. The present study examines vertical profiles of mean velocity and turbulence, as well as longitudinal profiles of velocity deficit and turbulence levels measured at the symmetry plane of a model three-blade axial flow turbine in a large open channel flow. Mean velocity and turbulence statistics are measured using an acoustic Doppler velocimeter (ADV) and a pulse coherent acoustic Doppler profiler (ADP). ADV and corrected-ADP derived values of mean velocity, turbulence intensity and root-mean-square velocity constitute a well-documented data set that can be used to validate numerical models simulating the effects of hydrokinetic turbine arrays. We found that 80% of the flow recovery occurred about ten diameters downstream from the rotor plane, which suggests that practical values for longitudinal spacing of turbines should be between ten and fifteen diameters. Significant errors observed in mean velocity and turbulence statistics derived from ADP measurements in the near wake region raise concerns on the use of these instruments for such measurements in lab and field studies. Although the cause of some of the errors requires further investigation, we show that errors in turbulence intensity can be successfully corrected with supplemental ADV measurements.

LAGRANGIAN MEASUREMENTS OF TURBULENT DISSIPATION OVER A SHALLOW TIDAL FLAT FROM PULSE COHERENT ACOUSTIC DOPPLER PROFILERS

We present high resolution (25 mm spatial, 8 Hz temporal) profiles of velocity measured over a shallow tidal flat using pulse-coherent Acoustic Doppler Profilers mounted on surface drifters. The use of Lagrangian measurements mitigated the problem of resolving velocity ambiguities, a problem which often limits the application of high-resolution pulse-coherent profilers. Turbulent dissipation rates were estimated from second-order structure functions of measured velocity. Drifters were advected towards, and subsequently trapped on, a convergent surface front which marked the edge of a freshwater plume. Measured dissipation rates increased as a drifter deployed within the plume approached the front. A drifter then propagated with and along the front as the fresh plume spread across the tidal flats. Near-surface turbulent dissipation measured at the front roughly matched a theoretical mean-shear-cubed relationship, whereas dissipation measured in the stratified plume behind the front was suppressed. After removal of estimates affected by surface waves, near-bed dissipation matched the velocity cubed relationship, although scatter was substantial. Dissipation rates appeared to be enhanced when the drifter propagated across small subtidal channels.

Wave Breaking Dissipation Observed with “SWIFT” Drifters

Abstract Energy dissipation rates during ocean wave breaking are estimated from high-resolution profiles of turbulent velocities collected within 1 m of the surface. The velocity profiles are obtained from a pulse-coherent acoustic Doppler sonar on a wave-following platform, termed a Surface Wave Instrument Float with Tracking (SWIFT), and the dissipation rates are estimated from the structure function of the velocity profiles. The purpose of the SWIFT is to maintain a constant range to the time-varying surface and thereby observe the turbulence in breaking crests (i.e., above the mean still water level). The Lagrangian quality is also useful to prefilter wave orbital motions and mean currents from the velocity measurements, which are limited in magnitude by phase wrapping in the coherent Doppler processing. Field testing and examples from both offshore whitecaps and nearshore surf breaking are presented. Dissipation rates are elevated (up to 10−3 m2 s−3) during strong breaking conditions, which are confirmed using surface videos recorded on board SWIFT. Although some velocity contamination is present from platform tilting and heaving, the structure of the velocity profiles is dominated by a turbulent cascade of eddies (i.e., the inertial subrange). The noise, or uncertainty, in the dissipation estimates is shown to be normally distributed and uncorrelated with platform motion. Aggregated SWIFT measurements are shown to be useful in mapping wave-breaking dissipation in space and time.

Wave dissipation by flexible vegetation

[1] Dissipation of waves propagating through natural salt marsh vegetation was about half the dissipation expected for rigid vegetation. This low dissipation was predicted by a theoretical model that accounts for bending of vegetation motions. A transect of 3 pulse-coherent Acoustic Doppler Profilers recorded water velocity and pressure (at 8 Hz) within the dense (650 stems/m2) canopy of semi-flexible single-stem vegetation (Schoenoplectus americanus). Most wave energy (56–81%) was dissipated within 19 m of the marsh edge. Two dissipation models, the first assuming rigid vegetation, and the second simulating wave-forced vegetation motion using the theory for bending of linearly-elastic beams, were tested. After choosing optimal drag coefficients, both models yielded a good fit to the observed dissipation (skill score = 0.96–0.99). However, fitted drag coefficients for the rigid model (0.58–0.78) were below the range (0.98–2.2) expected for the observed Reynolds numbers (13–450) and canopy densities (accounting for interactions between stem wakes), whereas drag coefficients for the flexible model (0.97–1.6) were nearer the expected range, indicating that prediction of wave dissipation was improved by simulating vegetation motion.

Determining suspended sediment concentration and settling velocity from PC-ADP measurements in the Beibu Gulf, China

Modeling sediment transport depends on several parameters, such as suspended sediment concentration (SSC), shear stress, and settling velocity. To assess the ability of Pulse-Coherent Acoustic Doppler Profiling (PC-ADP) to non-intrusively quantify spatial and temporal SSC and settling velocity at seabed, a field experiment was conducted in the Beibu Gulf (Tonkin Gulf), in the South China Sea. The spatial profiles and temporal variations in SSC at 1 m above bottom were derived from PC-ADP acoustic backscatter intensity determinations after being calibrated with the optical backscatter sensor (OBS) measurements at the same elevation. The PC-ADP and OBS results agreed well. The temporal settling velocity obtained from Rouse profiles agreed well with the Soulsby formula based on size information by LISST (laser in situ scattering and transmissometry). Tides and tidal currents are diurnal in the gulf. SSC increased with increasing ebb and flood flow, and it rapidly decreased with the increase of distance from the seabed. The maximum SSC at 0.16 m and 1.3 m above bottom reached 816 mg/L and 490 mg/L during spring tides, respectively. The sediments consisted of mineral particles 23–162 μm in diameter and 0.05–2.04 cm/s in settling velocity. Generally, both the SSC and settling velocity followed variations in the bottom friction. Results suggest that PC-ADP is able to provide reasonable SSC and settling velocity measurements of both profiles and time series for a long study period.

A high‐resolution hydrodynamic investigation of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redds

AbstractHigh‐resolution velocity measurements were taken over a series of redds on a gravel‐bed stream using a Pulse Coherent Acoustic Doppler Profiler (PCADP) to quantify the hydrodynamics of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redds. On redds studied, over 4500 velocity measurements per redd were acquired per day to quantify the flow velocity, flow depth and related fluid mechanics metrics of Reynolds numbers, Froude numbers and turbulent kinetic energy per unit area. Results showed that velocity and Froude numbers varied widely at the redd scale, but consistently showed higher velocities and Froude numbers over the tailspill regions relative to the surrounding study limits. Results of Reynolds numbers calculations showed no apparent correlations to spawning location preference and redd structure. Turbulent kinetic energy per unit area consistently demonstrated a strong correlation with redd locations. The metric maintained low values (i.e. unidirectional flow with little turbulence) where all redds and attempted redds were observed. The study also demonstrates that a number of hydraulic metrics and several spatial scales will likely be necessary to understand any inherent relationship between river hydraulics and redd placement. Copyright © 2010 John Wiley & Sons, Ltd.

The influence of current speed and vegetation density on flow structure in two macrotidal eelgrass canopies

Lay AbstractThe influence of eelgrass (Zostera marina) on near‐bed currents, turbulence, and drag was investigated at three sites in two eelgrass canopies of differing density and at one unvegetated site in the San Juan archipelago of Puget Sound, Washington, USA. The data were compared to the canopy shear‐layer model, which has been shown to describe flow through submerged vegetation in laboratory studies. Although most of the data were consistent with the model, velocity profiles from the site with lowest eelgrass density were not, particularly when currents were strong. The eelgrass attenuated tidal currents by at least 40%. Attenuation decreased with increasing current speed, and was greater than 70% at the site with greatest eelgrass density. The attenuation produces a region of low velocities within the canopy that provides a sheltered environment for zooplankton, benthic invertebrates, juvenile fishes, and microalgae. In contrast, the top of the canopy is a region of strong turbulent mixing, exhibiting large‐scale velocity fluctuations. Friction velocity (an indicator of turbulence intensity) at the canopy top was 1.5–2 times greater than at the sea floor of the unvegetated site. This strong turbulence creates a very different habitat type than within the canopy and promotes exchange of oxygen, nutrients, and organic matter between the canopy and the water above it. The eelgrass canopies generated significantly more roughness and drag than the unvegetated sea floor. The drag coefficient CD for flow over the canopies was 3–8 times greater than at the unvegetated site (0.01–0.023 vs. 2.9 × 10− 3).

A hydrodynamic investigation of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redd selection at the riffle scale

ABSTRACTHigh‐resolution velocity profile measurements were taken over a series of riffles on a gravel‐bed stream using a Pulse Coherent Acoustic Doppler Profiler (PCADP) to quantify the fluid structure of riffles and nests (redds) where brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) spawned. Velocity profiles were obtained on a highly discretized planometric scale ranging between 20 and 40 cm grid spacings, with vertical observations occurring every 1.6 cm. From the velocity profiles, between 15 000 and 38 000 velocity measurements were obtained over each riffle on any given day of measurement. Velocity profiles were converted to streamwise velocity magnitude, flow depth, Reynolds number, Froude number, shear stress, vertical velocity components and turbulent kinetic energy per unit area to evaluate the spatial structure of the riffles and the spatial structure of redds (pits and tailspills) relative to the surrounding riffle structure. Semi‐variograms were employed to evaluate the persistence of the fluid structure based upon the metrics evaluated.Results showed that discrete velocity observations poorly described the spatial structure of the flow system and poorly correlated with redd locations. Reynolds number analysis identified a relatively consistent fluid property for distances typically 2–3 times the longitudinal length of redds. Turbulent kinetic energy per unit area consistently identified common regions on all riffles studied that corresponded with the location selections for redds where flow was identified as essentially uni‐directional. Froude number was found to be insensitive in predicting the fluid spatial structure in wadeable flow depths and relating it to the fluid structure of redds. Results indicated that a series of metrics at varying spatial scales of turbulence may be necessary to understand the spatial complexity of redd selection. Copyright © 2011 John Wiley & Sons, Ltd.

Estimation of high-resolution sediment concentration profiles in bottom boundary layer using pulse-coherent acoustic Doppler current profilers

The capability of two acoustic profilers — SonTek's 1.5-MHz pulse-coherent acoustic Doppler profiler (PC-ADP) and RDI's 600-kHz acoustic Doppler current profiler (ADCP) with pulse-coherent mode — was evaluated for estimating high-resolution suspended sediment concentration (SSC) profiles in bottom boundary layer. In the laboratory measurements with a PC-ADP, two types of sediments were tested to study acoustic responses to grain size. A natural sediment sample from Clay Bank, a mixture of clay and very fine sand, showed a good linear relationship between range-corrected volume scattering (Sv¯) and backscattered strength (E) until SSC increased up to about 10gl−1. In contrast, a commercially available kaolinite exhibited earlier signal saturation and non-unified linear regressions between Sv¯ and E, most likely because the particle size is much smaller than the transmitted acoustic wavelength. Using a pulse-coherent ADCP, the field measurement results from Mobile Bay, Alabama showed that the acoustically-derived SSC profiles were well matched with the optically-derived outcomes although slight discrepancies were noted. The overestimation of acoustically-derived SSC near the bed may be related to the side lobe interference near the bed and the enhanced acoustic sensitivity by coarser particles and denser aggregates eroded from the bed. Mean absolute error of acoustic estimates was within 4.1–7.3% of the optically-derived SSC range, which is attributable to the different acoustic and optical scattering responses to given sediment size spectra. Despite some error sources in acoustic inversion, the results from laboratory and field experiments suggest that the pulse-coherent acoustic profiler is able to reveal the evolution of in-situ near-bed SSC profiles with high vertical and temporal resolutions.

Estimation of suspended sediment concentrations using Pulse-coherent Acoustic Doppler Profiler (PCADP)

The objective of the study is to investigate the suitability of using Pulse-coherent Acoustic Doppler Profiler (PCADP) to estimate suspended sediment concentration (SSC). The acoustic backscatter intensity was corrected for spreading and absorption loss, then calibrated with OBS and finally converted to SSC. The results show that there is a good correlation between SSC and backscatter intensity with R value of 0.74. The mean relative error is 22.4%. Then the time span of little particle size variation was also analyzed to exclude the influence of size variation. The correlation coefficient increased to 0.81 and the error decreased to 18.9%. Our results suggest that the PCADP can meet the requirement of other professional instruments to estimate SSC with the errors between 20% and 50%, and can satisfy the need of dynamics study of suspended particles.

Shear velocity estimates on the inner shelf off Grays Harbor, Washington, USA

Shear velocity was estimated from current measurements near the bottom off Grays Harbor, Washington between May 4 and June 6, 2001 under mostly wave-dominated conditions. A downward-looking pulse-coherent acoustic Doppler profiler (PCADP) and two acoustic-Doppler velocimeters (field version; ADVFs) were deployed on a tripod at 9-m water depth. Measurements from these instruments were used to estimate shear velocity with (1) a modified eddy-correlation (EC) technique, (2) the log-profile (LP) method, and (3) a dissipation-rate method. Although values produced by the three methods agreed reasonably well (within their broad ranges of uncertainty), there were important systematic differences. Estimates from the EC method were generally lowest, followed by those from the inertial-dissipation method. The LP method produced the highest values and the greatest scatter. We show that these results are consistent with boundary-layer theory when sediment-induced stratification is present. The EC method provides the most fundamental estimate of kinematic stress near the bottom, and stratification causes the LP method to overestimate bottom stress. These results remind us that the methods are not equivalent and that comparison among sites and with models should be made carefully.

Application of Coherent ADCP for Turbulence Measurements in the Bottom Boundary Layer

Abstract This paper presents a novel approach for estimating the rate of turbulent kinetic energy dissipation from pulse-coherent acoustic Doppler current profiler (ADCP) measurements using the inertial dissipation method. Although the inertial dissipation technique is widely accepted and used in oceanographic and atmospheric research, its application to ADCP data is limited by the loss of directional information for high-wavenumber velocity fluctuations. However, measurements in the bottom boundary layer of a lake revealed astonishing agreement between dissipation rates estimated from temperature microstructure profiles and those estimated by applying the inertial dissipation method to data from two different brands of ADCPs.