Acoustic Doppler Profiler Research Articles

A hydrographic time series station in the Wadden Sea (southern North Sea)

In the tidal inlet between the East Frisian islands of Langeoog and Spiekeroog, southern North Sea, a time-series station was set up in autumn 2002 as part of the research programme BioGeoChemistry of Tidal Flats run by the University of Oldenburg. The purpose of the station is to provide continuous data on physical, biological and chemical parameters. In addition to instruments recording basic hydrographic and meteorological parameters, the time-series station is equipped with acoustic Doppler profilers for measuring surface waves and current profiles. Compact optical spectrometers are being used for spectral measurements of seawater transmission and for daylight reflectance. Additional sensors were installed for measuring oxygen, nutrients and methane in the seawater. The data shall help to quantify the flux of dissolved and suspended matter between the backbarrier tidal flat and the open sea and to characterise the material transformation in the tidal flat area by biogeochemical processes over the tidal cycle. Due to its novel design, operation of the station is also possible during winter and under extreme weather conditions (gales, storm surges, and sea ice) when data sampling with conventional platforms such as research vessels, buoys, or smaller poles could not be performed in the past. In this way, time series of data are obtained, which include events that are most relevant to the evolution of this coastal area. The performance of the station and its equipment are presented with data covering 6 years of operation. Time series of air and water temperature as well as seawater salinity demonstrate the multiyear dynamics of these parameters in the East Frisian Wadden Sea. Hydrographic data collected under specific meteorological conditions such as gales and storm surges exemplify the all-weather capabilities of the station and its value for studying hydrographic processes in the Wadden Sea.

Technological advances in suspended‐sediment surrogate monitoring

Surrogate technologies to continuously monitor suspended sediment show promise toward supplanting traditional data collection methods requiring routine collection and analysis of water samples. Commercially available instruments operating on bulk optic (turbidity), laser optic, pressure difference, and acoustic backscatter principles are evaluated based on cost, reliability, robustness, accuracy, sample volume, susceptibility to biological fouling, and suitable range of mass concentration and particle size distribution. In situ turbidimeters are widely used. They provide reliable data where the point measurements can be reliably correlated to the river's mean cross section concentration value, effects of biological fouling can be minimized, and concentrations remain below the sensor's upper measurement limit. In situ laser diffraction instruments have similar limitations and can cost 6 times the approximate $5000 purchase price of a turbidimeter. However, laser diffraction instruments provide volumetric‐concentration data in 32 size classes. Pressure differential instruments measure mass density in a water column, thus integrating substantially more streamflow than a point measurement. They are designed for monitoring medium‐to‐large concentrations, are generally unaffected by biological fouling, and cost about the same as a turbidimeter. However, their performance has been marginal in field applications. Acoustic Doppler profilers use acoustic backscatter to measure suspended sediment concentrations in orders of magnitude more streamflow than do instruments that rely on point measurements. The technology is relatively robust and generally immune to effects of biological fouling. Cost of a single‐frequency device is about double that of a turbidimeter. Multifrequency arrays also provide the potential to resolve concentrations by clay silt versus sand size fractions. Multifrequency hydroacoustics shows the most promise for revolutionizing collection of continuous suspended sediment data by instruments that require only periodic calibration for correlation to mean concentrations in river cross sections. Broad application of proven suspended sediment surrogate technologies has the potential to revolutionize fluvial sediment monitoring. Once applied, benefits could be enormous, providing for safer, more frequent and consistent, arguably more accurate, and ultimately less expensive sediment data for managing the world's sedimentary resources.

Determination of the wave climate for the southern Brazilian shelf

The main objective of this study is the characterization of the wave climate in the Southern Brazilian Shelf (SBS) based on a thorough review of existing field data and on numerical modeling experiments. A quantitative knowledge about the wave climate of this area is important to understand the mechanisms driving episodic mud bank attachments to the sandy shore, and the interaction of these banks with the flow and waves. The statistical analysis of existent data on the wave climate throughout the SBS indicates that the predominant wave directions are 100° and 160° (E–SE), with wave heights varying between 1 and 1.50 m. The wave period varies between 6 and 14 s, with predominance of mean wave period of 8 s (sea conditions) and 12 s (swell conditions). The spectral wave model SWAN version 40.41 [Booij, N., Haagsma, I.J.G., Holthuijsen, L.H., Kieftenburg, A.T.M.M., Ris, R.C., van der Westhuysen, A.J., Zijlema, M., 2004. SWAN Cycle III Version 40.41 Users Manual, Delft University of Technology, Delft, The Netherlands, p. 118, http://fluidmechanics.tudelft.nl/swan/index.htm] is used to simulate the wave climate for the region. Special attention is given to Cassino Beach, describing the wave climate observed during the Cassino Experiment, carried out in 2005. The verification of the standard SWAN model was carried out based on the comparison between numerical modeling results and available data of significant wave height, peak period, mean wave direction and energy density for the period relative to February of 1998. Results showed satisfactory model predictions of significant wave height and reasonably accurate predictions of peak spectral wave period and direction. The model performance is also considered satisfactory in the representation of the wave climate of the region when the wave spectrum has only one spectral peak, but presents limitations for bimodal wave spectrum. When two spectral peaks are observed, the SWAN model agrees with the spectral level observed in the low frequency, but underestimates the spectral level in the high-frequency band. When considering the presence of mud deposits in the area, model results predict that although the presence of mud attenuates most of the wave energy on the low frequency peak, it has a smaller effect in attenuating the wave energy on the high frequency peak. The comparison between significant wave heights calculated by the SWAN model (with and without mud) and measured with an Nortek acoustic doppler profiler (NDP) for the NDP localization point, and with data from a waverider relative to the 2005 survey for the studied period showed that all the time series have a similar pattern. Observed and calculated results without mud are in agreement, following the expected behavior. By considering the presence of mud in the whole domain, the model with mud shows a clear decrease of the significant wave height in relation to the NDP localization point. Furthermore, the significant wave height is underestimated due to the consideration of a constant mud thickness, extent, density and viscosity in the whole domain.

Glider surveillance of physics and biology in the southern California Current System

Since 2005, Spray underwater gliders have been used to sample sections across the southern California Current System (CCS). Properties measured are depth‐averaged velocity and profiles of temperature, salinity, Acoustic Doppler Profiler shear, chlorophyll a fluorescence, and 750‐kHz acoustic backscatter. Although slow and carrying a relatively small sensor suite, gliders observed multiple 350‐700‐km sections that reveal details of known elements of the CCS circulation like the California Current and Undercurrent as well as a previously undescribed undercurrent 100‐200 km offshore. The long time series with high spatial resolution disclose the close link between fronts in physical and biological variables, both the main pycnocline front and secondary weaker but sharp fronts farther offshore. These fronts appear to organize chlorophyll and acoustic backscatter fields, particularly during spring. Much of the patchiness of chlorophyll is associated with vertical displacements of fields that are much smoother within potential density surfaces. The horizontal structure of both chlorophyll and spice (the dynamically passive variable formed from temperature and salinity) appear to be formed by mesoscale stirring.

Tidal and Meteorological Forcing of Suspended Sediment Flux in a Muddy Mesotidal Estuary

Although the supply and fate of suspended sediment is of fundamental importance to the functioning and morphological evolution of muddy estuaries, reliable sediment budgets have been established in only a few cases. Especially for smaller estuaries, inadequate bathymetric surveys and a lack of intertidal sedimentation data often preclude estimation of the sediment budget from morphological change, while instrument-derived residual fluxes typically lie well within the errors associated with measurement of much larger gross tidal transports. Given suitably long-term records of continuously monitored suspended sediment concentration (SSC), however, analysis of the major scales of variation in sediment transport and their relation to hydrodynamic and meteorological forcing permits qualitative testing of hypotheses suggested by directly measured residual fluxes. This paper analyzes data from a 1-year acoustic Doppler profiler deployment in the Blyth estuary, a muddy mesotidal barrier-enclosed system on the UK east coast. Flux calculations indicate a small sediment import equivalent to just 1.5% of the gross flood tide transport. Little confidence can be assigned to either the magnitude or direction of such a small residual when considered in isolation. However, the inference that the sediment regime is finely balanced is qualitatively supported by the close similarity between flood-tide and ebb-tide SSC values. Singular spectrum analysis of the SSC time series shows the expectedly large contributions to the variance in SSC at intratidal and subtidal (semimonthly and monthly) scales but also picks out intermittent variability that is initially attributed to a combination of non-tidal surge and wind stress forcing. Closer examination of the data through cross-correlograms and event-scale analysis indicates that local meteorological forcing is the major factor. Acting through the resuspension of intertidal mudflat sediments at times of strong westerlies, meteorological forcing is directly implicated in episodic sediment export from the estuary. Thresholding of tide-averaged fluxes using a range of critical wind stress values further indicates that ‘tide-dominated’ (i.e., low wind stress) and ‘wave-dominated’ (high wind stress) conditions are associated with sediment import and export. Sediment balance is potentially sensitive to the frequency of high wind stress events, since the associated sediment exports are several times larger than the average import under calm conditions. Intermittent meteorological forcing may thus exert an important control on the sedimentary balance of otherwise tidally dominated muddy estuarine systems, and the role of wind climate should not be overlooked in studies of estuary response to environmental change.

Frontal structure and transport in the northwestern Weddell Sea

Hydrographic data from the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project are analyzed to determine the frontal structure and transport along a section across the continental shelf and slope in the northwestern Weddell Sea. The flow is dominated by three barotropic northward flowing currents: the Antarctic Coastal Current, the Antarctic Slope Front and the Weddell Front. The strongest baroclinic flows are confined to the region between the Slope Front and the Weddell Front over the steepest part of the continental slope. The Antarctic Coastal Current flows over the continental shelf near a local steepening in the bathymetry and has a transport of ∼ 1.3 Sv . The Antarctic Slope Front is found approximately 25 km offshore of the shelf break in 800 m of water. The Slope Front, which is associated with a transport of ∼ 4 Sv , exhibits peak velocities above the bottom that reach 35 cm s - 1 as detected by lowered acoustic Doppler profiler (LADCP) measurements. A third northward current is found between the 2500 and 3000 m isobaths, corresponding to a local break in the topography. Potential temperature–salinity diagrams show that the change in water mass properties across the deep front is similar to the change found across the Weddell Front in the northern Weddell Sea. This suggests that the deep front is a crossing of the Weddell Front further upstream in the Weddell Gyre. The Weddell Front accounts for ∼ 17 Sv of northward transport across the section. A deep outflow is observed all along the continental slope between the Slope Front and the Weddell Front. Transport within the deep outflow is localized in two to three distinct cores that are tied to topographical features. The total transport across the ADELIE section is 46 ± 8 Sv . This value exceeds previous estimates because the full-depth and de-tided LADCP measurements allowed the narrow ( ∼ 20 km ) frontal currents to be resolved, leading to more accurate estimates of the barotropic component of the flow. We discuss the physical processes that may lead to the formation and maintenance of these fronts.

Tidal current observation in the southern Yellow Sea in the summers of 2001 and 2003

Direct current observations in the Yellow Sea interior are very scarce due to intense fishing and trawling activities. Most previous studies on tides in the area were based on coastal measurements or satellite altimeter sea levels and have not been rigorously compared with direct measurements. In this paper, tidal currents are studied with current profiles from three bottom-moored Sontek Acoustic Doppler Profilers (ADPs) deployed in the southern Yellow Sea in summer of 2001 and 2003. The measured current series were dominated by tidal currents. Maximum velocities are between 40–80 cm/s at the mooring stations. M2 current is the most dominant primary tidal constituent, while MS4 and M4 are the most significant shallow water tides with much smaller amplitudes than the primary tides.

Physics of early summer ice/ocean exchanges in the western Weddell Sea during ISPOL

Abstract Early summer exchanges of momentum, heat, and salt between a multiyear ice floe and the underlying upper ocean in the western Weddell Sea were investigated during the Ice Station Polarstern (ISPOL) drift. Instrumentation clusters including three-dimensional acoustic current meters, temperature and conductivity meters were deployed at up to three levels beneath two different sites on the ISPOL floe, at depths ranging from 3 to 10 m below the ice/ocean interface. ISPOL presented a complex physical environment with the floe comprising a mixture of ice types, responding to relatively strong tidal and internal ice forcing, drifting in a matrix of rapidly shifting open-water areas. Turbulence measured during the drift was generally moderately energetic during spring tides early and late in the drift, with maximum interfacial stress approaching 0.2 Pa. Measured turbulent heat flux at the instrument levels, which varied from about 20 upward to 70 W m−2 downward, responded to both solar radiative and horizontal advective effects, therefore did not necessarily correspond to interface heat flux values. Currents measured with an acoustic Doppler profiler showed consistent Ekman turning in the water column between 10 and 30 m. A novel application of modeling based on temperature and salinity profiles plus relative current measured at 20 m provided an estimate of the representative undersurface hydraulic roughness for the floe: z0=0.04 m. By solving the numerical model used to establish z0 for all 3-h average segments when relative current at 20 m exceeded 0.03 m s−1, interface conditions were estimated for the entire drift. Average values evaluated at the ice/water interface included turbulent stress of about 0.05 Pa; turbulent heat flux of about 15 W m−2 (implying a maximum bottom ablation of around 15 cm); and salinity flux of about 1.5×10−6 psu m s−1.

Observation of currents in the southern Yellow Sea in the summers of 2001 and 2003

Long term current observations in the southern Yellow Sea are very scarce because of the intense fishing and trawling activities. Most of the previous studies on tides and circulation were not rigorously validated with direct current measurements. In this study, tidal and sub-tidal currents were examined using current profiles from three bottom-moored Sontek Acoustic Doppler Profilers (ADPs) deployed in the southern Yellow Sea in the summers of 2001 and 2003. The measured current time series were dominated by tidal currents. The maximum velocities were between 40–80 cm s−1 at the mooring stations. The M2 current was the dominant primary tidal constituent, while the MS4 and M4 components produced the most significant shallow water tidal currents with much weaker amplitudes. The measured mean sub-tidal velocities were less than 5 cm s−1. The mean flows in the lower layer implied that an anti-cyclonic circulation pattern might exist in the deeper central Yellow Sea. However, the previously expected cyclonic circulation pattern in the upper layer was not clearly shown by the observations.

Seasonal Variability and Estuary–Shelf Interactions in Circulation Dynamics of a River-dominated Estuary

The long-term response of circulation processes to external forcing has been quantified for the Columbia River estuary using in situ data from an existing coastal observatory. Circulation patterns were determined from four Acoustic Doppler Profilers (ADP) and several conductivity–temperature sensors placed in the two main channels. Because of the very strong river discharge, baroclinic processes play a crucial role in the circulation dynamics, and the interaction of the tidal and subtidal baroclinic pressure gradients plays a major role in structuring the velocity field. The input of river flow and the resulting low-frequency flow dynamics in the two channels are quite distinct. Current and salinity data were analyzed on two time scales—subtidal (or residual) and tidal (both diurnal and semidiurnal components). The residual currents in both channels usually showed a classical two-layer baroclinic circulation system with inflow at the bottom and outflow near the surface. However, this two-layer system is transient and breaks down under strong discharge and tidal conditions because of enhanced vertical mixing. Influence of shelf winds on estuarine processes was also observed via the interactions with upwelling and downwelling processes and coastal plume transport. The transient nature of residual inflow affects the long-term transport characteristics of the estuary. Effects of vertical mixing could also be seen at the tidal time scale. Tidal velocities were separated into their diurnal and semidiurnal components using continuous wavelet transforms to account for the nonstationary nature of velocity amplitudes. The vertical structure of velocity amplitudes were considerably altered by baroclinic gradients. This was particularly true for the diurnal components, where tidal asymmetry led to stronger tidal velocities near the bottom.

Comparison of Fixed- and Moving-Vessel Flow Measurements with an aDp in a Large River

A comparison of three-dimensional flow velocity measurements, made with an acoustic Doppler profiler (aDp) from fixed and moving vessels at cross sections of the Paraná River, Argentina, was performed. The purpose was to design a rapid and reliable procedure for quantifying the velocity field, and related parameters such as bed shear velocity and the identification of secondary circulations, in large rivers using an aDp. The fixed-vessel measurements were performed over a period of 10min at three vertical profiles along two of the sections. These data were then compared with the results of ten moving-vessel repeat transects made at each of the sections, and which intersected the fixed-vessel sampling locations, using a number of different aDp setup configurations. From the velocity profiles obtained with both fixed- and moving-vessel measurements, total bed shear velocity values were computed by applying the law-of-the-wall. The results indicate there can be significant differences between velocities obtained using the moving-vessel method and fixed-vessel measurements averaged over 10min. These differences in horizontal velocity can be significantly reduced by averaging five, or more, moving-vessel transects, with corresponding shear velocities calculated from five-transect averages showing differences ranging between 10 and 15%, dependent on the aDp configuration. Location of the at-a-point vertical velocity profile in relation to large-scale bed roughness may also be an influential factor, and ideally the bed morphology should be quantified together with the aDp-derived velocities. When using the aDp to identify secondary flow cells, it was found that although one cross-section transect can provide a reasonable overall picture, an average of five cross sections is necessary to resolve the finer details of flow. The implications for applications that use moving-vessel techniques for measurement and analysis of three-dimensional flow structures, including secondary flows, are highlighted.

Errors in Acoustic Doppler Profiler Velocity Measurements Caused by Flow Disturbance

Acoustic Doppler current profilers (ADCPs) are commonly used to measure streamflow and water velocities in rivers and streams. This paper presents laboratory, field, and numerical model evidence of errors in ADCP measurements caused by flow disturbance. A state-of-the-art three-dimensional computational fluid dynamic model is validated with and used to complement field and laboratory observations of flow disturbance and its effect on measured velocities. Results show that near the instrument, flow velocities measured by the ADCP are neither the undisturbed stream velocity nor the velocity of the flow field around the ADCP. The velocities measured by the ADCP are biased low due to the downward flow near the upstream face of the ADCP and upward recovering flow in the path of downstream transducer, which violate the flow homogeneity assumption used to transform beam velocities into Cartesian velocity components. The magnitude of the bias is dependent on the deployment configuration, the diameter of the instrument, and the approach velocity, and was observed to range from more than 25% at 5 cm from the transducers to less than 1% at about 50 cm from the transducers for the scenarios simulated.

Observations of sediment transport on the continental shelf off the mouth of the Waiapu River, New Zealand: Evidence for current-supported gravity flows

Instrumented tripods deployed at depths of 40 and 60 m on the shelf off the mouth of the Waiapu River on the east coast of New Zealand's North Island recorded data on waves, currents, and sediment fluxes from May 22 to August 10, 2004. Three major flood events and several wave events occurred during the deployment. Data from acoustic Doppler velocimeters and profilers revealed that downslope sediment fluxes accompanied a flood event of late June, during which near-bed downslope current speeds approached 0.5 m s −1. The most pronounced downslope transport within the benthic layer occurred the day after peak flood but coincided with strong isobath-parallel currents. Suspended sediment concentrations about 1 m above the bed were on the order of 2–4 g l −1 at times of maximum seaward flow. Suspension of freshly discharged sediment within the hyperpycnal layer over much of the profile was maintained by the high bed stresses associated with the strong benthic currents, in contrast to the wave-supported sediment gravity flows recently reported on other river-nourished shelf systems. Nonetheless, observed concentrations and velocities were largely consistent with the dynamics of critically stratified sediment gravity flows based on equations previously applied elsewhere to wave-supported cases. Despite lower concentrations in current-supported gravity currents, the greater thickness results in similar total loads. In further contrast to earlier results, our data suggest that as sediment off the Waiapu flowed into deeper water across the seaward steepening bottom profile, auto-suspension may have aided in thickening and accelerating the gravity current.

Rotary spectrum analysis of tidal current in the southern Yellow Sea

A complete set of one-month Acoustic Doppler Profiler (ADP) current data at a station in the southern Yellow Sea (SYS) is analyzed using the rotary spectrum method. The results revealed different rotary properties between barotropic and baroclinic tidal currents. The barotropic and baroclinic tidal currents rotate elliptically counter-clockwise and clockwise, respectively. Meanwhile, baroclinic bottom tidal currents are almost along-isobath. The baroclinic cross-isobath velocities attenuate quickly at the bottom, implying important effects of bottom topography on the cross-isobath motions.

Estimation of TKE dissipation rates in dense bottom plumes using a Pulse Coherent Acoustic Doppler Profiler (PC-ADP) — Structure function approach

During a hydrographic survey in January 2006 the spreading of inflowing saline water was observed in the Arkona Basin (Western Baltic Sea). Two bottom mounted ‘pulse coherent’ acoustic Doppler profilers (PC-ADP) were used to measure the near-bottom current field of the dense plume with a high temporal (1 s) and spatial resolution (5 cm). In order to estimate the dissipation rate of turbulent kinetic energy ( ɛ) a structure function approach was applied to the beam velocity data. Simultaneous measurements with a microstructure shear profiler (MSS) and an acoustic Doppler velocimeter (ADV) supplied independent data for the verification of the structure function method. Additional measurements with standard CTD, near-bottom towed and vessel mounted acoustic Doppler current profilers (ADCP) completed the data set. The estimated dissipation rates from the structure function approach fit well with the values derived from the ADV and the MSS probe. It is shown that the structure function approach is a reliable and easily applicable method to derive estimates of TKE dissipation rates from PC-ADP beam velocities. The observed dissipation rates ranged between 5 · 10 − 6 and 1 · 10 − 8 W kg − 1 depending on the hydrographic conditions. Inside the plume the dissipation rates exceeded that of the overlaying brackish water by two orders of magnitude. Since the noise level of velocity data in pulse coherent mode is considerably lower than in the Doppler mode the PC-ADP can also be used for ɛ estimates in marine environments with low turbulence level. Reynolds stresses estimated from the PC-ADP and the ADV agreed well at the same depth level. TKE production derived from PC-ADP measurements compared reasonably well with the dissipation rate of TKE in a varying environment.

Field Survey on Resuspension of Fine Sediments and Turbulence Structures in a Bottom Boundary Layer

3-D near-bottom velocities were measured with high resolution in space and time to elucidate dynamics in the bottom boundary layer and resuspension process of fine bottom sediment at a coast off the Port of Kumamoto. Acoustic devices, including Acoustic Doppler Velocimeters (ADV) and Acoustic Doppler Profilers (ADP), were introduced to the velocity measurement and they were compared with ambient suspended solid concentrations monitored at the same time. Tidal asymmetry was observed in the data both of current velocities and suspended solid concentrations near the bed and higher concentration appeared during flood tide with higher turbulence near the bed.

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.

Dispersion of disposed dredged slurry in the meso-tidal Changjiang (Yangtze River) Estuary

To understand the dispersal pattern of sediment plume and its controlling processes, a field experiment of concentrated slurry dispersal created by a dredger was conducted in the Changjiang (Yangtze River) Estuary during the 2002 flood season. An acoustic suspended sediment concentration profiler and an acoustic Doppler profiler were deployed to simultaneously observe suspended sediment concentrations (SSC) and tidal currents at the pre-selected sections shortly following the release of dredged materials. Water sampling, grab sampling and shallow coring were simultaneously carried out to obtain the SSC and grain-size texture. High-resolution SSC profiler observations showed that two distinct sediment plumes (middle level- and near-bed plumes) occurred during the intermediate tidal phase between the spring and neap due to differential settling of the sediment mixture, whereas only a benthic plume occurred due to rapid flocculation settling during the neap tide. Three subsequent stages can be identified during the dispersal of the sediment plume: (1) initially stable stage before the release; (2) unstable stage shortly following the release as a settling cloud; and (3) stable stage after the formation of a primary lutocline or a benthic plume. Enhanced mixing due to oscillatory shear flows could raise only the elevation of the lutocline in the slurry, but could not enhance the transport capacity of suspension. In the presence of high concentration, the fate of bottom sediment plume was controlled by the bottom stress, independent of the interfacial mixing.

A model for high-frequency acoustic Doppler current profiler backscatter from suspended sediment in strong currents

Acoustic current Doppler profilers (ADCP's) have been used to measure currents for over 20 years. Although ADCP's are not designed to measure acoustic backscatter intensity accurately, several researchers have (successfully) related the backscatter intensity to suspended sediment concentration (SSC) using a random phase acoustic backscatter model. However, this paper shows experimental evidence that in a tidal inlet during high tidal current velocities, a random phase acoustic backscatter model overestimates the SSC by factors up to 60, which could not be explained by a time-varying particle size distribution. An acoustic backscatter model is developed that includes the effect of acoustic backscatter enhanced by coherence in the particles’ spatial distribution as a result of turbulence-induced sediment fluctuations. The model results are compared with field measurements, showing a good correspondence between measured and modelled SSC, including the strong high current conditions for which the random phase acoustic backscatter model was shown to fail.

Field measurements and monitoring of wastewater discharge in sea water

This paper reports a study of the field measurements and monitoring of wastewater discharge in sea water at Bari East (Italy). A wastewater sea outfall system is an integral and fundamental part of each wastewater treatment with ultimate sink in the sea water. The design of a water treatment plant and wastewater outfall must take into account the use of the environmental water, the values of physicochemical parameters to be respected in order to safeguard the use itself and the quality of the environmental water where wastewater is issued. In the present study measurements of sea current velocity components were carried out with a VM-ADP (Vessel-Mounted Acoustic Doppler Profiler). Salinity, wind directions and velocities were assessed with, for one survey, the total faecal coliforms and other biochemical parameters. It was emphasized that the measurements necessary for monitoring cannot be concentrated in the wastewater outfall pipe zone only, but should be extended to a neighbouring area of the outfall pipe, with an extension depending on the wastewater discharge, the polluting charge and the magnitude of the sea currents and the winds typical of the zone of interest. The analyses presented in this paper confirm that the sea zones close to the wastewater outfall pipe are particularly sensitive and vulnerable. Such results must be considered in the planning of a wastewater outfall pipe.