Acoustic Doppler Profiler Research Articles

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.

Bedform characteristics during falling flood stage and morphodynamic interpretation of the middle–lower Changjiang (Yangtze) River channel, China

A survey was conducted to examine river bedforms using side-scan sonar, sub-bottom profiler and an Acoustic Doppler Profiler in the middle–lower (mid-lower) Changjiang (Yangtze) River (downstream of Wuhan), China, during August 14–20, 2003 when the discharge was 27,000–36,000m3s−1. The results show bedforms of a range of sizes occur in the channel and their heights and lengths follow an exponential correlation below Flemming's maximum line. Not all bedforms were formed under the flow conditions experienced during the survey, as shown by comparison to three existing models in the literature. Small-scale bedforms with a length <60m, defined as flat bed, mega-ripple and small dune in the present study, are mostly in equilibrium and are probably formed under the existing flow conditions. The large dunes with a length of >60m are not in equilibrium and are believed to be the relicts of previous large discharges now undergoing adjustment. Different bedform types occur in the five river reaches in this section of the Changjiang (Wuhan–Tianjiazhen, Tianjiazhen–Anqing, Anqing–Maanshan, Maanshan–Jiangyin and Jiangyin–estuary downstream). In the narrow and single channel reaches above Anqing, large/small dunes are limited to the channel with higher water surface slope where stream power is strong enough for riverbed sediments to be moved frequently. In the downstream reaches between Anqing and Jiangyin, characterized by anabranching channels, small-scale bedforms superimposed on large dunes occur widely due to intensive riverbed sediment transport from frequent diverging/converging flow during both floods and low-flow periods. Mega-ripples and flat beds prevail in the lowermost channel below Jiangyin where wider cross-sections result in dispersed stream power.

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.

Fine vortex columns on the shelf in the area of bottom convection influence

This paper analyzes the results of investigations into extremely large releases of current velocities registered by an acoustic Doppler profiler during experiments performed on the Hawaii shelf. An integral analysis of the variability of current fields relies on the profile data of current velocities, temperature, and acoustic-scattering layers. Based on these investigations, we proposed a hypothesis stating that the small-scale variability of currents in profiler data is conditioned by the passage of fine vortices emerging in the outflow and rise of freshened waters from a subsurface sink near the shelf edge. To confirm this hypothesis on the emergence mechanism of the observed releases of current velocities, we conducted a mathematical simulation of the response that the Doppler meter has to the passage of fine vortex, which revealed a good agreement between model calculations and experimental data. Similar phenomena were also found in the Gelendzhik Bay of the Black Sea.

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 &amp; 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).

Flow measurement with multi‐instrumentation in a tidal‐affected river

AbstractFlow measurements in a tidal‐affected river provide important information of cyclic flow pattern characteristics in temporal and spatial variations. Reliability and accuracy of measured flow data can provide for numerical model calibration and verification. In the present study, we measured tidally affected river flow in the Danshui River of northern Taiwan during the 2008 summer and fall. The measured flow data, produced by both state of the art and conventional instruments including particle tracking velocimetry (PTV), acoustic Doppler profiler (ADP), horizontal acoustic Doppler current profiler (H‐ADCP) and Price‐type current metre (PCM) were compared and correlated during several tidal cycles. The mean flows by ADP were similar to that by PCM and highly correlated with those by PTV and H‐ADCP. Index velocity rating between ADP and H‐ADCP separately computed during the flood and ebb tides shows good correlation for the ebb tide and poor correlation for the flood tide due to subsurface maximum velocity in the middle‐ to near‐bottom layers.

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 &amp; 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.

Helical cell motions in a small ice‐covered meander river reach

AbstractThe investigation of the flow field with a pulse‐coherent acoustic Doppler profiler has led to new high resolution observations of the secondary flow pattern occurring in a natural ice‐covered meander reach. Surveys were conducted during two successive winter periods with different ice conditions. Massive frazil ice accumulation was present during one of the survey and its influence on the flow pattern could be assessed. Results show that the primary flow is clearly deflected towards the outer bend. Secondary flows are one order of magnitude less than the primary flow and they display two stacked counter‐rotating helical cells pattern occurring at the entrance of the bend. This pattern is associated with the parabolic shape of the velocity profiles entering the bend. The pattern rapidly evolves downstream, reducing to one helical cell rotating in an opposite direction than what is observed in open channel flows. Flow mixing and morphological non‐uniformity are potential factors governing the development of the helical cells throughout the bend. Our observations show that a similar coherent flow pattern rapidly forms downstream of a massive frazil ice obstruction in the bend. Frazil ice does not constrain the formation of helical flow pattern in river bends. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Wind Speed Dependence of Single-Site Wave-Height Retrievals from High-Frequency Radars

Abstract Wave-height observations derived from single-site high-frequency (HF) radar backscattered Doppler spectra are generally recognized to be less accurate than overlapping radar techniques but can provide significantly larger sampling regions. The larger available wave-sampling region may have important implications for observing system design. Comparison of HF radar–derived wave heights with acoustic Doppler profiler and buoy data revealed that the scale separation between the Bragg scattering waves and the peak energy-containing waves may contribute to errors in the single-site estimates in light-to-moderate winds. A wave-height correction factor was developed that explicitly considers this scale separation and eliminates the trend of increasing errors with increasing wind speed.

OBSERVATION OF CURRENTS AND SUSPENDED SEDIMENT CONCENTRATION OFF NORTHERN JIANGSU COAST, CHINA

The abandoned Yellow River delta off Jiangsu coast is an important sediment source area,where erosion occurs and large amounts of suspended materials are carried seaward. In order to understand the hydrodynamic process here,a 3-vessel simultaneous measurement was conducted with ADP (acoustic Doppler profiler) and water samplers in autumn of 2006. The analyses of survey data show suspended sediment fluxes,the changes of currents and suspended sediment concentrations. The net suspended sediment fluxes of a tidal cycle range from 103 kg m-1 to 104 kg m-1,mainly to the south. First order estimation of yearly fluxes through the area is similar to a long-term average estimation. The investigation shows the currents are controlled by tides. The suspended sediment concentrations change within a tidal cycle. There are peaks of suspended sediments concentrations corresponding approximately to the time just after the maximum current velocity. Data show that velocity gradients become larger when velocity is bigger. High velocity gradients mean high bed shear stress. The high bed shear stress in turn causes the erosion and resuspension of seabed sediments,resulting in a high suspended sediment concentration in the water column.

Integrated monitoring of the hydro-morphodynamics of a beach protected by low crested detached breakwaters

The aim of this paper is to propose an integrated low cost system for monitoring the performance of beach defence works during storms. For this purpose, the site of Igea Marina, Northern Adriatic Sea, Italy, which is protected by low crested detached breakwaters, is selected. The monitoring system is composed by a video installation, for the identification of the shoreline position and of the intertidal bathymetry at high space and time resolution, and a 2DH numerical model, to reconstruct the hydrodynamics induced by coastal defenses. The accuracy of the monitoring system is verified against available measurements of waves and currents performed during a field campaign with acoustic Doppler profilers. Assimilation of data on the shoreline position in 2DH model operatively working is challenging in the improvement of the production of risk maps. These show the current intensities and flooded areas during forecasted storms. First promising results suggest the possibility of setting up an early warning tool.

Flood flows and channel conveyance in the Athabasca River Delta, Canada

Channel conveyance, channel size and water levels in the Athabasca River Delta were investigated in detail during three summer seasons. The Athabasca Delta is between 58°N and 59°N, in Alberta, Canada, and is formed immediately upstream of where the river enters Lake Athabasca. Ice-free flow rates in the river and in the various distributary channels were measured by acoustic Doppler profiler, and water surface elevations by differential GPS and pressure transducer recorders. Approximate values were measured for river bank heights, and a profile of elevation for the river bank for a distance of 80 km upstream of the lake was established. Computed bankfull flows for cross-sections starting at the delta apex were found to reduce significantly in the downstream direction. High values for bankfull conveyance at the head of the delta, equivalent to 50-year return period flood flows, were noted. For medium Athabasca River flows (about 800–1100 m3/s), a comparison was made of water surface elevations at the delta apex during the survey, and data from 1971. A significant increase of water surface height for the same flow rate, by as much as 0.8 m, is apparent, indicating rapid channel aggradation.

Acoustic profilers and urban pollutant fluxes

ABSTRACT A prudent management of networks, along with the minimisation of pollution discharged into receiving waters, requires an accurate knowledge of flow rates and pollutant loads conveyed in sewers. Conventional methods are based on well-known techniques, including acoustic devices and the sampling and analysis of total suspended solids. Although such methods yield satisfactory results for a typical gauging campaign, sensor implementation proves to be rather difficult. This study is intended to test the capabilities of acoustic Doppler profilers (ADP); the paper will present a PC-ADP profiler and the set of accompanying experiments. Initial results demonstrate great improvement offered by profiling techniques, in terms of both simultaneity and time savings.

Storm surge induced flux through multiple tidal passes of Lake Pontchartrain estuary during Hurricanes Gustav and Ike

In September 2008, Hurricanes Gustav and Ike generated major storm surges which impacted the Lake Pontchartrain estuary in Louisiana. This paper presents analyses of in situ measurements acquired during these storm events. The main data used in the analyses were from three bottom mounted moorings equipped with conductivity, temperature, and depth sensors, acoustic Doppler current profilers (ADCPs), and a semi-permanent laterally mounted horizontal acoustic Doppler profiler (ADP). These moorings were deployed in the three major tidal channels that connect Lake Pontchartrain with the coastal ocean. A process similar to tidal straining was observed: the vertical shear of the horizontal velocity was negligible during the inundation stage, but a shear of 0.8 m/s over a less than 5 m water column was recorded during the receding stage, 2–3 times the normal tidal oscillations. The surge reached its peak in the Industrial Canal 1.4–2.1 h before those in the other two channels. The inward flux of water lasted for a shorter time period than that of the outward flux. The inward flux was also observed to have much smaller magnitude than the outward flux (∼960–1200 vs. 2100–3100 million m 3). The imbalance was believed to have been caused by the additional water into Lake Pontchartrain through some small rivers and inundation over the land plus rainfall from the hurricanes. The flux through the Industrial Canal was 8–12%, while the flux through the other two tidal passes ranged between 17% and 70% of the total, but mostly split roughly half-half of the remaining (∼88–92% of the total).

Morphodynamics of the exit of a cutoff meander: experimental findings from field and laboratory studies

AbstractThe morphological evolution of the entrances and exits of abandoned river channels governs their hydrological connectivity. The study focusses on flow and sediment dynamics in the exit of a cutoff meander where the downstream entrance is still connected to the main channel, but the upstream entrance is closed. Two similar field and laboratory cases were investigated using innovative velocimetry techniques (acoustic Doppler profiling, image analysis). Laboratory experiments were conducted with a mobile‐bed physical model of the Morava River (Slovakia). Field measurements were performed in the exit of the Port‐Galland cutoff meander, Ain River (France). Both cases yielded consistent and complementary results from which a generic scheme for flow patterns and morphological evolution was derived. A simple analogy with flows in rectangular side cavities was used to explain the recirculating flow patterns which developed in the exit. A decelerating inflow deposits bedload in the downstream part of the cavity, while the upstream part is eroded by an accelerating outflow, leading to the retreat of the upstream bank. In the field, strong secondary currents were observed, especially in the inflow, which may enhance the scouring of the downstream corner of the cavity. Also, fine sediment deposits constituted a silt layer in a transitional zone, located between the mouth of the abandoned channel and the oxbow‐lake within the cutoff meander. Attempts at morphological prediction should consider not only the flow and sediment conditions in the cavity, but also the dynamics of the main channel. Copyright © 2010 John Wiley &amp; Sons, Ltd

High-resolution Monitoring of Sedimentary Processes in an Estuarine Channel

The erosion and deposition dynamics in a highly turbid estuarine channel were successfully surveyed using an echo sounder in combination with a high-resolution acoustic Doppler profiler. While the surface level of the channel bed was stable during neap tide, it underwent fluctuations as the current velocity increased. The bed sediment was eroded by the flood tide and was found as suspended sediment. During slack high tide, the suspended sediment was deposited on the channel bed and fluid mud was formed rapidly. The fluid mud was gradually eroded by the ebb tide, and the fluid mud thickness was approximately 0.05 m per semi-diurnal cycle during spring tide. Lastly, the height of the channel bed increased by 0.8 m during a semi-lunar tide.

Tidal movements of East Pacific green turtle Chelonia mydas at a foraging area in Baja California Sur, México

We tracked East Pacific green turtles Chelonia mydas using GPS-VHF telemetry in Estero Banderitas, a tidally-influenced foraging area in Bahia Magdalena, Baja California Sur, Mexico. Tidal currents were measured with a bottom-mounted Acoustic Doppler Profiler (ADP) and the data used to predict tidal current speed and direction at the location and time during which turtles were tracked. Twenty-nine turtles were tracked in the summers of 2000 to 2003. Vagility (mean ± SD; 18.6 ± 11.4 km d -1 ) and speed (0.83 ± 0.47 km h -1 ) of turtles was the greatest so far reported for green turtles at foraging areas. Turtles displayed highly linear movements, and move- ment patterns were circatidal. Vector correlation was used to compare turtle speed and direction with tidal speed and direction. Correlation coefficients were significant for 11 out of 13 tracks, indicating significant linear interdependence between turtles and tides. Speed and direction contributed equally to the correlation. Results indicated a new paradigm for green turtles in feeding areas, where turtles are transported on continual tides that allow them to exploit a patchy and seasonal distribution of algae, their main diet component. This tidal transport is markedly different than Selective Tidal Stream Transport, in which animals use either the ebb or flood tide for transport. Tidal currents may be an accurate indicator of turtle movement in tidal areas, and this transport system has implications for foraging ecology, energetics, and growth.

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.