Acoustic Doppler Current Profiler Surveys Research Articles

An Assessment of the Potential, Suitability and Sustainability of the Sand Mining Site in the Kemaman River Basin, Terengganu Using Acoustic Doppler Current Profiler

Sand mining from the catchment basin for building is a worldwide issue. The increasing demand for sand in the construction industry has led sand suppliers to look for alternative methods by which they can obtain source of sand from the riverbed. Floodplain and river slabs can be used as new sources of sand mining. In Sungai Kemaman, during September until March a high precipitation can cause high flow in river. This high flow can cause riverbank erosion which leads to instability of river. Therefore, river erosion can probably be reduced by identifying the potential area for sand mining. This research was conducted to analyse sand capability on floodplain and riverbed by integrating resistivity method and sediment transport loads using Acoustic Doppler Current Profiler (ADCP). Resistivity survey is used in determining the availability of potential soil at the study area and the equipment could measure subsurface profile up to 80 meters depth. Meanwhile, ADCP survey is utilized to make river profiler in term of velocity meshes and riverbed depth. The primary data collected was from 20 January 2014 to 19 February 2014. The findings found that the samples trapped in the Helly-Smith grabber were majority of the samples consisted more than 93% of gravel and sand materials and from the resistivity analysis, it is verified that the surrounding materials along the Sungai Kemaman is sandy material and high potential of the sand mining site.

Comparative Evaluation of Volumetric Current Measurements in a Tidally Dominated Coastal Setting: A Virtual Field Experiment

AbstractHigh-resolution, four-dimensional mapping of currents in tidally dominated coastal settings can be conducted with a range of instrumentation. Here, we assess four approaches to data collection: an X-band radar, a stationary (bottom mounted) acoustic Doppler current profiler (ADCP), a mobile (vessel based) ADCP, and a swarm of Lagrangian floats. Using the output from a hydrodynamic simulation, a virtual field campaign was performed at 24 locations in Admiralty Inlet, Puget Sound, Washington, during spring and neap tidal exchanges. A reconstruction of the volumetric currents was generated for each platform every 15 min and evaluated against the true currents to assess accuracy over a horizontal extent of 400 m × 500 m at 5 m resolution and vertically through the entire water column (20–80 m) at 2 m resolution. Results demonstrate that, for this survey extent and resolution, a vessel-based ADCP survey is most accurate, followed closely by the float swarm. The overall performance hierarchy persists over most locations and times. Thus, if mapping currents at high resolution (<10 m) and short time scales (<1 day) is the primary scientific objective, vessel-based ADCP surveys are likely the best option. For longer-duration surveys, a combined deployment with a stationary ADCP and X-band radar system is the best choice. Last, if in situ measurements of scalar properties (e.g., salinity, temperature, dissolved oxygen) are also desired, float swarms can simultaneously sample these while surveying currents with accuracy comparable to mobile ADCPs.

Rock Control of River Geometry: The Fraser Canyons

AbstractA 524‐km reach of Fraser River, British Columbia, was traversed, where the channel alternates between gravel‐bedded reaches that are incised into semiconsolidated glacial deposits and bedrock‐bound reaches, including Fraser Canyon. Continuous centerline acoustic Doppler current profiler survey and observations of bedrock confinement, supplemented with measurements on air photos, reveal the influence of bedrock on channel properties. The surveyed reach was subdivided into 10 morphological reaches, ranging from dominance by alluvial gravel‐bed channels to dominantly rock‐walled canyons. Measured data were grouped within each reach based on the presence of bedrock on both banks, one bank, or neither bank. From alluvial to mixed to canyon‐dominated reaches, river widths decreased while water depths, flow velocities, water surface slopes, and bed shear stresses increased. Within reaches, subreaches with bedrock on both banks were narrower and deeper with higher bed shear stresses but, at the observed stage, had lower depth‐averaged velocity and water surface slope than partly confined and alluvial (unconfined) subreaches. A conceptual model is presented representing a bedrock river as an alternating series of relatively low gradient, deep, rock‐walled pools and steep semialluvial or alluvial sections. Downstream hydraulic geometry relations have scaling exponents expected for alluvial channels provided that the alluvial and rock‐influenced sections are analyzed separately. In Fraser River, canyons become relatively narrower downstream, apparently in response to increasing rock uplift during the Quaternary period. The observations are compared against prior semitheoretical frameworks established to explain variations in width, slope, and channel shape along rockbound channels.

Evaluation of the Logarithmic Law of the Wall for River Flows

AbstractThe logarithmic law of the wall is commonly used to determine the shear stress applied to the river bed by the flow field. The shear stress calculation requires a velocity profile at the location of interest—data that can be obtained with a boat‐mounted acoustic Doppler current profiler (ADCP). ADCP survey procedures use either a fixed‐vessel (FV) or moving‐vessel (MV) with each providing different spatial and temporal resolution. MV procedures require significantly less field effort but the data lack the temporal resolution of FV measurements. This fact has motivated investigators to seek MV procedures and analysis techniques that provide equivalent results to FV measurements. This study compares results of the two survey procedures for depth‐averaged quantities (velocity and flow direction), mean velocity profiles, shear velocity and equivalent sand grain roughness. Mean velocity profiles are produced by time averaging for the FV measurements and spatial averaging for the MV measurements. Shear velocity and equivalent sand grain roughness are determined using simple linear regression applied to the logarithmic law of the wall. These procedures are demonstrated using data obtained during bankfull flow on the lower Roanoke River, North Carolina, USA. The results indicate comparable estimates of depth‐averaged quantities from both survey procedures. Estimates of the mean velocity profiles were found to be more variable. The agreement for shear velocity and equivalent sand grain roughness was generally poor indicating that further work is necessary to produce comparable results with MV survey procedures. Copyright © 2015 John Wiley & Sons, Ltd.

Flow in bedrock canyons

Bedrock erosion in rivers sets the pace of landscape evolution, influences the evolution of orogens and determines the size, shape and relief of mountains. A variety of models link fluid flow and sediment transport processes to bedrock incision in canyons. The model components that represent sediment transport processes are increasingly well developed. In contrast, the model components being used to represent fluid flow are largely untested because there are no observations of the flow structure in bedrock canyons. Here we present a 524-kilometre, continuous centreline, acoustic Doppler current profiler survey of the Fraser Canyon in western Canada, which includes 42 individual bedrock canyons. Our observations of three-dimensional flow structure reveal that, as water enters the canyons, a high-velocity core follows the bed surface, causing a velocity inversion (high velocities near the bed and low velocities at the surface). The plunging water then upwells along the canyon walls, resulting in counter-rotating, along-stream coherent flow structures that diverge near the bed. The resulting flow structure promotes deep scour in the bedrock channel floor and undercutting of the canyon walls. This provides a mechanism for channel widening and ensures that the base of the walls is swept clear of the debris that is often deposited there, keeping the walls nearly vertical. These observations reveal that the flow structure in bedrock canyons is more complex than assumed in the models presently used. Fluid flow models that capture the essence of the three-dimensional flow field, using simple phenomenological rules that are computationally tractable, are required to capture the dynamic coupling between flow, bedrock erosion and solid-Earth dynamics.

Modelling hydrodynamics in the Rio Paraná, Argentina: An evaluation and inter-comparison of reduced-complexity and physics based models applied to a large sand-bed river

Depth-averaged velocities and unit discharges within a 30km reach of one of the world's largest rivers, the Rio Paraná, Argentina, were simulated using three hydrodynamic models with different process representations: a reduced complexity (RC) model that neglects most of the physics governing fluid flow, a two-dimensional model based on the shallow water equations, and a three-dimensional model based on the Reynolds-averaged Navier–Stokes equations. Flow characteristics simulated using all three models were compared with data obtained by acoustic Doppler current profiler surveys at four cross sections within the study reach. This analysis demonstrates that, surprisingly, the performance of the RC model is generally equal to, and in some instances better than, that of the physics based models in terms of the statistical agreement between simulated and measured flow properties. In addition, in contrast to previous applications of RC models, the present study demonstrates that the RC model can successfully predict measured flow velocities. The strong performance of the RC model reflects, in part, the simplicity of the depth-averaged mean flow patterns within the study reach and the dominant role of channel-scale topographic features in controlling the flow dynamics. Moreover, the very low water surface slopes that typify large sand-bed rivers enable flow depths to be estimated reliably in the RC model using a simple fixed-lid planar water surface approximation. This approach overcomes a major problem encountered in the application of RC models in environments characterised by shallow flows and steep bed gradients. The RC model is four orders of magnitude faster than the physics based models when performing steady-state hydrodynamic calculations. However, the iterative nature of the RC model calculations implies a reduction in computational efficiency relative to some other RC models. A further implication of this is that, if used to simulate channel morphodynamics, the present RC model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations. These observations illustrate the trade off between model realism and efficiency that is a key consideration in RC modelling. Moreover, this outcome highlights a need to rethink the use of RC morphodynamic models in fluvial geomorphology and to move away from existing grid-based approaches, such as the popular cellular automata (CA) models, that remain essentially reductionist in nature. In the case of the world's largest sand-bed rivers, this might be achieved by implementing the RC model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales.

Physical structure of the Barents Sea Polar Front near Storbanken in August 2007

The Polar Front separating Atlantic Water (AW) and Arctic Water (ArW) is one of the most dominant meso- and large scale features in the Barents Sea. Here, the results of submeso-mesoscale (1–10km) variability in physical fields associated with the Barents Sea Polar Front (BSPF) are reported from a high-resolution ADCP (Acoustic Doppler Current Profiler) and CTD (Conductivity–Temperature–Depth sensors) survey near Storbanken in August 2007. A surface front separating AW and melt water with a strong salinity gradient was present, while the subsurface BSPF was characterized by a strong temperature gradient and thermohaline compensation. Isopycnal mixing leading to the formation of Polar Front Water (PFW) was observed. The dominant flow was a barotropic southeastward along-frontal jet with two cores, coinciding with the surface front and the BSPF. This gives new insights into the circulation at the BSPF. Small-scale variability in the hydrographic and dynamic structures was observed, which were rarely resolved in previous cruises. Such submeso–mesoscale physical processes can potentially have significant impacts on the biogeochemistry and biology in the area, indicating the importance of parametrizing the processes in future climate models.

Considerations of a horizontal axis tidal turbine

This paper considers the performance of a horizontal axis tidal turbine, at two sites off the Welsh coast of the UK, one in the Severn estuary and the second off Anglesey. Acoustic Doppler current profiler surveys were performed to investigate local velocity profiles. A small water plane area twin hull (Swath) bathymetric survey established the local bathymetry of the area. Energy extraction from a three-bladed 10 m diameter turbine was studied using a quasi-static computational fluid dynamics model and velocity profile data from the acoustic Doppler current profiler surveys. The acoustic Doppler current profiler data were rescaled allowing further insight to the attenuation of power density through the water column at a higher tidal velocity. The velocity profile has a significant effect on power attenuation reducing the extracted power to 30–40% of peak power. The turbine performance calculation is highly dependent on the velocity used; the velocity should be monitored around five turbine diameters upstream, at the depth of its axis of rotation. The axial load on the turbine peaks at the freewheeling velocity dropping to 88% at peak power. It is suggested that the velocity at the Severn estuary site is, unlike the Anglesey site, unlikely to provide a suitable level of power to be viable.

Observations on the circulation of the Saronikos Gulf: A Mediterranean embayment sea border of Athens, Greece

The Saronikos Gulf three‐dimensional flow structure is mapped by objective analysis in 12 acoustic Doppler current profiler surveys (4 seasonal surveys per period for the periods May 1998–May 1999, May 2000–May 2001, May 2003–May 2004) under various winds. Robust seasonal flows throughout the Gulf are basically induced by thermohaline effects (winter water subduction, different responses to atmospheric heating of shallow and deeper areas of the Gulf) and density contrasts with inflowing Aegean waters. Inner Gulf observations show that the seasonal flows are modified by the wind, while recurrent structures (cyclonic and anticyclonic) appear between the seasonal flows and the coast of Attica. In summer an anticyclonic and a cyclonic flow exists throughout the Gulf above and below the pycnocline, respectively, with a jet observed to meander within the Inner Gulf. In winter and early spring an anticyclonic flow prevails in the upper ∼100 m. In late spring‐early summer cyclonic and anticyclonic flow occurs in the upper (∼0–40 m) and deeper (∼60–100 m) layers, respectively. The predominant northerly winds in summer and winter push the Inner Gulf eastward seasonal jet to the south and favor the formation of a recurrent cyclonic upwelling structure between this jet and Attica. Northwesterly, westerly, and southerly winds favor the northward meandering of the seasonal jet in the Inner Gulf and no recurrent structures appear in the limited space between this jet and Attica. In the southeast area of the Inner Gulf, the wind‐driven flow fluctuations at 30 m have time scales of 3.5–4 days. Observed meandering period is ∼13 days; meandering characteristics are close to instability predictions. Time scales of flows due to recurrent structures are ∼8 days.

Evaluation of Tidal Removal Method Using Phase Average Technique from ADCP Surveys along the Peng-Hu Channel in the Taiwan Strait

Three cruises with shipboard Acoustic Doppler Current Profiler (ADCP) were performed along a transect across the Peng-hu Channel (PHC) in the Taiwan Strait during 2003-2004 in order to investigate the feasibility and accuracy of the phase-averaging method to eliminate tidal components from shipboard ADCP measurement of currents. In each cruise measurement was repeated a number of times along the transect with a specified time lag of either 5, 6.21, or 8 hr, and the repeated data at the same location were averaged to eliminate the tidal currents; this is the so-called ”phase-averaging method”. We employed 5-phase-averaging, 4-phase-averaging, 3-phase-averaging, and 2-phase-averaging methods in this study. The residual currents and volume transport of the PHC derived from various phase-averaging methods were intercompared and were also compared with results of the least-square harmonic reduction method proposed by Simpson et al. (1990) and the least-square interpolation method using Gaussian function (Wang et al. 2004). The estimated uncertainty of the residual flow through the PHC derived from the 5-phase-averaging, 4-phase-averaging, 3-phase-averaging, and 2-phase-averaging methods is 0.3, 0.3, 1.3, and 4.6 cm s-1, respectively. Procedures for choosing a best phase average method to remove tidal currents in any particular region are also suggested.

Flow‐field observations of a tidally driven island wake used by marine mammals in the Bay of Fundy, Canada

AbstractCorrelations between fine‐scale oceanographic features and aggregations of marine mammals are frequently reported, but the physical forces shaping these relationships are rarely explored. We conducted a series of oceanographic observations and remote sensing surveys of an oceanographic feature near Grand Manan Island known to attract marine mammals on flood tides. We tracked drift drogues from cliff‐top with a theodolite and conducted box‐type surveys with an acoustic Doppler current profiler (ADCP) to assess flow patterns within the oceanographic feature. The feature was also visualized with Synthetic Aperture Radar (SAR) scenes. Drift drogues were advected towards a shear line originating near the northern tip of the island and entrained in one or more eddies downstream. ADCP surveys confirmed the presence of the shear line between rapid easterly flow and slower return flow. As the tide progressed, the shear line extended and manifested a single anti‐cyclonic eddy at its distal end. As the flood tide progressed, northerly flow along the eastern shore of the island intensified and deflected the shear line northwards, shedding the eddy at slack high water. SAR images confirmed the presence of the shearline and eddy system, illustrating the evolution of a wake behind the island on flood tides. Profiles of flow direction and acoustic backscatter revealed secondary flows within the wake consistent with models and observations of other wakes. Oceanographic and remote sensing observations confirm that an island wake is generated by tidal flow past Grand Manan Island and provide an ecological context for the predictable aggregations of odontocete and mysticete cetaceans observed foraging within this region.

Acoustic Doppler current profiler surveys along the Yangtze River

An acoustic Doppler current profiler is used to characterize the river velocity against the morphology of the Yangtze River from Chonqing to the sea. High flow velocities occur in the Three Gorges section and lower velocities in the middle and lower reaches of the river. This is largely due to the change in river pattern from a high gradient deeply-cut valley to a flat fluvial plain. Flow velocities fluctuate in the middle Yangtze due to the presence of meander bends of different length. There are numerous smaller velocity fluctuations in the lower Yangtze channel that reflect multichannel pattern with numerous sand bars and a river morphology affected by bedrock outcrops. Water depths of 40–100 m occur in the Three Gorges valley but decrease to 15–40 m in the middle and lower Yangtze. At the Gezhou Reservoir, 30 km downstream of the Three Gorges damsite velocity drops to low (< 1.0 m s − 1 ) 20 km reach. A second low velocity (< 0.5 m s − 1 ) zone, about 20 km in length, is located in the lower Yangtze near the coast probably due to the tidal influence. The results from this research will serve as a datum for evaluating changes to the river once the Three Gorges dam is completed in 2009.

Advancing from pattern to process in Hawaii’s near-shore pelagic ecosystem

Micronekton comprising a near-shore sound-scattering layer around the Hawaiian Islands have been shown to exhibit diel horizontal migrations, moving onshore until midnight and them moving offshore into deep waters where they remain in the day. The reasons for this daily movement pattern have remained unexamined. Moored echosounders were used to describe micronekton migration over a 5-week period. A moored acoustic Doppler current profiler (ADCP), thermistor chains, point current meters, and an autonomous vertical profiler provided a description of the physical circulation. Periodic four-frequency echosounder and ADCP surveys were used to further characterize the micronekton’s movement along with water column physics. During surveys, vertical profiles measured zooplankton abundance using the Tracor acoustic profiling system (TAPS) and integrated net tows, micronekton identity and size using an optical imaging system, and primary productivity with a fluorometer. It was found that micronekton are not simply tracers of water mass movement indicating active migration. Near-shore waters had significantly higher biomass of zooplankton during nighttime hours than offshore waters, suggesting micronekton have greater access to food resources as a result of their migration. Primary productivity was also significantly higher inshore, indicating bottom-up control of this system. These results suggest the underlying reasons behind this daily horizontal movement.

The front on the Northern Flank of Georges Bank in spring: 1. Tidal and subtidal variability

During March–April 1999, 2 weeks of undulating CTD and shipboard acoustic Doppler current profiler surveys revealed the variability of the intense internal tide on the northern edge of Georges Bank. The nature of the internal tide was modulated by episodic surface intrusions of cool, fresh Scotian Shelf Water (SSW), stratifying the otherwise vertically well‐mixed outer bank. The introduction of SSW created a system analogous to that in summer, when the outer bank is thermally stratified and separated from well‐mixed regions by a tidal mixing front. During SSW intrusions, internal tidal behavior is characterized by tidal advection that is significantly faster than internal wave propagation speeds on the bank (supercritical flow) and slower than the lowest internal modes in deeper water to the north (subcritical flow). A large‐amplitude internal lee wave develops over the slope during off‐bank tidal flow. This stalled energy is released to propagate on‐bank as a high‐frequency internal wave when the tide reverses. It is suggested, by analogy with the summer case, that a portion of this energy is used by mixing at the on‐bank limit of SSW. The presence of SSW thus modifies the internal tidal response, enabling a mechanism that contributes to its assimilation with ambient bank water. The off‐bank‐propagating internal tide is dominated by a mode 1 internal wave at the M2 frequency, with cross‐bank velocities comparable to the barotropic tide.

Enhanced marine CH4 emissions to the atmosphere off Oregon caused by coastal upwelling

Methane in surface waters and marine air off Oregon (44°24′N–44°54′N, 124°36′W–125°24′W) was continuously surveyed in July 1999. During a high‐resolution survey after a period of steady winds from the north, CH4 concentrations were high in the northeastern region, near the shelf edge. The highest CH4 concentrations were 2.5 times higher than equilibrium with the atmospheric partial pressure. In contrast, concentrations were near equilibrium in the western part of the survey area, the Hydrate Ridge. The increase in CH4 from southwest to northeast correlates with a drop in sea surface temperature (SST), from 16.5°C to &lt;13.5°C, toward the shelf edge. The observed SST pattern was caused by summer upwelling off Oregon. The results suggest that CH4 derived from bottom sources near the shelf/slope break and methane found in connection with shallow (100–300 m) turbidity layers is transported to the surface by coastal upwelling, which causes an enhanced net flux of CH4 to the atmosphere. Vertical profiles of the methane distribution on the shelf in October demonstrate the accumulation of methane introduced by shelf sources. Surface concentrations at these stations in October (during nonupwelling conditions) were lower than in July (during upwelling) and were only slightly oversaturated with respect to the atmosphere. An acoustic Doppler current profiler survey indicates that the observed trend cannot be attributed to a surface flow reversal in the area. The low‐salinity waters in the core of the Columbia River plume (S &lt; 31) showed no enhanced CH4 concentrations. The trend of higher CH4 concentrations at lower temperatures existed over the whole 17‐day survey, but large spatial and temporal variations existed. The presence of methane sources in regions of coastal upwelling worldwide, such as shallow seeps, gas hydrates, and intermediate nepheloid layers, suggests that the enhancement of CH4 fluxes to the atmosphere by coastal upwelling occurs on a global scale.

Freshwater and temperature transports through the Tsushima‐Korea Straits

Using acoustic Doppler current profiler (ADCP) and conductivity‐temperature‐depth (CTD) data sets obtained from 1991 to 1993, volume, freshwater, and temperature transports through the Tsushima‐Korea Straits are estimated. To remove tidal currents contained in raw ADCP data, tidal currents are calculated using harmonic constants obtained by ADCP surveys from 1998 to 1999. The annual mean and range of the volume transport through the straits are 2.4 Sv (1 Sv = 106 m3/s) and 0.8 Sv, respectively. These values are consistent with those of previous studies. The annually averaged freshwater transport is 33 × 106 kg/s, which is the same as the total river discharge around the Yellow and East China Seas. This suggests that the total river discharge and freshwater transport through the Tsushima‐Korea Straits are the prominent two components in the freshwater budget, averaged annually in the Yellow and East China Seas. The annually averaged temperature transport through the Tsushima‐Korea Straits is 0.17 PW.

Mesoscale Currents on the Inner New Jersey Shelf Driven by the Interaction of Buoyancy and Wind Forcing

Abstract Shipboard hydrographic and acoustic Doppler current profiler surveys conducted in August 1996 on the New Jersey inner shelf revealed a buoyant intrusion advancing southward along the coast. This buoyant intrusion originated from the Hudson estuary more than 100 km upshelf and appeared as a bulge of less saline water with a sharp across-shelf frontal zone at its leading edge. During this time, the study area was also forced by a brief upwelling-favorable wind event opposing the direction of buoyant flow propagation. The interaction of buoyancy and wind forcing generated a spatially variable velocity field. In particular, across-shelf currents were comparable to their alongshelf counterparts. Variability in the alongshelf direction occurred on the scales of the order of the baroclinic Rossby radius. Intensive across-shelf currents reached speeds of 20– 40 cm s−1 and appeared as spatially localized mesoscale flows with a width of O(10 km). They were generated at the leading edge of the buoyant intru...

Effect of the Earth's rotation on the circulation in regions of freshwater influence

Recent surveys in various regions of freshwater influence have shown considerably different flow patterns in each region. An analytical model including viscous effects and the Earth's rotation is proposed to examine the along‐channel flow pattern and to explain the differences. Model results show that the flow pattern is strongly dependent on the Ekman number E. With a large Ekman number (E &gt; 1) the system is governed by gravitational circulation, and thus horizontal density gradients in along‐channel direction are important. The whole water column is in the Ekman layer and consequent jet inflow penetrates the surface over a deep depression if the bottom topography varies in a cross‐channel direction. With an intermediate Ekman number (E ∼ 0.1) this jet inflow concentrates in the lower layer in which the viscosity still plays an important role. The flow in the upper layer is, on the other hand, determined by the geostrophic balance. The contribution by the geostrophic flow becomes larger so that the cross‐channel density gradients are important when die Ekman number is small (E &lt; 0.01). Since the Ekman layer is clung to the bottom, the jet inflow exists only in the thin bottom layer. The hydrographic and acoustic Doppler current profiler surveys were conducted in Ise Bay, Japan. Both the density structure and the flow pattern were different from those observed in many drowned river valleys. A strong jet inflow existed in the lower layer over the depression while the flow in the upper layer suggested anticyclonic circulation. The estimated Ekman number is 0.07 in the bay and thus the observed pattern is consistent with the model result when E = 0.1.

A Portable Retractable ADCP Boom-Mount for Small Boats

This paper describes the design, testing, and application of a portable and retractable shipboard acoustic Doppler current profiler (ADCP) boom-mount. The boom is specifically adapted for small fiberglass boats working with a minimal crew. The design permits the rapid collection of ADCP data on discontinuous transects which would be difficult or impossible using a large displacement hull vessel or with a towed vehicle. This capability is particularly useful in shallow wind-driven estuaries and in tidal channels where flow time scales are often on the order of several hours. Tests of the boom show that high quality ADCP data (as measured by percent good) can be obtained at boat speeds up to 4.0 m s−1 and that data quality depends on transducer depth. The utility of the retractable design is demonstrated with an 8 m boat on two nearly synoptic ADCP surveys of a shallow estuary. With minor modifications to accommodate different vessel geometries, the design could be readily adapted for use on similar vessels.

Detiding Three-Dimensional Velocity Survey Data in Coastal Waters*

A three-dimensional data interpolation technique is proposed that efficiently removes tidal currents from spatial velocity surveys. The least squares method extends prior two-dimensional detiding methods to three spatial dimensions using biharmonic splines. Biharmonic splines are fitted to velocity data from acoustic Doppler current profiler (ADCP) surveys, moorings, and ocean surface current radar (OSCR). The data are used to predict diurnal and semidiurnal tidal currents on the inner shelf off New Jersey that vary between 1 and 15 cm s−1 at spatial scales of about 20 km. The (tidal) signal to (subtidal) noise is thus O(1) in the study area. Although the main task of this study is to remove tidal variance from the ADCP survey data, an attempt is made to accurately“predict” tidal currents from the data. The latter task is more difficult. Both artificial data with known signal-to-noise properties and actual measurements indicate that the method estimates both diurnal and semidiurnal tidal currents to within about 3.5 cm s−1 rms, or 30% of the true tidal signals. While the biharmonic splines remove tidal currents successfully, the prediction of the vertical structure of tidal currents is only fair. Some experimentation guided by physical intuition and prior knowledge of the tidal fields is necessary in order to obtain an accurate and stable solution. While this ambiguity constitutes the main disadvantage of the method, its simple algebraic expression to predict tidal currents in space and time is its main advantage. Properly weighting velocity data from different sources, such as moorings, surface current radar, and ADCP surveys of different quality, improves the quality of the fit.