Acoustic Doppler Current Profiler System Research Articles

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.

Instrumentation for direct measurements of wave‐driven flow over a fringing reef crest

AbstractShallow dynamic flows are very important processes in environmental systems, yet they are notoriously difficult to measure. For example, on coral reefs, shallow flows over the reef crest typically range from 0 to 1 m depth and velocity up to 1–2 m s−1 including oscillatory motion of waves. To directly measure depth and velocity in this challenging environment, we retrofitted a vertical acoustic Doppler current profiler (ADCP, Teledyne, RDI) designed for streams into a new shallow water acoustic Doppler current profiler system (SW‐ADCP) to expand memory and power limitations and allow for deployment in wave‐dominated environments. We captured variations in shallow reef crest depth and flow over a period of 3 weeks in the reef/lagoon system of Ofu Island, American Samoa. The new SW‐ADCPs recorded water depth and three‐dimensional velocity in 3 cm bins every 3 s. We then used velocity profiles to estimate volumetric flow and drag coefficients and verified these estimates using boundary layer theory. We observe that the mean velocity profile is well approximated by a log‐layer formulation with z0 of 3.5 cm, despite the shallow depths, strong flows, and breaking waves. Our observations validated the use of SW‐ADCPs as a tool for measuring flows in shallow (0.1–1 m), dynamic coastal marine environments.

Monitoring tidal currents with a towed ADCP system

The tidal circulation in the semi-enclosed Boulogne harbour (eastern English Channel) is measured during the various stages of the tidal cycle with a low-cost towed Acoustic Doppler Current Profiler (ADCP) system for the first time. The system is equipped with an interpolation algorithm which allows reconstructing space-time evolution of the velocity field for surveys whose duration is comparable or larger than the typical time of tidal variation (1–2 h). The method employs space-time velocity covariances derived from a numerical simulation of the surveyed area by a high-resolution relocatable model “Model for Applications on Regional Scale” (MARS). The covariances are utilized by the optimal interpolation algorithm to obtain the most likely evolution of the velocity field under the constraints provided by the ADCP observations and their error statistics. Technically, the MARS model run provides the first guess (background) evolution of the velocity field in the surveyed area which is then corrected by the data in a statistically consistent manner as it explicitly takes into the account both observational and modeling errors. The quality of the velocity reconstruction was validated against independent bottom-mounted ADCP data, the background model evolution, and against the results of spatial interpolation by Kriging technique. All tests demonstrated significant (30 to 60 %) reduction of the model-data misfit for the velocity field obtained as a result of space-time optimal interpolation. Although the method was applied to recover surface circulation, it can be extended for assessment of the full 4D tidal flow dynamics using the data recorded throughout the entire water column.

A Comparison of Vessel-Mounted Acoustic Doppler Current Profiler and Satellite Altimeter Estimates of Sea Surface Height and Transports between New Jersey and Bermuda along the CMV Oleander Route

Abstract In recent years the acoustic Doppler current profiler (ADCP) has found increasing use on commercial vessels to measure currents and their variability along selected routes in the ocean. One such dataset, in operation since late 1992, is the ADCP record from the Container Motor Vessel (CMV) Oleander, which operates between New Jersey and Bermuda. Because the Oleander ADCP system measures upper-ocean currents of O(10−2) m s−1 accuracy every 2.5 km, it provides excellent coverage of the mesoscale and submesoscale velocity field, and also of transport. The question addressed here is how well do estimates of fluxes between the continental shelf break and Bermuda compare with corresponding geostrophic estimates derived from satellite altimeter measurements of sea level extracted from weekly mapped fields along the same route. The Oleander route spans three distinct deep-sea regions: the Slope Sea, the Gulf Stream, and the Sargasso Sea. Agreement in sea surface height variability depends principally upon the length of the section being compared, and not upon eddy kinetic energy levels. Thus, yearly averages for short subsections such as across the quiet Slope Sea and energetic Gulf Stream both have correlation coefficients in excess of 0.9, whereas across the longer Sargasso Sea the correlation coefficient drops to 0.64 and to 0.58 for the 950-km-long Slope–Bermuda section. The principal cause of decrease in correlation with increasing distance appears to be due to ageostrophic flow, principally the Ekman layer and inertial motion, measured by the ADCP but not represented in the altimeter-derived geostrophic fluxes.

Limitations in Extraction of Survey Data from Real-Time Kinematic GPS ADCP Systems

A total of 595 real-time kinematic global positioning system (RTK-GPS) water surface elevations (WSE) were measured using georectified and nongeorectified survey systems along a 21-km-long river reach to evaluate vertical coordinate accuracy of a nongeorectified RTK-GPS acoustic Doppler current profiler (ADCP) system. A georectified survey of a series of quality control benchmarks produced a vertical root-mean-square error (RMSE) of 0.033 m, which was considered globally accurate for river survey purposes. The WSE measured simultaneously at the same locations comparing the nongeorectified to the georectified system resulted in a RMSE=0.741 m. Evaluation of intersetup precision of the nongeorectified WSE confirmed relative accuracy to each initialized arbitrary vertical datum but were found not to be globally accurate. Only 7% of the nongeorectified base station setups resulted in RMSE less than the combined manufacturer’s tolerances when compared to the georectified system at the same spatial locations. When vertical datum corrections were applied to the nongeorectified system from the globally accurate RTK-DGPS system, 80% of the base station setups demonstrated RMSE within the combined manufacturer’s tolerances. Datum corrections varied greatly, ranging from −2.43 to +1.80 m. This study demonstrates that when using nongeorectified RTK-GPS ADCP systems for purposes other than discharge measurement, such as in the extraction of bathymetry data for use in hydraulic assessments and models, great care must be exercised when verifying WSEs

A near‐inertial mode observed within a Gulf Stream warm‐core ring

Layering of ocean velocity “fine structure” has been coherently observed across the entire extent of a Gulf Stream warm‐core ring using a shipboard acoustic Doppler current profiler system in September 2009 and independently sampled as the ring transited a moored array. Lines of constant velocity phase generally followed isopycnals as they deepened within the ring center. We also observed a clear separation of the vertical structure of the flows associated with the ring (of order 0.5 m/s) with the shorter (200 m) and less energetic (~0.2 m/s) flows of the velocity fine structure, which was further observed to rotate clockwise with increasing depth, consistent with downward propagating near‐inertial waves (NIWs). Observations are consistent with a ring‐scale NIW packet, probably wind forced, that shows enhanced NIW energy within the sloping pycnocline at depths of 300–700 m. Evidence of wind‐forced NIWs within anticylonic eddies in a numerical simulation shows some similar features to our observations, which we try to understand physically with basic WKB‐type wave/current dynamics along the lines of previously published work and a new calculation of NIW trapping within an isolated, baroclinic vortex.

Practical aspects of ADCP data use for quantification of mean river flow characteristics; Part I: moving-vessel measurements

Acoustic Doppler Current Profiler (ADCP) measurements have gained considerable popularity because of their capabilities to efficiently measure discharges from boats moving across river transects. The accuracy of the estimated discharges is relatively good compared with the existing measurement alternatives. Currently, there is an increased demand for measurement of the mean velocity field in rivers for applications where mean and turbulence quantities play important roles. Given the good estimates for discharges, it can be inferred that velocity profiles collected during transects can be potentially used for characterization of the average mean velocity field. Instrument and operational limitations, as well as environmental factors result in large scattering of the instantaneous velocity profiles collected from moving boats during transects, making it difficult to readily use the collected data for reporting mean velocities and turbulence parameters across the depth or in cross-sections. The paper presents several methods for enhancing the significance of the ADCP velocity distribution profiles collected from moving boats and proposes complementary software to further process and visualize the ADCP data for better support of hydraulic investigation requirements. The paper analyzes first the assumptions and error sources involved in the measurement process. The appearance of the velocity profiles collected with ADCP operated from moving vessels and post-processing algorithms for data conditioning and smoothing are tested and evaluated next. The relevance of data collected from moving vessels along transects for description of the mean velocity profiles is inferred by comparison with data obtained with the same ADCP system and settings, collected from fixed locations along the transect route. Velocities enhanced using the optimum tested post-processing algorithms are used to further determine depth-averaged velocities, velocity components in river coordinates, and river discharge estimates.

Using ADCP Background Sound Levels to Estimate Wind Speed

Abstract It is well known that ambient sound is generated by wind through the process of wave breaking and bubble injection. The resulting sound levels are highly correlated with wind speed and, even though the physical process is not fully understood, sound levels can be used to estimate wind speeds with accuracies comparable to other marine wind measurement techniques. It has been noted by several researchers that background sound levels in acoustic Doppler current profiler (ADCP) systems are correlated to wind speeds; however, conventional wisdom would suggest that this signal should be dominated by thermal noise. In this report, background sound levels in 75-, 150-, and 300-kHz ADCP systems have been investigated. Techniques required to convert raw data into absolute sound levels and to adjust these values to estimate representative surface sound levels are presented. Only the background sound levels in the 150-kHz ADCP retain a signal from the surface-generated ambient sound. For these systems, deplo...

TRANSIENT STORAGE ASSESSMENTS OF DYE‐TRACER INJECTIONS IN RIVERS OF THE WILLAMETTE BASIN, OREGON1

ABSTRACT: Rhodamine WT dye‐tracer injections in rivers of the Willamette Basin yield concentration‐time curves with characteristically long recession times suggestive of active transient storage processes. The scale of drainage areas contributing to the stream reaches studied in the Willamette Basin ranges from 10 to 12,000 km2. A transient storage assessment of the tracer studies has been completed using the U.S. Geological Survey's One‐dimensional Transport with Inflow and Storage (OTIS) model, which incorporates storage exchange and decay functions along with the traditional dispersion and advection transport equation. The analysis estimates solute transport of the dye. It identifies first‐order decay coefficients to be on the order of 10−5/sec for the nonconservative Rhodamine W.T. On an individual subreach basis, the first‐order decay is slower (typically by an order of magnitude) than the transient storage process, indicating that nonconservative tracers may be used to evaluate transient storage in rivers. In the transient storage analysis, a dimensionless parameter (As/A) expresses the spatial extent of storage zone area relative to stream cross section. In certain reaches of Willamette Basin pool‐and‐riffle, gravel‐bed rivers, this parameter was as large as 0.5. A measure of the storage exchange flux was calculated for each stream subreach in the simulation analysis. This storage exchange is shown subjectively to be higher at higher stream discharges. Hyporheic linkage between streams and subsurface flows is the probable physical mechanism contributing to a significant part of this inferred active transient storage. Hyporheic linkages are further suggested by detailed measurements of river discharge with an Acoustic Doppler Current Profiler system delineating zones in two large rivers where water alternately enters and leaves the surface channels through gravel‐and‐cobble riverbeds. Measurements show patterns of hyporheic exchange that are highly variable in time and space.

TOLEX‐ADCP monitoring

Since February 1991, the Physical Oceanography Group, Tohoku University has conducted a program to monitor the oceanic current field across the Kuroshio over the Izu Ridge, south of Japan, using the Acoustic Doppler Current Profiler (ADCP) system installed on the ferry Ogasawara Maru. This ferry shuttles 58 times in a usual year between Tokyo and Chichijima in the Ogasawara (Bonin) Islands. This ADCP monitoring is regarded as one component of the Tokyo‐Ogasawara Line Experiment (TOLEX). In this paper, we will briefly describe our ADCP system and data processing procedures, and will present several aspects of the current fields using the data obtained through January 1996. During the period, the Kuroshio took the bimodal paths of 34°N and 32°30′N as mean positions on the ship course. In the sea south of the Kuroshio, many cyclonic and anticyclonic eddies with a scale of 200–400 km in diameter were observed with passing time of two–four months.

Performance and Calibration of an Acoustic Doppler Current Profiler Towed below the Surface

Abstract A towed acoustic Doppler current profiler (ADCP) system was tested. The instrument was deployed from ships of opportunity and towed at depths between 5 and 25 m. The towed system carries upward- and downward-looking ADCPs. The instrument platform is stable in most operating conditions at ship speeds up to 4.5 m s−1. Large discrepancies are found, however, between the ship's velocity obtained from bottom-tracking ADCP pulses and that from navigational data. These are explained with a magnetic compass bias that varies with the ship's heading direction. Both the ship and the tow platform induce magnetic fields that bias the ADCP compass. An in situ compass calibration scheme is thus necessary and requires accurate navigational data. In our main study area, it is found that the Global Position System provides absolute and relative positions to within 88 and 4 m, respectively. These accuracies are sufficient for calibration purposes. With our calibration scheme the towed ADCP system performs as well a...

Towed ADCP fish with Depth and Roll controllable wings and its application to the Kuroshio observation

An intelligent ADCP (Acoustic Doppler Current Profiler) fish, called “DRAKE” (Depth and Roll Adjustable Kite for Energy flux measurements) was developed with the controllable wings which can adjust the submerging depth of the fish and stabilize its roll motion. The Kuroshio west of Okinawa was measured in a roundtrip course on the same traverse line by the ADCP fish. The forward survey obtained the maximum submerging depth of 208 m at a fish operation speed of 2.9 ms−1. The maximum fish speed of 5.4 ms−1 was achieved at a submerging depth of 48 m in the return survey. The data in the overlapped area of data acquisition between depths 212 and 276 m were used to examine the accuracy of velocity measurement for the towed ADCP system. The summation of both survey data made it possible to estimate a sectional velocity structure and transport of the upper 600 m corresponding roughly to the whole section of the Kuroshio in this region.

Single ping ADCP data

Because acoustic Doppler current profiler (ADCP) data are inherently noisy, they are usually presented in an averaged form. But a great deal can be learned, particularly about the ADCP system itself, by looking at single ping data. The author presents examples of analyses done on single ping data and shows how these data can be used to understand system sampling, bin-to-bin independence, system noise, and the character of a secondary parameter-AGC. Some discussion of how these results are applied to processing procedures and their verification are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>