Near-Transducer Errors in ADCP Measurements: Experimental Findings

Acoustic Doppler current profilers (ADCPs) are not able to accurately determine velocity near their transducers and near the bed. These limitations have restricted the use of ADCPs to flow depths that are large enough to allow acquisition of few directly measured velocity data that can be subsequently used to accurately estimate vertical velocity profiles and flow discharge in cross sections. While the causes that make ADCPs unable to collect data in the near-bed region are relatively well documented, the causes of near-transducer errors have not yet been fully understood and are only partly documented. We present results from an experimental study aimed at characterizing the systematic errors due to the combined effect of acoustic interference and instrument-induced flow disturbance near a Janus-configured ADCP. The study comprises: (1) concurrent measurements with an ADCP and an acoustic Doppler velocimeter (ADV) under the ADCP; (2) measurements of the flow disturbance produced by the ADCP in the vertical and horizontal planes; and (3) ADV measurements along the path of the acoustic beams ensonified by the ADCP during a measurement. Results suggest that ADCPs bias low the velocity profiles with respect to the undisturbed velocity profiles, mostly because of the flow disturbance induced by the ADCP, with acoustic effects playing a secondary role. For the range of flows we studied, both undisturbed and disturbed profiles exhibit similar shapes when plotted in dimensionless form, with the bulk flow velocity and the ADCP diameter (D) as characteristic scales. The differences between the undisturbed and the ADCP-disturbed profiles extend up to a distance of about 1.5D from the ADCP, except for the profiles measured at locations where the flow depth is close to D for which the boundary layer induced by the ADCP interacts with the one induced by the flume bed.     

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.     

Investigation of Two Elemental Error Sources in Boat-Mounted Acoustic Doppler Current Profiler Measurements by Large Eddy Simulations

Measurements of water discharge and flow velocities in riverine and tidal environments are commonly made with acoustic Doppler current profilers (ADCPs) mounted on a moving boat. This paper presents results of high-resolution Large Eddy simulations (LES) conducted to investigate two elemental error sources in ADCP measurements from a moving boat. One of these errors is due to the flow disturbance induced by the boat-mounted ADCP. The other error is due to the lack of flow homogeneity in horizontal layers assumed by the ADCP algorithm to compute orthogonal velocities from the measured radial velocities along the acoustic beams. The first error is investigated by comparing LES results for an undisturbed flow field with LES results for a flow field disturbed by a boat-mounted ADCP. The second error is investigated by comparing the velocities beneath the ADCP simulated by LES with virtual ADCP velocities, which are obtained by applying the ADCP algorithm to LES velocities data mined along the path of the acoustic beams of the virtual profiler. The distribution of the Reynolds stresses beneath the ADCP estimated with the ADCP algorithm from the virtual ADCP velocity data are also compared with those obtained from the LES solutions for both the undisturbed and ADCP-disturbed flows. Results show that the boat significantly disturbs the flow field and that the disturbed flow field is qualitatively different from the flow fields observed around an isolated ADCP (no boat).     

Analyzing Turbulence Intensity in Gravel Bed Channels

In this paper, the results of an experimental investigation of the turbulence intensity in gravel bed channels are described. The runs were carried out by measuring, with an acoustic Doppler velocimeter, the turbulence intensity profile along six verticals of a given cross section in a laboratory flume. The analysis of the measured intensity distributions has shown the existence of two different regions, above and below the tops of the roughness elements, in which different intensity profiles occur. Furthermore, the measured profiles have shown a maximum of the turbulence intensity that decreases for increasing values of the roughness height, confirming that the turbulence damping efficiency increases when the roughness elements protrude inside the flow. The applicability of Nezu’s relationship (derived for a hydraulically smooth bed) for the experimental intensity profiles above the roughness elements is positively tested. Finally a new intensity distribution for a rough bed, applicable to the whole water depth, is proposed. In this profile, two coefficients having a known physical meaning (the maximum turbulence intensity and the depth at which this maximum is located) appear.     

Turbulent Effects on the Settling Velocity of Suspended Sediment

The mean settling velocities of suspended sediments in turbulence have been examined. The settling velocities in a flume are directly measured by using an acoustic Doppler velocimeter. The results indicate the same trend as previous work in homogeneous isotropic turbulence. In addition to the flume experiment, the numerical experiments were conducted in the velocity field of homogeneous isotropic turbulence simulated by Kraichnan’s technique. The experimental and numerical results show that at high turbulence intensity the relative settling velocity increases with the increasing relative turbulence intensity regardless of the Stokes number. At intermediate turbulence intensity, it seems that the settling data bifurcate, i.e., the particles at the large Stokes number tend to be slowed, whereas the settling velocity of particles is increased at the small Stokes number.     

Validation of Streamflow Measurements Made with Acoustic Doppler Current Profilers

The U.S. Geological Survey and other international agencies have collaborated to conduct laboratory and field validations of acoustic Doppler current profiler (ADCP) measurements of streamflow. Laboratory validations made in a large towing basin show that the mean differences between tow cart velocity and ADCP bottom-track and water-track velocities were ?0.51 and ?1.10%, respectively. Field validations of commercially available ADCPs were conducted by comparing streamflow measurements made with ADCPs to reference streamflow measurements obtained from concurrent mechanical current-meter measurements, stable rating curves, salt-dilution measurements, or acoustic velocity meters. Data from 1,032 transects, comprising 100 discharge measurements, were analyzed from 22 sites in the United States, Canada, Sweden, and The Netherlands. Results of these analyses show that broadband ADCP streamflow measurements are unbiased when compared to the reference discharges regardless of the water mode used for making the measurement. Measurement duration is more important than the number of transects for reducing the uncertainty of the ADCP streamflow measurement.     

Field Assessment of Alternative Bed-Load Transport Estimators

Measurement of near-bed sediment velocities with acoustic Doppler current profilers (ADCPs) is an emerging approach for quantifying bed-load sediment fluxes in rivers. Previous investigations of the technique have relied on conventional physical bed-load sampling to provide reference transport information with which to validate the ADCP measurements. However, physical samples are subject to substantial errors, especially under field conditions in which surrogate methods are most needed. Comparisons between ADCP bed velocity measurements with bed-load transport rates estimated from bed-form migration rates in the lower Missouri River show a strong correlation between the two surrogate measures over a wide range of mild to moderately intense sediment transporting conditions. The correlation between the ADCP measurements and physical bed-load samples is comparatively poor, suggesting that physical bed-load sampling is ineffective for ground-truthing alternative techniques in large sand-bed rivers. Bed velocities measured in this study became more variable with increasing bed-form wavelength at higher shear stresses. Under these conditions, bed-form dimensions greatly exceed the region of the bed ensonified by the ADCP, and the magnitude of the acoustic measurements depends on instrument location with respect to bed-form crests and troughs. Alternative algorithms for estimating bed-load transport from paired longitudinal profiles of bed topography were evaluated. An algorithm based on the routing of local erosion and deposition volumes that eliminates the need to identify individual bed forms was found to give results similar to those of more conventional dune-tracking methods. This method is particularly useful in cases where complex bed-form morphology makes delineation of individual bed forms difficult.     

Turbulence Measurement in Nonuniform Open-Channel Flow Using Acoustic Doppler Velocimeter (ADV)

Measurements of the mean and turbulence characteristics in nonuniform open-channel flows were carried out using a 3D acoustic Doppler velocimeter. Both accelerating and decelerating flows were investigated. Analyses based on the Reynolds equation and the continuity equation of 2D open-channel flow show that a flow will be in equilibrium if the pressure-gradient parameter β is a constant at different sections along the flow direction. The experimental data show that all the flows investigated are in equilibrium. The effect of the flow nonuniformity on the mean velocity and turbulence profiles was also examined. The study shows that (1) the log law is still valid for both accelerating and decelerating flows in the inner region. The Coles law can be used for the entire region, but the wake-strength parameter Π depends on the β-value; and (2) the turbulence intensities and the Reynolds stress decrease in accelerating flow and increase in decelerating flow, when compared with those in uniform flow. Finally, using the Reynolds equation and the continuity equation of 2D open-channel flow, the theoretical expressions for the distribution of vertical velocity and the Reynolds stress have been developed. The measured vertical velocity and the Reynolds stress profile compare well with the derived expressions.     

Response of Velocity and Turbulence to Sudden Change of Bed Roughness in Open-Channel Flow

The experimental study shows how an open-channel flow would respond to a sudden change (from smooth to rough) in bed roughness. Using a two-dimensional acoustic Doppler velocimeter and a laser Doppler velocimeter, the velocity, turbulent intensities, and Reynolds stress profiles at different locations along a laboratory flume were measured. Additionally, the water surface profile was also measured using a capacitance-type wave height meter. The experimental data show the formation of an internal boundary layer as a result of the step change in bed roughness. The data show that this boundary layer grows much more rapidly than that formed in close-conduit flows. The results also show that the equivalent bed roughness, bed-shear stress, turbulent intensities, and Reynolds stress change gradually over a transitional region, although the bed roughness changes abruptly. The behavior is different from that observed in close-conduit flows, where an overshooting property—which describes the ability of the bed-shear stress to attain a high-peak value over the section with the larger roughness, was reported. A possible reason for the difference is the variation of the water surface profile when an open-channel flow is subjected to a sudden change in bed roughness.     

Turbulent flow field over fluvial obstacle marks generated in a laboratory flume

The study is aimed at investigating the mean flow and turbulence characteristics in scour geometry developed near a circular cylinder of length 10cm placed over the sand bed transverse to the flow. The obstacle placed on a sand bed, on the way of a unidirectional flow, develops a crescent-shaped scour mark on the bed. The scour is caused by generation of vortex developed on the upstream side of the obstacle. Sand grains eroded by this vortex, are deposited on the downstream side of the obstacle as wakes. The turbulent flow field within the scour mark was measured in a laboratory flume using an Acoustic Doppler Velocimeter (ADV). The scour marks named as current crescents preserved in geological record are traditionally used as indicators of palaeocurrent direction. The distribution of mean velocity components, turbulent intensities and Reynolds stresses at different positions of the mark are presented. The experimental evidence also shows that the geometric characteristics of the scour mark (width) depend primarily on the cylinder aspect ratio, cylinder Reynolds number and sediment Froude number.     

Turbulence Characteristics of Open-Channel Flow with Bed Suction

The influence of bed suction on the characteristics of turbulent open channel flow is studied in a laboratory flume using a two-component laser Doppler velocimeter. The experimental results show how bed suction significantly affects the mean flow properties, turbulence levels, and Reynolds stress distributions. The data reveal the presence of a more negative vertical (downward) velocity. The results also show how the horizontal and vertical turbulence intensities and Reynolds shear stresses respond to suction. All these properties are found to reduce with increasing relative suctions: decreasing more rapidly around the bed region than that near the free surface. In the downstream direction, the flow structure in the suction zone undergoes a process of rapid readjustment within a transitional region. Beyond this region, the turbulence flow structures asymptotes toward an “equilibrium” region.     

Unobstructed and Obstructed Turbulent Flow in Gravel Bed Rivers

An acoustic Doppler velocimeter was used to characterize turbulence in two gravel bed rivers. Data were collected in unobstructed flow and compared to recent investigations. Additional data collected in the wake of emergent boulders indicate that mean flow velocity, turbulent kinetic energy, gradients in the streamwise velocity, and Reynolds stress downstream from large rocks deviate from unobstructed flow results, but similar turbulence patterns are found behind each boulder. Results of this study are discussed with regard to natural channel design and fish habitat.     

Characteristics of Submerged Jets in Evolving Scour Hole Downstream of an Apron

This paper reports an experimental investigation on the velocity and turbulence characteristics in an evolving scour hole downstream of an apron due to submerged jets issuing from a sluice opening detected by an acoustic Doppler velocimeter. Experiments were carried out for the conditions of submerged jets, having submergence factors from 0.96 to 1.85 and jet Froude numbers from 2.58 to 4.87, over sediment beds downstream of a rigid apron. The distributions of time-averaged velocity vectors, turbulence intensities, and Reynolds stress at different streamwise distances are plotted for the conditions of initial flat bed, intermediate scour holes, and equilibrium scour hole downstream of an apron. Vector plots of the flow field show that the rate of decay of the submerged jet velocity increases with an increase in scour hole dimension. The bed-shear stresses are determined from the Reynolds stress distributions. The flow characteristics in evolving scour holes are analyzed in the context of self-preservation, growth of the length scale, and decay of the velocity and turbulence characteristics scales. The most significant observation is that the flow in the scour holes (intermediate and equilibrium) is found to be plausibly self-preserving.     

Wall-Wake Flows Downstream of a Sphere Placed on a Plane Rough Wall

The time-averaged characteristics of turbulent wall-wake flows downstream of a sphere placed on a rough wall are studied. The profiles of the defect of streamwise velocity, Reynolds shear stress, and turbulence intensities exhibit some degree of similarities when they are scaled by their respective peak defect values. For the velocity defect profiles, the vertical distances are scaled by the height of the location of the half-peak velocity defect. However, for the defect profiles of the Reynolds shear stress and the turbulence intensities, the vertical distances are scaled by the height of the location of the half-peak Reynolds shear stress defect. The magnitudes of the peak defect of all the quantities diminish with the distance downstream of the sphere characterizing the recovery of their undisturbed profiles. Additionally, the theoretical similarity solution for the velocity defect profiles is obtained. The third-order correlations imply that in the inner layer of wall wakes, a streamwise acceleration is prevalent and associated with a downward flux, suggesting sweeps. In contrast, in the outer layer, a streamwise deceleration exists and is associated with an upward flux, suggesting ejections. The profiles of the energy budget show that the turbulent and pressure energy diffusions oppose each other. The turbulent production has a positive peak, and the pressure energy diffusion has a negative peak, indicating a large gain in turbulence production in the wall-wake flows. The quadrant analysis confirms that in wall-wake flows, sweeps are the governing mechanism resulting from an inrush of fluid streaks. The bursting events have shorter duration, but they are more frequent than those in upstream.     

Confinement Effects in Shallow-Water Jets

This paper documents measurements of the mean velocity field and turbulence statistics of an isothermal, round jet entering a shallow layer of water. The lower boundary of the jet was a solid wall and the upper boundary a free surface. The jet axis was midway between the solid wall and the free surface in all cases. Experiments were performed at a Reynolds number of 22,500 for water layer depths 15, 10, and 5?times the jet exit diameter (9?mm). Particle image velocimetry measurements were made on vertical and horizontal planes—both containing the axis of the jet. The measurements were taken from 10 to 80 jet diameters downstream. Results showed that, for the highly confined cases at downstream locations, the axial velocity was quite uniform over the depth, with a mild peak below the jet axis. In the horizontal plane, the velocity profiles were slightly narrower than the free jet profile, but in the vertical plane, they were wider. The mean vertical velocity profiles showed that entrainment was suppressed in the vertical direction. At the same time, the lateral velocity profiles indicate that fluid flows from the sides toward the jet centerline. For the shallow cases, the mean vertical velocity becomes negative over most of the depth at downstream locations, indicating that this inflow from the sides is directed downward toward the solid wall. The relative turbulence intensity results were suppressed in the axial and vertical directions and mildly enhanced in the lateral direction. As well, the Reynolds shear stress in the vertical plane was significantly reduced by the vertical confinement, while in the horizontal plane it was only slightly affected by the confinement.     

Turbulent Flow near Vertical Angled Fish Screen

Mean and turbulent flow characteristics on the upstream and downstream sides of the screen in a flow diversion channel have important implications for operation and maintenance (e.g., sedimentation) and for assessing fish behavior related to flow turbulence. This technical note extends an earlier study on mean flow near screens to turbulence characteristics. Acoustic Doppler velocimeter was used to explore three-dimensional mean and turbulent flow characteristics on the upstream and downstream sides of vertical angled fish screens. The present study confirms the two-dimensional mean velocity observations of the previous experimental work and shows that the vertical mean velocities are less than 10% of the local magnitudes of longitudinal velocity and hence can be ignored. Horizontal components of the mean velocity on the downstream side of the screen were relatively small, but the turbulent velocity fluctuations were two to three times as intense as those measured on the upstream side.     

Surface Roughness Effects in Near-Bed Turbulence: Implications to Sediment Entrainment

In this study, the characteristics of near-bed turbulence were experimentally investigated for three distinct roughness regimes, namely (1) isolated; (2) wake interference; and (3) skimming. Spherical particles of the same size and density were placed upon a rough sediment bed to simulate the three regimes. Experimental runs for the aforementioned regimes were performed in a tilting water-recirculating flume. Flow measurements atop the spherical particles were performed by means of a 3D laser Doppler velocimeter. The aim of the tests was to provide further evidence that the structure of turbulence is affected throughout the boundary layer by the presence of roughness geometry. The measurements reported here include velocity profiles of the mean streamwise and vertical velocity components and of the Reynolds shear stress distribution. To further quantify the differences in turbulent structure under various surface roughnesses, a quadrant analysis was performed.     

Laboratory Measurements of Flow and Turbulence in Discontinuous Distributions of Ligulate Seagrass

Turbulent flow characteristics were investigated in laboratory flume studies of a ligulate plant canopy interrupted by a gap representing discontinuities observed in seagrass prairies. The reliability of velocity measurements obtained using an acoustic Doppler velocimeter within the canopy was shown using specifically designed experiments. In relatively fast flow (mean velocity 5.5?cm?s?1), the mean flow profile was logarithmic above the canopy, had an inflection point near its top, and uniformly low values within it. Within the gap, a recirculation cell formed. Reynolds stress maxima were approximately coincident with the mean flow inflection point. Quadrant analysis revealed an ejection-dominated upper layer, a sweep-dominated region around the top of the canopy and within the gap, and no dominant quadrant within the canopy. In slower flow (mean velocity 1.7?cm?s?1) the plants were quasiemergent and the flow fields more uniform. Sweeps similarly dominated the region near the top of the canopy and within the gap. In both flows, autocorrelation of longitudinal velocity fluctuations showed a Lagrangian time scale maximum at the downstream end of the gap.