Kevin A. Oberg

Measuring river velocity and discharge with acoustic Doppler profilers

Abstract Acoustic Doppler profilers and associated software packages presently are being used to measure water velocity, channel bathymetry, and river discharge. The instruments have various configurations and frequencies; choice of the appropriate instrument depends on various factors including depth, width, and sediment load of the rivers being measured. The acoustic Doppler profilers are mounted on powerboats or small remote-controlled or tethered rafts or catamarans. Profilers enable users to make fast, accurate, and economical discharge measurements on large rivers and rivers with unsteady flow conditions because of flooding or irregular releases from reservoirs. This article describes the principles of operation, application of acoustic Doppler profilers to the measurement of velocity and discharge, and calibration and verification issues.

Measurement of Turbulence with Acoustic Doppler Current Profilers - Sources of Error and Laboratory Results

Acoustic Doppler current profilers (ADCPs) provide a promising method for measuring surface-water turbulence because they can provide data from a large spatial range in a relatively short time with relative ease. Some potential sources of errors in turbulence measurements made with ADCPs include inaccuracy of Doppler-shift measurements, poor temporal and spatial measurement resolution, and inaccuracy of multi-dimensional velocities resolved from one-dimensional velocities measured at separate locations. Results from laboratory measurements of mean velocity and turbulence statistics made with two pulse-coherent ADCPs in 0.87 meters of water are used to illustrate several of inherent sources of error in ADCP turbulence measurements. Results show that processing algorithms and beam configurations have important effects on turbulence measurements. ADCPs can provide reasonable estimates of many turbulence parameters; however, the accuracy of turbulence measurements made with commercially available ADCPs is often poor in comparison to standard measurement techniques.

Density currents in the Chicago River: Characterization, effects on water quality, and potential sources

Bidirectional flows in a river system can occur under stratified flow conditions and in addition to creating significant errors in discharge estimates, the upstream propagating currents are capable of transporting contaminants and affecting water quality. Detailed field observations of bidirectional flows were made in the Chicago River in Chicago, Illinois in the winter of 2005-06. Using multiple acoustic Doppler current profilers simultaneously with a water-quality profiler, the formation of upstream propagating density currents within the Chicago River both as an underflow and an overflow was observed on three occasions. Density differences driving the flow primarily arise from salinity differences between intersecting branches of the Chicago River, whereas water temperature is secondary in the creation of these currents. Deicing salts appear to be the primary source of salinity in the North Branch of the Chicago River, entering the waterway through direct runoff and effluent from a wastewater-treatment plant in a large metropolitan area primarily served by combined sewers. Water-quality assessments of the Chicago River may underestimate (or overestimate) the impairment of the river because standard water-quality monitoring practices do not account for density-driven underflows (or overflows). Chloride concentrations near the riverbed can significantly exceed concentrations at the river surface during underflows indicating that full-depth parameter profiles are necessary for accurate water-quality assessments in urban environments where application of deicing salt is common.

In Search of Easy-to-Use Methods for Calibrating ADCP's for Velocity and Discharge Measurements

Acoustic Doppler current profilers 2 (ADCP’s) have become a common tool for measuring streamflow and profiles of water velocity. Despite their widespread use, no standard procedure has been adopted or accepted for calibration of ADCP’s. Limitations of existing facilities for testing point-velocity meters, the complexity of ADCP instruments, and rapid changes in ADCP technology are some of the reasons that a standard procedure has not been adopted. This paper outlines various methods for calibrating ADCP’s, discusses the advantages and disadvantages of these methods, and presents a simple, cost-effective procedure for calibrating an ADCP in the field. Standard methods for the calibration of current meters involve towing a meter in a tow tank at various known speeds. This method has also been used to calibrate ADCPs. Disadvantages to this method include lack of adequate and uniform backscattering material, lack of flowing water in the testing facility, and inability to use the ADCP’s internal flux-gate compass. Use of flumes for ADCP calibration is not practical for many ADCP’s due to width and depth restrictions associated with the instruments. ADCP’s and conventional methods for measuring velocity and discharge have also been compared. However, these field comparisons are costly and conventional velocity and discharge measurements may be subject to relatively large uncertainties. The USGS is investigating a new method for ADCP calibration. This method requires the use of differential global positioning system (DPGS) with sub-meter accuracy and standard software for collecting ADCP data. The method involves traversing a long (400 – 800 meter) course at a constant compass heading and speed, while collecting simultaneous DGPS and ADCP data. This process is repeated several times and the ratio of the course length measured by means of the ADCP to the course length measured by means of DGPS is computed. When this ratio is less than 0.995, measurements made with RD Instruments’ Rio Grande ADCP most likely have a negative bias error and when it is greater than 1.003 the ADCP most likely has a positive bias error. It is estimated that this procedure can be completed in 2 hours or less, and can be done by anyone with access to a sub-meter DGPS.

Validation of Streamflow Measurements Made with M9 and RiverRay Acoustic Doppler Current Profilers

AbstractThe USGS Office of Surface Water (OSW) previously validated the use of Teledyne RD Instruments (TRDI) Rio Grande (in 2007), StreamPro (in 2006), and Broadband (in 1996) acoustic Doppler current profilers (ADCPs) for streamflow (discharge) measurements made by the USGS. Two new ADCPs, the SonTek M9 and the TRDI RiverRay, were first used in the USGS Water Mission Area programs in 2009. Since 2009, the OSW and USGS Water Science Centers (WSCs) have been conducting field measurements as part of their stream-gauging program using these ADCPs. The purpose of this paper is to document the results of USGS OSW analyses for validation of M9 and RiverRay ADCP streamflow measurements. The OSW required each participating WSC to make comparison measurements over the range of operating conditions in which the instruments were used until sufficient measurements were available. The performance of these ADCPs was evaluated for validation and to identify any present and potential problems. Statistical analyses of st...

Measuring Discharge with ADCPs: Inferences from Synthetic Velocity Profiles

Synthetic velocity profiles are used to determine guidelines for sampling discharge with acoustic Doppler current profilers (ADCPs). The analysis allows the effects of instrument characteristics, sampling parameters, and properties of the flow to be studied systematically. For mid-section measurements, the averaging time required for a single profile measurement always exceeded the 40 s usually recommended for velocity measurements, and it increased with increasing sample interval and increasing time scale of the large eddies. Similarly, simulations of transect measurements show that discharge error decreases as the number of large eddies sampled increases. The simulations allow sampling criteria that account for the physics of the flow to be developed.

Measuring suspended sediment concentrations and flow velocities using multibeam sonar.

Modern data handling and storage technologies facilitate the logging of the large quantity of water‐column backscatter information received by multibeam sonars. Methods of using these data to derive estimates of the mass concentration and flow velocities of suspended sediment flow structures have been developed. The results obtained by the application of these methodologies to data collected at the confluence of the Parana and Paraguay rivers in Argentina and the confluence of the Mississippi and Missouri rivers in the United States will be presented. An analysis of those data in conjunction with a set of experimental data collected in a large‐scale test facility will be also given. The applicability and limitations of the use of multibeam sonar for deriving suspended sediment concentrations will be discussed. By enabling the simultaneous measurements of suspended sediment concentration, flow velocities, and bathymetric data, multibeam echo‐sounders are demonstrated to be a versatile tool for the surveyin...

Characterizing a December 2005 Density Current Event in the Chicago River , Chicago, Illinois

During the winter months, the Chicago River in Chicago, Illinois is subject to bi dire ctional flows , and density currents are thought to be responsible for the se flow variations. This paper presents detailed field measurements using three acoustic Doppler current profiler instruments and simultaneous water -quality measurements made during December 2005 . Observations indicate that the formation of density currents within the Chicago Riv er and density differences are mostly due to salinity differences between the North Branch and the main stem o f the Chicago Rive r, wh ereas temperature difference does not appreciably affect the creation of density currents. Sources of higher water temperat ure, conductivity, and salinity values should be addressed in future studies.