Effect of Temporal Resolution on the Accuracy of ADCP Measurements

Abstract

The application of acoustic Doppler current profilers (ADCP's) in river flow measurements is promoting a great deal of progress in hydrometry. ADCP's not only require shorter times to collect data than traditional current meters, but also allow streamflow measurements at sites where the use of conventional meters is either very expensive, unsafe, or simply not possible. Moreover, ADCP's seem to offer a means for collecting flow data with spatial and temporal resolutions that cannot be achieved with traditional current-meters. High-resolution data is essential to characterize the mean flow and turbulence structure of streams, which can in turn lead to a better understanding of the hydrodynamic and transport processes in rivers. However, to properly characterize the mean flow and turbulence intensities of stationary flows in natural turbulent boundary layers, velocities need to be sampled over a long-enough time span. The question then arises, how long should velocities be sampled in the flow field to achieve an adequate temporal resolution? Theoretically, since velocities cannot be sampled over an infinitely long time interval, the error due to finite integration time must be considered. This error can be estimated using the integral time scale. The integral time scale is not only a measure of the time interval over which a fluctuating function is correlated with itself but also a measure of the time span over which the function is dependent on itself. This time scale, however, is not a constant but varies spatially in the flow field. In this paper we present an analysis of the effect of the temporal resolution (sampling time span) on the accuracy of ADCP measurements based on the integral time scale. Single ping velocity profiles collected with frequencies of 1 Hz in the Chicago River at Columbus Drive using an uplooking 600 kHz ADCP are used in this analysis. The integral time scale at different depths is estimated based on the autocorrelation function of the velocity fluctuations and is used to evaluate the mean-square error as a function of the integration time. The implications of these errors in typical ADCP measurements for discharge estimates in natural streams are discussed. (Gordon, 1996) is known to most ADCP users. It has long been recognized that the sampling time also affects the accuracy of mean velocity estimates. However, only qualitative guidelines to select the time span needed to achieve meaningful estimates of the mean velocity based on ADCP measurements have been reported (e.g., Droz et al., 1998, Barua y Rahman, 1998, González et al. 1996. Droz et al. (1998) have estimated that a sampling time of 5 minutes should be used for estimating mean streamwise velocities with an ADCP. Baru and Rahman (1998) reported that to estimate turbulent intensities for flows in the Jamuna River in Bangladesh from broadband ADCP velocity records with a resolution frequency of about 2 Hz, 15-minute averaging was necessary. González et al. (1996), based on fixed broadband ADCP measurements collected at ...

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

“…Similar analyses are available for measurements with fixed ADCPs in man-made canals [7], laboratory [21], or coastal areas [4,19]. These studies reveal widely different conclusions regarding the potential of ADCP to measure turbulence parameters.…”

“…A more rigorous assessment of the sampling time length required for accurately capturing the temporal randomness of turbulence process is obtained by observing the variation of the normalized mean square error (NMSE) defined by Gonzalez-Castro et al [7] as…”

“…For example, [25] conclude that an averaging time of approximately 15 minutes is needed to estimate higher order moments of velocity statistics samples in a river with similar characteristics. In another investigation, Gonzalez-Castro et al [7] recommend record lengths between 5 and 30 min (errors ranging from 13% to 2%) for ADCP measurements in a man-made large channel.…”

Flow Analysis of a River Confluence with Field Measurements and Rans Model with Nested Grid Approach

“…In order to determine the required time interval, the variation of the normalized mean square error was used, as defined by Gonzalez‐Castro et al . (). Time series of raw velocity data from each profile in different measurement layers (so‐called bins) were postprocessed and analysed.…”

Emplois de l’aDcp en rivière : une revue de synthèse

“…En faisant l'hypothèse d'homogénéité des vitesses par couches horizontales dans le champ des faisceaux, l'intensité turbulente peut être calculée à partir des fluctuations de la vitesse mesurée et reconstituée par l'aDcp [Barua et al 1998, Muste et al 2004b. En régime permanent, vitesse moyenne et intensité turbulente se stabilisent pour des séries de mesure suffisamment longues, typiquement 15 min pour une fréquence de mesure de l'ordre du Hertz [Barua et al 1998, Gonzalez-Castro et al 2002, Muste et al 2004b.…”