Data evaluation for acoustic Doppler current profiler measurements obtained at fixed locations in a natural river

Abstract

[1] Methodologies are presented to (1) evaluate the effect of acoustic Doppler current profiler (ADCP) motion on velocity measurements, (2) assess the stationarity of velocity time series, and (3) ensure that a sample record is of sufficient length for measurements taken with an ADCP at a fixed location in a natural river. ADCP motion occurring on the timescale of an ensemble is investigated by comparing single-ping and multiping measurements obtained at the same location under the same flow conditions. Large-scale motion occurring over the course of the entire measurement is studied by first examining a set of indicators to determine if motion may have resulted in the ADCP measuring in regions of different flow characteristics and then dividing the record into segments representing the potentially different regions for comparison. If boat motion is significant, the ADCP record is edited accordingly. Stationarity is assessed using statistical trend tests with guidance provided for selecting parameters based on instrument physics and flow conditions. Mean streamwise and spanwise velocity profiles can be calculated after confirming the reliability of a measurement. A set of criteria are presented to evaluate the appropriateness of an assumption of the law of the wall for streamwise velocity profiles. The methodology is demonstrated using 67 fixed-vessel measurements obtained at two sites on the lower Roanoke River for two discharges. The results indicate that an appropriate procedure was used for securing the ADCP in the channel and that the sample records were sufficient to provide both accurate measurements of mean quantities and assess the stationarity of velocity.

Characterisation of Tidal Flows at the European Marine Energy Centre in the Absence of Ocean Waves

“…An empirical study was conducted, as per [44,45], to ascertain a suitable stationarity period, and as with [44], a five minute period was selected. Whilst five-and ten-minute stationarity periods have become standard durations on which many TEC application analyses are conducted, the selection of this period for such complex flows is challenging and is discussed further in Section 6.1.…”

Evaluation of the Logarithmic Law of the Wall for River Flows

“…Application of the log law requires a mean velocity profile at the location of interest; therefore, the sample record length of FV measurements must be sufficient to describe the mean velocity in each bin. Petrie et al () demonstrate that the minimum record length of 1200 s is sufficient to describe the mean velocity profile at this field site. The log law parameters are determined by the slope of the mean velocity profile making it unclear whether this sample record is also sufficient for the log law parameters u τ and k s .…”

“…Considerations when applying the log law to field measurements include that the measured velocity profile may not capture the logarithmic region or a logarithmic region may not exist. Field (Petrie et al , , ) and laboratory (Diplas, ) measurements have confirmed deviations from the log law as elevation increases. To address this limitation, the extent of the logarithmic region can be visually determined using a semi‐logarithmic plot of the velocity profile.…”

“…An additional consideration is that the FV velocity time series are not statistically independent. The procedure of Petrie et al () is followed to generate an independent sample. Independent sample records are created using every m ensemble from the original long‐term record, where m is larger than the number of ensemble lags required for the autocorrelation coefficient to go to zero.…”

“…For FV measurements, ensembles were created from 20 sub‐pings sent 50 ms apart. Petrie et al () demonstrate that mean velocities from the multiping setting are equivalent to those obtained with a single ping setting. For MV measurements, the settings recommended by WinRiver II for the field conditions were used.…”