Three-dimensional measurement of river channel flow processes using acoustic doppler velocimetry

Abstract

This paper describes and assesses: (i) the use of a new instrument for the determination of three-dimensional flow velocities in natural rivers, the acoustic Doppler velocimeter (ADV); and (ii) a method for positioning and orienting such measurements relative to a single local coordinate system to relate flow velocity vectors with the bed and water surface. The ADV uses the Doppler shift principle to measure the velocity of small particles, assumed to be moving at velocities similar to the fluid. Velocity is resolved into three orthogonal components, and measured in a volume 5 cm below the sensor head, minimizing interference of the flow field, and allowing measurements to be made close to the bed. A simple method for positioning and orienting the instrument using digital tacheometry is described, and is used to obtain velocity measurements concurrently with measurements of both bed and water surface topography. The paper includes a preliminary field assessment of the ADV by comparing velocity profiles with those generated from Marsh McBirney electromagnetic current meters, and a full field assessment of the position and orientation methodology. These results suggest that the recommended methods in combination with an ADV are able to provide reliable mean three-dimensional velocity field information and accurate bed and surface topography.

Flow, turbulence, and drag associated with engineered log jams in a fixed-bed experimental channel

“…At all sampling locations, velocities were recorded for 120 s at 75 Hz. When processing, all data with correlations b 0.7 were rejected (WinADV; Lane et al, 1998;Wahl, 2000), and all erroneous spikes were removed (phasespace threshold method; Goring and Nikora, 2002). These methods typically deleted about 4% of the observations.…”

Effect of accelerating and decelerating flows on incipient motion in sand bed streams

“…This likely is related to near bed interference in which the signal reflected from the sediment bed interferes directly with the return signal from the measuring volume. This problem is particularly problematic with down-looking probes [19]. It occurs when the time taken for one pulse to transfer from the sampling volume to the bed and back to the sampling volume is the same as the time between pulses.…”

The role of vortex shedding in the scour of pools

“…Signal processing was conducted on the files to remove data corrupted by communication errors, low signal-to-noise ratios (SNR) or low velocity correlations [12]. To process the mean velocity data and remove unreliable data points, a lower limit of 30 dB was used for SNR values and a lower limit of 0.70 was used for correlation values.…”

“…To process the mean velocity data and remove unreliable data points, a lower limit of 30 dB was used for SNR values and a lower limit of 0.70 was used for correlation values. Although a lower limit of 15 dB is often used to process ADV data [12], testing in the flume indicated that a higher standard was attainable and warranted. Finally, mean velocity files were checked to insure that at least 85% of the data was retained after processing.…”

“…Finally, mean velocity files were checked to insure that at least 85% of the data was retained after processing. This criteria is based on the observation that significant changes in average estimated velocity occur if less than 80% of the velocity values are retained and the root mean square (RMS) values are dampened with less than 85% of the values retained [12]. To minimize problems associated with aliasing, the velocity data was filtered with a Gaussian low-pass filter described by Lane et al [12] to a frequency of 12.5 Hz.…”