Acoustic Technology Being Used to Measure Delta Flows

Ultrasonic Velocity Meter (UVM) station. Channel top and cross-section views.
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A UVM (Laenen, 1985) transmits acoustic pulses back and forth across a channel and precisely measures the travel time of each pulse. The difference in travel time between a pair of back and forth pulses provides an average velocity (Vp) across the channel at the depth of the transducers. The measured velocity (Vp) is not an average cross-sectional velocity and is referred to as an "index velocity" (Vi) that is used when processing the data to determine an average cross-sectional velocity.

Acoustic Doppler Discharge Measuring System (ADDMS).
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An ADCP-based portable flow measuring system is used to make fast and accurate flow measurements of a channel for use in calibrating a UVM. Velocity and depth are measured and the flow is computed as the ADCP traverses the channel. Flow measurements of 600-foot wide channels can be made in 2 to 3 minutes with an accuracy of 2% (Simpson and Oltmann, 1993) using only a two-man crew.

A 15-minute interval UVM tidal flow record is computed by multiplying channel cross-sectional area by average channel cross-sectional velocity. Water-surface elevation is measured at the UVM station and converted to channel cross-sectional area by a relation defined from channel geometry surveys. Average channel cross-sectional velocities are determined from ADDMS measurements and are used to define a UVM index-velocity and average channel cross-sectional velocity relation.