Effects of Horizontal Velocity Variations
on Ultrasonic Velocity Measurements in Open Channels
Water-Resources Investigations Report 91-4200
By Eric D. Swain
ABSTRACT
Use of an ultrasonic
velocity meter to determine discharge in open channels involves measuring
the velocity in a line between transducers in the stream and relating that
velocity to the average velocity in the stream. The standard method of calculating
average velocity in the channel assumes that the velocity profile in the
channel can be represented by the one-dimensional von Karman universal velocity
profile. However, the velocity profile can be described by a two-dimensional
equation that accounts for the horizontal velocity variations induced by
the channel sides.
An equation to calculate
average velocity accounts for the two-dimensional variations in velocity
within a stream. The use of this new equation to calculate average velocity
was compared to the standard method in theoretical trapezoidal cross sections
and in the L-31N and Snapper Creek Extension Canals near Miami, Florida.
These comparisons indicate that the two-dimensional variations have the
most significant effect in narrow, deep channels. Also, the two-dimensional
effects may be significant in some field situations and need to be considered
when determining average velocity and discharge with an ultrasonic velocity
meter.
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