A series of high flow measurements made over 10 years showed a consistent departure from the standard rating curve. Smaller, in this context, is a flume that is less than one half of the channel width guideline normally used (which is itself based upon the tradeoff of flume cost versus installation cost – with larger flumes requiring less work to transition flow into / out of the flume).Īs part of their investigation on the field performance of over 220 Parshall flumes throughout Colorado, Ley et al took particular note of a 15-foot Parshall flume where approach velocities above a stage level of 3.00-feet, on average, exceeded 4 ft/s. Naturally this assumes that the smaller flume will handle the anticipated flow rates. Parshall suggested that a narrow throat width flume should be used, whereby the depth of the upstream water would be increased and the approach velocity is decreased. The Parshall measuring flume is primarily intended to operate under conditions where the (approach) velocities are moderate.įor applications where high velocities may be present, Dr. Parshall did not, however, suggest that the Parshall flume was insensitive to approach velocities, merely that in his limited investigations that the flume studied did not experience significant changes in the discharge, as the variations was less than the experimental error. His observation was that discharge through the flume was not significantly affected when approach velocities were almost 3 times the normal approach velocity of 1 ft/s. Parshall reported the results of a limited series of tests conducted on a 2-foot Parshall flume where the approach velocity was varied. In his 1936 report, The Parshall Flume, Dr.
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