Parshall Flume Free Flow

Ralph Parshall reworked the Venturi flume at Colorado Agricultural College in the 1920s into a shape that measures open-channel flow without any moving parts and, crucially, passes sediment instead of collecting it — which is why it is the standard primary device on wastewater plant influent to this day. Flow is a function of a single upstream depth measured at the Ha point, two-thirds of the way along the converging section. The USBR general free-flow equation for throats of 1 to 8 ft is $Q = 4WH^{1.522\,W^{0.026}}$, so a 2 ft flume under 1.5 ft of head passes about 15.0 cfs, and a 1 ft flume under exactly 1 ft of head passes exactly 4 cfs. Smaller 3, 6 and 9 inch flumes have their own published constants and are not covered by this form.

The word FREE in the name is the one that gets people. A Parshall flume reads correctly only while the downstream water is low enough that a hydraulic jump forms in the throat; once submergence — the ratio of downstream to upstream head — exceeds about 70%, backwater reaches upstream and the single-head reading over-reads badly, sometimes by 30% or more. That is the classic cause of a plant that appears to treat more water than the city sells. Check the flume dimensions with a tape rather than trusting the drawing, verify that the floor is level and the walls are plumb, and confirm that the ultrasonic transducer is truly at Ha and reading from a properly set zero: an installation error in a flume is a permanent, systematic billing error.