In general, most Parshall Flumes are designed to have free discharge (flow not restricted by downstream conditions). Sometimes, however, changes in the downstream hydraulics may submerge a flume, or a flume may be designed to operate under submerged conditions due to upstream head restrictions. 

Unfortunately measuring submerged flow is complicated by:

• The turbulent nature of flow in the throat of the flume
• The need for very precise measurement of the secondary point of measurement (H_b)
• The need to calculate the submergence value and apply the proper free or submerged flow equation

Turbulence and the need for precise measurement can be aided by using stilling wells.  Stilling wells at the points of measurement can help to eliminate surface turbulence and dampen sudden changes in the water level.  Level sensor can then be placed in / on the wells to provide high quality level measurements.

Ultrasonic Flow Meters

There are two approaches to using ultrasonic flow meters to measure submerged Parshall flume flows:  independent transducers sending level readings to a field controller or SCADA systems where the if-then math can be done or integrated flow meters which are factory configured for submerged flow math.

When independent transducers are used, the two output signals are sent to a field control or SCADA system which then determines the submergence ratio (H_b/H_a), determines if the flow is submerged, and then applies either the free flow or submerged flow equation as necessary.  Alternatively, the universal flow equation for Parshall flume flow may be preprogrammed so that the if-then logic of determining the submergence ration / correct flow equation is condensed into a single step.

A more elegant solution to measuring submerged flow is to use a flow meter that is factory configured to take two ultrasonic input signals, do the necessary math internally, and then display the corrected flow result.

Ametek Drexelbrook’s USonic-R flow meter is one factory solution implementing this second approach. The meter comes with two ultrasonic level transducers and a controller.  As described above, the meter can be configured to accept the two inputs, determine if the Parshall flume is submerged or operating under free-flow conditions, and then calculate the corrected flow rate.

It is important to note, and this is not documented particularly well in the meter’s manual, that BOTH readings use the elevation of the flat converging floor of the flume as their zero point.  The H_b reading is NOT taken to the floor of the throat of the flume. 

Image Credit:  Ametek Drexelbrook