Conventionally a Parshall ...

Conventionally a Parshall flume (open channel) is constructed just after the inlet screens and de-gritter.  This provides a direct relationship between depth of flow and flow velocity, and hence flow volume.  The parshall flume results in a change of flow from sub-critical to super-critical, with critical flow being the measuring point.  An ultrasonic meter is mounted above the flume just upstream of the critical flow point which measures depth.  This has proved fairly reliable as long as the meter is serviced, calibrated and maintained.