Adapting Parshall Flumes to Round Manholes

The Parshall flume is widely used to measure open channel flows both above and below grade. Above grade the flume is typically cast into a long, straight concrete channel or is free-standing with inlet / outlet end adapters and pipe stubs to connect the flume to piping. Below grade, however, the shape of the flume can make it problematical to install in a manhole.

While factory integration into a Packaged Metering Manhole is one option for below grade applications, there are times when cost or the need to reuse existing infrastructure means that the flume has to be installed in an existing pre-cast manhole.

The Parshall flume has a rectangular cross-section, which does not readily mate to the curve of manhole wall. For smaller Parshall flumes, this is not a problem as there is sufficient space to form transitions into / out of the flume. As the flume gets larger, though, there is insufficient space to properly form the transitions – leaving the installer to either skimp on the transitions or use a larger diameter manhole.

To bridge this middle ground, Openchannelflow has developed curved (or staged) end adapters for a variety of Parshall flume sizes / manhole diameters. With staged end adapters, the Parshall flume simply slides into place against the manhole wall. Depending upon the flume size and manhole diameter, staged end adapters can be configured for the inlet of the flume, the outlet of the flume, or for both the inlet and the outlet.

Parshall flume with staged end adapters to mate to a round concrete manhole

In configurations where staged end adapters are provided for both the inlet and the outlet of the flume, the outlet staged end adapter is usually bolted to the flume (while the inlet adapter is chemically bonded to the flume). Bolting allow the outlet end adapter to be removed if the manhole is smaller than anticipated – where a compromise between inlet flow conditioning and outlet flow direction needs to be made.