Parshall Flume Design

When viewed from the top, the Parshall Flume has an hourglass shape and, like the Venturi Flume upon which it is based, is composed of three distinct sections:  

• Uniformly converging inlet
• Short and narrow throat section
• Uniformly diverging discharge  

The floor of the Parshall Flume is: 

• Flat in the converging section (where the primary point of measurement is)
• Drops through the throat (creating supercritical flow and one of Dr. Parshall's more radical inventions)
• and then rises in the diverging section.  

The diverging section's outlet is always lower than the converging section's floor - the floor dips in at the beginning of the throat and only partly recovers at the end of the throat. 

A flat floor in the converging section (where the free flow point of measurement is) means that all that is necessary to determine the flow rate is to simply gauge or “stick” the water level at the point of measurement - measuring the depth of water from the surface to the flume floor.  Unlike Palmer-Bowlus or RBC Flumes, there is no ramp level to deduct from the reading. 

Unlike Palmer-Bowlus or Trapezoidal Flumes, the sidewalls of the Parshall flume are vertical.  Vertical walls mean that when a staff gauge is used, there is no need to check that the gauge has been calibrated to read true vertical depth. 

The flat converging section floor and the straight vertical sidewalls make the Parshall Flume one of the easiest flumes to use accurately. 

Parshall Flume Dimensional Standards

Dr. Parshall developed the dimensions for many Parshall Flume sizes.  Additional smaller and larger Parshall flume sizes were added after Dr. Parshall’s investigations had ended.

The dimensions and tolerances of the Parshall Flume have been standardized in several national / international standards and authoritative publications:

• ASTM D1941-91 (Reapproved 2013) Standard Test Method for Open Channel Flow Measurement of Water with the Parshall Flume
• ISO 9826:1992 Measurement of Liquid in Open Channels - Parshall and SANIIRI Flumes
• Water Measurement Manual, 3rd Edition, United States Department of the Interior, Bureau of Reclamation
• The Parshall Flume:  Instructions for Installation and Table of Discharge, State Engineer of Colorado
• JIS B7553 Parshall Flume Type Flowmeters

• IS 14371: Measurement of Liquid Flow in Open Channels - Parshall and SANIIRI flumes

It's critically important to note that although Parshall Flumes share a common layout, they are not scale models of each other – they are empirical devices for which investigations have been made to determine their specific flow rates.

The use of non-standard flume sizes is STRONGLY discouraged, and the dimensions and flow accuracies of such devices should be considered suspect.

Dimensional Tolerances

ASTM D1941 requires that Parshall Flumes have dimensions within +/-2% of nominal, while JIS B7553 requires tolerance of +/- 1 to 1.5% depending upon the flume size.  

Parshall Flumes with dimensions outside of these ranges are non-conforming and should be either replaced for field rated.

 

PARSHALL FLUME MASTER DIMENSIONS DRAWING