One of the most common questions that we receive is how much straight channel is required upstream of a Parshall flume.  The answer is quite simple:

• If the throat (control) width is greater than 50% of the channel width – somewhat uncommon with a Parshall flume – then 10 throat (control) widths of straight approach are required.
• If the throat (control) width is less than 50% of the channel width – much more common with Parshall flumes – then 20 throat (control) widths of straight approach are required.

Approach lengths shorter than the above may result in the velocity profile entering the flume not being uniform or there may be turbulence – both of which may affect measurement accuracy.

For a fuller discussion of installation conditions for Parshall flumes, visit our Parshall Flume Installation Instructions section.

 

Source: Water Measurement Manual, 3rd Edition, U.S. Department of the Interior, Bureau of Reclamation, 2001