Sidewall Heights of Small Parshall Flumes

One advantage of using the Parshall flume to measure open channel flow is that the layout and dimensions of the flume are standardized. For the range of sizes normally encountered in the field, the sidewalls heights of Parshall flumes are fixed and are:

24-inches [60.96 cm] for 6-inch Parshall flumes
30-inches [76.2 cm] for 9-inch Parshall flumes
36-inches [91.44 cm] for 12 to 96-inch Parshall flumes

The exceptions here are the smallest Parshall flumes - the 1-inch, 2-inch, and 3-inch sizes.

These flumes, the last of the 22 standard sizes of Parshall flumes to be developed, do not have fixed sidewall heights.

Small Flume Sidewall Heights

The sidewalls heights for these flumes range from:

1-inch Parshall flume, 6 to 9-inches [15.24 to 22.86 cm]
2-inch Parshall flume, 6 to 10-inches [15.24 to 25.4 cm]
3-inch Parshall flume, 12 to 24-inches [30.48 to 60.96 cm]

Needless to say with such a wide range of sidewall heights, there can be considerable differences in maximum rated flow rates for flumes of ostensibly the same size.

No clear reason for the variations in sidewall height is given in the research on the Parshall flume, either by Dr. Parshall or his colleagues, or as to why these smaller flumes vary in sidewall height while the other 19 standard sizes of Parshall flume do not.

With such a wide variation in sidewalls heights problems can arise both when applying Parshall flumes or when comparing between competing flume manufacturers' offerings.

Applying Small Parshall Flumes

One of the steps in selecting a primary device is determining if the flume can handle the minimum and maximum / peak flow rates expected at a site.

Usually this involves taking a site's flow numbers and comparing them against the discharge tables of various flumes. The assumption is that a manufacturer's discharge tables are calculated of their particular depth of Parshall flume. But this is not always the case!

In some instances, discharge tables have been copied by flume manufacturers from documents such as the Bureau of Reclamation's Water Measurement Manual or Teledyne ISCO's Open Channel Flow Measurement Manual without understanding the variations in sidewall heights of small Parshall flumes and whether or not the discharge tables copied from the source material actually apply to the depth of Parshall flume they product. These tables, then, can either over or understate the maximum flows a particular Parshall flume can measure.

As a worst case, a short sidewall height Parshall flume could overtop if it was sized based upon a flow rate that a taller height Parshall flume of the same size can handle so take care!

It is important to understand what sidewall height is being offered or supplied and verfiying that the sidewall height is sufficient to handle the maximum anticipated / peak flows the flume wil see.

Fiberglass vs. Sheet Metal Parshall Flumes

The trend among fiberglass Parshall flume manufacturers is to cluster around the upper end of the recommeded flume depths. With permanent / fixed fiberglass molds from whcih to manufacturer their products, fiberglass Parshall flume manufacturers seek to maximize the mold usage.

Sheet metal (galvanized or stainless steel) manufacturers tend to take a different appraoch. As their products do not require fixed molds from which to form their parts of, an emphasis is placed on reducing material costs. One way of doing this is to provide small Parshall flumes fabricated with the lower sidewall heights. These flumes are still "standard" - although not of the same depth a similarly sized fiberglass ones.