Openchannelflow does not offer non-standard throat width Parshall Flumes.
Per ASTM D1941-91 (2013) Standard Test Method for Open Channel Flow Measurement of Water with the Parshall Flume, the Parshall Flume is an empirical device and must be flow calibrated.
Interpolation between flow tables is not sufficient to develop an accurate flow.
As a result, Openchannelflow does not offer 4-inch, 21-inch, or 30-inch Parshall Flumes.