When it comes to installing a flume, one of the things occasionally overlooked is how wide the channel actually needs to install it.  For the Parshall flume, the inlet of the flume is the widest portion and it is from here that the channel should start to be sized from.

Keep in mind that the dimensions shown on most dimensional prints for flume are the INSIDE (flow surface) dimensions. They do not account for stiffening ribs, support angles, etc.  As such, if a channel is only sized based upon the inside dimensions it will be too small.

For most flumes, the stiffening ribs, top/end flanges, and support angles extend about 3-inches from the inside of the flume (on each side).  That means that at a bare minimum, the channel should be at least 6-inches wider than the flume.  That’s a MINUMUM.

In reality, the need to backfill under the flume and up / around the sides means that more area should be left.  For smaller Parshall flumes, 6-inches may be sufficient, but for larger Parshall flumes (12-inches and 96-inches) 9-12 inches should be left on each side.  Flumes larger than this should have a minimum of 12-inches on each side.