One question that comes up again and again is: Can Parshall flumes have the flow spill off the end? The answer is yes!
While initially designed for installation in irrigation channels and canals, the Parshall flume has been shown to be well suited for use on free-spilling discharges.
In fact, the flume has been so frequently used that a modification to the Parshall has been developed just for this application: the Montana (Short Parshall) flume.
Whether a full length Parshall flume or short length Montana flume is used, the standard Parshall flume free-flow equations / discharge tables can be used. Free-spilling discharge does not affect the flow rate or accuracy of the flume (unlike the Palmer-Bowlus which is affected).
Another advantage of using a Parshall or Montana flume for free-spilling discharge is that almost all operators are familiar with the flume and its usage.
Also, when the shorter Montana flume is used, considerable cost savings can be realized and the space required for installation can be halved. Further, length of the Montana flume means that it is readily adaptable to mounting at the end of a pipe or channel with only minimal support structure. Spaced constrained applications - like drop manholes - can often use Montana flumes when full length Parshall flumes won't fit.