One of the requirements of accurate flow measurement with a Parshall flume is a tranquil flow approaching the flume.  Poorly balanced, surging, or turbulent flows will adversly affect the systems accuracy for both instantaneous and totalized flow.

Here we have an example of highly turbulent (and energetic) flow exiting a pipe and entering a Parshall flume.

The flow is so energetic and turbulent that even with a rise into the flume, the flow very nearly overtops the converging section of the flume.  Even with a stilling well, the flow would be far too turbulent for any sort of accurate flow measurement to occur.

With the location of the flume fixed and the operator having little ability to moderate the volume of flow, the flow would have to be conditioned prior to it entering the flume. 

An energy absorber place just at the exit of the pipe would help to slow the flow

and tranquilizing racks could then straighten out and evenly distribute the flow as it enters the converging section of the flume.