The streamlines through a Parshall flume are assumed to be both parallel to the flume walls and continuous (Parshall 1936).  Many things can disrupt parallel streamlines as they enter the flume, including:  upstream debris, bank vegetation growing into the upstream channel, and poorly formed upstream transitions.  Of these, poorly formed upstream transitions are the easiest to correct IF the correction is done during the design and installation of the flume.

If flow is disrupted as it enters the flume and is not parallel to the sidewalls of the flume as it passes the point of measurement, the correct head will not be measured.  This is particularly true of applications where the stilling wells are used to assist in accurately measure the flow.

This error typically represents and under reading of the true head will result in inaccurate flow measurement. 

It is for this reason that the EPA recommends that the channel upstream of a Parshall flume be straight for 10-20 throat widths.  For pipe configurations, the rule-of-thumb is 15 pipe diameters upstream of the flume. 

Additionally, when transitioning from a pipe into a Parshall flume, there must be an engineered approach (end adapter) to calm the flow stream and straighten out the streamlines entering the flume.  Too short or abrupt and the flow will stay channelized and pass the point of measurement before it equalizes across the width of the flume.

A proper Parshall flume inlet end adapter should horizontally direct the flow into the flume though the use of curved walls, while at the same time the floor of the end adapter should rise at no more than a 1:4 slope to avoid boundary layer separation of the flow from the floor of the end adapter.

 

Sources:  NPDES Compliance Inspection Video Workbook:  Inspecting a Parshall Flume, EPA 300-B-95-017, September 1995, R.L. Parshall, The Parshall Measuring Flume, Bulletin No. 423, Agricultural Experiment Station, Colorado Agricultural College, Fort Collins, CO, March 1936