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User Daniel Kelly has posted two videos on Youtube that show problems with insufficienlty conditioned flow entering a Parshall flume and an attempt to fix it. The flume (and flow meter), measuring treated effluent, was under-recording the effluent flows.

Prior to the attempted fix, the flow through the flume was very channelized and never fully developed across the width of the flume.  From the video below, it can be seen that the flow from the pipe jets through, while the flows on either side of the centerline of the flume swirl around.

The attempt to elliminate the flow descrepancey included the addtion of a 2X12 installed across the 36" pipe as it enters the flume inlet channel.  The result was that the flow pattern out of the pipe was disrupted and more normally spread across with the full width of the inlet of the flume.  While it should be noted that this appears to solve the issue at the lower flows, it remains to be seen what would occur at higher flows.