Flumes are designed to develop a relationship between water level in the flume and the flow rate through the flume.  They do this by accelerating slow, subcritical flow to fast, supercritical flow.  

This can be accomplished by:  

• A change in floor elevation
• A contraction of the side walls
• Or a combination of the two  

The change in geometry is usually such that the flume has to be installed so that flow moves through the flume in a specific direction.  

Here a contractor has installed a Small 60 Degree V Trapezoidal Flume backwards as evidenced by the staff gauge and original mounting base for the ultrasonic sensor being downstream - where the flow has already been accelerated.

Instead of removing and reinstalling the flume, the contractor has decided to simply fabricate a new ultrasonic bracket and put it upstream – approximately where the flume’s point of measurement would be if it had been installed correctly.

Luckily the contraction geometry is the same on the inlet as it is the outlet of a Trapezoidal Flume.  The only difference being that the inlet of the flume is somewhat longer – and provides more flow conditioning – than the outlet.