Accurate flow rate measurement is highly dependent upon the approaching flow conditions. The approaching flow should be well distributed, laminar, and free of surface waves. To aid in the transition of flow from what is typically a wider approach channel into the flume, the use of curved wingwalls or flared transitions should be used to guide the flow into the converging section of the flume without creating eddies or surface waves. Right angles should always be avoided.
In addition, the approaching velocity should be subcritical. To determine if your approach velocity is too high there are a few basic calculations you can perform.
- Critical velocity through the throat of the flume = (Gravity X Head )1/2
Where Gravity = 32.3 fps
- Recommended approach velocity should be less than half the critical velocity. Calculated by:
fps = 0.5√(32.2) (Head)
In some cases the use of tranquilizer racks or energy absorbing plates can be used to reduce the velocity immediately upstream of a flume. The following video demonstrates how tranquilizer racks can be used to dissipate the energy caused by excessive approach velocity and create laminar flow.