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Where to Install a Palmer-Bowlus Flume

When selecting a site in which to install a Palmer-Bowlus flume there are several points to consider:

Upstream of the Flume 

  • The Froude number (Fr)  for flow entering a flume should not exceed 0.5 and should never exceed 0.99.
    • Surface turbulence may be encountered for Froude numbers above 0.5.
    • For a flume to accurately measure flow, that flow must be sub-critical (Fr<0.99).
    • If the approaching flow is critical (Fr = 1.0) or supercritical (Fr > 1.0), then a hydraulic jump must be formed at least 30 times the maximum anticipated head (Hmax) upstream of the entrance to the flume to slow the flow (or energy absorbers / flow straighteners must be used).
  • The flow entering the flume should be smooth and well distributed across the channel for 25 D (D = flume size).  
  • The site should allow flow to smoothly transition into the flume.
  • The upstream channel should be clear of vegetative growth.
  • Open channel (non-full pipe) flow must be present under all flow conditions.


  • The flume must be able to be set so that the top of the throat ramp is level from front-to-back and from side-to-side.
  • The throat ramp of the flume must be set downstream.

  • The flume must be centered in the flow stream.

  • All of the flow must go through the flume – there should be no bypass.

Downstream of the Flume

  • EPA guidelines call for the downstream channel to be straight for 5 to 20 throat widths – although flow spilling freely off the end of the flume can eliminate this requirement.
  • Where an outlet end adapter is used to transition the flow out of the flume, the connecting pipe must of sufficient size, slope, and straight run so as to ensure that flow does not back up into the flume.

  • The downstream channel should be armored (riprap) or otherwise protected so that scour does not occur.
  • The downstream channel must be clear of vegetative growth or the collection of debris so that flow does not back up into the flume.