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Where to Install a Trapezoidal Flume

A successful Trapezoidal flume installation requires that certain site selection criteria be met:

Upstream of the Flume 

  • Flow entering the flume MUST be sub-critical.  
  • 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, tranquil, and well distributed across the channel.  
    • ASTM D 1941 indicates that 10 to 20 times the throat width will usually meat the necessary inlet conditions.
  • The approaching channel should be straight so that the velocity profile is uniform.  Surging, turbulent, or unbalanced flows must be conditioned before the flow enters the flume.

  • Any bends, dips, elbows, or flow junctions upstream of the flume must be sufficiently far upstream so that the flow has is well distributed and non-turbulent.

  • While corrections can be made for improper installations or flume settlement, they should be avoided where at all possible.

  • Trapezoidal flumes have been successfully used in applications where the flow rises up a uniform vertical column and then enters the flume.

  • The site should allow flow to smoothly transition into / out of the flume (or allow the use of wing walls to do so).

  • When connecting to inlet piping, observations have shown that the pipe should be straight and without bends for at least 15 pipe diameters.

  • The upstream channel should be clear of vegetative growth.
  • Open channel (non-full pipe) flow must be present under all flow conditions.


  • Trapezoidal flumes must be set so that the floor is level from front-to-back and from side-to-side.

  • The somewhat longer straight section at the end of the flume goes upstream.

  • When Trapezoidal flumes are installed in earthen channels and furrows, care should be taken to ensure that a stable bottom elevation is present and that the elevation does not change during dry / wet seasons or low-flow periods.

  • 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

  • For a Trapezoidal flume to operate under free-flow conditions the downstream channel must be of sufficient size / configuration so that flow does not back up into the flume – slowing the discharge out of the flume.

    • While Trapezoidal flumes have relatively high submergence transitions (75 to 85%) corrections for submerged flow are not available.  It is important, therefore, that Trapezoidal flumes be installed so that they operate under free-flow conditions.

  • The downstream channel should be straight for 5 to 20 throat widths. 

  • 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.