Site Selection

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 (H_max) 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.

FLUME LOCATION

• 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.