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.
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.