Site Selection

Where to Install a H Flume

In selecting a site to install an H Flume there are several points to consider:

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.  
• 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.
• The upstream channel should be straight for at least 3 to 5 times the maximum anticipated head (H_max).  
• H Flumes have been successfully used in applications where the flow rises up a uniform vertical column and then enters the flume.
• Where the channel is wider than the inlet of the flume, wing walls should be formed to smoothly direct the flow into the flume.    
• 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

• The flume must be able to be set so that the flume is level from front-to-back and from side-to-side.
• When H 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.
• The narrow opening of the flume is set downstream.
• All of the flow must go through the flume – there should be no bypass.
• Gwinn notes that if sediment is a concern, the approach section can be provided with a sloped floor (side-to-side) so that channelization does not occur. The slope should not exceed 1:8.

Downstream of the Flume

• Flow must freely spill off the end of an H 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.