Where to Install an RBC Flume

In selecting a site in which to install an RBC 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 (H_max) upstream of the entrance to the flume.
• The flow entering the flume should be smooth, tranquil, and well distributed across the channel.  
  • ASTM D1941 indicates that 10 to 20 times the throat with usually will meet the necessary inlet conditions. 
• The site should allow flow to smoothly transition into / out of the flume (or allow the use of wing walls to do so).
• The upstream channel should be clear of vegetative growth.

The Flume Installation 

• RBC flumes must be set so that the floor of the flume is level from front-to-back and from side-to-side.
• The ramp of the flume must be set downstream.

• When RBC 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

• EPA guidelines call for the downstream channel to 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.