Where to Install a Parshall Flume
In selecting a site in which to install a Parshall flume there are several points to consider:
Upstream of the Flume
- Flow entering the Parshall 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.
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.
EPA recommendations for upstream channel runs are conservatively the same as for long throated flume – 25 throat widths, while ASTM D1941 indicates that 10 to 20 times the throat with usually will meet the necessary inlet conditions.
While corrections can be made for improper installations or flume settlement, they should be avoided where at all possible.
Parshall 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 Parshall flume, wing walls should be formed to smoothly direct the flow into the flume. The inlet wing walls should be of a constant radius and should end tangent to the inlet walls of 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.
- The flume must be able to be set so that the flat converging section is level from front-to-back and from side-to-side.
When Parshall 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.
Where a Parshall flume must be set above the floor of a channel, a 1:4 (rise:run) slope should be formed into the flume. Slopes greater than this should be avoided as they can cause turbulence as the flow separates at the junction of the ramp and the inlet of the flume.
- All of the flow must go through the flume – there should be no bypass.
Downstream of the Flume
For a Parshall 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.
When flow out of the Parshall flume is returning to a channel or pipe, the EPA recommends that the channel be straight and unobstructed for 5-20 throat widths – although flow spilling freely off the end of the flume can eliminate this requirement.
To transition the flow out of a Parshall flume, wing walls should be used. These walls can be flat and perpendicular to the flume (to save space and money) or they can extend from the flume’s discharge at some angle or radius sufficient to transition the flow as desired. Unlike the upstream of the flume, the outlet wing walls do not need to (generally) conform to a specific shape. The only caveat being that in earthen or natural channels the transition should be as gradual as practical to minimize downstream scour.
- 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.