Flumes are specially shaped, engineered structures that are used to measure the flow of water in open channels. Flumes are static in nature - having no moving parts - relying on restricting the flow of water in such a way so as to develop a relationship between the water level in the flume at the point of measurement and the flow rate.
Flumes can use:
- A change in elevation
- A contraction of the sidewalls
- Or a combination of the two
To accelerate slow, sub-critical (Fr<1) flow to a supercritical state (Fr>1). This acceleration creates upstream conditions where the flow rate can be determined by measuring the water level at a single point.
When properly installed, flumes can be accurate to within +/-2-5% (for the flume itself) - with overall system accuracy for a typical installation being +/-10% (when all factors are considered). The site accuracy of flumes is similar to that of weirs, with the accuracy of weirs being slightly better under ideal circumstances.
While similar in accuracy, flumes have distinct advantages over weirs, including:
- The ability to measure higher flow rates than a comparably sized weir
- Less head loss (generally 1/4)
- The ability to pass debris more readily
- Wide range of styles and sizes
- Off-the-shelf availability
- Smaller installation footprint
- Less rigorous maintenance requirements
Also, most styles of flumes are resistant to changes or restrictions in the downstream hydraulics (submergence) - something not found with weirs.
Sections of a Flume
Most flume styles consist of three parts: a converging section (inlet), a throat, and a discharge section (outlet).
- Flow is restricted in the converging section and is accelerated as it passes into the throat. Flow in the converging section must enter at a sub-critical state [typically with a Froude number (Fr) no more than 0.5] and it is here that the point of measurement (Ha) is located.
- The throat of the flume accelerates the flow to a critical / super-critical state.
- The discharge section slows the now energetic super-critical flow and allows it to transition into the downstream channel.
Classes and Styles of Flumes
In the approximately 100 years since Dr. Parshall began his investigation on his Improved Venturi flume, two distinct classes (and numerous styles) of flumes have been developed.
Short does not refer to the flume length but to the fact that flow is controlled in a very specific region of the flume to produce the level-to-flow relationship.
Openchannelflow manufactures the following styles of short-throated flumes:
- Montana (modified Parshall)
- USGS Portable (modified Parshall)
- HS / H / HL
Short-throated flumes have a single, defined point of measurement and are empirical devices.
Intermediate or non-standard sizes of short-throated flumes must be individually rates. Their use in not recommended.
Long-throated flumes control flow in a throat that is long enough to cause parallel flow lines in the section of flow control.
Openchannelflow manufactures the following styles of long-throated flumes:
Long-throated flumes are somewhat more accurate than short-throated ones and can be computer designed (eliminating the need for costly laboratory rating).
Additionally, the point of measurement is not fixed, but can be taken anywhere upstream of the draw down zone of the throat.