Flumes are specially shaped, static structures that are used to restrict the flow of free surface waters in such a way so as to develop a relationship between the water level and the flow rate.
Flumes 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 of flow 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 expected to provide flow reading accurate to within +/-2-5% (for the flume itself), depending upon the flume type, with overall system accuracy for a typical installation being +/-10% when all factors are considered. The accuracy of flumes is similar to that of weirs, with the accuracy of weirs being slightly better under ideal circumstance.
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, the ability to pass debris more readily, wide range of styles and sizes, off-the-shelf availability, smaller installation footprint, and less rigorous maintenance requirements.
Also, most styles of flumes are resistant (to varying degrees) to changes or restrictions in the downstream hydraulics (submergence).
Sections of a Flume
Most flume styles consist of three parts: a converging section, a throat, and a discharge section.
Flow is restricted in the converging section and is accelerated as it passed into the throat. Flow in the converging section must enter at a sub-critical state (typically with a Froude number of 0.5) and it is in this section that the where the point of measurement is.
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. While not all flumes have discharge sections (H / Montana flumes), the lack of this section means that the downstream channel may be susceptible to scour and erosion.
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:
- 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 rated in a laboratory or in the field.
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 (without the need for costly laboratory rating). Additionally, the point of measurement is not fixed, but can be taken upstream of the draw down zone of the throat.