Flumes are shaped, open-channel flow sections in which flow is accelerated by converging the sidewalls, raising the bottom, or a combination of both.
When only the bottom is raised with no side contractions, the flume is commonly called a broad-crested weir.
When the downstream depth is shallow and enough convergence exists between the upstream and downstream channels, the flow passes through critical depth. This condition is known as free flow. For this case, upstream heads at one location relative to the control bottom elevation near the region of critical depth can be used to determine a usable head versus discharge relationship for flow measurement.
Flume head loss is less than about one-fourth of that needed to operate a sharp-crested weir having the same control width, and in some long-throated flumes, may be as low as one-tenth. Another advantage compared to most standard weirs is that for a properly designed and installed flume, the velocity of approach is a part of the calibration equations. Velocity of flow can usually be designed to minimize sediment deposition within the structure. Gradual convergence sections at the entrance tend to improve velocity distribution of approach flow and the passage of floating debris.
The two basic classes or forms of flumes are;
Long-throated flumes control discharge rate in a throat that is long enough to cause nearly parallel flow lines in the region of flow control. Long-throated flumes can have nearly any desired cross-sectional shape and can be custom fitted into most canal-site geometries. The modified broad-crested weirs, also called ramp flumes, are styles of long-throated flumes.
Short-throated flumes are considered short because they control flow in a region that produces curvilinear flow. Although they may be termed short throated, the overall specified length of the finished structure, including transitions, may be relatively long. The Parshall flume is the most common example of this type of flume. calibrations for short-throated flumes are determined empirically by comparison with other more precise and accurate water measuring systems.
Other special uses flumes are;
H-Flumes are made of simple trapezoidal flat surfaces. These surfaces are placed to form vertical converging sidewalls. The downstream edges of the trapezoidal sides slope upward toward the upstream approach, forming a notch that gets progressively wider with distance from the bottom. These flumes should not be submerged more than 30 percent. This group of flumes, including H-flumes, HS-flumes, and the HL-flumes have been used mostly on small agricultural watersheds and have not found extensive use in irrigation flow measurements.
Cutthroat flumes are so named because they resemble Parshall flumes with the throat “cut out.” They are formed by directly connecting a 6:1 converging section to a similar diverging section. Thus, they consist of a converging level inlet section with vertical sidewalls and a diverging level outlet section also with vertical sidewalls. They do not have any parallel walls forming a straight throat and, thus, belong to a class of throatless flumes. The converging and diverging walls do not necessarily match those of other flumes in either converging or diverging slope or length. The primary objective of their development was construction simplicity compared to Parshall flumes.
Palmer-Bowles flumes are frequently made as inserts with circular bottoms that conveniently fit into U-shaped channels or partially full pipes. These flumes make a transition from a circular bottom section to a raised trapezoidal throat and transition back to a circular bottom section. These flumes are of the long-throated type and can be calibrated by theoretical analysis.
Flat-Bottomed Trapezoidal Flumes
Flat-bottomed trapezoidal short-form flumes were first developed to be placed in canals and to conform more closely to usual small canal shapes. Therefore, if possible, the cross section of the canal and the start of the converging portion of the flume should match. They were designed to set flush with respect to the bottom of the incoming channels in an effort to assist sediment movement and allow the canal to drain dry between uses. Although the latter objective was achieved, the authors did not establish whether sediment movement was a function of upstream velocity or floor configuration.