CHAPTER 8 - FLUMES
Flumes are shaped, open-channel flow sections that force flow to accelerate. Acceleration is produced 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. Therefore, flumes are sometimes called critical-depth flumes. When flow passes through critical depth, a unique water surface profile occurs within the flume or broad-crested weir for each discharge. 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.
Flumes range in size from very small-1 inch (in) wide-to large structures over 50 feet (ft) wide that are installed in ditches, laterals, and large canals to measure flow. These flumes cover a corresponding discharge range of 0.03 to over 3,000 cubic feet per second (ft3/s) although no particular upper size limit exists. Commonly, irrigation channels are designed to operate at near bank full to extend delivery coverage and, when the landscape is flat, to minimize earthwork involved in bank height construction. Therefore, some flumes have been calibrated for the condition when the critical depth has been nearly drowned by downstream backwater either purposely or by later increase of downstream flow resistance. To measure discharge with high levels of submergence, two head measurements are required, which results in significant loss of accuracy compared to free-flow measurements.
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. Unauthorized altering of the dimensions of constructed flumes to obtain an unfair share of water is difficult and, therefore, not likely. 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. Some flumes can be more expensive than sharp-crested weirs or submerged orifices in unlined channels.