2. Flume Classes

Many kinds of flumes are in use. The two basic classes or forms of flumes are discussed below.

(a) Long-Throated Flumes

Long-throated flumes (figure 8-1) control discharge rate in a throat that is long enough to cause nearly parallel flow lines in the region of flow control. Parallel flow allows these flumes to be accurately rated by analysis using fluid flow concepts. The energy principle, critical depth relationships, and boundary layer theory are combined to rate flumes and broad-crested weirs by Ackers et al. (1978) and Bos et al. (1991). Thus, the long-throated flumes and modified broad-crested weirs are amenable to computer calibrations. 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 (Replogle, 1975; Bos et al., 1991), also called ramp flumes (Dodge, 1983), are styles of long-throated flumes.

(a) Long-throated flume (broad-crested weir) under construction.

(b) The long-throated flume (broad-crested weir) with approximatly 1,200 ft3/s.
Figure 8-1 -- Large long-throated flume for left to right flow in Arizona canal (courtesy of U.S. Water Conservation Laboratory, Phoenix, Arizona)..

(b) Short-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 (figure 8-2). These flumes would require detailed and accurate knowledge of the individual streamline curvatures for calculated ratings, which is usually considered impractical. Thus, calibrations for short­throated flumes are determined empirically by comparison with other more precise and accurate water measuring systems.

Figure 8-2 -- Four-foot, short-form Parshall flume, discharging 62 ft3/s under free-flow conditions. Scour Protection is needed for this much drop.