CHAPTER 8 - FLUMES
3. Other Special Flumes
Many flumes have been designed for organizational preferences and special uses. Many of these are considered short-form flumes. Some examples are shown on figure 8-3, and some are briefly discussed in this section.
H-flumes, developed by the Natural Resources Conservation Service (former known as the Soil Conservation Service) (Brakensiek, et al., 1979; Kulin et al., 1975), 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 (Brakensiek et al., 1979; Gwinn and Parsons, 1976) have been used mostly on small agricultural watersheds and have not found extensive use in irrigation flow measurements.
(b) Cutthroat Flumes
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 (Skogerboe et al., 1973) 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.
However, the prescribed head measuring location, which may be in a zone of separation, and conditions of the upstream channel in which it is placed, along with variable conditions of the sharp connection of the convergence and the divergence, have caused considerable variability in calibrations. Because of these complexities in hydraulic behavior, several authors do not recommend their use (Ackers et al., 1978; Bos, 1989).
(c) Palmer-Bowles Flumes
Palmer-Bowles flumes (Wells and Gotaas, 1958) 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.
(d) 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 (Robinson and Chamberlain, 1960). 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.
Although these early versions were laboratory calibrated, more recently, they were found to conform to the analysis procedures for long-throated flumes for heads less than about 50 percent of maximum. For higher depths, their throats become too short for precise long-throated flume analysis, and the laboratory ratings should be used. Except for those already in existence, and for which the user may need calibrations, these flumes are generally being replaced with versions having longer throats, making them long-throated flumes that can be calibrated by analysis.
(e) Special Flumes for Passing Sediment
Where sediment is a major problem, special flume types have been designed to obtain flow measurements in these adverse conditions. Included in these devices are the trapezoidal supercritical-flow flume, the Walnut Gulch flume, and the San Dimas flumes which slope in the direction of flow (Brakensiek et al., 1979). These flumes are considered to be a class of supercritical flumes. Usually, they are characterized as requiring extensive head drop to operate and find limited application in irrigation.