CHAPTER 8 - FLUMES

8. Long-Throated Measurement Flumes

Long-throated flumes are coming into general use because they can be easily fitted into complex channel shapes as well as simple shapes (Replogle, 1975; Bos et al., 1991). Long-throated flumes have many advantages compared to other measuring devices, including Parshall flumes. Long-throated flumes are more accurate, cost less, have better technical performance, and can be computer designed and calibrated. Thus, long-throated flumes are preferred over Parshall flumes for new installations. However, some states may have laws or compact agreements mandating the use of Parshall flumes in certain situations.

(a) Characteristics of Long-Throated Flumes

The cross-sectional flexibility of long-throated flumes allows them to fit various channel shapes more conveniently than short-throated flumes, which have fixed sizes and shapes. Because of the ability to match the channel shape, the construction of forms is usually simplified. In contrast, the fixed geometry of short-throated (including Parshall) flumes usually makes upstream and downstream transitions necessary and may require long wingwalls. Because of their flexibility and capability to fit any channel shape, long-throated flumes have more gradual transitions. Thus, floating debris presents fewer problems. Also, field observations have shown that the structure can be designed to pass sediment transported by channels with subcritical flow.

A simple type of long-throated flume developed and described by Replogle et al. (1991) consists of a flat raised sill or crest across a trapezoidal channel with an approach ramp transition from the approach channel invert. The crest drops vertically at the downstream end back to the downstream canal invert. These flumes (figure 8-5) have been called Replogle flumes, modified broad-crested weirs, and ramp flumes. This simple version of the long-throated flume is formed with only two bottom planes. An optional third plane can be used for maximum head recovery. The lined canal shape serves as the flume approach section, compared to constructing 9 to 12 planes for Parshall flumes. It is usually easier to construct the two to three planes of the long-throated flumes.

Some confusion of terminology exists here. Some investigators would consider the ramp flume a broad-crested weir because the flow constriction is produced from a bottom transition alone, whereas a flume would depend to some extent on side convergence. Both long-throated flumes and broad-crested weirs can be accurately rated by analysis using fluid flow concepts. The energy principle, critical depth relationships, and boundary layer theory are combined when computer calibrating these flumes and weirs. Because of this close connection, this manual will consider and call both the long-throated flumes and broad-crested weirs long­throated measurement structures.

Cost estimates for a large 930­ft³/s ramp flume varied from about 45 to 60 percent of that for a Parshall flume in a retrofit situation. Clemmens and Replogle (1980) cited costs of one­tenth to one­third of equivalent Parshall flumes for a small ramp-type, long-throated flume. Some of the cost differences between small and large structures result from the need for service roads, foundation differences, and repair of approach channel surfaces in retrofit designs.

Long-throated flumes can be computer calibrated to within +2 percent plus head measurement error and have submergence limits up to 90 percent. Even when the listed submergence limits are near 70 percent, the absolute head loss or water surface drop through the long-throated flumes may be smaller than the older structures, depending on the particular design selection from among the many choices of shape.

Short-throated flumes can measure free flow accurately in the range of +3 to +5 percent plus head measurement error and have submergence limits from 50 to 80 percent. Increased uncertainty occurs when using flow corrections to obtain discharge rates beyond submergence limits commonly up to 95 percent. However, Peck (1988) found large correction errors caused by hysteresis shifts of the downstream wave front at a submergence of 90 percent. Correction is frequently done above 90-percent submergence with Parshall flume measurements. Using submergence corrections commonly results in discharge errors ranging from  7 to +20 percent, and possibly much more, as differences in upstream and downstream measuring heads become small.

With most flumes, close adherence to tolerances during construction is required to rely on empirical equations and calibrations provided for each specific short-throated flume. Dimensional errors and slippage of the forms frequently cause unacceptable errors that are difficult to resolve without laborious field calibrations. Field calibrations for submergence correction are very cumbersome and time consuming because of the usual project operational limitations, difficulties of controlling heads, and the need for long lag times for heads to settle to asymptotic levels. However, long-throated flumes can usually be computer recalibrated using as-built dimensions if form slippage has not caused crest slope in the direction of flow. Even then, crest correction may be practical and relatively inexpensive.

The measured heads in the short-throated flumes do not always indicate system head loss. For example, the upstream measured head of a Parshall flume is located about one-third of the way into its converging crest section, and the water surface may have a considerable drawdown from the approach canal surface elevation. This factor makes size selection and crest elevation setting more complicated than for long-throated flumes that approximate existing channel dimensions and shape.

Because long-throated flumes have greater tolerance to submergence than short-form flumes and weirs, they can deliver more discharge without having to consider the effects of submergence, which usually requires observation of a downstream depth. For example, Parshall flumes require 3 to 4 times the absolute water surface fall through the structure for free-flow measurements than long-throated flumes (Bos et al., 1991). Long­throated flumes, with tolerances for high submergence ratios, require only one head measurement. They are considered to be more accurate and economical than, for example, extending Parshall flume measurement range by submerging up to comparable long-throated flume submergence limits and making corrections using two head measurements.

Because long-throated flumes fit nicely into existing flow channels, they are convenient for making portable measurement devices. Portable long-throated devices for flow rates up to about 2 ft³/s are described in Bos et al. (1991), for trapezoidal and rectangular cross sections.

(b) Summary of Long-Throated Flume Advantages

The main advantages of long-throated flumes are:

(1) Provided that critical flow occurs in the throat (not excessively submerged), a rating table can be calculated with an error less than +2 percent. This calculation can be done for any combination of a prismatic throat and an arbitrarily shaped approach channel.

(2) Long-throated flumes can have nearly any desired cross-sectional shape and can be custom fitted into most canal-site geometries. The throat cross section can be shaped in such a way that the complete range of discharge can be measured accurately.

(3) Long-throated flumes can be made into portable devices that fit conveniently into open channels with considerably less complicated construction forming.

(4) The required head loss over the long-throated flume to obtain a unique relationship between the upstream sill-referenced head and the discharge is small. This head-loss requirement may be estimated with sufficient accuracy for any of these flumes placed in any channel.

(5) Because of their gradual converging transition, these flumes have few problems with floating debris and sediment. Field observations have shown that the flume can be designed to pass sediment transported by channels with subcritical flow.

(6) Provided that the throat is horizontal in the direction of flow, a rating table can be produced that is based on postconstruction dimensions. This horizontal orientation is required to allow an accurate rating table to be made to compensate for deviations from design.

(7) Under similar hydraulic and other boundary conditions, long-throated flumes are usually the most economical of all structures for accurately measuring flow.

(8) Long-throated flumes are amenable to selection, design, and calibration by computer techniques.

(c) General Design Procedures for Long-Throated Flumes

The major steps of the design process for long-throated flumes are: (l) selection of site, (2) selection of head measurement techniques previously discussed, and (3) selection of an appropriate structure. Design is an iterative process between these steps. The order and importance of these steps depend on the specific conditions encountered.

To properly select and design a measurement structure, all demands and operational requirements to be made on the structure should be listed and matched with the properties of the known structures. These demands and operational requirements originate from four sources: (1) the hydraulic performance, (2) the construction or installation cost, (3) the ease with which the structure can be operated, and (4) the cost of maintenance. The imposed demands will be discussed in more detail. Factors that affect design and selection such as submergence, site characteristics, workmanship, and head measurement systems are discussed earlier in this chapter.

(d) Determining Shape and Size of the Structure

Long-throated flumes operate by using a channel contraction to cause critical flow. Insufficient contraction will prevent critical flow. Under this condition, flow is then nonmodular or submerged and gage readings are meaningless. Too much contraction may raise the water surface upstream and cause canal overtopping or sediment deposition problems. The designer's problem is to select the shape of the control section or throat such that critical flow occurs through-out the full range of discharge measurement and produces required accuracy. Also, the designer must provide acceptable head reading sensitivity. Usually, the sensitivity of the structure at maximum flow is selected such that a change in measurement head, h₁, of about 0.03 ft causes less than a 10-percent change in discharge. Achieving these design requirements may seem difficult, but existing design aids and rating tables make this task more manageable.

(e) Computer Design Versus Sets of Precalibrated Long-Throated Flumes

A thorough treatment of the computational process and several precomputed, standard-size, long-throated flumes for a variety of canals and natural channels are presented in Clemmens et al. (2001) and Bos et al. (1991). The U.S. Water Conservation Laboratory, Agriculture Research Service, U.S. Department of Agriculture, developed the first computer programs for designing and calibrating long-throated flumes.

WinFlume (Wahl et al. 2000) is the most advanced software for analysis of long-throated flumes.  The program is Windows-based and can be downloaded from www.usbr.gov/pmts/hydraulics_lab/winflume.  Ratings are determined by numerical solution of the critical-flow equations, accounting for boundary friction and other losses.  The program includes a module that simplifies and accelerates the process of developing acceptable flume designs.

Clemmens et al. (2001) and Bos et al. (1991) provides calibration tables in metric (S.I.) and English units for a set of long-throated flume dimensions that covers a discharge range from about 2.8 to 280 ft³/s for trapezoidal channel shapes with side slopes of 1:1 to 1:1.5 horizontal and with bottom widths from about 1 to 5 ft. They also provide instructions for construction and field placement. Calibration tables for long-throated rectangular flumes are also presented. The S.I. tables are reproduced in Cheremisinoff et al. (1988). Statistically fitted equations in S.I. units that closely reproduce the computed tables are presented in Hoffman et al. (1991).

The above references and this manual provide design and calibration tables for selecting and sizing long-throated structures from sets of predetermined, dimensioned, and precalibrated structures or from dimensionless design tables for some special structures. However, computer techniques are much preferred for all installations, designs, and calibrations for long-throated flumes. Thus, when practical, long-throated flumes should be designed using the WinFlume computer program (www.usbr.gov/pmts/hydraulics_lab/winflume). Using restricted sets of dimensions reduces the capability of more exact custom fitting to shapes of existing channels, which can make it difficult to attain discharge range requirements. The need for computer techniques becomes much more important for large long-throated structures.