Long‐throated flumes and broad‐crested weirs have become accepted standards for open‐channel flow measurement during the past two decades. These structures offer the accuracy and reliability of critical‐depth flow measurement, theoretically based calibrations, the lowest head loss requirement of any critical flow device, and extraordinary design and construction flexibility. Computer software developed in recent years has streamlined the design and calibration process. The software, WinFlume, has been described in several papers and a recent text. Although WinFlume is very easy to use, there is still a need for simplified design and calibration tools for situations where use of the computer model is not possible or desirable. This paper combines several previous efforts to provide such tools in both metric and English units for the most typical measurement applications encountered in irrigation and drainage systems. Pre‐computed designs for trapezoidal broad‐crested weirs, long‐throated flumes with rectangular control sections, broad‐crested weirs in circular pipes, V‐shaped long‐throated flumes, and portable RBC flumes are presented in easy‐to‐use tables that provide head and discharge ranges, construction dimensions, head loss requirements, and flume rating equation parameters. The use of the tables is demonstrated with examples, and construction methods are illustrated. Copyright © 2005 John Wiley & Sons, Ltd.

Flap gates are commonly used at the end of pipe drains and pump outlets to prevent backflows of water and entry of small animals. Flap gates are relatively inexpensive, with low maintenance costs, but can trap debris in their hinge systems. Many texts refer to studies performed on flap gates at the University of Iowa in 1936, which may be limited in value because they are for specific "lightweight" gates. More recent studies in England have attempted to generalize the characteristics for pin-hinged flap gates and place the Iowa studies into a broader perspective. The flap-gate backpressure effect on a gate with free outfall appears to be small. However, under certain submerged conditions, a flap gate will increase the upstream backwater levels. This may be critical in sewerage situations and some surface land drainage cases. Recently, at least one manufacturer sells a rubber-coated steel gate cover with a flexure hinge made of the same rubber material. While it reduces the opportunity for trash to catch, such as may happen on a pinned-hinge type of pivot, this rubber hinge arrangement essentially becomes a spring-loaded gate with the force of closure due to both the weight of the gate and the elastic properties of the hinge. Users have questioned whether this arrangement introduces significant backpressure. We therefore tested a rubber-hinged flap gate to verify whether these gates fit into the general pattern of the limited previous studies on pin-hinged gates, which is to exhibit a continuous decrease in backpressure with increasing flow rate, and hence gate opening. The design information for pin-hinged flap gates is also updated to make it more readily available for design uses. The rubber hinge resulted in a slight deviation towards more head loss, approximately 3 mm (0.1 in.) at larger gate openings compared to about 1 mm (0.025 in.) for pin-hinged gates, which is attributed to the flexure strength of the rubber hinge. Thus, for free-flow outlet applications, flap gates of either the pinned-hinge or the flexure style add small head losses that amount to about 1% to 2% of the pipe diameter These studies and the review of previous work allow users to evaluate whether flap gates of either the pinned hinge design or the rubberized flexure design can cause detrimental backpressure on a drainage system under free outfall situations.

Calibration equations for free-flowing radial gates typically provide sufficient accuracy for irrigation district operations. However, many water purveyors have difficulty in determining accurate discharges when the downstream water level begins to submerge the gate. Based on experimental laboratory studies, we have developed a new calibration method for free-flowing and submerged radial gates that allows for multiple gates and widely varying upstream and downstream channel conditions. The method uses the energy equation on the upstream side of the structure and the momentum equation on the downstream side, and thus is called the Energy-Momentum Method. An iterative solution is required to solve these two equations, but this allows calibration from free flow to submerged flow continuously through the transition. Adjustments to the energy equation for free flow are described, along with an additional energy adjustment for the transition to submerged flow. An application is used to describe the new procedure and how it overcomes the limitations of current energy-based methods.

Long-throated flumes and broad-crested weirs provide a practical, low-cost, flexible means of measuring open-channel flows in new and existing irrigation systems, with distinct advantages over other flume and weir devices. A primary advantage is the fact that these structures can be custom-designed and calibrated with a computer program based on well-established hydraulic theory. This allows the design of structures that meet unique operational and site requirements, and eliminates the need for laboratory calibration. To facilitate future use of these devices, the Bureau of Reclamation and the Agricultural Research Service have recently developed the Windows-based, WinFlume computer program described in this paper.

State-of-the-art flow metering developments for irrigation are reviewed. Selected older methods are updated with recent improvements. These include orifice meters, venturi meters, critical-flow flumes, sonic methods, and vortex-shedding meters.

Laboratory studies are conducted on a rectangular broad‐crested weir to determine the effects of entrance and exit conditions on the discharge and required energy loss. In doing so, a method for visually determining whether downstream conditions affect the flume calibration is developed. The current mathematical model of these flumes satisfactorily predicts both discharge and required head losses. Approach‐ramp slopes of 3:1 and 2:1 are shown to be acceptable for accurate discharge prediction, although a slight rounding of the approach ramp corners will improve predictions. At high ratios of energy head to throat length and with a sudden downstream transition, the model slightly underpredicts the required energy loss. As long as the water‐surface profile is dropping continuously from the approach ramp to the end of the throat, the downstream water level is not influencing discharge prediction. Field observations and measurements support these laboratory results, which should apply to most long‐throated flumes, since a rectangular broad‐crested weir represents the worst‐case conditions for head losses.

Constant water‐level controllers of various kinds are used within many irrigation canal distribution systems. In a companion paper, simple transfer‐function equations were developed for one style of automatic water‐level controller, the dual acting controlled leak system. Such information can be used to study the dynamic response of proposed designs. In this paper, these transfer‐function equations are used to evaluate the stability and dynamic response of the system. It is shown that these systems are always stable. Response times and control decrement are well modeled.

Control schemes which are available for the operation of canal networks for irrigation projects are examined. These control schemes include upstream control, downstream control, controlled volume control, dynamic regulation, and flow rate control. The latter method is emphasized in particular because of its usefulness in managing water. The type of control and type of structure needed at each canal bifurcation should be carefully examined and matched with the type of delivery scheme and operational constraints. The result may be that different control schemes and devices may be used in different areas of the same project. Demand deliveries are generally not feasible for most large projects, unless the project is broken down into independently controlled subunits. Arranged schedules with upstream control are currently more common. An alternative is to use constant‐flow‐rate control at the head of lateral canals with upstream control within the lateral. Deliveries are then arranged within the lateral unit with the lateral obtaining water essentially on demand, thus reducing delivery lead times. This improves regulation on the lateral (and thus to the farm) and somewhat reduces control requirements on the main canal since constant water levels are not required. Control mechanisms are available which make accurate regulation of lateral discharges feasible.

Tests were conducted to study the effects of nonlevel placement or construction on the calibration of long‐throated flumes. These flumes have previously been mathematically modeled to provide accurate (±2%) calibrations for a variety of cross‐sectional shapes. While cross‐slope can be handled successfully by the modeling procedure, no assured procedure was apparent for longitudinal slope corrections. Laboratory tests were directed to determining errors that a user might experience if a flume crest were placed at various positive and negative longitudinal slopes. The magnitude of systematic errors are shown for portable flumes with either a sidewall gage or a translocated stilling well, and for permanently placed flumes. Correction procedures are suggested.

A control device has been developed for canal structures, which can be used to control upstream water levels, downstream water level, or off‐take canal discharges. The system uses no electrical power or electronics. It is powered by the available drop in water level across the structure. It can control water levels to within a very narrow band [±3mm (0.01 ft) for the systems tested], which is not feasible with electrically controlled motorized gates. Since this level of water surface control is often necessary for accurate flow rate control when weirs and flumes are used, this device, called dual‐acting controlled‐leak, or DACL, is particularly suited to control of canal flow rates. The DACL system was studied in a hydraulics laboratory to determine the relevant factors affecting control. A field test was conducted on a radial gate [1.9‐m (6.2‐ft) radius] to study the system's reliability in actual field operation.

A new approach to the design of flumes and weir for open channels is developed. The approach is based on matching the flume to approach channel conditions thus providing an appropriate Froude number in the approach channel for the passage of sediment and for producing a stable readable water surface at the head measurement station. The procedure is graphical and is applicable to approach sections and control sections of any shape.

Based on extensive experience one family of structures is recommended for flow measurement in open channels. The advantages of this family of broad-crested weirs and long-throated flumes are given. A 14-step design procedure is introduced that yields the appropriate weir or flume design for any channel. The head vs. discharge rating can be calculated by use of a programmable calculator method. A design example is given.

ABSTRACT A new style of portable flow-measuring flume has been designed for furrows and unlined channels. These flumes are relatively easy to install and operate. Sensitivity to leveling has been decreased by placing the stilling well near the flume throat. They are long-throated flumes and require very little head loss for satisfactory operation. They are simple to construct since they consist of a short trapezoidal section with a contraction inserted in the flume bottom. Rating tables are provided for several standard sizes with capacities ranging from 1.5 to 50 L/s (0.05 to 1.8 ftVs).

In recent years, significant advancements have been made in the practical application of long-throated flumes and broad-crested weirs for flow measurements in irrigation canals. The modified RBC (Replogle-Bos-Clemmens) broad-crested weir has many advantages over related open-channel flow devices. These include high accuracy and reliability for a wide variety of shapes, low head-loss requirements which are predictable, and relatively simple inexpensive construction. In this paper we have extended the application of these weirs to circular pipes flowing partially full. The theoretical equations are presented for ideal flow from which approximate ratings can be obtained to within a reasonable accuracy with an empirical discharge coefficient, However, a mathematical model is available which accurately predicts these ratings by directly accounting for the effects of friction. The ratings for a wide variety of shapes and sizes of these weirs were computed with the model and fit to an empirical equation. The constants for this equation are plotted graphically for easy use. The resulting ratings should be well within ±3%. Design examples are given which show how to select the flume dimensions for maintaining free-flowing conditions (modular flow) and for minimizing sediment deposition. Once constructed, the rating for a given flume can be determined even when not constructed as planned.

Good water management in irrigated agriculture requires that irrigation water be accurately measured. Presented herein are design procedures and rating tables for modified broad‐crested weirs (which act hydraulically as long‐throated flumes) that have rectangular control sections. The ratings are presented in terms of discharge per unit width so that rectangular broad‐crested weirs of a variety of sizes can be used. This makes it easier for the designer to adapt the flume to the site rather than vice versa. These weirs or flumes can be placed in lined and unlined canals of any shape. The rating tables are presented for rectangular lined canals with given values of sill or crest height. An adjustment procedure is given to develop rating tables for other approach area and sill height combinations. The head loss across the weir required to maintain a unique head discharge relationship is also presented. Finally, a design example is given for a rectangular lined canal. With this information, a rectangular measuring flume can be designed for a wide variety of site conditions.