John A. Replogle

Design of long-throated structures for flow measurement

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.

New Developments in Design and Application of Long-Throated Flumes

Long-throated flumes provide economical and flexible water measurement capabilities for a wide variety of open-channel flow situations. Primary advantages include minimal headloss, low construction cost, adaptability to a variety of channel types, and ability to measure wide ranges of flows with custom-designed structures. Long-throated flumes can be calibrated using computer programs that apply proven hydraulic theory, thus eliminating the need for laboratory calibration. This paper describes recent advances in the computer software available for design and calibration of long-throated flumes, and highlights two potential flume design issues that have recently come to light. Specifically, there have been field observations of an apparent suction effect below flumes having a vertical drop at the end of the control section; this can lead to significant differences between the actual and theoretical head-discharge rating curve. Second, in flumes that are primarily width-contracted—as opposed to those with a sill that creates a bottom contraction—there is potential for non-modular flow in the throat section when the width to crest length ratio is large, despite the fact that traditional hydraulic theory predicts critical flow. Again, this can cause a significant discrepancy between the actual and theoretical rating curves of a structure.

Practical technologies for irrigation flow control and measurement

Practical technologies can encourage farmers to adopt practices that support sustainable irrigated agriculture. Important among these are convenient water measurement and control techniques. Many simple constructions or operating procedures are available that can bring considerable convenience to farmers and irrigation delivery system operators. Some are new technologies and some are improvements on older technologies. Many can be implemented with small expense. Some are superior replacements for current practices. The techniques and devices discussed included: (a) accurate and convenient zero setting for weirs and flumes (b) pressure-transducer field checks, (c) easy-to-use scales for orifice and Venturi meters, (d) flow-profile improvers to assist accurate meter operations in irrigation pipelines, (e) floor sills and wave suppressors for canals that usually flow at variable depths of flow, (f) water surface slope measurements–based on static-pressure tubes, and (g) field checks of flow velocity profiles to evaluate flow conditioning using rising-bubble techniques for flow-profile visualization. Many of the concepts are demonstrated in a summary illustration showing several items in a typical stilling well and broad-crested weir (long-throated flume) that need attention, and offers suggestions for correcting the deficiencies.

Evaluating the Measurement Accuracy of Surface Water Flows and Accumulated Volumes

Many water users have been strongly encouraged to reduce the amount of their diversions through improved water management practices. To identify opportunities for improving a users water management practices, an estimate of the accuracy of the flow rates and accumulated volumes withdrawn by the user must be made. Typically, water users measure flow at a given interval (e.g., daily) and then integrate them over time to estimate the volume measurement. In the past, estimates of the uncertainties for individual flow measurements were obtained using standard statistical methods; however, estimates of the error for the accumulated volumes have been difficult. In this study, a procedure is outlined that identifies the random and systematic error components for individual flow measurements. These estimates of the individual error components can be combined to yield an estimate of the uncertainty for an individual flow rate measurement. The uncertainty for the accumulated volume that passes through a given site is then estimated by using 1) the uncertainty associated with the individual flow measurements, 2) the errors associated with integration over time, 3) and the errors introduced into the system during the time between individual flow measurements. These uncertainty estimates can then be used to improve water users flow measurement strategies.

Portable and permanent flumes with adjustable throats

The utility of developing vertically-adjustable flumes for canal flow measurement systems, is important to several problems in field practice. One involves the perception by some canal water users that flumes and weirs significantly and harmfully restrict flow. These perceptions, correct only sometimes, occur often enough to cause resistance to flow measurements and impedance to proper irrigation management. Parshall flumes and Cutthroat flumes require ponding depths upstream equal to about 40% of head reading while long-throated flumes and the related broad-crested weirs require only 10 to 15%. The actual head drop through all of these flumes is greater than hydraulically necessary for all but the maximum design discharge. The highly obvious excessive ponding is often misunderstood as a harmful restriction to flow. Also, the velocities at the low flows are reduced by this excessive ponding, which can aggravate sediment accumulation. The system described herein allows control of the ponding restriction from nearly zero to just enough restriction to gain measurement control of the flow at nearly all flow rates in the design range of a particular size. This reduces the amount of visible restriction. For a small structure size, with a control section less than 1 m wide and flowing under a head of less than about 25 to 30 cm, this restriction is about 10% to 15% of the head reading, or about 3 to 5 cm at maximum head. This maintains relatively high velocities in the approach channel for assisting sediment movement. The device is applicable to measuring flow rates in unlined and lined canals. The system described permits adjustment of the canal flow levels, reduces the perception of ponding, and minimizes the induced sediment problems.