PORTABLE AND PERMANENT FLUMES WITH ADJUSTABLE THROATS

Authors: Replogle, John; Wahlin, Brian

Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Accurate measurement of irrigation water is essential for managing the combination of natural resources and conservation techniques involved in modern farm production that emphasizes environmental concerns. A common false perception by many irrigation water users is that flow measuring flumes and weirs significantly and harmfully restrict flow, because they see ponding upstream of the flume or weir. These perceptions cause opposition to flow measurements and thwarts proper irrigation water management. In most instances, proper size selection and installation reduce any possibility of real harm, but do not address the false perceptions. To address this problem, a flow-measuring device (flume) was developed that could be adjusted vertically after installation in the field. We designed, tested and verified that such flumes with a movable flow control restriction, called the throat section, were practical and easy to use. The adjustment allows the minimum amount of ponding for all flow rates and not just the maximum, as is the case with previous non-adjustable styles. A mathematical model that was previously developed allows easy design of many sizes to match a variety of field situations. The reduction of ponding at nearly all flow rates makes it more acceptable to the user. These devices are applicable to measuring flow rates in field ditches, lined and unlined canals, and sewer flows. Canal operators, researchers, and farm irrigators dealing in water management and water measurement in canal systems, both large and small, should be active candidates for using these structures.

Technical Abstract: 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 create resistance to flow measurement and impede 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 allows control of the ponding restriction from nearly zer to just enough to gain measurement control of the flow for all flow rates for each flume 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.