Submerged venturi flume

Gill, Tom, author
Einhellig, Robert, author
U.S. Committee on Irrigation and Drainage, publisher
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Improvement in canal operating efficiency begins with establishing the ability to measure flow at key points in the delivery system. The lack of available head has been a constraint limiting the ability to measure flow using traditional critical flow measurement structures at many locations. Engineers at Reclamation's Water Resources Research Laboratory (WRRL) have been investigating the viability of measuring flow where limited head is available using a submerged venturi flume. The term "venturi flume" is used in flow measurement literature to describe a broad range of measurement structures. The geometry being referred to as a venturi flume in this paper is a flat bottomed-structure with prismatic upstream and downstream sections, a gradual contraction leading to a prismatic throat of narrowed width, followed by a gradual expansion to the downstream section. Sidewalls may be either sloped or vertical. Flumes of this geometry with sloped sides have commonly been called trapezoidal flumes. Venturi flumes have been used for many years as a critical-flow measurement device that will perform with greater tolerance for submergence than Parshall flumes. When functioning as a critical flow device, discharge through a venturi flume is a function of only the upstream water level. For submerged-flow measurement, water levels both upstream and at the throat section must be known. Laboratory tests were performed using a long-throated flume installed in series with, but downstream from a venturi flume. The long-throated flume both created submergence on the venturi flume and provided a means of comparative flow measurement. In initial laboratory testing, agreement between flows measured with the long-throated flume and the submerged venturi flume was within 4% over a discharge range from 0.5 ft3/s to 5.0 ft3/s. Key to practical field use of the submerged venturi is identification of an affordable means of obtaining accurate measurement of small head differential in an efficient manner. WRRL engineers are working to develop an affordable, robust differential-head sensing system to field test with the submerged venturi flume during the 2005 irrigation season.
Presented at SCADA and related technologies for irrigation district modernization: a USCID water management conference on October 26-29, 2005 in Vancouver, Washington.
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