H., Fundamentals of Fluid Mechanics, 4th Ed., New York: John Wiley and Sons, Inc, 2002. Bengtson, Harlan H., Open Channel Flow III - Sharp Crested Weirs_, an online continuing education course for PDH credit.ģ. of the Interior, Bureau of Reclamation, 2001 revised, 1997 third edition, Water Measurement Manual.Ģ. The article, “ Excel Templates for Rectangular Weir Calculations,” has Excel spreadsheet templates for making both suppressed and contracted rectangular weir calculations available for download. So B - L > 4H – OK P = 1.2 ft & 2H = 1 ft, so P > 2H – OK. Show that the conditions for use of the fully contracted rectangular weir equation are met and calculate the water flow rate for the 0.5 ft head over the weir.
Problem: Consider a contracted rectangular weir in a rectangular channel with B = 6 ft, L = 2.4 ft, P = 1.2 ft, and H = 0.5 ft. This weir equation is subject to the same conditions given above. L is the weir length, Hmax is the maximum head over the weir, and H, B, & P are as identified above.įor S.I units the fully contracted rectangular weir equation is: Q = 1.84(L - 0.2H)H3/2, where Q is the water flow rate in m3/sec, and H is in m. Use of this equation is subject to the condition that H/P 4 Hmax, and P > 2Hmax. Q = 3.33 B H3/2, where Q is the water flow rate in ft3/sec, B is the length of the weir (and the channel width) in ft, and H is the head over the weir in ft. The equation recommended by the Bureau of Reclamation in their Water Measurement Manual, for use with a suppressed rectangular weir is: The suppressed rectangular weir in the picture at the left is being used to meter flow of water in an open channel. Use a Rectangular Weir to Measure Water Flow Rate in an Open Channel Suppressed Rectangular Weir Equation Equations for calculating the water flow rate over a suppressed rectangular weir and over a contracted rectangular weir will be covered in the next two sections. The diagram at the right shows figures of the two types of rectangular weirs to be covered in this article, a suppressed rectangular weir, which has the weir opening across the entire channel width, and a contracted rectangular weir, which has a weir opening that is shorter than the channel width. General background on the sharp crested weir is given in the article, “ Open Channel Flow Measurement 4: the V Notch Weir.” Theĭiagram at the left summarizes some terminology and parameters used in connection with sharp crested weirs. An introduction to the rectangular weir and several other types of weirs and flumes for measuring open channel flow rate is given in the first article of this series, “ Open Channel Flow Measurement 1: Introduction to the Weir and Flume.” Sharp Crested Rectangular Weir Background It is used to meter flow of water over the weir (and through the open channel) by measuring the head of water over the weir crest. A sharp crested, rectangular weir is simply a flat plate obstruction in an open channel flow path, with a straight, level opening to allow water flow over the weir, as shown in the pictures and diagrams in the rest of this article.
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