# Use a Rectangular Weir to Measure Water Flow Rate in an Open Channel

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## Introduction

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. 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. 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

General background on the sharp crested weir is given in the article, “Open Channel Flow Measurement 4: the V Notch Weir.” The

diagram at the left summarizes some terminology and parameters used in connection with sharp crested weirs. 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. 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.

## Suppressed Rectangular Weir Equation

The suppressed rectangular weir in the picture at the left is being used to meter flow of water in an open channel. The equation recommended by the Bureau of Reclamation in their Water Measurement Manual, for use with a suppressed rectangular weir is:

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.

Use of this equation is subject to the condition that H/P < 0.33 and H/B < 0.33.

Note from the diagrams above that P is the height of the weir crest above the bottom of the channel, and B is the channel width.

For S.I. units the suppressed rectangular weir equation becomes Q = 1.84 B H3/2, where Q is the water flow rate in m3/sec, B is the length of the weir (and the channel width) in m, and H is the head over the weir in m. The same condition for H/P and H/B apply.

## Contracted Rectangular Weir Equation

Both images in this section show a contracted rectangular weir being used to meter flow in an open channel. The equation recommended by the Bureau of Reclamation in their Water Measurement Manual, for use with a fully contracted rectangular weir is:

Q = 3.33(L - 0.2H)H3/2, where Q is the water flow rate in ft3/sec and H is the head over the weir in ft.

Use of this fully contracted rectangular weir equation is subject to the conditions that H/L < 0.33, B - L > 4 Hmax, and P > 2Hmax. L is the weir length, Hmax is the maximum head over the weir, and H, B, & P are as identified above.

For 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. This weir equation is subject to the same conditions given above.

## Example Calculation

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. 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.

Solution: Check on conditions: H/L = 0.5/2.4 = 0.21 – OK; B - L = 6 - 2.4 = 3.6 & 4H = 2,

so B - L > 4H – OK; P = 1.2 ft & 2H = 1 ft, so P > 2H – OK.

Now calculate Q: Q = 3.33(2.4 - 0.2*0.5)0.53/2 = 2.708 cfs

The article, “Excel Templates for Rectangular Weir Calculations,” has Excel spreadsheet templates for making both suppressed and contracted rectangular weir calculations available for download.

## Reference and Image Credits

References for Further Information:

1. U.S. Dept. of the Interior, Bureau of Reclamation, 2001 revised, 1997 third edition, Water Measurement Manual.

2. Bengtson, Harlan H., Open Channel Flow III - Sharp Crested Weirs__, an online continuing education course for PDH credit.

3. Munson, B. R., Young, D. F., & Okiishi, T. H., Fundamentals of Fluid Mechanics, 4th Ed., New York: John Wiley and Sons, Inc, 2002.

Images:

Suppressed Rectangular Weir: U.S. Dept. of the Interior, Bureau of Reclamation, 2001 revised, 1997 third edition, Water Measurement Manual.

Contracted Rectangular Weir: Food and Agricultural Organization of the United Nations. https://www.fao.org/docrep/R4082E/r4082e06.htm

Rectangular, Sharp-Crested Weir: flowmeterdirectory.co.uk

## This post is part of the series: Open Channel Flow Measurement

Open channel flow rate measurement is usually done by measuring a change in water depth. It can be done with a weir or flume. Common types are the sharp crested weir (including V-notch weir, rectangular weir, and cipolletti weir), the broad crested weir, the Parshall flume and venturi flume.