Open Channel Hydraulics III – Sharp-crested Weirs

Harlan H. Bengtson, Ph.D., P.E.

Course Outline

A weir, widely used for measurement of open channel flow rate, consists of an obstruction in the path of flow.  Water rises above the obstruction to flow over it, and the height of water above the obstruction can be correlated with the flow rate.  The top of the weir, over which the liquid flows, is called the crest of the weir.  Two commonly used types of weir are the sharp-crested weir and broad-crested weir.  The sharp-crested weir will be covered in this course.   The emphasis will be on calculations used for the various types of sharp-crested weirs, but there will also be information on guidelines for installation and use of sharp-crested weirs.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

• Be familiar with standard terminology used in connection with sharp-crested weirs for open channel flow measurement;
• Be able to use the Kindsvater-Carter equation to calculate the flow rate over a suppressed rectangular weir for given head over the weir and weir dimensions;
• Be able to use the Kindsvater-Carter equation to calculate the flow rate over a contracted rectangular weir for given head over the weir and weir dimensions;
• Know the conditions required in order to use a simpler equation instead of the Kindsvater-Carter equation to calculate the flow rate over a suppressed rectangular weir and over a contracted rectangular weir for given head over the weir and weir dimensions;
• Know the conditions required in order to use the equation,  Q  =  2.49 H2.48,  to calculate the flow rate over a V-notch weir for given head over the weir and weir dimensions;
• Be able to use the Kindsvater-Carter equation to calculate the flow rate over a V-notch weir for notch angles other than 90 degrees, given head over the weir and weir dimensions; and
• Be familiar with installation and use guidelines for sharp-crested weirs for open channel flow measurement.

Intended Audience

This course is intended for hydrologists, civil engineers, hydraulic engineers, highway engineers and environmental engineers.  This course is intended to be taken after the course, H138, “Open Channel Hydraulics I – Uniform Flow.”  It will be assumed that anyone taking this course is familiar with the major classifications used for open channel flow (steady or unsteady state, laminar or turbulent flow, uniform or non-uniform flow, and critical, subcritical or supercritical flow) and with equations and calculations for uniform open channel flow.

Benefits to Attendee

An attendee of this course will gain knowledge about calculations and installation & measurement guidelines for sharp-crested weirs as used to measure flow rate in open channels.  Upon completing this course, the student will be prepared to study additional open channel flow measurement topics.

Course Introduction

A weir, widely used for measurement of open channel flow rate, consists of an obstruction in the path of flow.  Water rises above the obstruction to flow over it, and the height of water above the obstruction can be correlated with the flow rate.  The top of the weir, over which the liquid flows, is called the crest of the weir.  Two commonly used types of weir are the sharp-crested weir and broad-crested weir.  The sharp-crested weir will be covered in this course.   The emphasis will be on calculations used for the various types of sharp-crested weirs, but there will also be information on installation and measurement guidelines.

Course Content

The course content is in the following PDF file:

Open Channel Hydraulics III – Sharp-crested Weirs

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Course Summary

Sharp-crested weirs are commonly used for flow rate measurement in open channels.  Three types of sharp-crested weirs:  suppressed rectangular, contracted rectangular, and V-notch, are covered in detail in this course.  Emphasis is on calculation of flow rate over a weir for given head over the weir and weir/channel dimensions.  For each of the three types of sharp-crested weir, a general equation with a wide range of applicability is presented and discussed along with equations and/or graphs as needed for use with the main equation.  Also, for each of the three types of sharp-crested weir, a simpler equation is presented along with a set of conditions under which the simpler equation can be used.  Several worked examples are included covering all three types of weirs.  Practical installation and use guidelines for sharp-crested weirs are presented.

References:

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

2.  Chow, V. T., Open Channel Hydraulics, New York: McGraw-Hill, 1959.

3.  Kindsvater, C.E., and R.W. Carter, "Discharge Characteristics of Rectangular Thin-Plate Weirs," Paper No. 3001, Transactions, American Society of Civil Engineers, vol. 124, 1959.

Websites:

1. U.S. Dept. of the Interior, Bureau of Reclamation, 2001 revised, 1997 third edition, Water Measurement Manual, available for on-line use or download at: http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/index.htm

2.  LMNO Engineering, Research and Software, Ltd website.  Contains equations and graphs for open channel flow measurement.  http://www.lmnoeng.com/Weirs

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.