Open Channel Hydraulics I – Uniform Flow

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

Course Outline

Open channel flow occurs whenever the flowing liquid has a free surface at atmospheric pressure.  For example, this may be in a natural river channel, in a manmade concrete channel for transporting wastewater, or in a closed conduit, such as a storm sewer, which is flowing partially full.  The driving force for open channel flow must be gravity, since the flow, which is open to the atmosphere, cannot be pressurized.  In contrast, the primary driving force for flow in pressurized, closed conduit flow is usually pressure.  There may be a gravity component in pressurized, closed conduit flow as well, but, in fact, the flow is often against gravity, as when the fluid is being pumped upward.  In this introductory course several aspects of open channel flow will be presented, discussed and illustrated with examples.  The major topics included in this introductory course are:  i) classifications of open channel flow and ii) uniform flow in open channels.

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:

• Know the differences between laminar & turbulent, steady state & unsteady state, and uniform & non-uniform open channel flow;
• Be able to calculate the hydraulic radius for flow of a specified depth in an open channel with specified cross-sectional shape and size;
• Be able to calculate the Reynolds Number for a specified open channel flow and determine whether the flow will be laminar or turbulent flow;
• Be able to use tables such as the examples given in this course to determine a value for Manning roughness coefficient for flow in a manmade or natural open channel;
• Be able to use the Manning Equation to calculate volumetric flow rate, average velocity, Manning roughness coefficient, or channel bottom slope, if given adequate information about a reach of open channel flow; and
• Be able to use the Manning Equation, with an iterative procedure, to calculate normal depth for specified volumetric flow rate, channel bottom slope, channel shape & size, and Manning roughness coefficient for a reach of open channel flow.

Intended Audience

This course is intended for hydrologists, civil engineers, hydraulic engineers, highway engineers and environmental engineers.

Benefits to Attendee

An attendee of this course will gain knowledge about the basic nature of flow in open channels and the common ways of classifying open channel flow (laminar or turbulent, steady state or unsteady state, uniform or non-uniform, and critical, subcritical or supercritical).  Practice in the use of the Manning equation for a variety of uniform open channel flow calculations will be gained through several worked examples.  Upon completing this course, the student will be prepared to take more advanced open channel hydraulics courses.

Course Introduction

Open channel flow occurs whenever the flowing liquid has a free surface at atmospheric pressure.  For example, this may be in a natural river channel, in a manmade concrete channel for transporting wastewater, or in a closed conduit, such as a storm sewer, which is flowing partially full.  The driving force for open channel flow must be gravity, since the flow, which is open to the atmosphere, cannot be pressurized.  In contrast, the primary driving force for flow in pressurized, closed conduit flow is usually pressure.  There may be a gravity component in pressurized, closed conduit flow as well, but, in fact, the flow is often against gravity, as when the fluid is being pumped upward.  In this course several aspects of open channel flow will be presented, discussed and illustrated with examples.  The major topics included in this introductory course are:  i) classifications of open channel flow and ii) uniform flow in open channels.

Course Content

The course content is in the following PDF file:

Open Channel Hydraulics I – Uniform Flow

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Course Summary

Open channel flow, having a free surface at atmospheric pressure, occurs in a variety of natural and man-made settings.  Open channel flow may be classified as i) laminar or turbulent, ii) steady state or unsteady state, iii) uniform or non-uniform, and iv) critical, subcritical, or supercritical flow.  Much practical open channel flow can be treated as turbulent, steady state, uniform flow.  Several parameters of interest are related through the empirical Manning Equation, for turbulent, uniform open channel flow (Q = (1.49/n)A(Rh2/3)S1/2).  Through worked examples in this course, the use of the Manning equation for uniform open channel flow calculations and the calculation of parameters in the equation, such as cross-sectional area and hydraulic radius, are illustrated.

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.

Websites:

1.  Indiana Department of Transportation Design Manual, available on the internet at: http://www.in.gov/dot/div/contracts/standards/dm/index.html

2.  Illinois Department of Transportation Drainage Manual, available on the internet at:  http://dot.state.il.us/bridges/brmanuals.html

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.