Open-channel flow

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Open-channel flow, a branch of hydraulics and fluid mechanics, is a type of liquid flow within a conduit with a free surface, known as a channel. The other type of flow within a conduit is pipe flow. These two types of flow are similar in many ways but differ in one important respect: the free surface. Open-channel flow has a free surface, whereas pipe flow does not.

Classifications of flow[edit]

Open-channel flow can be classified and described in various ways based on the change in flow depth with respect to time and space. The fundamental types of flow dealt with in open-channel hydraulics are:

Time as the criterion
Steady flow – The depth of flow does not change over time, or if it can be assumed to be constant during the time interval under consideration.
Unsteady flow – The depth of flow does change with time.
Space as the criterion
Uniform flow – The depth of flow is the same at every section of the channel. Uniform flow can be steady or unsteady, depending on whether or not the depth changes with time, (although unsteady uniform flow is rare).
Varied flow – The depth of flow changes along the length of the channel. Varied flow technically may be either steady or unsteady.
Varied flow can be further classified as either rapidly or gradually varied.
Rapidly varied flow – The depth changes abruptly over a comparatively short distance. Rapidly varied flow is known as a local phenomenon. Examples are the hydraulic jump and the hydraulic drop.
Gradually varied flow – The depth changes over a long distance.
Continuous flow – The discharge is constant throughout the reach of the channel under consideration. This is often the case with a steady flow. This flow is considered continuous and therefore can be described using the continuity equation for continuous steady flow.
Spatially varied or discontinuous flow – The discharge of a steady flow is non-uniform along a channel. This happens when water enters and/or leaves the channel along the course of flow. An example of flow entering a channel would be a road side gutter. An example of flow leaving a channel would be an irrigation channel. This flow can be described using the continuity equation for continuous unsteady flow requires the consideration of the time effect and includes a time element as a variable.

Summary of the classification of open-channel flow[edit]

Steady flow
Uniform flow
Varied flow
Gradually varied flow
Rapidly varied flow
Unsteady flow
Unsteady uniform flow (rare)
Unsteady flow (“unsteady varied flow” should be here, but it is a practically impossible condition, so it is known simply as unsteady flow)
Gradually varied unsteady flow
Rapidly varied unsteady flow

States of flow[edit]

The behavior of open channel flow is governed by the effects of viscosity and gravity relative to the inertial forces of the flow. Surface tension has a minor contribution but does not play a significant enough role in most circumstances to be a governing factor. Depending on the effect of viscosity relative to inertia, as represented by the Reynolds number, the flow can be either laminar, turbulent, or transitional.

See also[edit]

Fields of study
Fluid dynamics
Types of fluid flow
Pipe flow
Laminar flow
Turbulent flow
Transitional flow
Fluid properties
Froude number
Reynolds number
Other related articles
Hydraulic jump
Manning formula
Chézy formula
Darcy–Weisbach equation
Standard step method


  • Chow, V. T. (2008). Open-channel hydraulics. Caldwell, New Jersey: Blackburn Press.