Open Channel Flow


Open channel flow means a flow-through channel that is open to the atmosphere and has a free surface, the flow is produced because of gravity that is obtained by providing a bed slope, and the pressure on the free surface is atmospheric.

Flow in a close conduit can also be classified as an open channel flow if the fluid level falls below the crown of the pipe.

Here we will learn about open channel flow, types of open channel flows & much more.


Introduction to open channel flow:

An open channel is a waterway, canal, or conduit by which a liquid flows along a free surface.

Examples of this flow are streams, rivers, artificial canals, irrigation ditches, waterfalls, pipe-lines, culverts, tunnels, sewer lines, gutters, domestic drainage panels.

This channel flow is needed to study the following purposes:

  • To estimate the discharge in a river or canal.
  • To develop the relationship between the depth of flow and discharge in a channel.
  • To Estimate the submergence area due to dam construction on a river.

Classification of Open Flow Channel:

Steady and Unstable Flow:

Flow is steady or unsteady according to whether the velocity and depth at a particular point on the channel varies with time (= temporal variations)

Steady flow:


Flow is one in which the velocity, pressure, and cross-section conditions can vary from point to point, but do not change over time.

Unstable flow:

If at any point the situation changes with time. (Although most Open Flow Channel are considered static)

Uniform and non-uniform flow:

Uniform Flow:

When different quantities do not change from point to point in a particular area – at particular times.

The average velocities in the gradual cross-section of a channel are the same; this only occurs when the liquid surface is parallel to the base of the channel.

Non-uniform flow:

It occurs when different quantities change from point to point in a specific area – at a particular point in time.

When the depth or velocity changes in the direction of either flow or straightening at a distance.

Laminar and turbulent flow:

Laminar flow:


At low velocities, fluid particles move in a straight line through the velocity of the particles along each line may not necessarily be the same – the move in layers or laminar.

Turbulent flow:

At high velocities, fluid particles no longer move in a straight path, but intersect and cross each other in a disorganized and disordered manner.

Turbulent flow is considered in the analysis of open channel flow.

Even the surface of the flowing liquid appears smooth and bright, it is no indication that turbulent flow is not present below.

Tranquil and rapid flow:

Tranquil and rapid flows depend entirely on the number of graded flows.

Froude number: A dimensionless value that describes the different flow rules of open channels.

Tranquil flow: (Fr <1.0)

When the flow velocity is small, the disturbance is possible to travel against the flow and thus affect the upward conditions.

Rapid flow: (Fr> 1.0)

When the flow velocity is high that a small disturbance cannot travel (propagate) against the flow, it cannot affect the situation.

Pipe Flow vs. Open Channel Flow:

Pipe FlowOpen Channel Flow
Flow occurs due to the pressure difference between different sections.The flow occurs due to the downward slope of the bed of the channel.
The pressure on different sections is different.The pressure on different sections is equal and equal to atmospheric pressure.
The cross-section is always circular.The cross-section may be circular or of any other shape.
The entire cross-sectional area does not flow.The open channel flow occurs over the entire cross-sectional area.




Open channels involves the flow of liquid in a drains that is not completely filled.

A free surface exists between the fluid present and the fluid above it.


Now I’d like to hear from you, which section was new to you or maybe I missed something. Either way, let me know by leaving a comment below.

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