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Open channel flow is liquid flowing through a conduit with a free surface area exposed to the open air. It can be steady or unsteady, uniform or nonuniform, and gradually or rapidly varying. Wastewater and water are common liquids found in open channels, while pipe flow is completely closed. Special devices are needed to measure open channel flow.
Open channel flow is a condition whereby liquid flowing through a conduit has a free surface area on which the only pressure is gravity. Essentially, the flow is only partially surrounded by boundaries. Another characteristic of this type of flow is that it is often subject to variations from the external environment, due to exposure to the open air. It is also known as free gravity flow. The term is used in the field of hydraulics.
In many cases, open channel flow flows through a conduit with nothing physically impeding it. This can include natural bodies of water such as rivers and streams. The term can also apply to pipes that are only partially filled with water, as part of the flow has free surface area.
There are several types of open channel flow, each of which has an opposite correspondent. For example, there is steady flow, which is characterized by no change in depth, and unsteady flow, which is a condition where the depth changes based on changes in the environment. Other types include uniform and nonuniform flow, of which the former is a constant depth of flow measured for a specific portion of a channel and the latter exhibits greater depth variance over the same distance. There are also gradually varying flows, where the depth changes slowly over a long channel length, and rapidly varying flows, where the changes occur within a shorter distance.
The open channel flow location can also vary. Surface flow is any movement of liquid through conduits above the ground. Ground flow is liquid that moves under the earth’s surface.
Wastewater and water are the two liquids most frequently found in open channel flow. In many cases the water in open channels is conveyed to plants where it can be purified or to places where it can be discarded. It is also common to use this type of stream to provide water for crops or livestock.
The opposite of open channel flow is pipe flow. This is a condition where the flow is completely closed. It usually comes in a completely filled tube.
It is common for engineers to measure the flow of water through various conduits to judge system efficiency, the amount of water treated, and to plan maintenance. Unlike the flow of pipes, it is not possible to measure the flow in an open channel by measuring the pressure. For this reason it is often measured with special radar or ultrasound devices.
