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Open channel flow is a condition by which liquid flowing through a conduit has a free surface upon which the only pressure is gravity. In essence, the flow is only partially surrounded by boundaries. Another characteristic of this type of flow is that it is often subject to change from the external environment, due to exposure to 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 runs through a conduit with nothing to physically impede 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 a free surface.
There are several different kinds of open channel flow, each of which has a corresponding opposite. For example, there is steady flow, which is characterized by no changes in depth, and unsteady flow, which is a condition where depth changes according to variances in the environment. Other types include uniform and non-uniform flow, of which the former is a constant flow depth measured for a specific portion of a channel, and the latter shows greater variance in depth over the same distance. There are also gradually varied flows, in which depth slowly changes over a long length of channel and rapidly varied flow where changes happen within a shorter distance.
The location of open channel flow can also differ. Surface flow is any movement of liquid through conduits above the ground. Ground flow is liquid that moves below the earth’s surface.
Wastewater and water are the two liquids most frequently found in open channel flow. In many cases water in open channels is funneled towards plants where it can be purified or towards places where it can be discarded. It is also common to use this type of flow to deliver water to crops or cattle.
The opposite of open channel flow is pipe flow. This is a condition where the flow is fully-enclosed. It is typically found in a pipe that is completely full.
It is common for engineers to measure water flow through various conduits in order to judge the efficiency of the system, the amount of water processed, and to plan maintenance. Unlike pipe flow, it is not possible to measure open channel flow by gauging pressure. For this reason it is often measured with special radar or ultra-sonic devices.