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Articles about Open Channel

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About Open Channel Flow

Growing environmental regulations and the increasing importance of water as a natural resource are generating growing interest in measuring flow in open channels.  Flow in open channels, or conduits, is contrasted with flow in closed pipes.  Measurement of flow in closed pipes is very common in industrial and process control environments, and many different types of meters are used for this purpose.  Open channel flow measurement occurs in industrial applications, but it also is very common in towns and municipalities that monitor drinking water and wastewater treatment flows.

Open channel flow occurs when liquid flows in a conduit or channel with a free surface.  Rivers, streams, canals, and irrigation ditches provide examples of open channel flow.  What is slightly confusing about this terminology is that that the flow of liquids in partially filled pipes, when not under pressure, is also considered open channel flow.  For example, water flowing through a culvert running underneath a street is considered open channel f low.  Likewise, flow in sewers and tunnels are classified as open channel flows, along with other closed channels that flow partly filled.  Other examples of open channel flow include flow in water treatment plants, storm and sanitary sewer systems, industrial waste applications, sewage treatment plants, and irrigation systems.

One way to understand the difference between open channel and closed pipe flow is to think of it as the difference between gravity-induced and pressurized flow.  Flow in uncovered channels such as irrigation ditches depends on gravity.  Likewise, flow in partially filled closed conduits, such as culverts and drain pipes, also is gravity-dependent.  Flow in closed pipes for industrial applications occurs under pressure.  So open channel flow might be called gravitational flow, while closed pipe flow could be called pressurized flow.  This explains why flows in both uncovered conduits and partially filled pipes are considered to be open channel flow: they are both examples of gravitational flow.

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