A covered or uncovered conduit in which liquid (usually water) flows with its top surface bounded by the atmosphere. Typical open channels are rivers, streams, canals, flumes, reclamation or drainage ditches, sewers, and water-supply or hydropower aqueducts.
Open-channel flow is classified according to steadiness, a condition in relation to time, and to uniformity, a condition in relation to distance. Flow is steady when the velocity at any point of observation does not change with time; if it changes from instant to instant, flow is unsteady. At every instant, if the velocity is the same at all points along the channel, flow is uniform; if it is not the same, flow is nonuniform. Nonuniform flow which is steady is called varied; nonuniform flow which is unsteady is called variable.
Flow occurs from a higher to a lower elevation by action of gravity. If the phenomenon is short, wall friction is small or negligible, and gravity shapes the flow behavior. Gravity phenomena are local; they include the hydraulic jump, flow over weirs, spillways, or sills, flow under sluices, and flow into culvert entrances.
If the phenomenon is long, friction shapes the flow behavior. Friction phenomena include flows in rivers, streams, canals, flumes, and sewers.