Flows in open channels

Open-channel flow is the flow of a liquid in a conduit with a free surface. There are many practical examples, both artificial (flumes, spillways, canals, weirs, drainage ditches, culverts) and natural (streams, rivers, estuaries, floodplains). Such flows are dominated by the effects of gravity. The presence of the free surface, which is essentially at atmospheric pressure, both helps and hurts the analysis. It helps because the pressure can be taken constant along the free surface, which therefore is equivalent to the hydraulic grade line of the flow. Unlike flow in closed ducts, the pressure gradient is not a direct factor in open channel flow, where the balance of forces is confined to gravity and friction. But the free surface complicates the analysis because its shape is a priori unknown. The depth profile changes with conditions and must be computed as part of the problem, especially in unsteady problems involving wave motion.
Open-channel flow is generally turbulent, due to its large scale and small kinematic viscosity, and is three-dimensional, sometimes unsteady, and often surprisingly complex due to geometric effects.