Basic Principles

Classification of Flows Flow in open channel can be classified based on time t and space x. For time criterion, the flow may be steady or unsteady. Steady flow has a water depth constant with time at a particular point on the channel, while the depth in unsteady flow changes with time at a particular point on the channel. For space criterion, the flow may be either uniform or nonuniform. Uniform flow has a constant water depth along the reach of the channel, but the depth of water in nonuniform flow changes along the reach of the channel. Nonuniform flows, also termed varied flow, further be classified as rapidly varied and gradually varied flows. For rapidly varied flow, the water depth changes significantly over a relatively short distance such as the case for a hydraulic jump or a hydraulic drop. For gradually varied flow, the depth changes rather slowly over a relatively long distance such as the case of a reservoir upstream of a dam.      According to this classification, four combinations of flows can be considered. Steady uniform flow has a constant water depth that does not change with time and space, and is found practically only in lab flume experiments. Although this case is rarely found in nature, it is widely used for the design of open channels due to its simplicity. Unsteady uniform flow has a depth function of time which requires the water surface fluctuating with time while remaining parallel to the channel bottom. Obviously, this case is very rare and cannot be found in nature. Therefore, the term uniform flow is used hereafter to refer only to steady uniform flow. Steady nonuniform flow has a depth function of space, such as a reservoir behind a dam. Unsteady nonuniform flow has a depth function of both space and time and is very common in nature, such as waves and hydraulic bores1. State of Flows The state or behavior of an open channel flow is governed by the effects of gravity and viscosity forces relative to the inertial force

low1, it is difficult to have laminar flows in open channels. Usually the flow in open channel is turbulent with high Reynolds numbers. Thus, the viscous forces would not be needed for the dynamic similarity of open channel flows. However, when a high viscosity liquid flows in an open channel or when water flows with relatively low Reynolds numbers, the effect of viscosity should be taken into consideration. Both gravity and inertial forces emphasize the dynamic similarity in open channel flow. The effect of gravity as the driving force in open channel flow is present due to the existence of the free surface. The ratio of inertial to gravity forces is used to develop the Froude number as