Several recognized types of flow are theoretically possible in open channels. The methods of analysis as well as certain necessary assumptions depend on the type of flow under study. Open channel flow is usually classified as uniform or non-uniform, steady or unsteady, or or critical or supercritical.

Non-uniform, unsteady, subcritical flow is the most common type of flow in open channels in Texas. Due to the complexity and difficulty involved in the analysis of non-uniform, unsteady flow, most hydraulic computations are made with certain simplifying assumptions which allow the application of steady, uniform, or gradually varied flow principles and one-dimensional methods of analysis.

Steady, Uniform Flow.

Steady flow implies that the discharge at a point does not change with time, and uniform flow requires no change in the magnitude or direction of velocity with distance along a streamline such that the depth of flow does not change with distance along a channel. Steady, uniform flow is an idealized concept of open channel flow that seldom occurs in natural channels and is difficult to obtain even in model channels. However, for practical highway applications, the flow is steady, and changes in width, depth, or direction (resulting in non-uniform flow) are sufficiently small so that flow can be considered uniform. A further assumption of rigid, uniform boundary conditions is necessary to satisfy the conditions of constant flow depth along the channel. Alluvial, sand bed channels do not exhibit rigid boundary characteristics.

Steady, Non-uniform Flow.

Changes in channel characteristics often occur over a long distance so that the flow is non-uniform and gradually varied. Consideration of such flow conditions is usually reasonable for calculation of water surface profiles in Texas streams, especially for the hydraulic design of bridges.

Subcritical/Supercritical Flow.

Most Texas streams flow in what is regarded as a subcritical flow regime. Subcritical flow occurs when the actual flow depth is higher than critical depth. A Froude Number less than 1.0 indicates subcritical flow. This type of flow is tranquil and slow and implies flow control from the downstream direction. Therefore, the analysis calculations are carried out from downstream to upstream. In contrast, supercritical flow is often characterized as rapid or shooting, with flow depths less than critical depth. A Froude Number greater than 1.0 indicates supercritical flow. The location of control sections and the method of analysis depend on which type of flow prevails within the channel reach under study. A Froude number equal to, or close to, 1.0 indicates instability in the channel or model. A Froude number of 1.0 should be avoided if at all possible.