Based on the gradually varied flow analysis, different portions of any given pipe may be under different flow regimes.

Pressure Flow

When a pipe is surcharged, headlosses are simply based on the full barrel area and wetted perimeter. Because these characteristics are all functions of the section shape and size, friction loss calculations are greatly simplified by pressurized conditions.

Uniform Flow and Normal Depth

Uniform flow refers to a hydraulic condition where the discharge and cross-sectional area, and therefore the velocity, are constant throughout the length of the channel or pipe. For a pipe flowing full, all that this requires is that the pipe be straight and have no contractions or expansions. For a non-full section, however, there are a few additional points of interest:

For the cross-sectional area to remain the same, the depth of flow must be constant throughout the length of the channel. This requires that the friction slope equal the constructed slope. This depth is called normal depth. Since the hydraulic grade line parallels the invert of the section and the velocity does not change, the energy grade line is parallel to both the hydraulic grade line and the section invert under uniform flow conditions. In prismatic channels, flow conditions will typically approach normal depth if the channel is sufficiently long.

Critical Flow, Critical Depth, and Critical Slope

Critical Flow occurs when the specific energy of the section is at a minimum. This condition is defined by the situation where:

A³/T = Q²/g

This is a relatively simple computation for simple geometric shapes, but can require iterative calculation for more complex shapes (such as arches). Some sections may even have several valid critical depths, making numerical convergence more difficult.

Critical depth refers to the depth of water in a channel for which the specific energy is at its minimum. Critical slope refers to the slope at which the critical depth of a pipe would be equal to the normal depth.

Subcritical Flow

Subcritical flow refers to any flow condition where the Froude number is less than 1.0. For this condition, the depth is above critical depth, and the velocity is below the critical depth velocity.

Supercritical Flow

Supercritical flow refers to any condition where the Froude number, or the ratio of internal forces to gravity forces, is greater than 1.0. For this condition, the depth is below critical depth, and the velocity is above the critical depth velocity.