The culvert routines allow for supercritical and mixed flow regimes inside the culvert barrel. During outlet control computations, the program first makes a subcritical flow pass through the culvert, from downstream to upstream. If the culvert barrel is on a steep slope, the program may default to critical depth inside of the culvert barrel. If this occurs, a supercritical forewater calculation is made from upstream to downstream, starting with the assumption of critical depth at the culvert inlet. During the forewater calculations, the program is continually checking the specific force of the flow, and comparing it to the specific force of the flow from the subcritical flow pass. If the specific force of the subcritical flow is larger than the supercritical answer, the program assumes that a hydraulic jump will occur at that location. Otherwise, a supercritical flow profile is calculated all the way through and out of the culvert barrel.

For inlet control, it is assumed that the water surface passes through critical depth near the upstream end of the barrel. The first step is to find the "Vena Contracta" water surface and location. As flow passes through critical depth at the upstream end of the barrel, it goes into the supercritical flow regime. The most constricted depth that is achieved will depend on the barrel entrance shape and the flow rate. The program uses an empirical equation to estimate the location and depth of the Vena Contracta (supercritical) water surface elevation. Once this depth and location are estimated, hydraulic forewater computations are performed to get the water surface profile through the barrel. The program will evaluate and compute a hydraulic jump if either the downstream tailwater is controlling, or if the slope of the barrel is flat enough and long enough that a jump would occur due to barrel control. If a hydraulic jump occurs, and that jump produces a water surface that will fill the barrel, it is assumed that any air pocket in the barrel would burp out and that outflow control will ultimately dictate the upstream energy and water surface elevation.