The classic solution of gravity flow hydraulics is via a backwater analysis. This type of analysis starts at the network outlet under free discharge, submerged, or tailwater control, and proceeds in an upstream direction. Steep pipes tend to "interrupt" the backwater analysis, and reset the hydraulic control to critical depth at the upstream end of the steep pipe. A frontwater analysis may be needed for a steep profile (such as an S₂), with the backwater analysis recommencing from the upstream structure.

Free Outfall

This program lets you define the tailwater condition at the outlet as either Free Outfall, Crown Elevation or User-Specified. For a pipe with a hydraulically steep slope, the Free Outfall condition will yield a starting depth equal to normal depth in the pipe. For a pipe with a hydraulically mild slope, the Free Outfall condition will yield a starting depth equal to critical depth. When an outlet has multiple incoming pipes, the Free Outfall condition yields a starting elevation equal to the lowest of the individual computed elevations. The Crown condition should be used when the pipe discharges to an outlet where the water surface elevation is equal to the elevation of the top of the pipe.

Structure Flooding

Flooding occurs whenever the elevation of water is above the structure rim elevation. When this occurs, the backwater analysis will continue by resetting the hydraulic grade to the structure rim elevation or ground elevation, whichever is higher. However, if a structure is defined with a bolted cover, the hydraulic grade is not reset to the rim elevation.

In actual flooding situations, flows may be diverted away from the junction structure and out of the system, or attenuated due to surcharged storage. In this program, even though the governing downstream boundary for the next conduit is artificially lowered to prevent the propagation of an incorrect backwater, the peak discharges at the structure are conserved and are not reduced by the occurrence of flooding at a junction.

Frontwater Analysis

The program will perform a frontwater analysis in a steep pipe operating under supercritical flow, since these pipes are typically entrance controlled. The hydraulic control is at the upstream end of the conduit, and the gradually varied flow analysis will proceed in a downstream direction until either the normal depth is achieved, a hydraulic jump occurs, or the end of the pipe is encountered.

The program's algorithm is fundamentally based on backwater analysis. As a result, a continuous frontwater analysis is not performed through two or more consecutive steep pipes.