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Typically StormCAD uses a backwater analysis. This type of analysis starts at the network outlet under free discharge, submerged, or tailwater control, and proceeds in the 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. At this point StormCAD performs a frontwater analysis for the steep profile, with the backwater analysis recommencing from the upstream structure.
An example of a steep pipe where frontwater analysis will take place:
StormCAD 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 reached. Even though outlet control rarely occurs in supercritical flow situations, the frontwater analysis is still performed for purposes of determining exit velocity.
In a case such as this, the software will use inlet control at the upstream end of the steep pipe. A steep pipe with inlet control is able to transport a greater discharge than the inlet accepts. Since the control section is just inside the pipe at its entrance, the flow profile passes through critical depth at this location. The downstream pipe has no affect on the steep pipe. The flow in the steep pipe will approach normal depth as it moves along. There may be pressure flow at the downstream end of the pipe. This may cause a hydraulic jump just inside the end of the pipe. This will provide a transition between the supercritical and pressure flow.
StormCAD analyzes one pipe at a time as there could be headloss in the transition or there could be a slope change from one pipe to another. If a user has two steeps pipes running continuously with the same characteristics, it would be better to make them one pipe.
Since the program's algorithm is fundamentally based on backwater analysis, a continuous frontwater analysis is not performed through two or more consecutive steep pipes.
This is a performance trade-off that has little impact in evaluating performance of the collection system in most situations. The assumption of critical depth at the upstream end results in a conservative depth in all cases, and is exactly correct at the point of the steep run furthest upstream.
Rapidly varied flow is turbulent flow resulting from the abrupt and pronounced curvature of flow streamlines into or out of a hydraulic control structure. Examples of rapidly varied flow include hydraulic jumps, bends, and bridge contractions.
The hydraulic phenomenon that occurs when the flow passes rapidly from supercritical to subcritical flow is called a hydraulic jump. The most common occurrence of this within a gravity flow network occurs when there is a steep pipe charging into a particularly high tailwater.
StormCAD allows you to define the tailwater condition at the outlet as either Free Outfall, Crown Elevation or User-Specified.
In 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. If 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.
If you have flooding at manholes in SewerCAD or at inlets in StormCAD (the elevation of water is above the structure rim elevation), 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.