Problem

What is the "Boundary Condition Type" for an outfall, what do each of the options mean and which options are available for each product and solver?

Solution

The "Boundary Condition Type" field in the properties of an outfall enables the modeler to define the tailwater conditions that influence the hydraulic grade at the outfall location, which can then influence upstream hydraulics.

Boundary Condition Type Definitions

• Free Outfall: This is used in cases where water flows freely out of the system with no tailwater effects. For example a pipe spilling into a channel where the channel water surface does not rise to the outfall invert.
• User Defined Tailwater: Enables the modeler to enter a custom tailwater elevation. For example if you would like to assume a certain submerged water surface elevation that the outfall discharges into, or a specific downstream river elevation (assumed to be fixed), you would use this option.
• Boundary Element: Enables you to specify an element (typically a pond) to receive flow that discharges out of the outfall. This is used when you need to direct outfall flow to another part of the model instead of assuming that it exits the system. For example if you wish to discharge pipe or channel flow to a pond, you would use an outfall with this boundary condition type, then choose the pond as the "boundary element". If you want the flow out of the end of a pipe to discharge into a channel, you would not use the outfall element but instead connect a conduit element to a channel element using a Cross Section node element.
• Elevation-Flow Curve: enables you to define a tailwater elevation that varies with flow, by entering a table of elevation vs. flow. Based on the flow approaching the outfall, the respective elevation will be used as the tailwater elevation. This could be used for example to model downstream conditions such as a channel whose depth depends on the flow and can act as a tailwater condition for the upstream system.
• Time-Elevation Curve: 
• Crown: Automatically sets the outfall tailwater elevation to the top of the connecting link element. For example for a 1 foot diameter conduit connected to the outfall at an elevation of 100 ft, this option will set the tailwater elevation to 101 ft.
• Normal: Automatically sets the outfall tailwater elevation to Normal depth in the last channel or conduit. Meaning, it will solve for normal depth in the connecting conduit or channel and use that as the tailwater depth at the outfall. In most cases this will likely be similar to using Free Outfall.
• Tidal: 

Solver Availability

• GVF-Rational (StormCAD): 
• GVF-Convex (SewerCAD): 
• Implicit Dynamic:
• Explicit (SWMM) Dynamic: 

See Also