# Problem

What is the "Boundary Condition Type" for an outfall and what do each of the options mean?

# 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. If the last conduit or channel (connecting to the outfall) has a hydraulically steep slope, the Free Outfall condition will assume a tailwater depth (at the outfall location) equal to normal depth in that conduit or channel. If it has a hydraulically mild slope, Free Outfall will assume critical depth at the outfall location. When an outlet has multiple incoming pipes, the Free Outfall condition uses an elevation equal to the lowest of the individual computed elevations. With the GVF-Rational (StormCAD) and GVF-Convex (SewerCAD) solvers, you can check if the pipe is mild or steep by viewing the "Profile Description" result field, and there are also result fields available to show the calculated Critical Depth and Normal Depth as references.
• User Defined Tailwater: Enables the modeler to enter a custom fixed 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. If you want to have a custom tailwater elevation that changes over time, use the Time-Elevation or Tidal options defined below.
• Time-Elevation Curve: Enables the modeler to define a custom tailwater elevation that varies with time. For example a river or body of water where you know the specific elevation changes that occur over time. When selected, you must enter a table of time vs. elevation.
• Tidal: Similar to Time-Elevation, but the pattern repeats. So, it can be used to model tidal cycles. When selected, you must enter a table of time vs. elevation.
• 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.
• Crown: Automatically sets the outfall tailwater elevation to the top of the connecting link element. This can be useful to model an outlet that is submerged, with water surface elevation equal to the top of the pipe. 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. This option is not supported with the Implicit dynamic solver.
• 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.
• Boundary Element: Enables you to specify an element (typically a pond) to receive flow that discharges out of the outfall (and the element whose hydraulic grade will act as the tailwater condition at the outfall location). 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", which facilitates the connection between the channel or conduit link element and the polygon pond element (see more here). 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.

Note: Normal is an option for use with Implicit/Explicit solver, and “Free Outfall” is the nearly equivalent choice to use with our GVF solvers.  With the GVF solvers, Free Outfall means that hydraulically steep pipes will have minimum tailwater of normal depth and mild sloped pipes will have a minimum depth of critical depth.  Whereas the Implicit and Explicit (SWMM) solvers do an approximated solve of depths as they vary along a pipe at just their downstream, mid, and upstream ends.

In other words, the GVF solvers cannot use Normal Depth for the boundary type of an outfall (or headwall acting as an outfall) because the Free Outfall option essentially already enforces this.