Step-by-step:

An outfall is the location where water can leave the network. For designs where it is necessary to meet an existing pipe network or water course, the outfall from the network you are designing cannot be any lower than the system you are connecting to, unless the outfall is to be pumped. It is therefore necessary to define a minimum invert level for the outfall.

1. In the Tree View, duplicate the existing phase and name the new phase “Fixed outfall”.
2. Exclude the existing phase from analysis and turn OFF its visibility.
3. Set the new Fixed outfall phase as the current phase.
4. In the Tree View, double-click Connections.
5. In the Connections dialog box, click Preferences to customize the data in the Connections table.
   
6. In the Visible Columns dialog box, scroll down the list and enable Minimum Downstream Invert Elevation.
   
7. Click OK.
8. Back in the Connections dialog box, specify a Downstream Invert Elevation of 37.00 for Pipe (6).
   
9. On the ribbon, Preliminary Sizing tab, open the Network Design Criteria.
10. In the Network Design Criteria dialog box, click the Design Options tab; the dialog box expands to show more options.
11. Enable the Minimum Downstream Invert Elevation.
    

A warning appears because in some situations, it is not possible to meet all the design criteria. For example, on very flat sites, where the system to be connected to is close to the surface, it may not be possible to provide sufficient slope and sufficient cover further up the system. In this situation, the minimum cover rule is broken, and a warning is provided.

12. Click OK to close the dialog box.
13. On the ribbon, Preliminary Sizing tab, Network Design panel, click Network Design Wizard.
14. On the Select Flow Path page, click Next.
15. On the Enter Flow Criteria page, expand the Return Period drop-down and select 30.
16. On the Enter Design Options page, click Next.
17. On the Summary of Changes page, review the pipes information. You may see some pipes highlighted with warnings.