Click the scenario “Existing” under the Drainage Network Scenarios in the Project Manager. Then, click the Duplicate button 
on the Project Manager. Give a name “Post” for the duplicate scenario and uncheck Runs.
We will remove the background on the Schematic view, because the post-development map will look different from the existing conditions. Right-click on the empty place on the Schematic view to open the context menu and select Background -> Remove Background.
In the post-development scenario, the original subcatchment NasHyd will be divided into two subcatchments by half. One half is still modelled as a NasHyd command, whereas the other half is to be modelled as a StandHyd command to represent the new development.
Add a StandHyd command to the Schematic view.
Select the NasHyd command on the Schematic view and change the AREA in the Properties tab to 14 ha, which is half of the NasHyd command in the “Existing” scenario, and change the TP to 0.4.
Select the StandHyd command. In the Properties tab, change the parameters as below.
