Can a catchment be the outflow element for a catchment?

Hello Brian,

If the lawn is going to receive the runoff from the roof "only" then you can try modeling it as a pond. Hence your connection would be;

Roof (Catchment) > Lawn (Pond) > Street > Catch-basin.

I tried connecting catchment with catchment but it is an invalid connection.

You can also look into modeling Low Impact Development controls which can help you here. See the below article to understand more about it;

Low Impact Development (LID)

See if this helps.

Our agency requires calculating the Q for each C and then adding them, hence I have broken each out. Now the trick is getting a time of concentration to add together each area that a catchment flows through in a somewhat automated way. I am not sure I can do the lawn as a pond as it has a runoff. I have attached a sketch that maybe shows the scenario.

PDF

Hi Brian,

With the SWMM solver available in SewerGEMS and CivilStorm, you can use the EPA-SWMM runoff method's subarea routing feature as described here, the Subarea option with the the SCS CN loss method as described here, or advanced Low Impact Development controls as described here (see also video at the bottom).

However for the Rational Method for peak flow analysis, you may need to use a single catchment and estimate the appropriate Tc and Rational C, to get an appropriate peak flow. You can select "multiple subareas" in the "Area defined by" dropdown, which can help you determine a composite Rational C based on the different parts of your overall drainage area. When selecting "Composite Tc" as the "Tc Input type", the "Tc Data collection" dialog offers many different methods of calculating the Tc, which may help in your case (even though you may need to calculate each segment and add them up to determine the Tc of the longest path).

Our book Stormwater Conveyance Modeling and Design goes into some detail on this subject, such as on page 426.