Can a catchment be the outflow element for a catchment?

Question

I have a StormCad model broken up with each catchment material. In this example, I have an area for a roof, a grass lawn, and an urban street. The roof drains to the lawn which drains to the street which drains to a catch basin. For the roof outflow element, it allows me to select the lawn. Is this possible? The calculator seems to error out. Thanks. 
 Stormcad connect update 2

Yashodhan Joshi · Answer

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.

Jesse Dringoli · Answer

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.