This memo documents how to model a Purple Roof using TR-20 methodology in HydroCAD. The critical components are the soil, needled mineral wool, and honeycomb storage layer. The following sections detail the steps for defining the profile, calculating curve numbers, and setting up the model.
Step 1: Define the Profile
The profile consists of soil (GRS), mineral wool (NMW), and a honeycomb (HC) storage layer. ASTM porosities are provided as defaults but can be adjusted in the generator.
Step 2: Calculations - Defining Curve Number Values
Curve numbers (CN) for each profile are determined by calculating 'S', the potential maximum soil moisture retention. The honeycomb detention storage is not factored in; only the soil and needled mineral wool are. Three area types exist: the greened area, the ballast/tributary area, and the paver area.
Step 5: Define Outlet Control Structure
The size of the drain box opening orifices is defined. There are typically four orifices per drain box of equal width. A low discharge coefficient (0.1) is used because the lower fabric layer goes through the orifice. An auxiliary overflow can be modeled with a weir set to the elevation of the drain box.
