Subgrades aren't really assigned CNs. B soils under good condition woods have a different CN than B soils under poor grass cover.
I've used several different approaches for modeling pervious pavement, that vary with the pavement section, the application, and the reviewer. I highly recommend talking to your review agency first and finding out how they want it modeled, because however you choose to do it now won't matter when you submit for review if they want it done differently.
In the City of Atlanta for instance, you're allowed to use it, but you must model it as regular pavement. (so basically there's no point in using it) Their rationale is they have no guarantee the owner won't pave over it later. In other areas I've been required to use it, and take credit for it in the water quality model, but not take credit for it in the hydrology model.
For the cases where I've taken some credit for it in the hydrology model, I've usually either called it gravel, or I've adjusted my initial abstraction to include an additional birdbath effect, or I've treated it as a storage/routing device.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
