I'm not versed in rain gardens. However, in my local, design of bioswales boils down to travel time in the swale and depth of flow for the quality event, and capacity for larger events.
I model the swale as a reach. Hydrocad will give the travel time in the swale.
I do a "save as" of the file to create a quality model and a model for other events. This is so that I can utilize one mannings N for quality (higher N...shallow flow) for quality vs. the other events (deeper flow...flattened grass etc.).
If the raingarden has similar criteria as bioretention, I have modeled bioretention as a pond with the voids option utilized in the storage dialog. This representing the biorentention mix filling the bioretention basin. One can then route to infiltration or even through the soil mix to underdrains if called for.
You should be able to use a single file for all events. If you use a custom reach cross-section you can enter a different Mannuing's value for each segment of the profile. That will handle the different depths appropriately.
I took a quick look, but I have not sufficiently played with the custom cross section in Hydrocad. (Just recently upgraded version).
Without checking all the calcs...I am unclear whether this will acheive my goal: To have an n value of 0.20 when flow depth is less than 0.25', and to have an n value of 0.030 for deeper flows. It appears I want to use one of the subdivided flow methods for mannings in the custom cross section dialog.
The custom reach section will allow you to use a different Manning's value for different parts of the section (such as the main channel vs. the flood plain). But it sounds like you're trying to change the Manning's value for a specific segment based on depth - I don't see an easy way to do that at present.
