CREATE AND EXPORT GIS DATA FROM AUTOCADI was able to use ADEDEFDATA and ADEATTACHDATA to create an arbitrarily named data table with a column arbitrarily named Roughness like in this earlier post. I drew polylines representing roughness areas (including buildings with a roughness of 10) and edited their data. Then I used MAPEXPORT as in my earlier post linked above.
IMPORT MAP INTO HEC-RASIn this earlier post I explained that you have to skip setting a projection before you import Civil 3D terrain, or the import will fail. Well, the opposite is the case when you import your Land Cover Map. Before you go to Tools, New Land Cover... you have to go to Tools, Set Projection for Project (even if a projection was brought in with your terrain) and select the projection (prj file) that was exported from AutoCAD along with your Land Cover Map.
The next time you want to import your terrain after tweaking it, you will need to go into Tools, Set Projection for Project again and delete the project you picked. And thus you have to go back and forth so you can import terrain and land cover.