RiverGIS can prepare pure 2D HEC-RAS geometry. Just create 2D tables (2D Flow Areas and, optionally, breaklines and breakpoints), create 2D points and then you have two options:
1. choose an existing geometry (for example, with other 2D area) or
2. give a name for a new geometry file.
In either case you will have to import the geometry from the main HEC-RAS window using File > Import HEC-RAS Data...
Hope that helps,
Radek Pasiok