HEC-RAS 3.0 and multiple Manning's n inside of culvert

[Alexis Dufour]

Hi,
I am trying to design a culvert for fish passage using HEC-RAS 3.0. This version of HEC-RAS allows to enter 2 Manning's n values inside of the culvert, one for the top and sides, and a second for the culvert bottom. You can then simulate a natural bottom stream inside the culvert. However, I have been unable to make this new feature work properly. The simulation only uses the Manning's n for the top and sides, and the n value for the bottom has no effect on the results. It is not suppose to be that way. Anybody used this feature before? Any suggestions?
Thanks for your help.

Alexis