Dear Kanstad Stian Bue :
Note that this option has been placed in the HEC-RAS interface, but has not yet been implemented within the HEC-RAS SNET 2.2.1 analysis engine. That is why this data entry is grayed out. Note, however, you can adjust the ground geometry using a multiplication factor. Perhaps this can help some.
If you have any additional questions or need any further information, please contact me again.
Sincerely,
Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com