I’ve tried using the geotiff directly exported from Civil3D but get the same result. The reason I tried converting was I found this document on the autodesk website:
https://forums.autodesk.com/autodesk/attachments/autodesk/66/286834/1/Civil%203D%20with%20HEC%20RAS.pdf
Page 12 of that document it details the process I’ve tried.