You may find that you have very different land conditions in one or both of your overbanks. For example, for one of the overbanks, you could have riparian forest next to the main channel and farmland further out in the overbank. Two very different n-values. You can also have different n-values within the main channel, although RAS has autmated methods for compositing main channel n values into one single n value, under certain circumstances. To put in more than the standard "3" n-vlaues, go to options in your cross section editor, and select "Horizontal Variation in n Values". Then fill in the new column called n Value, that will show up in your Cross Section Coordinates table.