How does HEC-RAS apply n values between cross sections? For example, if a section cuts through grass but there is forest immediately upstream or downstream, what n value should be used in that overbank? Will RAS apply n value toward the next section u/s or d/s, or just immediately at that location?
HEC-RAS has no awareness of the n values in-between the cross sections. The n value at a cross section should be representative of the conditions half-way between it and the upstream section to half-way between it and the downstream section. If there are significant n value changes then it would be advisable to add cross sections.