The section and its roughness

The calculation works out the section this way at every water level it needs, from the normal depth to the rating curve.

The section at a water level

At a given water level, segments wholly below the surface are wet, and those wholly above it are dry. A segment that crosses it is cut at the crossing, by linear interpolation, and only its wet part counts, with its own $n$. Each wet part adds its length to the wetted perimeter $P$, its horizontal width to the top width $T$, and the area between it and the water surface to the flow area $A$. A vertical wall adds only to $P$.

Parts and their roughness

The section is divided at every vertex where $n$ changes, and at a dry stretch between two pools. A part made only of steep segments, steeper than 1 vertical to 5 horizontal (a channel’s side, a wall with its own $n$), joins the part it runs down to, as HEC-RAS keeps a main channel with steep sides whole. A part with more than one $n$ takes Horton–Einstein’s equivalent, which assumes all of it moves at one mean velocity:

$$n = \left( \frac{\Sigma P_i n_i^{1.5}}{P} \right)^{2/3}$$

The wetted segments change with the water level, so the parts and their $n$ are worked out again at every level. When the wet section is one part, the report gives its equivalent $n$; divided, it gives $V = Q/A$ instead, since one $n$ for the whole of it would be lower than any of its parts’.