Bank-full capacity and the rating curve

Both use Manning’s equation, with the section divided into parts where its roughness changes.

Bank-full capacity

The water can rise only to the lower bank; above it, it leaves the section. $Q_{bank}$ is Manning’s discharge with the water at that level. The check compares discharges because a depth above the banks has no meaning: the section ends there. When $Q > Q_{bank}$, nothing above the banks is reported, not even the normal depth or the regime.

Rating curve

The rating curve is Manning’s discharge at 21 water levels, equally spaced from $10^{-5}$ m above the bed to the lower bank; the last one gives $Q_{bank}$. With the section divided, the discharge grows with the water level. When one $n$ covers a floodplain and its channel alike, nothing divides them, and the discharge can still drop as the water first spreads over the floodplain, its wetted perimeter growing much faster than its area: Rischio warns of it.