Calculating capacity in ditch section with uphill slope

Backwater analysis.
Assume a flowrate. Starting at downstream end, should be some controlling water level, or free-flow. Break ditch into sections of similar cross-section. For each, calculate HGL to achieve assumed Q, such as with Mannings equation. Carry elevations upstream. End result is starting water elevation for that Q.

Don’t know clear meaning of lost capacity for certain sections. If cross-section is enlarged, will get more Q through that portion for a given HGL.

The high points in the ditch are going to control the available section below the grade line, and the low points are going to set the maximum water level.

Using these constraints I would lay out grade line estimate a scaled profile drawing.

It likely would be less effort to model in USEPA Storm Water Management Model (SWMM) as open channel flow.