Every gravity sewer, storm drain, and open channel relies on one thing to move water: slope, not pressure. Predicting how fast — and how much — water will flow down that slope requires the single most important formula in open-channel hydraulics: Manning's equation. At the top license tier, you're expected to not just plug numbers into it, but to understand what each term represents and why it behaves the way it does, including the elegant (and exam-favorite) fact that a circular pipe flowing half full carries exactly half the flow of that same pipe flowing full.
Why Open Channels Need a Different Equation
Everything up to this point — Q = VA, the continuity equation — tells you the relationship between flow, velocity, and area, but it doesn't tell you what velocity a channel will actually produce on its own. In a pressurized pipe, a pump provides the energy. In a gravity sewe