What is a key functional difference between open-channel flow and closed-conduit flow in hydraulics?

• A. Open-channel flow has a free surface exposed to atmosphere; closed conduits are pressurized and have energy primarily in pressure head.
• B. Open-channel flow is pressurized; closed conduits have a free surface.
• C. Closed conduit flow has a free surface; open-channel does not.
• D. Open-channel and closed-conduit flows are the same.

Answer: (A)

Open-channel flow has a free surface that is exposed to the atmosphere, so the surface pressure is atmospheric and the flow depth sets the boundary. In contrast, closed-conduit flow occurs inside a fully enclosed conduit and is typically pressurized, with no free surface; the fluid energy within a pipe is largely carried as pressure head rather than surface or atmospheric pressure. This boundary condition difference is what distinguishes the two: open-channel flow has a free surface, while closed conduits are pressurized, with energy mainly in pressure head.