What is Manning's n and where is it used?

• A. It is the roughness coefficient used in open-channel flow to relate channel geometry and roughness to discharge.
• B. It is a dimensionless factor used in closed conduit flow calculations.
• C. It is a coefficient describing the roughness of a channel surface in hydrology.
• D. It is the roughness coefficient used in open-channel flow to relate channel cross-section area to velocity.

Answer: (A)

Manning's n is a roughness coefficient used in open-channel hydraulics. It appears in Manning’s equation, which links discharge to channel geometry and slope: Q = (1/n) A R^(2/3) S^(1/2), where A is the cross-sectional area, R is the hydraulic radius, and S is the energy slope. The n value captures the resistance created by the channel surface—bed texture, vegetation, and irregularities. A rougher channel (larger n) slows the flow and reduces discharge for a given area and slope; a smoother channel (smaller n) allows more discharge.

This concept is specific to open channels like rivers, canals, and drainage ditches. It isn’t used for flow in closed conduits (pipes), which rely on other friction factors. Also, while Manning’s equation relates discharge to area and the roughness through R and S, velocity is obtained from V = Q/A, so n affects how much discharge you get for a given cross-section rather than directly dictating velocity from area alone.