In Manning's equation, what does the roughness coefficient n represent?

• A. It represents surface roughness and flow resistance.
• B. It represents channel slope.
• C. It represents cross-sectional area.
• D. It represents the hydraulic radius.

Answer: (A)

In Manning's equation, the roughness coefficient n represents how rough the channel boundary is and the friction this roughness creates to flow. It’s an empirical factor that captures the texture of the bed and walls—irregularities, vegetation, sediment, and other obstructions that slow the flow. In the equation Q = (1/n) A R^(2/3) S^(1/2), a larger n means more friction and a smaller discharge for the same cross-sectional area, hydraulic radius, and slope, while a smoother channel (smaller n) allows more flow. The other terms in the equation describe geometry and slope, not the friction due to surface roughness.