Manning’s roughness coefficient n represents what aspect of open-channel flow?

Manning’s roughness coefficient n represents what aspect of open-channel flow?

• A. It is a measure of channel roughness
• B. It is the hydraulic radius
• C. It is the cross-sectional area
• D. It is the slope

Answer: (A)

Manning’s n represents how rough the channel boundary is and the friction it imposes on the flow. It’s an empirical parameter that lumps together the effects of surface texture, vegetation, and obstructions along the bed and walls. In open-channel flow, velocity is proportional to (1/n) times a function of hydraulic radius and slope, so a higher n means more friction and slower flow for the same channel geometry and grade. A smoother boundary (smaller n) yields faster flow, while a rough boundary (larger n) slows it down. This is why the correct choice is that n measures channel roughness, not the hydraulic radius, cross-sectional area, or slope.

Manning’s n represents how rough the channel boundary is and the friction it imposes on the flow. It’s an empirical parameter that lumps together the effects of surface texture, vegetation, and obstructions along the bed and walls. In open-channel flow, velocity is proportional to (1/n) times a function of hydraulic radius and slope, so a higher n means more friction and slower flow for the same channel geometry and grade. A smoother boundary (smaller n) yields faster flow, while a rough boundary (larger n) slows it down. This is why the correct choice is that n measures channel roughness, not the hydraulic radius, cross-sectional area, or slope.