Which statement best describes the effect of increasing roughness on the internal surface of pipes?

• A. Carrying capacity increases
• B. Carrying capacity decreases
• C. Carrying capacity remains the same
• D. Carrying capacity becomes unpredictable

Answer: (B)

Roughness controls how much energy is lost to friction as water moves through a pipe. In closed conduit flow, the friction factor rises with roughness, especially in turbulent conditions. More friction means more head loss per unit length, leaving less energy to push the water and a lower velocity for the same slope and pipe size. Since discharge depends on velocity, the overall carrying capacity drops as roughness increases. (In very smooth, laminar cases roughness has little effect, but wastewater networks are typically turbulent, so rougher walls reduce capacity.)