Which of the following factors affects the flow capacity of an installed sewer line?

• A. Burial Depth, Size, Manhole Spacing
• B. Backfill Material, Manhole Spacing, Pipe Size
• C. Pipe Material, Invert Elevation, Pipe Size
• D. Pipe Size, Pipe Material, Grade

Answer: (D)

Flow capacity is governed by cross-sectional area, friction losses, and the driving head along the line. The pipe size (diameter) sets how much water can physically pass—the larger the diameter, the greater the capacity. The pipe material determines interior roughness: smoother materials reduce friction losses, increasing capacity for a given slope, while rougher materials slow flow. The grade (slope) provides the gravitational force that moves the wastewater; a steeper slope increases flow for the same cross-section and roughness.

Factors like burial depth, backfill material, and manhole spacing affect installation, protection, and maintenance but not the pipe’s fundamental ability to carry a given volume of flow under gravity. Invert elevation influences alignment and starting head but isn’t the primary determinant of capacity once slope is set.