What kind of shape should the channel bottom have and at what grade?

• A. U-shaped and at the same grade as connecting pipes.
• B. Flat bottom and higher grade than connecting pipes.
• C. V-shaped bottom at a lower grade than connecting pipes.
• D. Circular bottom at matching grade.

Answer: (A)

Ensuring a smooth flow path and a consistent slope from the connecting pipes into the channel is essential for gravity-fed sewer systems. A U-shaped channel bottom provides a stable, uniform cross-section that supports efficient flow and easier self-cleaning, reducing areas where sediments can settle and pipes can snag debris. Keeping the channel bottom at the same grade as the connecting pipes means there is no vertical step or mismatch at the junction, so the flow remains driven purely by gravity without abrupt changes that could cause backwater, turbulence, or headloss. Other shapes or grades would create discontinuities: a flat bottom at a higher grade can leave a higher inverted surface that traps sediment; a V-shaped bottom at a lower grade concentrates flow in a narrow throat and promotes turbulence and deposition; a circular bottom is not typical for open-channel flow and complicates connections. Together, a U-shaped bottom with matching grade promotes a smooth, predictable flow profile and reliable sewer performance.