Design conventions for gravity sewers: Which of the following statements are typically adopted in practice for size and velocity criteria?

Introduction / Context:
Gravity sewers are sized not only for capacity but also for maintenance. Designers conventionally avoid flowing “just full” in larger sewers to leave air space, reduce odor issues, and accommodate surges, while ensuring self-cleansing velocities to limit sediment deposition.

Given Data / Assumptions:

• Small pipes: < 0.4 m diameter.
• Large pipes: ≥ 0.4 m diameter.
• Target minimum velocity near 0.8 m/s for self-cleansing (textbook value ranges 0.6–0.9 m/s).

Concept / Approach:

At design peak, small pipes are often checked “running full” due to practical constraints. Larger pipes are checked at a fraction (about 2/3–3/4) of full depth to provide reserve capacity and ventilation. A minimum velocity criterion reduces grit accumulation and maintenance burden.

Step-by-Step Solution:

Verification / Alternative check:

Utility standards commonly list similar criteria; local codes may specify exact minimum velocities and depth fractions.

Why Other Options Are Wrong:

All three statements reflect standard practice; hence (d) “All of the above” is the most inclusive and correct option.

Common Pitfalls:

Ignoring future peak factors; undersized ventilation leading to septicity; relying on too low self-cleansing velocities for gritty catchments.

Final Answer:

All of the above