Combined sewers cross-section: Which cross-section is most suitable for carrying combined flow (sanitary + storm) over a wide range of discharges?

Introduction / Context:Combined sewers must perform satisfactorily at both low dry-weather flows and high wet-weather flows. Cross-section choice affects velocity distribution, self-cleansing, and surcharge behavior across this wide operating range.

Given Data / Assumptions:

• Combined sewer (sanitary + stormwater).
• Need stable velocities at low flows and adequate capacity at peaks.

Concept / Approach:The egg-shaped section narrows at the invert, producing higher velocities for low flows to limit deposition, yet provides large capacity higher up for storm events. This dual benefit makes it a classic choice for combined systems compared with simple circular or rectangular sections.

Step-by-Step Solution:Assess low-flow behavior: egg-shaped has smaller wetted width → higher velocity.Assess peak behavior: taller crown provides storage/conveyance for storms.Select egg-shaped as the balanced solution.

Verification / Alternative check:Historical sewer design literature consistently recommends egg-shaped sections for combined sewers to maintain self-cleansing across flow regimes.

Why Other Options Are Wrong:Circular: Good general choice but less effective than egg-shaped at very low flows.Rectangular/horse-shoe/triangular: Have disadvantages in sediment control or structural efficiency for buried conduits.

Common Pitfalls:

• Assuming circular always suffices; combined flow needs special attention at low flows.
• Ignoring maintenance issues tied to sediment deposition.

Final Answer:Egg-shaped