For a combined sewerage system handling both sanitary sewage and storm runoff, which conduit cross-section is considered the most hydraulically efficient and self-cleansing over a wide range of discharges?

Introduction / Context:
Combined sewerage systems must perform adequately at very low dry-weather flows and very high storm flows. The choice of cross-section strongly affects hydraulic efficiency, self-cleansing behavior, and construction practicality.

Given Data / Assumptions:

• Combined sewers must convey a wide flow range.
• Self-cleansing at low flow depths is desirable.
• Structural feasibility and ease of construction are considered.

Concept / Approach:
The “new egged” (modified egg-shaped) section provides a narrow invert for higher velocities during low flows, aiding self-cleansing, while the upper widening accommodates peak storm discharges. Circular or rectangular sections are simpler but do not offer the same low-flow velocity advantage. Parabolic and horseshoe shapes see more specialized use and are less common for municipal combined sewers.

Step-by-Step Solution:
1) Identify low-flow requirement: maintain velocity high enough to prevent deposition.2) Identify high-flow requirement: provide adequate area and freeboard.3) Recognize that the new egged section best satisfies both by geometry: small hydraulic radius at invert for DWF and large area above for storms.

Verification / Alternative check:
Standard sanitary engineering texts cite egg-shaped sections as the preferred geometry for combined sewers, with “new egged” refinements improving constructability and performance.

Why Other Options Are Wrong:

• Circular: Adequate at full or near-full flow, but poorer self-cleansing at small depths.
• Rectangular: Structural and deposition issues at low depths.
• Parabolic / horseshoe: Not standard for combined sewer networks.

Common Pitfalls:
Assuming the simplest shape (circular) is best for all conditions; combined systems require geometry tuned for extremes.

Final Answer:
new egged section