Egg-shaped sewer geometry: If D denotes the diameter of the circular upper portion of a standard egg-shaped sewer, what is the overall vertical depth of the section?

Introduction / Context:

Egg-shaped (ovoform) sewers are favored for combined systems because they provide better self-cleansing velocities at low flows and adequate capacity at high flows. Their proportions are standardized so that hydraulic performance is predictable and formwork or precast moulds can be repeated.

Given Data / Assumptions:

• D is defined as the diameter of the upper circular arc of the egg-shaped section.
• Standard proportions are assumed (not custom/modified ovoforms).

Concept / Approach:

In the standard form, the overall depth (crown to invert) is 1.5 times D. This ensures a narrower lower portion for velocity maintenance during dry-weather flow while the broader upper portion accommodates storm peaks.

Step-by-Step Solution:

Verification / Alternative check:

Handbooks on sewer design provide proportional templates: height-to-upper-diameter ratio H/D ≈ 1.5 for the common standard egg shape.

Why Other Options Are Wrong:

• D and 1.25 D underestimate the height; 1.75 D and 2 D exceed standard proportions and would alter hydraulics.

Common Pitfalls:

• Confusing D (upper circular diameter) with overall height H; they are not equal.

Final Answer:

1.5 D