For designing circular sewer pipes having diameters from 0.4 m to 0.9 m at maximum design flow, what depth of flow is generally assumed in practice so that some freeboard and ventilation are available?

Introduction / Context:
Sanitary sewers are gravity conduits that also function as open channels when not flowing full. Designers rarely size them to run full at peak because some freeboard is desirable for ventilation, surcharge allowance, and to reduce odor and pressurization risks. A common rule-of-thumb is to assume a fraction of the diameter at peak design flow.

Given Data / Assumptions:

• Circular sewers with diameters between 0.4 m and 0.9 m.
• Conventional municipal design practice for peak (maximum) design flow.
• Need to choose a practical depth fraction for design capacity.

Concept / Approach:

Although a circular section gives maximum discharge when running full from strict hydraulics, sewer design includes serviceability concerns. Operating “part-full” at design peak leaves headroom for inflow/infiltration, surges, and ensures air movement. Many practice manuals adopt two-thirds depth (about 0.67D) as the standard design assumption for peak flow sizing of medium sewers.

Step-by-Step Solution:

Verification / Alternative check:

Two-thirds full corresponds to a large proportion of full-flow capacity while leaving headspace. Numerous standard problems and exam syllabi reference this assumption for circular sewers in the 0.4–0.9 m range.

Why Other Options Are Wrong:

Full depth sacrifices freeboard; half or one-third full are overly conservative; three-fourths full leaves less ventilation margin than the common standard.

Common Pitfalls:

Confusing “hydraulic maximum discharge” with “design practice”; ignoring operational needs like aeration and surcharge allowance.

Final Answer:

two-thirds full