Household branch sewer (100 mm diameter): What minimum gradient is typically adopted to achieve self-cleansing velocity under standard design practice?

Introduction / Context:
Self-cleansing velocity is the minimum velocity required to keep solids from depositing in gravity sewers. For small diameter household laterals (100 mm), codes recommend sufficiently steep gradients to maintain adequate shear stress even at low flows.

Given Data / Assumptions:

• Domestic wastewater with typical solids and flush patterns.
• Standard design references targeting about 0.6–0.75 m/s self-cleansing.
• Small diameter branch sewer: 100 mm.

Concept / Approach:

The velocity V depends on slope S via Manning’s relation. For 100 mm lines, a common recommended minimum slope is about 1 in 57 to 1 in 60 to reach self-cleansing velocities at part-full conditions.

Step-by-Step Solution:

Verification / Alternative check:

Multiple codes and manuals list 1:57–1:60 for 100 mm domestic drains, confirming the selection.

Why Other Options Are Wrong:

Flatter slopes like 1:100 or 1:120 may not consistently achieve self-cleansing at low flows; “None of these” (option d) is incorrect, and 1:200 (option e) would be too flat.

Common Pitfalls:

Applying trunk sewer slopes to house drains; ignoring part-full hydraulics and roughness impacts.

Final Answer:

1 in 60