In large sewers, the recommended self-cleaning (non-silting) velocity under Indian practice, to avoid deposition on the invert and sides, is approximately:

Introduction / Context:Sewer hydraulics aims to maintain velocities high enough to keep solids in motion, preventing deposition and odor issues. The self-cleaning velocity is a rule-of-thumb minimum velocity to achieve this at design flow conditions.

Given Data / Assumptions:

• Relatively large municipal gravity sewers.
• Indian design practice for self-cleansing criteria.
• Typical mixed domestic sewage with suspended and settleable solids.

Concept / Approach:Empirical recommendations indicate that velocities near or above about 0.6 to 0.75 m/s help prevent settlement of typical sewage solids. For large sewers and Indian conditions, 0.75 m/s is commonly adopted as a practical design value for self-cleansing under dry weather flow.

Step-by-Step Solution:Identify the objective: avoid grit and organic deposition.Select empirical minimum velocity: 0.75 m/s for Indian practice for large sewers.Check that at peak factors, actual velocities will exceed this minimum, further improving scouring.

Verification / Alternative check:Design guides and solved examples for sewer design repeatedly cite 0.75 m/s as a benchmark minimum to deter siltation in large conduits under DWF conditions.

Why Other Options Are Wrong:0.50–0.70 m/s: sometimes acceptable in small sewers or under specific conditions, but 0.75 m/s is the widely cited Indian recommendation for large sewers.1.00 m/s: conservative; may raise excavation depth or diameters unnecessarily.

Common Pitfalls:

• Using peak flow velocity instead of minimum DWF velocity for self-cleaning check.
• Ignoring upstream flat slopes that can create local silting pockets.

Final Answer:0.75 m/s