Self-cleansing velocity criterion: When a sewer is designed for self-cleansing velocity, what should happen at the invert (bottom) of the conduit under design flow?

Introduction / Context:
Self-cleansing velocity is the minimum velocity (or shear stress) in a sewer that prevents deposition of suspended and bed materials while avoiding damage to the pipe. It is a key check for minimum-flow conditions in sanitary and combined systems.

Given Data / Assumptions:

• Design target is to prevent deposition during low flows.
• Pipe material should not be eroded by routine operation.
• Velocity or shear stress is set based on sediment characteristics.

Concept / Approach:
At self-cleansing velocity, the hydraulic forces at the invert are just adequate to keep solids from settling; they are not so high as to scour the pipe wall or protective coatings. Thus, the desired condition is absence of both silting and scouring under design flow.

Step-by-Step Reasoning:
Set a threshold for bed shear/velocity based on sediment size and specific gravity.Check velocity at minimum flow; ensure it meets or exceeds this threshold.Confirm that the selected velocity is still below erosive limits for the pipe material.

Verification / Alternative check:
Standards often recommend minimum velocities around 0.6–0.75 m/s for sanitary sewers (varies by code, sediment load, and slope), specifically to maintain self-cleansing without erosion.

Why Other Options Are Wrong:
Silting or continuous scouring indicates the velocity is too low or too high, respectively.Simultaneous silting and scouring contradicts a stable operating point.Selective movement of only gross solids does not guarantee a clean invert.

Common Pitfalls:

• Using a single velocity value irrespective of sediment characteristics.
• Ignoring pipe roughness and slope, which affect shear stress.

Final Answer:
Neither silting nor scouring occurs at the bottom