Hydraulics of self-cleansing velocity in sewers: At the self-cleansing velocity threshold, what happens at the sewer invert (bottom)?

Introduction / Context:
Self-cleansing velocity is the minimum mean flow velocity required to prevent sediment deposition in sewers. Design manuals specify values (often around 0.6–0.75 m/s for sanitary sewers) to ensure that shear stress at the invert is sufficient to mobilize and remove settleable solids.

Given Data / Assumptions:

• Steady uniform flow condition considered.
• Granular settleable solids are present.
• Velocity at or just above the self-cleansing limit.

Concept / Approach:

At the self-cleansing threshold, boundary shear equals the critical value needed to initiate motion of typical sewer solids. This produces scouring sufficient to prevent accumulation. Below this velocity, silting begins; well above it, excessive scouring may risk pipe wear.

Step-by-Step Solution:

Verification / Alternative check:

Field observation of clean invert and absence of deposits during peak dry-weather flows validates design; computational checks use Shields parameter for initiation of motion.

Why Other Options Are Wrong:

(a) Silting indicates velocity below threshold; (c) simultaneous strong silting and scouring is inconsistent; (d) some scouring is needed to remain self-cleansing.

Common Pitfalls:

Confusing self-cleansing with average velocity only; neglecting low-flow periods; ignoring grit sizes and slopes that affect required velocity.

Final Answer:

Scouring occurs at bottom