Hydraulics of sewers: In a flowing sewer, where is the scouring action (abrasive wear by grit and high velocities) typically the most pronounced?

Introduction / Context:

Scouring refers to abrasive wear caused by grit and suspended solids moving with the flow. Understanding where scouring is most severe guides material choice and maintenance of sewer lines.

Given Data / Assumptions:

• Gravity sewers with bed shear stress acting along the invert (bottom).
• Presence of grit (sand, small stones) that travels along the bed.

Concept / Approach:

Under gravity flow, the highest contact between moving particulates and pipe occurs along the invert. Bed shear, rolling and sliding of particles, and occasional saltation cause maximum abrasion at the bottom. The crown (top) experiences corrosion risks (e.g., H2S to sulfuric acid) but not scouring.

Step-by-Step Solution:

Verification / Alternative check:

Field inspections often show invert wear and need for hard inverts or improved self-cleansing velocities in grit-prone networks.

Why Other Options Are Wrong:

• Top side / horizontal side / all sides: Do not match the mechanics of bed load abrasion.
• Only at manholes: Local turbulence exists, but scouring dominance is along the invert in pipe reaches.

Common Pitfalls:

• Confusing chemical crown corrosion with mechanical scouring.

Final Answer:

bottom side