Drop manholes: In sewer networks, “drop manholes” are provided primarily when…

Introduction / Context:
Where a branch sewer joins a deeper main at a substantial level difference, direct plunging inflow can cause turbulence, odor, and structural issues. “Drop manholes” use a vertical drop connection to dissipate energy and improve hydraulics and safety.

Given Data / Assumptions:

• Two or more sewers with different invert elevations.
• Need to connect higher-level flow to a lower-level main safely.

Concept / Approach:
A drop manhole routes the higher sewer through a vertical or near-vertical drop pipe into the manhole, discharging near the benching level. This reduces free-fall splashing inside the shaft, controls scouring, and protects the structure.

Step-by-Step Solution:
Identify the condition: different levels (inverts) at junctions.Engineering response: provide a drop (inside or outside) to handle elevation difference.Select the option describing drains at different levels.

Verification / Alternative check:
Design manuals illustrate internal or external drop pipes where level differences warrant, confirming the purpose of drop manholes.

Why Other Options Are Wrong:
Ladders/covers: Safety and access features do not define “drop” manholes.Depth > 3.5 m alone is not the defining criterion; it is the level difference at junction.Diameter changes do not necessitate a drop unless levels differ.

Common Pitfalls:

• Confusing deep manholes with drop manholes; the key is level difference and drop connections.
• Overlooking the need for energy dissipation and ventilation in drop configurations.

Final Answer:
Drains meet at significantly different invert levels