When is a drop manhole provided in a sewerage system? Select the condition that necessitates constructing a drop connection in a manhole.

Introduction / Context:
Manholes provide access, ventilation, and flow control in sewer networks. A special form, the drop manhole, safely conveys a branch flow entering at a higher elevation down to the main invert, preventing destructive free falls and turbulence inside the chamber.

Given Data / Assumptions:

• Branch sewer invert is substantially above the main sewer invert.
• Need to avoid erosion, odor release, and splashing in the manhole shaft.
• Urban gravity sewer with manholes at junctions.

Concept / Approach:

A vertical or near-vertical drop pipe is fitted to carry incoming flow down to the benching level, discharging smoothly into the main channel. This configuration is used when grade differences exceed the acceptable limit for direct junctions (commonly a drop of around 0.6 m or more, depending on standards).

Step-by-Step Solution:

Verification / Alternative check:

Design guides illustrate internal/external drop manholes to manage large invert differences and to mitigate turbulence and corrosion.

Why Other Options Are Wrong:

(a) is vague; continuous sewer grade changes are handled by slope design, not a drop manhole. (c) relates to maintenance lighting, not design geometry. (d) is invalid; (e) pertains to outfall hydraulics, not manhole junction geometry.

Common Pitfalls:

Using free fall within the manhole; neglecting corrosion risks from turbulence and H2S; omitting access for maintenance of the drop pipe.

Final Answer:

a branch sewer joins the main sewer at a considerably higher level