Sewer manholes: At which locations along a sewer line are manholes typically provided for access, inspection, and maintenance?

Introduction / Context:
Manholes are essential appurtenances in urban sewerage. They provide access for inspection, cleaning, maintenance, flow monitoring, and ventilation. Proper placement reduces blockages, shortens maintenance time, and improves safety for workers.

Given Data / Assumptions:

• Sewer lines include horizontal and vertical geometric changes.
• Flow may carry grit and debris that can settle at hydraulic transitions.
• Periodic access is required for cleaning and repairs.

Concept / Approach:

Hydraulic transitions and connections are the locations most prone to energy losses, turbulence, and deposition. Therefore, design practice places manholes at all points where grade, direction, or pipe size changes, and at junctions. Spacing is also limited (e.g., 30–100 m depending on size) to ensure routine access even on straight, uniform reaches.

Step-by-Step Solution:

Verification / Alternative check:

Operations records show that blockages cluster near bends, junctions, and slope changes. Providing manholes at these locations reduces time to clear blockages and improves safety.

Why Other Options Are Wrong:

Each of options (a), (b), and (c) is individually correct; the comprehensive correct choice is (d) because all listed locations require manholes.

Common Pitfalls:

Placing manholes too far apart on long straight runs; omitting manholes at diameter transitions; poor alignment causing difficult entry or unsafe benching inside the structure.

Final Answer:

All of these