Urban drainage design: drop manholes are typically provided in sewers in which type of locality?

Introduction / Context:
Where ground slopes are steep, conventional sewer gradients may become too steep, causing high velocities, scouring, and difficult depth transitions. Drop manholes are a practical detail to accommodate large elevation differences while controlling internal flow conditions.

Given Data / Assumptions:

• Gravity sewer system following natural topography.
• Need to connect higher incoming sewers to deeper downstream sewers.
• Objective: prevent excessive velocities and erosion inside manholes and pipes.

Concept / Approach:

A drop manhole allows the incoming sewer to fall vertically (or via a drop pipe) before joining the lower invert, dissipating energy and keeping the downstream grade manageable. Such configurations are common in hilly townships where street grades fall rapidly between successive manholes.

Step-by-Step Solution:

Verification / Alternative check:

Hydraulic grade line analysis shows controlled velocities and acceptable head losses compared with a continuous steep gradient that might exceed permissible velocity limits.

Why Other Options Are Wrong:

(a), (b), and (d) do not specifically require drop arrangements; flat areas can use mild gradients without drops.

Common Pitfalls:

Improper drop alignment leading to splashing and odor; inadequate corrosion-resistant materials; ignoring access and safety clearances.

Final Answer:

Hilly townships (steep ground levels)