Select the correct statement regarding water-supply mains versus gravity sewers in planning and layout.

Introduction / Context:
Network planning differs for pressurized water mains and gravity sewers. Understanding hydraulics, gradients, and solids behavior is crucial for reliable service and maintenance.

Given Data / Assumptions:

• Water-supply mains are pressurized, conveying treated water.
• Gravity sewers flow partially full and need sufficient slope for self-cleansing.
• Topography may vary across the corridor.

Concept / Approach:

Water mains are not “pure” in the absolute sense but contain low solids; they do not rely on a self-cleansing velocity concept like sewers. Sewers must maintain grade to transport settleable solids; their alignment may follow undulating ground using appropriate slopes, siphons, or aqueducts to cross valleys while maintaining hydraulic requirements.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals show long sewer profiles rising and falling with terrain, incorporating manholes and siphons where necessary.

Why Other Options Are Wrong:

(a) “Perfectly pure” is inaccurate; (b) self-cleansing velocity pertains to sewers; (d) sewers cannot be laid level—minimum slopes are required to avoid deposition; (e) is false because one statement is correct.

Common Pitfalls:

Applying sewer criteria to water mains; assuming sewer lines must mirror flat planes rather than hydraulic grade.

Final Answer:

Gravity sewers may be routed up and down hills and valleys while maintaining hydraulic gradient and adequate slope