Select the correct statement regarding water-supply mains versus gravity sewers in planning and layout.
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AWater-supply pipes carry perfectly pure water with no solids
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BWater-supply pipes clog if velocity is below self-cleansing velocity
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CGravity sewers may be routed up and down hills and valleys while maintaining hydraulic gradient and adequate slope
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DSewer pipes are generally laid along level hills without gradient
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ENone of these
Answer
Correct Answer: Gravity sewers may be routed up and down hills and valleys while maintaining hydraulic gradient and adequate slope
Explanation
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:
Reject absolute claims about purity or clogging rules for water mains.Recognize gravity sewer design depends on slope, not “level” laying.Select the statement acknowledging sewers can traverse varied topography with proper design.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