Design basis for distribution mains: a city’s distribution mains are normally sized for approximately what fraction of the average daily water requirement?

Introduction / Context:
Distribution mains must convey peak demands, not merely average flows. A common planning benchmark is the maximum day demand, often taken as a multiplier of the average day, to ensure adequate pressures and service during high-use periods and emergencies.

Given Data / Assumptions:

• Average daily requirement is the baseline.
• Peak factors translate average to design flow.

Concept / Approach:
Empirical peaking factors account for diurnal and seasonal variability. For many systems, maximum day ≈ 1.5 × average day, while maximum hour may be higher and handled by storage plus main capacity. Designing mains near 150% of the average daily requirement is a typical baseline before detailed modeling.

Step-by-Step Solution:
Identify the design driver: maximum day consumption. Apply peak factor ≈ 1.5 on average daily requirement. Select 150% as the appropriate choice.

Verification / Alternative check:
Hydraulic modeling with diurnal curves usually confirms that mains sized for the maximum day plus storage can meet maximum hour at acceptable pressures.

Why Other Options Are Wrong:

• 100%: Insufficient for peaks.
• 200% or 225%: Overly conservative for typical systems and uneconomical without justification.

Common Pitfalls:

• Confusing maximum day factor with maximum hour factor, which is higher but primarily met via storage.

Final Answer:
150% of the average daily requirement (maximum day demand).