Peaking for large trunk mains: For trunk sewers with diameters greater than 1.25 m, the ratio of maximum sewage flow to average sewage flow is typically:

Introduction / Context:
Large trunk mains experience flow attenuation due to substantial storage and long travel times. Consequently, the peaking factor (max/average) is lower than for small laterals or branch sewers.

Given Data / Assumptions:

• Diameter > 1.25 m implies significant hydraulic storage and wave smoothing.
• Diurnal peaks are damped by upstream networks and in-sewer storage.
• Standard peaking factors are used for preliminary design.

Concept / Approach:
Empirical peaking factors reduce with increasing contributing area and pipe size. For large trunks, a value near 1.5 is commonly adopted to reflect reduced peak-to-average variation.

Step-by-Step Solution:
Recognize that larger systems peak less sharply.Choose a conservative yet typical factor.Select 1.5 as a representative ratio for large trunks.

Verification / Alternative check:
Several design guides present peaking factors decreasing toward ~1.3–1.7 for major interceptors, depending on catchment size and infiltration/inflow characteristics.

Why Other Options Are Wrong:
2.0–4.0: represent small sewers or localized peaks; high for large trunks.1.2: optimistic and may under-design capacity.

Common Pitfalls:

• Applying small-system peaking factors to city-scale interceptors.
• Ignoring wet-weather inflow that can modify effective peaks.

Final Answer:
1.5