In municipal hydrology for storm-water design, the intensity of rainfall should be reported in which standard unit? (Assume conventional practice for intensity–duration–frequency curves and sewer design.)

Introduction / Context:
Rainfall intensity is a fundamental input for storm-water drainage, roof drain sizing, and combined sewer overflow analysis. Designers typically read intensity from intensity–duration–frequency (IDF) curves. Using a standard and consistent unit avoids conversion errors that can drastically affect calculated peak flows and conduit sizes.

Given Data / Assumptions:

• Conventional civil engineering practice for urban drainage.
• Intensity is defined as rainfall depth divided by the time over which it falls.
• No special regional unit system is imposed in the question; metric is used.

Concept / Approach:
Intensity I is depth/time. In metric hydrology, intensity is most commonly expressed as millimetres per hour or centimetres per hour. This aligns with typical storm durations (5–180 minutes) and allows easy integration with runoff methods (e.g., rational method where I is in consistent units with C and A).

Step-by-Step Solution:
Define intensity: I = rainfall depth / duration.Select a practical time base: hours are commonly used for IDF curves.Choose a depth unit suited to typical storm depths: millimetres or centimetres. The option provided that matches standard practice is cm/hour.

Verification / Alternative check:
Most IDF tables are given in mm/h (equivalent to cm/h divided by 10). Converting between mm/h and cm/h is straightforward and keeps dimensional consistency in runoff equations.

Why Other Options Are Wrong:

• cm/minute: Too fine a time base; uncommon and invites errors.
• cm/day: Appropriate for hydrology at basin scale, not urban drainage peaks.
• mm/second and litres/m²·hour: Nonstandard or awkward for IDF use in sewer design.

Common Pitfalls:
Mixing mm/h and mm/min in the rational formula; misreading IDF charts drawn for specific return periods; failing to convert if rainfall records are in different units.

Final Answer:
cm/hour