Rainfall description parameters – what fully characterizes a storm at a location? Which set of parameters is used to describe rainfall at a point for design and analysis purposes?

Introduction / Context:
Hydrologic design relies on characterizing rainfall using metrics that capture its severity and likelihood. Drainage sizing, detention storage, and flood risk assessments routinely reference intensity, duration, and frequency (IDF) relationships.

Given Data / Assumptions:

• Point rainfall at a site, not areal averages.
• Design relies on historical IDF curves or intensity-duration-frequency equations.
• Stationarity over the fitting window is assumed for frequency analysis, unless nonstationary methods are used.

Concept / Approach:
Intensity describes the rate (e.g., mm/h), duration is the storm length over which intensity is averaged, and frequency quantifies the statistical rarity via return period (e.g., 2-year, 10-year, 100-year). Design storms are specified by pairing duration and return period to read a corresponding intensity from IDF curves.

Step-by-Step Solution:
Recognize that intensity depends on the averaging duration; they are inseparable in IDF analysis.Frequency encodes exceedance probability, linking design risk to infrastructure class.Therefore, a complete description includes intensity, duration, and frequency together.

Verification / Alternative check:
Municipal stormwater manuals specify design intensities for multiple durations at prescribed return periods, confirming the IDF triad as standard.

Why Other Options Are Wrong:

• (a), (b), and (c) alone are incomplete descriptors.
• (e) is false because the combination in (d) is correct and widely used.

Common Pitfalls:
Using a single-duration intensity across all events; always match the critical duration to the watershed’s time of concentration for conservative design.

Final Answer:
All the above