Rainfall characterization for hydrologic design: which parameters are used to describe rainfall at a place for runoff estimation and stormwater design?

Introduction / Context:
Hydrologic design of drains, culverts, and detention requires quantifying storms by how hard it rains, for how long, and how often such storms occur. These three descriptors underpin IDF (Intensity–Duration–Frequency) curves used worldwide.

Given Data / Assumptions:

• Local IDF data or design storms available from agencies.
• Stationarity assumed for design, unless climate-adjusted factors are prescribed.
• Single event design for small urban works; continuous simulation for larger systems may be used.

Concept / Approach:

Intensity controls peak runoff; duration controls catchment response relative to time of concentration; frequency (return period) reflects risk tolerance and desired level of service. Together, they define the design storm used in Rational or hydrograph-based methods.

Step-by-Step Solution:

Verification / Alternative check:

Cross-check with historical storm records or regional guidelines; ensure consistency with floodplain and overland flow pathways.

Why Other Options Are Wrong:

Each of (a)–(c) is part of standard rainfall characterization, hence “All the above” is correct.

Common Pitfalls:

Confusing storm “direction” with “duration”; selecting intensity for the wrong duration; ignoring temporal distribution within the storm hyetograph.

Final Answer:

All the above.