Intensitydurationfrequency

Intensity-duration-frequency (IDF) is a statistical framework used in hydrology and civil engineering to describe the relationship between rainfall intensity, duration, and its return period. For a given duration, the model provides the rainfall intensity that is expected to be exceeded on average once every T years; conversely, for a chosen return period, it gives the corresponding intensity for various durations.

IDF curves are built from long-term rainfall records. Data are organized into events of different durations (for example 5 minutes, 1 hour, 6 hours, 24 hours). For each duration, an annual maximum rainfall series is formed and a probability distribution is fitted to estimate quantiles for selected return periods such as 2, 5, 10, 25, 50, and 100 years. The resulting curves are used to translate a short-term rainfall event into a design rainfall for civil works.

Applications include sizing of drainage systems, sewers, culverts, and detention basins, as well as urban flood risk assessment and stormwater planning. IDF data support hydraulic modeling and code-based design standards.

Limitations include assumptions of stationarity, uncertainties in data and distribution fitting, regional variability, and potential nonlinearity under climate change or rapid urbanization. Users should apply region-specific curves, consider uncertainty, and account for potential future changes when planning and designing infrastructure.