IDFcurves

IDF curves, or intensity–duration–frequency curves, are a foundational tool in hydrology and civil engineering used to estimate the characteristics of rainfall events for design and risk assessment. For a given location, an IDF curve expresses the expected rainfall intensity (mm/h) as a function of storm duration (minutes to hours) for a specified return period (e.g., 2-year, 10-year, 100-year). They underpin the selection of design storms for sizing drainage networks, culverts, and detention facilities, and are also used in rainfall–runoff modeling and flood risk assessments.

IDF curves are constructed from long-term rainfall records, typically by analyzing annual maximum rainfall for different durations. Statistical distributions such as Gumbel or generalized extreme value are fitted to derive a family of curves across return periods; or empirical relationships with semiparametric fits are used. The resulting curves are region-specific and are published by national meteorological services, hydrological agencies, or engineering design manuals.

To use, engineers choose location and return period, pick the duration of interest, and read the corresponding rainfall intensity from the curve. They may be provided as tables or plots and used directly or interpolated. It is common to present curves for a range of durations (e.g., 5–180 minutes).

Limitations include uncertainties from record length, measurement errors, non-stationarity due to climate change, and extrapolation beyond observed periods. Compatibility with local drainage design standards should be checked, and updates are periodically issued to reflect new data and methodologies. IDF curves remain central to urban drainage design and flood risk assessment in many regions.