Intensity-Duration-Frequency (IDF) curves are the foundation of drainage design hydrology. They relate rainfall intensity to storm duration for various return periods (frequencies) at a specific location. Every time you use the Rational Method, you need an IDF curve to determine the design rainfall intensity.
What Is an IDF Curve?
An IDF curve is a graph (or set of equations) that shows:
- Intensity (I): Rainfall rate, typically in inches per hour (in/hr)
- Duration (D): Length of the storm or averaging period (minutes or hours)
- Frequency (F): Return period — how often the event is expected to occur (e.g., 10-year, 100-year)
For any given location, shorter storms have higher intensities, and rarer storms (longer return periods) have higher intensities than more common storms of the same duration.
How to Read an IDF Curve
Step 1: Determine your design storm return period (e.g., 10-year).
Step 2: Calculate the time of concentration (Tc) for your drainage area. The design storm duration equals Tc.
Step 3: Find the curve for your return period on the IDF chart.
Step 4: Enter the chart at your duration (x-axis) and read up to the curve, then across to the intensity (y-axis).
Important: IDF data is location-specific. A 10-year, 15-minute storm in Miami produces roughly twice the intensity as the same storm in Seattle. Always use data for your specific project location.
Example IDF Values (Typical US Southeast)
The following are representative values only. Use NOAA Atlas 14 for your specific location.
| Duration | 2-yr (in/hr) | 10-yr (in/hr) | 25-yr (in/hr) | 100-yr (in/hr) |
|---|---|---|---|---|
| 5 min | 6.4 | 8.5 | 9.8 | 11.8 |
| 15 min | 4.5 | 6.2 | 7.2 | 8.8 |
| 30 min | 3.2 | 4.5 | 5.3 | 6.5 |
| 1 hour | 2.2 | 3.1 | 3.7 | 4.5 |
| 2 hours | 1.4 | 2.0 | 2.4 | 3.0 |
| 24 hours | 0.25 | 0.37 | 0.44 | 0.54 |
IDF Equation Form
IDF relationships are often expressed as empirical equations fitted to observed data:
i = a / (tc + b)c
Where i = intensity (in/hr), tc = duration (min), and a, b, c are fitted coefficients specific to the location and return period. Many jurisdictions publish these coefficients in their drainage design manuals.
Where to Find IDF Data
NOAA Atlas 14 (Recommended)
The NOAA Precipitation Frequency Data Server (PFDS) provides point precipitation frequency estimates for all US locations based on NOAA Atlas 14. Enter your coordinates or click on the map to get IDF data including 90% confidence intervals.
Available at: hdsc.nws.noaa.gov/pfds/
State and Local Sources
Many state DOTs and local agencies publish IDF curves or equations specific to their jurisdictions. These may differ from NOAA Atlas 14 and may be required for regulatory compliance. Always check which data source your reviewing agency requires.
Technical Paper 40 (TP-40) — Outdated
TP-40 (1961) was the predecessor to NOAA Atlas 14. While some older regulations still reference TP-40, it is based on outdated data and should be replaced with Atlas 14 values whenever permitted.
Design Storm Selection
| Application | Typical Return Period | Annual Exceedance Probability |
|---|---|---|
| Roof / building drainage | 100-year | 1% |
| Minor storm drains | 10-year | 10% |
| Major storm drains | 25-year | 4% |
| Bridge / culvert waterway | 50 to 100-year | 2% to 1% |
| Floodplain management | 100-year | 1% |
Primary Sources
- NOAA (2004–2022). Atlas 14: Precipitation-Frequency Atlas of the United States. National Weather Service, National Oceanic and Atmospheric Administration.
- Hershfield, D.M. (1961). Rainfall Frequency Atlas of the United States (Technical Paper No. 40). U.S. Weather Bureau.
- FHWA HEC-22 (2009). Urban Drainage Design Manual, 3rd Edition.