Rational Method: Intensity-Duration-Frequency (IDF) Curves
Intensity-duration-frequency (IDF) curves are graphical representations of the probability that a given average rainfall intensity will occur. They are commonly used when determining the rainfall rate for the rational method, modified rational method or the DeKalb rational method.
A standard IDF curve is plotted with duration on the horizontal axis and intensity on the vertical axis. Multiple curves are plotted on the same graph with each line representing the average frequency between occurrences for a given rainfall.
Example IDF curve for Madison, Wisconsin (Wisconsin DOT)
What some important things to remember about IDF curves?
When using IDF curves, it is important to remember the following:
- IDF curves are not plots of real storm data. They are the composite of multiple storms.
- IDF curve rainfall intensities represent average intensities.
- The duration is not the duration of an actual storm but most likely a shorter period of a longer storm during which the selected intensity occurred.
- IDF curves should not be used to obtain storm runoff volumes.
- The rational method illustrates a proper application of IDF curves.
How do I obtain or generate an IDF curve?
Location
specific IDF curves are often available through local and
state agencies. IDF curves can be generated using local rainfall
data and frequency analysis using an Extreme Value Type I
or Gumbel distribution. A detailed description of IDF curve
construction can be found in McPherson (1978). IDF curves
can also be quickly plotted using StormNET’s IDF Curve
Generator.
Where can I find rainfall data that can be used to generate IDF curves?
Precipitation data for use in IDF curve generation is provided by the National Weather Service (NWS) for several states. Other publications of interest include:
Rainfall Frequency Atlas of the Midwest
Short Duration Rainfall Frequency Relations for California
Short Duration Rainfall Relations for the Western United States
Technical Memorandum NWS Hydro 35 (Central and Eastern US)
How do I generate an IDF curve in StormNET?
When selecting the rational method, modified rational or DeKalb rational method in StormNET, an IDF rainfall value can be quickly selected using the IDF Curve Manager. The user can also tailor the IDF curve to meet the specific needs of a project. The IDF Curve Manger is accessible by selecting the Input Menu followed by IDF Curves from within StormNET.
StormNET’s IDF Curve Manager
What is the rational method?
The rational method is used to predict the peak runoff from a storm event. Despite being one of the oldest methods, it is still commonly used in the design of storm sewers. The peak runoff is calculated according to the following formula:
Q = kCiA
Q is the peak discharge (cfs or cms), k is a conversion factor (1.008 for SI and 0.00278 for metric), C is a runoff coefficient (dimensionless), i is the rainfall intensity (in/hr or mm/hr) and A is the drainage area (acres or hectare). The rainfall intensity (i) is determined using an IDF curve.
Sources:
Arkell,
R. E. and Richards, F., Short Duration Rainfall Relations
for the Western United States, Preprint Volume of the
Conference on Climate and Water Management, AMS (1986).
Bedient, P. B. and Huber, W.C., Hydrology and Floodplain
Analysis, 3rd edition, Prentice Hall, 2002.
V. T. Chow, David R. Maidment and L.W. Mays., Applied Hydrology,
McGraw-Hill, New York, 1988.
Frederic, R. H. and Miller, J. F., Short Duration Rainfall
Frequency Relation for California, Preprint from Third
Conference on Hydrometeorology, AMS (1979).
Huff, F. A. and Angel, J. R., Rainfall Frequency Atlas
of the Midwest, Bulletin 71, Midwest Climate Center,
1992.
McCuen, R. H., Hydrologic Analysis and Design, Pearson Prentice
Hall, New Jersey, 2007.
McPherson, M. B., Urban Runoff Control Planning,
EPA-600/9-78-035, Environmental Protection Agency, Washington
D.C., 1978.
NOAA, Five- to 60-minute Precipitation Frequency For the
Eastern and Central United States, NOAA Technical Memorandum
NWS HYDRO-35, Silver Spring, MD, 1977.
Wisconsin Department of Transportation, Facilities Development
Manual, Chapter 13 - Drainage.
