An IDF curve gives intensity as a function of duration, so you still need to determine the duration, which is commonly equal to the time-of-concentration (Tc).

Peter Smart
HydroCAD Software
www.hydrocad.net

An IDF curve is exactly that, a curve. It does not provide the Time of Concentration (T_c) or the Intensity (I). There is more to it than that.

Instead, an IDF curve shows the predicted relationship between Duration (=Time & =Time of Concentration) and Intensity (i.e. average rainfall intensity over the Duration in question). Also, T_c is the independent variable and I is the dependent variable. This relationship between Duration and Intensity is derived using actual precipitation data and statistics. An IDF chart typically includes multiple IDF curves to cover a range of rainfall probabilities (e.g. 2-yr, 10-yr, 25-yr, etc. events, representing annual probabilities of occurrence of 0.50, 0.10, 0.04, etc.).

For applying the Rational Method, we make T_c equal to Duration. So, to get the Intensity (typically inches per hour or millimeters per hour), you must first estimate T_c (typically minutes). There are many methods for estimating T_c. One of my hydrology books lists seven different methods for estimating T_c and I have seen several more that aren't in the book. The simplest method I know of is the Kirpich Equation, which requires knowing only the length and height of the watershed. However, its simplicity is also its weakness because there is more to a watershed than just its length and height. Other equations require more information about the watershed, such as average Runoff Coefficient, etc. Actually, the entire Rational Method is weak (that's another story), but it's also extremely useful.

For one site I recently worked on, the equation for the 2-year IDF curve is I₂ = 3.13 * T^c^(-0.49) and the equation for the 10-year IDF curve is I₁₀ = 5.86 * T_c^(-0.49). Please note that the family of IDF curves for a single site will have the same exponent and the IDF curve will plot as parallel lines on a log-log chart. So for a 2-year event, T_c = 10 min results in I = 1.01 in/hr, while T_c = 20 min results in I = 0.72 in/hr.

BTW, I derived the above equations from a paper chart by manually scaling two points on each IDF curve, then doing a power curve regression using my HP-42S calculator.

I hope this helps.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

The idf curve just tells you the rainfall intensity to use in the design storm.

The time of concentration is determined based on watershed characteristics. In my jurisdiction two methods are recommended based on size of watershed.

Those things are input into hydrology and hydraulic software which uses design storm distributions and rain on snow events. The software is then used to provide peak flows or storage requirements to size a pond, ditch, or sewer.