These design equations (Steel Equation), data and calculator will determine Rainfall Intensity and Peak Storm Water Runoff Rate for proper design of drainage requirements.
Using the STEEL equation ( i = a/(d + b)
NOTE: The example calculation was made for U.S. Region 3, which includes northwestern Illinois.
The calculated values for design rainfall intensity, i, and peak runoff rate, Q, are close to those calculated from IDF data for northwestern Illinois in the other worksheet, but not exactly the same. If IDF data is available for the location of interest, it would be a better choice than the Steel Equation.
Enter the Steel Equation constants ( a & b ) for your region of the U.S. (or a location with comparable rainfall characteristics) and the design return period - from the table on page: