How to Calculate Runoff Coefficient

Using the Rational Method

• 1

  The Rational Method is used for areas less than 50 acres. If the area you are assessing is more than 50 acres, you will have to use a coefficient table. Write down the following formula:

  Q = CiA

• 2

  Find the appropriate values for each variable. In the Rational Method equation, “C” is the runoff coefficient, meaning it is your unknown. “Q” is the value for the peak rate of runoff. This value is in CFS. One CFS is equal to one cubic foot of water passing a particular point over the course of a second. “A” equals the size of the area you are measuring. “A” is measured in acres.

• 3

  Calculate the value for “i.” This is a measurement of rainfall intensity. It is measured in inches per hour. This value is calculated using a Seelye chart (see Resources) and an IDF chart. To use a Seelye chart, you need to have the length of the land and its angle of grade. Once you have this value, you can use an IDF chart to determine “i.” IDF charts vary from area to area and are based on average rainfall intensities. IDF charts are typically available from the state government. The actual agency varies from state to state. For instance, in Tennessee, the Department of Transportation releases IDF charts.

• 4

  Isolate and solve for “C.” Your altered equation should look like this:

  C = Q/iA

Using a Runoff Coefficient Table

• 5

  Find a reputable runoff coefficient table (see Resources).

• 6

  Determine what type of land you have. Some examples of land include business districts, residential areas, roofs and agricultural lands.

• 7

  Use the runoff coefficient chart to determine the runoff coefficient (C).