Rain is a normal phenomenon for many mid-latitude areas. It creates runoff, a term for water movement. A runoff coefficient is a number that relates the rainfall rate and runoff rate. Using the runoff coefficient, scientists and hydrologists can calculate how much water passes over a given area per second. There are two ways you can find the runoff coefficient. One is by using the Rational Method, and the other is by using a runoff coefficient table.
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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
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.
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.
Isolate and solve for “C.” Your altered equation should look like this:
C = Q/iA
Find a reputable runoff coefficient table (see Resources).
Determine what type of land you have. Some examples of land include business districts, residential areas, roofs and agricultural lands.
Use the runoff coefficient chart to determine the runoff coefficient (C).
Tips and warnings
- If you use a runoff coefficient chart and you have different types of land, you will likely have to use a weighted average.
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- U.S. Army Corps of Engineers, Sediment and Erosion Control Planning, Design and Specification: Estimating Runoff Using the Rational Method
- Bonneville Power Administration, Power: Federal Columbia River Power System (FCRPS) Definition List
- Mountain Empire Community College, Water/Wastewater Distance Learning Website: Rational Method
- Tennessee Department of Transportation, Design Division: Drainage Manual