The analysis of storm sewers is usually based on testing the ability of the piping system to appropriately handle peak flows without flooding roadways or scouring the pipes. The rational method is a popular method for estimating peak flows, based on the size and runoff coefficient of a watershed, and the intensity of the storm event.

The fundamental rational formula is:

Other forms of the rational method are commonly used which incorporate values in different units to make the order of magnitude of parameters more suitable for hand calculations. For example, the rational formula is often used with watershed area in acres and rainfall intensity in inches per hour. However, using this formula as-is can result in common mistakes, such as omitting the required unit conversion from acre- inches per hour to cubic feet per second (1.008). Conversions such as these are automatically performed within Bentley SewerGEMS to give you the most accurate results possible.