Today I want to continue my discussion of hydrology by showing you the simplest of all runoff models, the Rational Method.
In the interests of decreasing the complexity, most modeling methods use a lumped parameter system. Rather than accounting for the different types of rainfall loss separately they’re all lumped into one variable. Simplicity is is where the Rational Method shines.
peak flow = curve number *rainfall intensity* drainage area
You can measure drainage area from a map, and rainfall intensity is easy to look up from all the precipitation data the National Weather Service has been collecting for over a century. The key to the Rational Method is the curve number. The curve number takes into account the soil type, ground vegetation, land use, and various other factors so it’s the ‘lumped parameter’ in this instance.
The origins of the curve number are shrouded in mystery. The Rational Method was one of the earliest attempts at calculating runoff and it dates back to the first half of the twentieth century. The curve numbers were originally derived by a lot of physically measuring of both rainfall and runoff in order to develop a relationship between the two. These days it comes from a table showing the pertinent curve number for various types of land use, soil type, etc…
Selection of the runoff modeling method is largely a matter of risk. The larger the drainage area, and the more valuable the property that may be flooded, the more complex the methodology. The Rational Method is a popular method due to its ease of use, and it is considered viable up to a drainage area of about 200 acres. It is best applied in urban environments with lots of pavement and it’s most often used for small scale applications like parking lots or highway drainage. It’s not a method you’re likely to see used for reservoir design or 100 year flood modeling.