Calculation of peak storm water runoff rate from a drainage area is often done with the Rational Method equation (Q = CiA).  Calculations with the Rational Method equation involve determination of the design rainfall intensity and the time of concentration of the watershed, as well.

This course includes a discussion of the equations and the variables for these types of calculations.  Example calculations, including the use of a spreadsheet for the Rational Method equation calculations and for the determination of the design rainfall intensity and the time of concentration of the drainage area, are presented and discussed in this course.  The parameters in the equations are defined with typical units for both U.S. and S.I. units. 

Upon completion of this course, the learner will:

• Know the parameters and their U.S. and S.I. units to be used in the Rational Method equation.
• Be able to calculate peak storm water runoff rate with the Rational Method equation, using either U.S. or S.I. units.
• Be able to place a given soil into one of the four SCS soil groups based on its measured minimum infiltration rate.
• Be able to place a given soil into one of the four SCS soil groups based on its description.
• Be able to estimate peak runoff rate from a drainage area using the Rational Method.
• Be able to determine a value of the Rational Method runoff coefficient based on land use, soil group, and slope of the watershed.
• Be able to calculate the watershed time of concentration using the Kirpich equation, the Kerby-Hathaway equation, the FAA equation, and the SCS Lag equation.
• Be able to calculate the watershed time of concentration using more than one equation, with the Kerby-Kirpich method and with the NRCS method.
• Be able to calculate the overland sheet flow travel time for a watershed using the Manning Kinematic equation.
• Be able to calculate the shallow concentrated flow travel time for a watershed using the NRCS method.
• Be able to calculate the open channel flow travel time for a watershed using the Manning equation.
• Know the form of the equation used for rainfall intensity as a function of storm duration for a specified return period.
• Be familiar with methods for using Excel spreadsheets to make the types of calculations discussed in this course.