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 often involve determination of the design rainfall intensity and the time of concentration of the watershed as well. Example calculations and examples using an Excel spreadsheet for Rational Method equation calculations and for 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. This course is intended primarily for civil engineers, hydraulic engineers, highway engineers, and environmental engineers. After completing this course you will be able to calculate peak storm water runoff rate using the Rational Method equation in either U.S. or S.I. units, including determination of runoff coefficient, calculation of time of concentration, and determination of design rainfall intensity. You will also be prepared to use Excel spreadsheets to efficiently make the calculations.

Learning Objectives

• 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 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 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 able to use an Excel spreadsheet to make the types of calculations discussed in this course.