Rational Equation Calculator
Compute peak discharge from a drainage basin using the Rational Equation Method
Units in Rational Equation calculation: ft3=cubic foot, m3=cubic meter, mm=millimeter, s=second
Rational Method Equation
Rational Equation: Q=ciA
Note that our calculation allows you to use a variety of units.
The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. A simplified table is shown below. See the references at the bottom of the page for more complete tables including impact of slope.
The Rainfall intensity (i) is typically found from Intensity/Duration/Frequency curves for rainfall events in the geographical region of interest. The duration is usually equivalent to the time of concentration of the drainage area. The storm frequency is typically stated by local authorities depending on the impact of the development. A 10-yr, 25-yr, 50-yr, or even 100-yr storm frequency may be specified.
Error Messages given by calculation
Chow, Ven Te, David R. Maidment, and Larry W. Mays. 1988. Applied Hydrology. McGraw-Hill.
Corbitt, Robert A. 1999. Standard Handbook of Environmental Engineering. McGraw-Hill. 2ed.
Lindsley, Ray K., Joseph B. Franzini, David L. Freyberg, and George Tchobanoglous. 1992. Water-Resources Engineering. McGraw-Hill. 4ed.
McCuen, Richard H. 1998. Hydrology Analysis and Design. Prentice-Hall. 2ed.
Singh, Vijay P. 1992. Elementary Hydrology. Prentice-Hall.
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Time of Concentration