Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Rational Method
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RATIONAL METHOD
TABLE OF CONTENTS
3 RATIONAL METHOD
3.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.2 RATIONAL EQUATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.3 ASSUMPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6
3.4 VOLUME CALCULATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
3.5 LIMITATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
3.6 APPLICATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
3.6.1 Peak Discharge Calculation ........................................................................ 3-7
3.6.2 Multiple Basin Approach ............................................................................. 3-8
3.1 GENERAL
The Rational Method was originally developed to estimate runoff from small areas and its use
should be generally limited to those conditions. For the purposes of this manual, its use should
be limited to areas of up to 160 acres. In such cases, the peak discharge and the volume of runoff
from rainfall events up to and including the 100-year, 2-hour duration storm falling within the
boundaries of the proposed development are to be retained. This is the required minimum criteria
for unincorporated areas of Maricopa County. If the development involves channel routing,
the procedures given in Chapters 4 through 6 should be used, since the peak discharge generated
by the Rational Method cannot be directly routed.
3.2 RATIONAL EQUATION
The Rational Equation relates rainfall intensity, a runoff coefficient and the watershed size to the
generated peak discharge. The following shows this relationship:
Q =
CiA
where:
(3.1)
Q = the peak discharge, in cfs, from a given area.
C = a coefficient relating the runoff to rainfall.
i = average rainfall intensity, in inches/hour, lasting for a T c .
T c = the time of concentration, in hours.
A = drainage area, in acres.
August 15, 2013 3-1