Water that falls close to the culvert will arrive at the outlet sooner; rain that falls at the farthest reaches of the
basin will arrive later.
For discharge at the outlet to be maximum, the runoff closest to the culvert must continue to arrive at the same
time runoff from the farthest (with respect to time) point of the watershed reaches the outlet.
Consequently, to get maximum runoff rate, the steady, uniform rain must last at least as long as the time it takes
water to flow from the farthest point of the watershed to the outlet.
The time it takes for water to move from the most remote part of the basin to the outlet is a very important
parameter in the Rational Method called the TOC.
The Rational Method is used only for small drainage basins (areas less than or equal to 1,000 acres). This is
because the idealized storm in the Rational Method approximates thunderstorms. These types of storms are small
in area, but move rapidly through an area.
The Rational Method should be used for areas without active streams. Also, the Rational Method should not be
used if there are ponds or lakes within the watershed. There are several reasons for this. Streams flowing through
the watershed greatly complicate the runoff estimation process, and are, therefore, beyond the scope of the
Rational Method. The Rational Method does not consider storage from lakes or ponds.
The Rational Method should be used only if the basin has a regular shape. If the basin is not of a regular shape,
then your TOC assumptions may not be accurate.
Sound judgement must be used to determine if the basin is regular in shape.
Before you use the Rational Method, it is important to remember that it is a standard design method for relating
probable precipitation over a small watershed to the probable maximum runoff rate at the outlet of the watershed.
It is a simple approximation of a very complex process and it cannot be regarded as a precise forecast. In reality,
uniform rainfall does not occur, TOCs do not always match peak flow at the outlet of the basin, and the
coefficient of runoff isn't really constant in time, even within the same storm. The method's accuracy cannot be
verified because the idealized storm does not exist and we don't have a lengthy record of measured precipitation
and runoff to verify the method statistically. In short, the Rational Method is not cut and dried. It will require
However, the Rational Method is simple to apply; it gives consistent results, and most importantly, it is the
standard method adopted by most state highway departments and the US Army.
RATIONAL METHOD DESIGN PROCEDURES
Use the following steps when doing design procedures:
STEP 1. Delineate the drainage basin. The area of rainfall contribution to your structure must be defined by
delineation. Instructions for delineation are found in Lesson 1, Part B.