Critical Tasks: 01-1990-20-1002
This course describes the proper procedures used to determine runoff using the Rational Method of runoff
TERMINAL LEARNING OBJECTIVE:
Calculate runoff using the Rational Method.
You will be given the subcourse, a pencil, a ruler, a calculator, and an ACCP examination
Demonstrate competency of the task skills and knowledge by responding correctly to 70
percent of the examination questions.
The material contained in this lesson was derived from the following publication: FM 5-
430-00-1/AFPAM 32-8013, Volume 1 (to be published within the next six months).
In Lesson 1 you learned some basic skills to help you solve drainage design problems. In Lesson 2 you will learn
the steps used in the Rational Method. The Rational Method is a procedure used by most state highway
departments and the military to estimate the peak flow or discharge for small watersheds. However, use by most
highway departments and the Army, the Rational Method is not an actual prediction of rainfall. You cannot use
data from an actual rainstorm and estimate the actual amount of runoff that will accumulate at your basin outlet.
The Rational Method is a design procedure only.
PART A - CALCULATE RUNOFF USING THE RATIONAL METHOD
The Rational Method is to be used only for estimating peak runoff to design a structure for a desired estimated
lifetime, based on watershed characteristics and statistically probable rainfall.