LESSON 3
CALCULATE RUNOFF FOR SUCCESSIVE AREAS
Critical Tasks: 01-1990-20-1002
OVERVIEW
Lesson Description:
This course describes the procedures used to calculate runoff in successive drainage basins.
Terminal Learning Objective:
ACTION:
Calculate the peak runoff (Q = CIA) for successive areas.
CONDITION:
Given this subcourse, a No 2 pencil, paper, and an ACCP examination response sheet
STANDARD:
Demonstrate competency of the task skills and knowledge by responding correctly to 70
percent of the examination questions.
REFERENCES:
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).
INTRODUCTION
Lesson 2 covered runoff for single areas, both simple and complex. This lesson covers calculating runoff for
successive areas. Often a drainage system will consist of a series of drainage areas, called successive areas, with
upstream areas discharging runoff into lower areas. Due to this action, the runoff accumulates and increases as it
passes through the system.
CALCULATION OF RUNOFF FOR SUCCESSIVE AREAS
Unfortunately, the increase in runoff is not the simple summation of the peak runoff of each individual area.
Various factors, such as storage and peak flow attenuation, act on the individual peak flows. Peak flow(s) from
the upstream area(s) will probably not arrive at the lower outlet simultaneously with the peak from the
downstream dependent area. Hence, the total peak flow is less than the sum of the individual peak flows. To
estimate with precision the quantity of accumulated runoff a procedure has been developed where TOC is
recalculated for each of the successive drainage areas. Naturally, as TOC increases, the rainfall intensity, I,
decreases.
The following terms are used in connection with computing Q = CIA for successive areas:
Independent area -- A single area which does not receive runoff from some upstream area(s).
Dependent area -- An area which receives runoff from upstream area(s).
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