# Rational Method for Pipe Design

Written by sachiko schott
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The Rational Method is an equation used to calculate runoff . The equation can be used when designing storm sewer pipes, because the calculation tells the designer how much water the pipes will have to carry.

## The Rational Method and Runoff

The equation for the Rational Method calculates the maximum rate of runoff using contributing basin area, the runoff coefficient, time and rainfall intensity. This equation is a simplification of actual conditions, because it assumes that rainfall intensity is constant for the duration of the storm and that there is no storage of the runoff.

## Applying the Rational Method to Pipe Design

The Rational Method can be used to determine what size pipe is required to carry runoff away from the drainage area. For example, in storm sewer design, the Rational Method can determine the required pipe diameter and slope by calculating how much water the pipe will need to accommodate.

## Limitations of the Rational Method

The Rational Method is simplistic and is usually invalid in the case of large drainage areas, according to the Hydraulic Design Manual. However, the Rational Method is considered a useful tool when planning storm sewers for a small drainage area.

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