Product:  Autodesk SSA  |   Subject: Storm and Sanitary Analysis

In this exercise, we will learn about designing stormwater sewers using the Rational Method 

Rational Method Storm Sewers Design (Peak Flows)

As we already mentioned, we have two main requirements for the design of our stormwater management system.

1. Design the storm sewers, which are the underground pipes and structures. Generally, the requirements are that no flooding or surcharge should occur during the 10-year storm events
2. Design the detention systems in a manner that the post-development flow is equal or less than the pre-development flows during the 100-year storm event.

First, let’s see how to design and meet the first requirements.

Introduction to the Rational Method in SSA

The most important requirement, when designing the storm sewers, is to use the Rational Method, not the Modified version, but the original one. The reason is simple. The Rational Method is a peak flow method. It gives the most flow rate because it combines peak flows of all catchments. While the extended flow methods (Modified Rational, SWMM, HEC-1, and others) also combined all catchments, but they use a hydrograph routing with peak flows occurring at different times for individual catchments.

An ancillary benefit of the Rational method is that it is very simple and can be used with a simple calculator or spreadsheet.

Its Q=CIA formula is simple, and all we need is the following information.

    A the Subcatchment area.

    C the runoff coefficient, and

    I the rainfall intensity, which we can obtain from IDF or Intensity-duration-frequency curves.

Let’s start the storm sewer design using the Rational Method.