Dual Drainage Examples

In this article we will use a few examples to understand how dual drainage work.

You can find the model on GitHub.

A simple dual drainage model,

  • A constant 3% slope stretch of street with sewer pipe underneath, the street is 20ft wide and 1ft tall, the sewer pipe is 1.5ft in diameter

Base Scenario: No inlets with 15cfs constant flow at upstream node (N1)

Without inlets, ,

  • Flow will fill in N1 and start to flow in the sewer pipe down stream
  • Since there is no surcharge, the flow never reached the street level

Apparently, this is not the desired behavior of a dual drainage system.

Scenario 15cfs with inlet 3cfs max flow

In this scenario,

  • 15cfs is loaded at N1
  • All the inlets have an max. capture of 3cfs
  • For N1, 3cfs goes into the sewer pipe, and 12 cfs goes down the street
  • At N2, another 3cfs goes into the sewer pipe, and 9 cfs goes down the street, etc.

The final results are shown below. Each time an inlet is passed, 3cfs is transferred from the street to the sewer pipe.

Note: ##.1 is the sewer pipe, ##.2 is the street, link 31->32..->36

Flow in the dual drainage system (cfs)

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