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)