You want to design a storm water management system. What you usually get is a rainfall-runoff model. In this specific subfield, the model is SWMM. EPA SWMM contains many features that were implemented to simulate urban storm water depletion network. Meaning that there are specific model's parameters set for that, and that a community gathered around this tool.
However SWMM is not a system for designing sewers. Designing requires that you repeat the modeling actions several times. At the outlet of any pipe, you have to:
- estimate the runoff under a "design rainfall" coming from some intensity-duration-frequency curves.
- get the maximum discharge with an assigned return period (say 10 years)
- Use simplified hydraulics for obtaining the size of the pipe apt to contain the maximum discharge
- repeat the operation for the pipes downhill, without leaving out uphill branches.
Operation 2 above requires a search algorithm to find the rainfall duration that is responsible for the maximum discharge. The complete theory is in Rigon et al., 2011.
The point is that SWMM does not do the sequence of operation above. This is one of the reasons we implemented JSWMM. To see what it does, click on the figure.