How to get the peak flow in/out of a pond


For a node with multiple incoming and outgoing links, getting the peak flow in and out of the node can be a little tricky for new modelers. In this article, we’ll explain how we should calculate the peak flows.

  • the peak flow is not the sum of the peak flow of the incoming pipes
  • the peak flow into the node is not the same and the peak flow out of the pipe

Steady state model

For a steady state model, the flow doesn’t change over time. This is the type of hydraulic analysis that can be easily done with a pencil, and commonly used in sewer and other hydraulic designs.

Unsteady state model

Steady state model is very simple, and in many situations can be easily solved with a pencil. However, it has some serious limitations when it comes to evaluating sewer capacities. Steady state model tends to oversize and ignores storage. A few obvious reasons:

  • the timing of the peak in different sewer branches can be different, therefore, the peak flow in the trunk line is usually less than the sum of the peak flow of all upstream branches
  • steady state ignores the storage of a sewer system, since the level doesn’t change over time, no storage of the system will be utilized
  • flow through pipe will attenuates, therefore, lower the peak and change the timing of the peak downstream

Unsteady state model with multiple inlets and outlets

Here is a more realistic example to understand the benefits of an unsteady state model. A model with multiple inlets and outlets for a storage node. The question is what is the peak flow in and out of the storage node.

  1. inflow pipes show no delay for the flow to reach steady state
  2. it takes the outlet pipes sometime to show flow before the storage is filled up, after that, they both reached steady state
  • Inflow =5+5= 10 cfs
  • Outflow=6.2+3.8=10 cfs
  • The two graphs on the left are the flows in the inflow pipes, both has 5cfs as peak flow, but one peaked at hour1, and another at hour 3
  • The two graphs on the right are the flows in the outflow pipes.
  1. It takes some time for the storage to fill up
  2. The first peak from inflow1 showed up
  3. The second peak as a results of inflow2+inflow1 showed up
  • Inflow = 6.66 cfs (10cfs if we sum the peak)
  • Outflow = 6.64 cfs (6.7cfs if we sum the peak, for this one is actually quite close, because the timing of the peak is the same for both outlet pipes)

Extract values

Depending on the software you are using, getting the peak values can be different. Some software packages report the routing flow time series in and out of a node, which will make it much easier to get the peak values. For the software that doesn’t report such data, flow time series will need to be exported for additional calculations.


Since XPSWMM doesn’t report node inflow/outflow time series, the method shown above will be needed to get the peak flow in/out of a node. Refer to this article for more information on extracting time series out of an XPSWMM model.


SWMM5 reports total inflow as time series, therefore, it can be directly accessed in the model.


ICM doesn’t report total inflow. However, ICM has more tools to export time series out of the model.

  • We can plot all the inflow time series, then export the data as a csv file
  • Or we can export all the time series using the CSV export tool.


ICM SWMM5 works very similar to ICM.



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