Surge analysis with SWMM
I am emailing a response to this older thread as the older thread does not mention the new Slot method in EPASWMM. In particular Mitch's comment specifically mentions the older Extran method which has been around since 1980 at least. This comment updates the topic. Here is the older thread which was last updated in 2018 https://www.openswmm.org/Topic/11269/surge-analysis-with-swmm
As always from the very good to great EPASWMM Help file "Surcharge Method Options - Selects which method will be used to handle surcharge conditions. The Extran option uses a variation of the Surcharge Algorithm from previous versions of SWMM to update nodal heads when all connecting links become full. The Slot option uses a Preissmann Slot to add a small amount of virtual top surface width to full flowing pipes so that SWMM's normal procedure for updating nodal heads can continue to be used."
Here is a link to another good thread about the new Preissmann Slot method from earlier in 2019.