What is the difference between the available options for the Surcharge Method calculation option when using the Explicit (SWMM) solver? - Preissmann slot method and Extran method?
When using the Explicit (SWMM) solver there are two choices for the Surcharge Method, for handling surcharged conditions; Slot and Extran. The Slot method is based on Preissmann slot methodology whereas Extran is based on the original SWMM methods.
Excess water at a junction can become partially pressurized while connecting conduits are surcharged and can either be lost from the system or be allowed to pond atop the junction and subsequently drain back into the junction.
The Preissmann slot method is used for simulating pressure or surcharged flows for gravity conduits, by adapting the conceptualization of pressurized flow to fit a free surface model. The slot extends vertically from pipe crown to infinity and over the entire length the pipe, and the width of the slot is usually 1% of the characteristic pipe dimension (diameter for a circular pipe) but not large than 0.02 ft. Read more about this in help documentation - search for "Pressure (Surcharged) Flow and Overflow (Street Flooding)".
EXTRAN is the original SWMM method to simulate pipes under pressurized conditions. When the nodes at both ends of a pipe have a water stage higher than the top of the pipe at that node, EXTRAN switches to equations for pressurized pipe flows.
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"Depth (Surcharged)" vs. Ponded Area surface storage