Observations on Surcharge Behavior: Comparing EXTRAN and Preissmann Slot Responses in Force Mains

In SWMM5 hydraulic computations, once a node becomes surcharged, the method of updating the hydraulic head at the node is replaced with a separate surcharge algorithm. The EXTRAN surcharge method is the default method used by SWMM5 which involves a node continuity approach. Under some conditions, the EXTRAN method was found to cause numerical instability during the transition from non-surcharged to surcharged flow. Later SWMM5 versions introduced the Slot method which allowed using the original free surface flow algorithm via the addition of an artificial Preissmann slot at the top of the conduit as an alternative to EXTRAN. This presentation shares some key observations from a series of simple tests conducted on force mains using the SWMM 5.2.4 engine and its two options for surcharge methods. These results were further compared with equivalent EPANET models.

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