The Storm Water Management Model (SWMM), currently in version 5.1, is a dynamic rainfall-runoff model that is often used to estimate runoff quantity and quality, primarily in urban areas. The model has an unsteady hydraulic algorithm based on an EXTRAN module that, due to its link-node solution, it is adequate for gradual filling scenarios but it does not represent correctly extreme situations such as rapid fillings associated with intense rain events. However, choosing an appropriate time step and artificial spatial discretization can improve significantly the SWMM capability of modeling such conditions. In 2002, during the development of SWMM5, a rigorous Quality Program was established to ensure that the results of the new version are compatible with the previous one (SWMM 4.4). The QA testing procedure involved running 20 situations for both versions with identical set ups and then comparing its results, particularly the flow continuity error. This QA study provided an excellent dataset to assess the advantages of artificial discretization in SWMM5 and to evaluate the potential improvements in SWMM 5.1 with the newly implemented surcharge algorithm based on the Preissmann Slot. Additionally, a time-step analysis was executed for each model using both time-step recommended by EXTRAN (1988) and Vasconcelos et al. (2018).
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