A large difference between SWMM 5 and SWMM 4 is how the Groundwater Aquifer interacts with the drainage network. In SWMM 4 since the hydrology was simulated in the Runoff Block, the results saved to an interface file and the hydraulics were simulated in the Extran Block it was not possible to have a time step to time step interaction between the Aquifer and the Open Channels. SWMM 5 has integrated hydrology and hydraulics so it is possible to use either a Fixed Surface Water Depth for each Subcatchment or the Receiving Nodes Node Depth Invert Elevation – the Aquifer Bottom Elevation. The groundwater thus flows either to a fixed boundary condition as in SWMM 4 or to a time varying boundary condition.
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