Surchage method |
From the help file (below) and a few past threads that should explain the difference between modeling surcharge with a Preissmann Slot and the original Extan3, SWMM4, SWMM5 Surcharge Algorithm:
https://www.openswmm.org/Topic/4583/manhole-surcharge-calculations
https://www.openswmm.org/Topic/6551/solution-of-saint-venant-equation-without-preissmann-slot
https://www.openswmm.org/Topic/14231/why-do-junction-nodes-sometimes-have-strange-maximum-depth-results
Surcharge Method (Help)
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.
EXTRAN is the classical way SWMM used 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.
The slot option is an implementation of the Preissmann slot. Preissmann observed that the shallow water equations used for simulating open channel flows and the flow equations for pressurized pipes are similar. He proposed to attach a thin slot at the top of a pipe. This slot always provides an open surface to the pipe. Then pipes can still be calculated using the shallow water equations. The water stage in the slot represents the pressure in the pipe.
The above mentioned for EXTRAN refers to pipes connected to junctions. If pipes are connected to storages, EXTRAN always assumes pipes to have a relatively large open surface area. For this case the slot option is theoretically more accurate.
Personally I prefer to use EXTRAN, even when pipes are connected to storages, since the slot option showed more wiggles in my results. You can simply test both options and see what you like more. Both methods simulate pipes under pressurized conditions.
Complementing the previous discussion: In our evaluation, both EXTRAN and SLOT approaches can have varying degrees of accuracy depending on whether there's additional discretization between two hydraulic structures (i.e. dummy nodes), the time step size, whether the inertial terms are kept in the full dynamic equation, and even the head convergence criteria. For pressurized flows, however, with adequate time step selection (typically small routing time steps), SLOT has been shown to be more accurate for fully-pressurized unsteady flow simulations.