Typically, in flow routing, when the flow at a junction exceeds the system's ability to carry it further downstream, the excess volume overflows the system and is lost. There is an option under (Swmm / General Options / Allow Flooding) to have excess volume stored on top of the junction instead, in a way accumulated, and be reintroduced into the system as capacity permits. In Kinematic Wave flow routing, accumulated water is simply stored as excess volume. For Dynamic Wave routing, which is influenced by water depths held at the nodes, the excess volume is considered as a lake over the node with a constant surface area. This amount of surface area is an input parameter given to the join.
Alternatively, the user may wish to represent the surface overflow system explicitly. In open channel systems, this can include road overflows at bridge or culvert crossings, as well as additional floodplain storage areas. In closed conduit systems, surface overflows can be carried via streets, alleys or other surface routes to the next available stormwater intake or open channel. Overflows can also be trapped in shallow depressions such as parking lots, backyards or other areas.
In sewer systems with pressurized pipes and power lines, the hydraulic head at the junction nodes can sometimes exceed the ground elevation under Dynamic Wave routing. This would normally result in an overflow which, as described above, can either be lost or accumulated. SWMM allows the user to specify an additional "surcharge" depth at the junction nodes which allows them to pressurize and prevents any outflow until this additional depth is exceeded. If both lagoon and pressurization are specified for a node, the lagoon takes precedence and the overflow depth is ignored. Neither accumulation nor pressurization apply to storage nodes.