SWMM 5 Simplification
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This procedure automates simplification of a SWMM5 model network by removing junctions that connect exactly two conduits where certain criteria are satisfied. The lengths of the conduits on either side of the deleted junction are added, and the properties of the upstream conduit are retained, with the exception of the downstream offset, which is obtained from the most downstream deleted conduit. This procedure must be used with care, as it does not check constraints such as differing Manning's Ns, presence of control rules, etc. An iterative process is used. The program selects a junction at random from among junctions meeting criteria for removal. It merges two pipes and then randomly selects another eligible junction. The process is repeated until no candidate junctions exist. The procedure does not remove any junction with fewer or more than one conduit connection, any storage junction, or any junction connected to a weir, pump, orifice, or multi-barrel conduit.


The user may optionally populate and edit the JunctionCodes table in the \NetSTORM\SWMMSimplification.mdb database. The program reads this table during the Identify step. Any junction listed that is not marked InvertVerified will be assigned high priority for merging. High merge priority is also assigned to any junction with invert less than -900.



Merge Rules

Height tolerance - Conduits with heights differing by more than this ratio will not be merged. Note that the program does not check conduit width or Manning's N. The comparison is strictly based upon the height parameter. E.g. a ratio of 0.4 would allow a 4 ft pipe to be merged with a 3 ft pipe but not with a 6 ft pipe.

Minimum junction depth (ft or m) - Junctions with less clearance between rim and invert will not be merged. This facilitates retention of potential ground flooding locations in the simplified model.

Maximum conduit length (ft or m) - Merged conduits will not exceed this length. 

Preserve all shape changes - checking this box ensures that conduits of different shapes will not be merged. E.g. a 4 foot circular pipe might otherwise be merged with a 4-foot high box culvert.

Preserve catchment inlets - checking this box ensures that subcatchment inlet junctions will not be eliminated. If this box is not  checked, the procedure will re-route deleted inlet loads to the next downstream junction.





Reference Table



Junctions / Links

removed during simplification



high priority removal candidate (Invert<-900 or unverified)


Junctions / Links

removal candidate



storage junctions and outfalls



pumps, weirs, and orifices



connected to weir, pump, orifice, or multi-barrel conduit



does not have exactly one upstream and one downstream link



pipe size changes exceeds user-specified tolerance



shape transition



user-specified shallow junctions unless sealed



catchment inflow location



connected to long pipe


The procedure re-writes the [JUNCTIONS], [CONDUITS], [XSECTIONS], [DWF], [LOSSES], [INFLOWS], and [SUBCATCHMENTS] sections of the input file. Any comments in these sections are lost. 


[DWF] loads are consolidated at the next downstream junction. Only one flow pattern is preserved though. 

[SUBCATCHMENT] loads to deleted nodes are shifted the next preserved downstream junction.

[INFLOWS] are consolidated at the next downstream junction. Scale Factors and Baseline values are summed. In some cases this may yield inappropriate results!

[CONTROLS] are not checked.


The [LABELS] sections is amended with the deleted junctions.

The [VERTICES] section is replaced with vertices of deleted junctions. Any existing vertices are deleted.


The program has received minimal testing on open channel models.