Stopping Tolerance in InfoSWMM, H2OMAP SWMM and SWMM5 Internal Units

Stopping Tolerance in InfoSWMM, H2OMAP SWMM and SWMM5 Internal Units

InfoSWMM, H2OMAP SWMM and SWMM 5 share the same underlying dynamic engine code but one small difference is that InfoSWMM and H2OMAP SWMM allows the user to select the node stopping tolerance instead of always using the default SWMM 5 stopping tolerance of 0.0005  feet.  SWMM 5 uses internal units of feet and shows the output in meters if you are using SI units, as does InfoSWMM and H2OMAP SWMM.  The following table shows how the stopping tolerance translates to inches and millimeters in the engine of a US and SI model.   The smaller the tolerance the larger the number of iterations used during the simulation but using a very small tolerance does not always mean a better simulation.  If possible, for example, with pumps it is better to use a small time step and a medium level tolerance – for example 1 millimeter is a good starting  value, but maybe 2 or 3 millimeters may help if you have a continuity error at a pump node. 

The nodes are considered converged if the depths between successive iterations is less than the stop tolerance of the program (the default stop tolerance is less than the stopping tolerance (Figure 1)

Stopping Tolerance	Inches	Millimeters
0.1000000	1.2000000	30.4800000
0.0500000	0.6000000	15.2400000
0.0100000	0.1200000	3.0480000
0.0050000	0.0600000	1.5240000
0.0001000	0.0012000	0.0304800
0.0005000	0.0060000	0.1524000
0.0000100	0.0001200	0.0030480
0.0000500	0.0006000	0.0152400
0.0000010	0.0000120	0.0003048
0.0000050	0.0000600	0.0015240
0.0000001	0.0000012	0.0000305

Figure 1  If the node depths between successive iterations are less than the stopping tolerance then the node is considered to be converged.