Stopping Tolerance in InfoSWMM, ICM SWMM and SWMM5 Internal Units

 Stopping Tolerance in InfoSWMM, ICM SWMM and SWMM5 Internal Units

InfoSWMM, ICM SWMM, and SWMM 5 all operate on the same core dynamic wave engine for hydraulic simulations. However, there's one nuanced difference:

• Node Stopping Tolerance Adjustment:
  • SWMM 5: Uses a fixed default stopping tolerance of 0.0005 feet for all simulations.
  • InfoSWMM and ICM SWMM: Allow users to customize this tolerance, providing more control over simulation accuracy and performance.

Unit Conversion and Impact:

• Units in SWMM 5 and InfoSWMM:
  • Both use feet internally for calculations but can display results in meters if SI units are selected. This consistency means that the stopping tolerance is always considered in feet for computation but can be interpreted in other units for user convenience.

Understanding Stopping Tolerance:

• The following table shows how the stopping tolerance translates between different units:
  
  

  Tolerance (feet)	Tolerance (inches)	Tolerance (millimeters)
  0.0005	0.006	0.1524
  0.001	0.012	0.3048
  0.002	0.024	0.6096
  0.003	0.036	0.9144

• Impact on Simulation:
  • Smaller Tolerance: Leads to more iterations, potentially increasing simulation accuracy but also computational time.
  • Larger Tolerance: Might reduce accuracy but speeds up the simulation.

Best Practices for Tolerance Setting:

• General Rule:
  • A smaller tolerance isn't always better; it depends on the context of the simulation.
  • For simulations involving pumps, where precision in flow control is critical, a strategy might involve:
    • Using a small time step to capture rapid changes.
    • Employing a medium level tolerance like 1 millimeter (0.00328 feet) as a starting point.
• Adjusting for Continuity Errors:
  • If continuity errors occur at pump nodes, slightly increasing the tolerance to 2 or 3 millimeters might resolve issues without overly compromising simulation fidelity.

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