Changes from SWMM Version 4 to XPSWMM

 Changes from SWMM Version 4 to XPSWMM

The transition from SWMM Version 4 to XPSWMM (extended and enhanced SWMM) introduced numerous improvements, bug fixes, and new features. These updates significantly expanded the capabilities of the software, including enhanced stability, improved accuracy, and more flexible user options for stormwater modeling. Below are the main changes and enhancements made from SWMM Version 4 to XPSWMM:

1. Bug Fixes:

• Several hundred bug fixes were implemented to address known issues in the original SWMM code, improving the overall stability and reliability of the program.

2. Vertically Differentiated Roughness:

• Roughness in conduits is now differentiated vertically. Each conduit can have two regions of roughness:
  • Higher roughness is applied when the flow depth is less than a predefined threshold.
  • Normal roughness is applied when the flow depth is above that threshold.

3. Shock Losses in Flow Transitions:

• Shock losses have been added to model the transition from subcritical flow to supercritical flow, providing more accurate flow modeling in these conditions.

4. Side Flow Weir Equations:

• The approach velocity in the side-flow weir equations is now properly accounted for, improving the accuracy of flow predictions in weirs with side flows.

5. EXTRAN Simulation Linking:

• Two EXTRAN simulations can now be linked via a stage boundary condition, allowing the time history of stage and flow from one simulation to be connected to the stage and flow history in another EXTRAN simulation.

6. Transition Between Open Channel and Pressure Flow:

• The transition between open channel flow and pressure flow at weirs has been enhanced to improve the stability of the flow solution, particularly in systems with varying flow conditions.

7. Demand Curve for Outfalls:

• A demand curve for outfalls, or a Q(t) boundary condition, has been added, allowing for more flexibility in modeling the flow and depth at outfalls.

8. Floodplain Modeling for Natural Channels:

• Natural channels now automatically include floodplain modeling to account for flows above the maximum channel depth, improving the model's ability to simulate realistic floodplain dynamics.

9. Ponded Water at EXTRAN Nodes:

• Ponded flood water at EXTRAN nodes can optionally be returned to the main system, providing more flexibility in the modeling of stormwater systems with ponded areas.

10. Enhanced Stability in EXTRAN:

• EXTRAN stability has been greatly enhanced, particularly in the solution of conduit flows and junction depths, making the model more robust and reliable for large and complex networks.

11. Flow Losses in Conduits:

• Expansion and contraction losses due to changes in cross-sectional area are now simulated in conduits, improving the accuracy of flow predictions in variable-section conduits.

12. Entrance/Exit Losses at Junctions:

• Entrance and exit losses at junctions are now simulated in conduits, providing more accurate flow modeling at these locations.

13. Outfall Depth Calculation:

• The depth at an outfall can now be calculated in three different ways:
  • Fixed depth at the normal depth (especially useful for natural channels).
  • Fixed at the critical depth.
  • Minimum of normal or critical depth, based on conduit flow.

14. Rating Curve Boundary Conditions:

• Rating curve boundary conditions for outfalls (Q(h)) have been added, providing greater flexibility in modeling outfalls with variable flow conditions.

15. Irregular Shaped Closed Conduits:

• Support for irregularly shaped closed conduits has been added, allowing for more complex conduit geometries in the stormwater model.

16. Water Quality Modeling in EXTRAN:

• Water quality modeling in EXTRAN has been started, enabling the model to simulate pollutant transport and water quality along with hydraulic flows.

17. User-Defined Weir Exponents:

• User-defined weir exponents allow for more flexibility in modeling weir flow, enabling users to customize the behavior of weirs based on empirical or site-specific data.

18. Weir Lengths as a Function of Depth:

• User-defined weir lengths that vary as a function of depth have been introduced, providing more realistic simulations for weirs with varying geometry.

19. Weir Rating Curves:

• Weir rating curves have been added, allowing users to define the flow through weirs based on depth.

20. Single Solution for EXTRAN:

• XPSWMM uses a single solution method that merges the previous solutions in EXTRAN 4, which helps reduce the complexity and maintenance required by the multiple solutions in older versions of SWMM.

21. Pollutants:

• The number of pollutants has been increased to 20 across all modules, with support for routing and tracking of these pollutants throughout the system.

22. Land Use Modeling:

• The number of land uses has been increased to 10, with the ability to route flow through all conduits, not just through specific types.

23. Storage/Treatment (STP) and BMP Integration:

• The Storage/Treatment module has been integrated with the Transport module, allowing for multiple STPs and BMPs within a single network, and no longer constraining them to the outlet of the system.

24. User-Defined Pollutant Removal Equations:

• User-defined pollutant removal equations have been introduced, allowing users to define custom equations for pollutant removal based on site-specific data or treatment processes.

25. Flexible Time Units:

• Time units in XPSWMM have been made more flexible, enabling better customization of time steps and durations in the simulation.

26. Enhanced Interface for Modules:

• Utilities (e.g., rainfall, temperature, wind, statistics) have been integrated within the overall interface, improving user experience and making it easier to manage multiple types of data.

27. SCS Hydrograph Generation:

• The SCS method of hydrograph generation has been provided, enabling the model to generate runoff hydrographs based on Soil Conservation Service guidelines.

28. EMC (Event Mean Concentration) Method:

• The EMC method is now available for pollutant generation, allowing users to model event-based pollutant concentrations based on observed or estimated values.

29. Flow Divide and Flow Paths:

• Flow divides now have explicitly defined flow paths, making it easier to track and model the flow distribution in systems with complex flow-dividing elements.

30. Flexible Pollutant Characteristics:

• Pollutant characteristics are now local, meaning that users can assign different characteristics to pollutants at different locations in the system, allowing for more detailed modeling.

31. Improved Hydraulic Elements:

• The number of hydraulic elements has been increased to 26 across all modules, improving the model’s ability to handle complex networks and diverse hydraulic conditions.

Conclusion:

The move from SWMM Version 4 to XPSWMM introduced substantial improvements in the flexibility, stability, and accuracy of the model. New features like the ability to model irregularly shaped conduits, user-defined pollutant removal and weir rating curves, and enhanced EXTRAN simulations significantly expanded the modeling capabilities of the software. Additionally, the integration of pollutant routing, land use modeling, and storage/treatment options made XPSWMM a much more comprehensive tool for simulating both hydrology and water quality in stormwater systems.