SWMM Code Archive

SWMM 4.30 represented a major advancement over earlier versions, introducing natural channel cross-sections, groundwater quantity simulation, variable time steps, and improved access to long-term meteorological data. Significant adaptations were made for microcomputer environments, including free-format input and improved usability. This version reflects the early evolution of SWMM toward physically based, flexible urban drainage modeling.

Numerical robustness was improved for rounded conduit shapes, along with added safeguards for handling large result files in SWMM 5.0.003. Corrections addressed issues related to RDII processing, temperature file handling, and rule-based weirs, while significant GUI performance optimizations improved editing, graphing, and long-term simulation workflows.

SWMM 5.0.007 update focused on improving hydraulic stability for non-standard conduit shapes and introduced an 'Ignore Rainfall' analysis option. Flow continuity reporting was refined to include ponded volume, and GUI performance was improved for reporting, profile plots, and group editing.

In SWMM 5.0.008 core simulation stability was improved through refinements to dynamic wave routing and pollutant mass balance calculations. Several issues related to infiltration, invert elevations, and unit conversions were corrected, and new summary tables for node inflows and outfall loads were added.

This maintenance release corrected multiple engine logic and routing issues, including RDII event triggering and dynamic routing for force mains and Type 3/4 pumps. Climate file parsing and profile plot display below zero elevation were also fixed in SWMM 5.0.009.

In SWMM 5.0.011, key computational issues related to control rules for weirs and outlets, along with corrections to weir flow calculations and dynamic wave ponded depth handling were resolved. Numerical stability was further improved for pollutant mixing on subcatchments, and GUI fixes addressed problems with Map Label rendering and backdrop file name handling.

SWMM 5.0.012 delivered extensive fixes to flow routing and infiltration stability while introducing a new PID controller and support for absolute elevation offsets. Status reports were expanded, and the GUI gained new tools including measurement, bookmarks, and enhanced status bar displays.

Modeling accuracy was improved in SWMM 5.0.013 through refinement of the PID controller and corrections to several core dynamic wave stability components. Enhancements included improved Froude number weighting, more accurate ponding calculations, and revised flow limit checks. Additional GUI safeguards were added to detect erroneous INI configuration values that could otherwise prevent graphs from displaying correctly.

Core engine logic of SWMM 5.0.014 was revised to improve dynamic wave routing, infiltration, and water quality mass balance. Modeling capabilities were expanded through the addition of a Minimum Slope option, support for culverts, and integration of external time series data. Corresponding GUI updates enabled full support for these features, extended Group Editing to include Snow Pack and Groundwater parameters, and delivered improvements to Profile Plotting and report handling.

Modeling capabilities of SWMM 5.0.015 were expanded with the addition of Green-Ampt infiltration for storage units and a Meander Modifier for transects to improve channel characterization. Critical limitations were addressed by removing the 2-GB output file size restriction, and dynamic wave routing accuracy was refined through improved normal flow checks and flooding calculations. GUI updates added support for the new infiltration and meander features and corrected the pre-run display of conduit offsets in profile plots.

Evaporation modeling capabilities in SWMM 5.0.016 were expanded through the addition of the Hargreaves method, enabling evaporation estimation from temperature data. Corrections addressed RDII inflow calculations and refined dynamic wave logic for ponding and storage nodes. GUI updates added Evaporation Rate as a viewable variable, supported the new evaporation option, and updated storage unit terminology by renaming Shape Curve to Storage Curve.

Continuity performance was improved in SWMM 5.0.017 by correcting the dynamic wave ponding routine, reducing mass balance errors during surcharge conditions. Additional refinements improved RDII time-step logic and the application of Curve Number infiltration capacity. Water quality modeling was strengthened through enhanced mass balance calculations and the introduction of a default dry-weather pollutant concentration, while a usability enhancement added a start-point indicator to the GUI Ruler tool.

In SWMM 5.0.018, reporting capabilities were expanded to include infiltration and evaporation results for storage units, and groundwater state information was added to hot start files for improved restart fidelity. Additional engine refinements strengthened Status Report logic and rain gage error checking, while GUI corrections resolved temporary report file cleanup issues and ensured add-on tools receive complete project input files.

Substantial refinements were applied to the LID module, correcting issues with conductivity definitions and infiltration calculations in SWMM 5.0.022. Overall robustness was improved through additional error checks targeting rainfall data, groundwater inputs, and conduit geometry, while the Pump Summary Report was expanded. GUI enhancements added a map variable for LID area percentage, strengthened LID Editor validation for fractional inputs and total capture area, and corrected issues with saving Time Pattern comments.

Core modeling capabilities were significantly expanded in SWMM 5.0.001 with the addition of Modified Horton infiltration, new Green Roof and Rain Garden LID types, user-defined groundwater outflow equations, and seepage losses for conduits and storage units. RDII workflows were improved through binary RDII file support and refined time-step selection, while hot start files were enhanced to preserve the complete system state. Numerous corrections strengthened snowmelt infiltration, Horton recovery, and hydraulic and mass balance accuracy across conduits, storage nodes, and LIDs. A major GUI modernization via the Delphi XE2 port introduced enhanced editors, expanded reporting, and improved multi-variable plotting.

SWMM 5.1.002 maintenance update fixed a critical issue that prevented projects from reading hot start files created with the latest format. Hydraulic accuracy was improved by refining the dynamic wave surcharge algorithm to exclude ponded area, while the GUI resolved a grid editor memory leak and improved startup window handling.

SWMM 5.1.003 enhanced aquifer modeling by introducing an Upper Zone Evaporation Pattern property and corrected issues with binary RDII flow file read/write operations. Several GUI refactoring issues were also resolved, improving compatibility with non-US Windows regional settings, restoring Group Delete functionality, and correcting form display hierarchy behavior.

Simulation continuity in SWMM 5.1.005 was improved in this release by resolving an issue with reading hydraulic results from hot start files. User interface refinements included an updated Open File dialog and correct enforcement of the read-only Ponded Area property for storage nodes.

SWMM 5.1.006 refined multiple LID processes, correcting issues related to soil water evaporation, permeable pavement infiltration, and an off-by-one error in LID result reporting. User interface fixes also ensured that edits made in the Options and Climatology dialogs are correctly recognized by the application.

New controls for climate adjustment and weir surcharging were introduced in SWMM 5.1.007, including monthly climate adjustment factors and support for GHCN-Daily data. Core fixes corrected Green-Ampt infiltration initialization and refined LID behavior under flooded conditions. The GUI was updated to restore the Object Toolbar and add editors supporting climate adjustments and Green-Ampt seepage loss for storage units.

New modeling capabilities were introduced in 5.1.008, including Rooftop Disconnection LID, monthly adjustments for infiltration and seepage, and the ability to route outflow from both LID drains and Outfall nodes. A major performance enhancement added parallel processing for Dynamic Wave routing, significantly improving execution speed. The engine also refined LID runon distribution, revised pollutant washoff to account for LID flow reduction, and updated mass balance terminology, while fixes addressed hot start reliability, snowmelt behavior, LID continuity errors, and pollutant mass balance for seepage and evaporation in links. The GUI was updated to restore the Object Toolbar, add the Rooftop Disconnection editor, and introduce Threads and Minimum Variable Time Step options.

Engine stability issues were resolved in SWMM 5.1.009, including the elimination of a refactoring bug that limited simulation duration to 68 years, corrections to control rule variable parsing, and fixes to runon reporting for subcatchments fully occupied by LIDs. The GUI was also refined with corrections to the Cross Section Editor, restoring the display of Transect and Shape Curve names, and improvements to default axis scaling for both time series and scatter plots.

New hydraulic and infiltration capabilities were introduced in SWMM 5.1.010, including a Modified Green-Ampt infiltration method and a ROADWAY weir formulation based on FHWA HDS-5 guidance. Engine fixes addressed issues with LID infiltration into groundwater and control rule variable testing, improving simulation reliability. The GUI was updated to support the new infiltration and weir options, add Potential Evapotranspiration as an output variable, and improve automatic plot scaling.

SWMM 5.1.011 extends SWMM’s event-driven modeling capabilities by allowing detailed flow routing to be confined to user-defined Event Periods and by introducing SIMULATION DAYOFYEAR for use in control rules. A related engine fix corrected how link settings are read from hot start files with pollutants, improving restart accuracy. The GUI enhancements include a new Event Editor and expanded Rain Garden seepage configuration options.

In SWMM 5.1.012, hydraulic computation accuracy was improved through corrections to conduit seepage conversion and governing formulations for weirs and culverts. Adjustments to LID processes ensured saturated rain garden units correctly restrict surface infiltration and produce smoother drain flow at shallow depths. Interface corrections resolved Profile Plot title update behavior and enforced zero Rain Garden Storage Layer Thickness.

Modeling capabilities were expanded through the introduction of monthly adjustment patterns for subcatchment properties in SWMM 5.1.013, allowing LIDs to treat pervious area runoff, and adding enhanced LID underdrain controls, including a pollutant removal option. Hydraulic representation was further advanced with the implementation of the Preissmann Slot (SLOT) surcharge method and a new Storage Unit Surcharge Depth parameter. Corresponding GUI updates added editors supporting all newly introduced features.

Model reliability was enhanced through comprehensive engine fixes, including corrections to shared rainfall time-series handling, elimination of LID-related execution failures, and resolution of errors in [REPORT] section parsing. Additional corrections in SWMM 5.1.014 addressed LID underdrain pollutant loads, stalled LID outflows, and inaccuracies in conduit seepage and evaporation calculations.

Modeling flexibility and physical consistency were improved by allowing mixed infiltration methods within a single project and ensuring that monthly depression storage adjustments apply only to pervious areas of subcatchments. Additional engine corrections in SWMM 5.1.015 resolved a mass balance error in shallow storage units and eliminated spurious groundwater summary values for fully impervious subcatchments.

Street runoff modeling was expanded with new Street, Inlet, and Inlet Usage objects, along with a dedicated Street Summary Report in SWMM 5.2.0. Engine enhancements added a Type 5 Variable Speed Pump, predefined Storage Curve shapes, and more flexible control rules using mathematical expressions and antecedent rainfall. Additional improvements included support for updated NOAA GHCN climate data, fixes to Curve Number runon and Green-Ampt validation, and corrected handling of LID drain pollutant loads, improving both hydraulic and water-quality results.

In SWMM 5.2.1, greater control and reliability were introduced for dynamic wave routing by making the Normal Flow Limited option configurable. Engine corrections resolved a refactoring issue that caused excessive runtimes in projects with control rules, refined egg-shaped cross-section geometry at shallow depths, and ensured dry node pollutant concentrations are no longer forced to zero when non-zero inflow is present.

Critical issues affecting control rule math expressions were corrected by fixing an error in exponent evaluation, preventing inaccurate simulation behavior. Additional engine improvements of SWMM 5.2.2 eliminated excessive runtimes when simulations exceeded the duration of an external time series, restored important warning messages, and corrected storage unit percentage loss reporting, improving both stability and result clarity.

Water balance accuracy for green roof LID units was improved by correcting an error that caused the initial moisture volume in the drainage mat layer to be double counted during simulation. This fix in SWMM 5.2.3 ensures more reliable representation of green roof hydrologic performance.

Result consistency was improved by correcting a mismatch between pollutant surface runoff and wet-weather inflow mass in the Status Report and fixing LID underdrain input validation. Additional corrections in SWMM 5.2.4 refined water flux calculations within LID layers, resolved issues with Street cross-sections (including depressed gutters), and improved how conduit evaporation and seepage losses are applied during dynamic wave routing.

Utility for converting SWMM 4 data files to SWMM 5 files is an official EPA tool for converting legacy SWMM 4 models into SWMM 5 format. It allows users to migrate models built with Runoff, Transport, and EXTRAN modules into the updated SWMM 5 environment. The tool preserves existing modeling work and reduces the need to rebuild older projects, ensuring continuity across evolving stormwater modeling workflows.