WinSLAMM Development and Release Notes

For a complete list of program modifications from v10.1 to v10.4.1 see WinSLAMM Updates. For a complete list of updates prior to v10.1, see the WinSLAMMLog.txt file for details.

Upcoming Features

WinSLAMM is continuously under development. Below are some of the planned modifications for upcoming versions of the model.

  • Leaf Pick-Up Program Quantification
  • Phosphorus Seasonality
  • Ability to import database information to create model files (Additional interaction with ArcSLAMM and ArcSLAMM Plus)

Is there something you'd really like to see integrated into the program? Email Caroline with suggestions for enhancements.

  • Added Wet Detention Pond design tools.
  • Able to store control practice configurations in a database to insert in other WinSLAMM models.
  • Modified street parameter form to include number of street edges.
  • Added option to link the output from one WinSLAMM file as input to another WinSLAMM file.
  • Biofilter - expanded and enhanced the media characteristics table.
  • Incorporated tree canopy runoff interception as a source area parameter.
  • Modified the orifice outflow calculation to more accurately represent discharges at lower stages.
  • Updated Isolator Row chamber types and made the underdrain discharge optional.

  • Interface, Data Input and Calculation Changes and Corrections.
  • Printing - Set up a printing option for .pdf files for input data and output summary data.
  • Added the Pipe input data to the printing input data option.
  • Added a scroll bar to the Output Summary Tab to allow users with small screens to scroll to see then entire summary output tab.
  • Corrected error that occurred when adding the isolator row to a file that was updated from a previous version.


  • Corrected an error caused by an improperly defined variable that caused an overflow during very long model runs.
  • Corrected a surface drain time calculation error on the Biofilter data entry form.
  • Corrected error that fixed orifice outflow rate above the top of the orifice pipe to the value at the top of the pipe.


  • Corrected subscript out of range error when applying Pipes, the Storm Filter and the Upflo Filter.

Command Processor Option

  • Added site description line to top of output option 5 for command line processing, and runoff per acre and load per acre.

Storm Filter

  • Corrected error that prevented the program from including bypass flow volumes around the device from the final effluent hydrograph.

Input and Output Print Files

  • Added "Other Device as Bypass" statement to input and output print files.

Isolator Row

  • Added the Isolator Row control practice.


  • Users can now modify source area names.
  • Run WinSLAMM from the command line, without invoking the user interface.


  • Corrected an error in the Biofilter caused by an improperly defined variable that caused an overflow during very long model runs.
  • Corrected a surface drain time calculation error on the Biofilter data entry from.
  • Corrected a minor inconsistency between Biofilter source area and Biofilter drainage system calculations.

Grass Swales

  • A Grass Swale manning’s n interpolation loop that occurred under unusual conditions was limited to 100 iterations and a maximum manning’s n value of 1.0000.

Hydrodynamic Device

  • Corrected the Hydrodynamic Device code that listed the rainfall events after a device was full as 100% removal rather than 0% removal.


  • Revised the way that percent load calculations for pollutants in the outfall summary was calculated to be consistent with the suspended solids calculations.
  • Modified stage‐discharge curve in Porous Pavement and Biofilter devices for heads less than the diameter of the discharge pipe to reflect the standard q/Q pipe flow curve for partially full pipes.


  • Users running the batch editor can now get Output Option 5 - One line per event output.

Wet Detention Ponds

  • Corrected sediment elevation calculation error in v 10.3.2.

Grass Swales

  • Corrected error in Mannings n calculation that occurred when very small values of the flow velocity times the hydraulic radius were applied to the Mannings n equation.

Interface, Data Input and Calculation Changes and Corrections

  • Corrected file update error when loading WinSLAMM v 10.2 and earlier files.
  • Corrected file update error that occurred when opening earlier file versions with a wet detention pond.
  • Created a command line input option.
  • Corrected calculation errors for high traffic urban source areas, where if there was more than just one soil type, the program did not add the previous soil type area fraction to the current soil type area fraction.

Wet Detention Ponds

  • WinSLAMM now calculates the scour reduction ratio by time step, not event. This allows users to calculate the performance of dry or extended detention ponds.


  • Added and corrected bypass calculations.


  • Simplified residence time calculation to account for the time it takes water to flow through an engineered media plus the time the media is all or partly submerged. Residence time calculations now do not vary by event; if no underdrain, the model uses the slowest of the media or native soil infiltration rate; otherwise the model uses the orifice flow rate calculated with the water surface at the top of the media.
  • Corrected problems with drawdown time calculation in input form because outflow device invert elevations were not set until after the calculation was performed.

Interface, Data Input and Calculation Changes and Corrections

  • Correct error that occurs when loading files with unconnected junctions.
  • Updated .mdb File Batch Change option to include changing files to compacted soil types in addition to normal soil types and additional biofilter outlet options.

Wet Detention Ponds

  • Added a pump discharge option as either another outlet or as a discharge to a wastewater treatment plant. Discharges to the WWTP are removed from the system.
  • Minor data entry grid problems and copy/paste problems corrected.
  • Modified the wet pond cumulative storage calculation to use the average area of each stage increment rather than the upper area of each stage increment. This may have a minor effect on the pond performance calculation.
  • Added a calculator to simplify the creation of wet pond stage-area increments.

Other Device

  • Added a filterable pollutant reduction fraction option to the device.
  • Added all particulate and filterable pollutants to the calculation modification process used to remove the off‐site drainage areas of a drainage basin.

Hydrodynamic Device

  • Corrected total area and drainage area calculations.
  • Removed the Particle Size Distribution file name requirement.


  • Removed the Particle Size Distribution file name requirement.
  • Prevented vertical standpipe from having a height less than 0.01 ft to prevent divide by zero errors.
  • Prevented the vertical standpipe discharge calculation variable c from going above 4.


  • Added the Pipe drainage system option to convey runoff between junctions with or without a time shift.

  • Added rain numbers to the file execution interface to allow the user to track model run progress.
  • Allow the user to rename the project file set run output file.
  • Corrected Land Use Icon deletion problem.
  • Interface modifications that bold the control practice selected in the drainage system map and the corresponding practice in the control practice list.

Filtering Control Practices

  • Added the Hydro International Upflo filter as a source area and drainage system control practice.
  • Added the Stormwater Management Stormfilter as a source area and drainage system control practice.

Wet Detention Ponds

  • Corrected scour calculation error.

Street Cleaning

  • Corrected street cleaning variables M and B initiation when the parameters affecting them were changed.

Hydro Dynamic Devices

  • Restricted sediment accumulation in the settling tank to the top of the tank, preventing a sediment accumulation overflow.
  • Added maximum water depth value for each model run to the Control Practice Summary Tab output.

Grass Swales

  • Forced grass swale Mannings n maximum value to equal 1.

Interface, Data Input and Calculation Changes and Corrections

  • Added the ability to import an image into the drainage system map area.
  • Added Land Use Type to land use output names for each land use tab in tabbed output.
  • Modified the Source Area Particle Size Distribution button on the Current File Data form to access and change all source area Particle Size Distribution files using the Particle Size Distribution file listed on the Current File Data form.
  • User-defined Land Use Label names are is now listed in input data printouts.
  • Applied the critical particle size calculation process to all appropriate control practices.
  • Corrected stormfilter database name error that occurred when opening a file. Error had no effect on calculations.
  • Changed the maximum allowable runoff for pervious areas due to soil compaction from the Rv row number for the pervious area soil type to Rv = 3, for paved parking areas, so that pervious areas with compacted soil would reflect the compaction.
  • Corrected procedure call error due to Mannings n function getting a zero V x R value.
  • Changed text references of void ratio to porosity.
  • User can now end execution during a model run when the program is processing control practices.


  • Biofilter stage‐discharge curve corrections.
  • Added Biofilter overflow warning to alert user that biofilter overflowed and results may be inaccurate if the biofilters is too small.
  • Added residence time calculation to the Biofilter control practice for each event (in the detailed output file) and for the model run (in the Control Practice Summary Tab).
  • Corrected Biofilter sharp crested weir error where program was using the .dat file variable format for the biofilter end contraction variable. Changed the .dat file import format to match the v 10 format.
  • Added reset variable to Biofilters to prevent any further reduction in time step values when the time increment value gets too small, and increased minimum time step from 0.1875 minutes to 0.75 minutes.

Porous Pavement

  • Added Porous Pavement icon to drainage system.
  • Corrected two minor Porous Pavement problems that caused differences between two identical PP practices in the same file.
  • Corrected Porous Pavement errors that allowed the control practice summary output to be different than the main summary output for runoff volume under certain conditions.
  • To calculate the PP area in the aggregate bedding if the pavement to base area ratio is different than 1, changed gPP(PPNm).Area * 43560 * mVoidRatio(a) * Ratio to gPP(PPNm).Area * 43560 * mVoidRatio(a) / Ratio, to correctly calculate the area of the aggregate bedding.

Wet Detention Ponds

  • Corrected subscript out of range error in Wet Detention Ponds when entering all 20 stage‐area values.
  • Changed Wet Detention Pond stage increments to equal 100 times the maximum stage depth.
  • Corrected Wet Detention Pond Other Outlet stage discharge error that prevented the other outlet stage discharge curve from calculating the discharge for the stage increment with 0 cfs and the next stage increment with the first discharge for the other outlet.
  • Added mass lost to natural seepage basin calculation.

Streets, High Traffic Urban Areas and Freeways

  • Added street width calculation to input data printout.
  • Updated v 9.4 parameter file and street dirt value update code to import .dat files into v 10.2.
  • Corrected mapping error in Freeway Land Use from runoff coefficient parameter file to source areas 19‐27.
  • Modified street cleaning detailed output #194, #195, #197, #198 and #199 to include project file name and path, to consistently save these parameter files in the project file directory.
  • Corrected the street cleaning problem that prevented user‐defined street cleaning coefficients m and B from being applied in the calculations.
  • Fixed high traffic urban source area parameter data entry issue that brought up message box unnecessarily. Added Source Area PSD file name to input file output listing, corrected divide by zero error when printing high traffic urban highway input data, removed unneeded text when printing input values, improved formatting when printing street width information, and corrected case range for high traffic urban streets.
  • Corrected error that prevented source area .cpz file updates done using button in Current File Data for Freeway land uses, source areas 1 to 14.
  • Added Grass Swales as High Traffic Urban source area control practice option.

Other Control Practice

  • Added feature to reduce the non‐control outfall load by the mass of sediment removed by Other Devices, when the other devices are used to remove the off‐site drainage areas of a drainage basin.
  • Changed Other Device significant figures from 2 decimal point to 4 decimal points to correct the problem where the event by event volume numbers are calculated correctly, but the hydrographs are reduced by the fraction 0.01 rather than 0.01 x 0.001.

Filter Strips

  • Changed the effective flow length test for Filter Strips from less than zero to less than or equal to zero to prevent calculation errors that occur when the effective filter strip length is exactly equalto zero.

Grass Swales

  • To accurately track mass loss, changed grass swale particulate reduction calculation approach to weigh the TSS reduction calculation by the incremental effluent volume rather than the incremental infiltrated volume.
  • Corrected error that allowed grass swale effluent hydrograph to not account for all the flow leaving the swale system if the fraction of the area served by swales was less than 1.0.

  • Changed wet detention pond stage increments to 0.01 feet per increment to provide more consistent algorithm performance.
  • Added warnings to notify users if errors occur that could invalidate output.
  • Added Grass Swales as a High Traffic Urban source area control practice option.
  • Modified street cleaning code to save street cleaning detailed output files in the project file directory.
  • Added street width calculation to input data printout.
  • Miscellaneous upgrades and bug fixes.
Note - Interim version numbers were test builds.

  • The last pollutant listed in the output summary and the output summary tab pollutant list prints correctly.
  • Open water pollutant calculation error corrected.
  • The peak flow value for each rainfall event is now calculated and listed in the Output Outfall/Runoff Volume tab.
  • Added default particle size and peak to average flow ratio file update to the "Use Default Values" button in the Current File Data form.
  • Added error checks for the biofilter outlet device variables and added and modified Source Area parameter error checks.
  • Corrected minor pollutant calculation and unit conversion errors.
  • Modified cost file format to allow program to perform an error check to determine if a .csv file is a cost file.
  • Wet Detention Pond data entry form modifications.
  • Added a number of error checks to prevent inadvertent data entry errors.
  • Minor bug fixes

  • Added 'Create Land Uses from Data File' Tool to create a set of land uses from a .csv file in one file.
  • Activated wet pond flow bypass option.
  • Modified output option 5 to include pollutants.
  • Converted particle size distribution weighting from a flow-weighted basis to a mass-weighted basis.
  • Miscellaneous minor error checks and bug fixes.
  • Particle size distribution files assigned to source areas rather than globally.
  • Added five more 'Other Impervious Areas' and 'Other Pervious Area' source areas.
  • Modified porous pavement control practice form and computation algorithms.
  • Added soil compaction factors for all pervious source areas.

Changed BF soil data form soils Peat and Compost to "50-50 Peak/Sand" and "50-50 Compost/Sand", respectively. Those two types can only have a ratio value of 1.0. This is a temporary change to reflect initial data in newest studies. The complete data set, with more flexibility, will be available in v 10.1. The soil parameters were changed to:

Porosity Field Cap Wilting Pt Infiltration Rate
50-50 peat/sand 58 33.5 3.8 13.0
50-50 compost/sand 49.5 31.5 3.8 13.0
  • Minor error checks added to the hydrodynamic device to address overflow weir issues.
  • Added note in control practice particle size distribution text boxes to note that the particle size distribution was not needed if particle size distributions were routed.
  • Removed the porous pavement control practice as a second source area control option. Corrected porous pavement surface infiltration reduction due to surface clogging calculations. A major restructuring of this practice will occur in v 10.1.
  • Added the missing catchbasin initial sediment depth variable. This variable will only be saved if the .mdb file was created in v 10.0.3 unless the user changes the variable from integer to single directly in the database. Files will update properly in v 10.1.
  • Added street length multiplier to group file modifications input form.
  • Porous pavement surface filtering added
  • Added street length multiplier to the File Modification form
  • Added a run time label to the main status bar
  • Added high traffic urban pervious areas to the standard land use options
  • Additional Error Checks and Bug Fixes
  • Corrected catchbasin inflow hydrograph calculations
  • Corrected PSC File editor problems
  • Corrected flow-weighted calculation problems in the output
  • Corrected cost file application problem
  • Added biofilter surface drain pipe outflow to the total outflow calculations
  • Added flow from biofilter drain tile to the outflow hydrograph

  • Activated wet pond flow bypass option.
  • Added help button to describe available options if a user is opening an earlier version file. Converted particle size distribution weighting from a flow-weighted basis to a mass-weighted basis.
  • Modified the hydrodynamic control practice device to prevent the user from accessing the proprietary device file if there is no proprietary device.
  • Added "Create Land Uses from Data File" Tool to create a set of land uses from a .csv file in one file.
  • In Biofilter soil characteristics form, corrected error trap that occurred when the user entered a fraction greater than one.
  • Corrected Filter Strip error that improperly calculated the incremental flow reduction leaving the device. The program calculated the filter strip reduction correctly.
  • Modified output option 5 to include pollutants.
  • Changed v 9.4.0 .dat file importer to treat engineered media soil as user defined rather than trying to update the soil type to match the new engineered media table format.
  • Added high traffic urban and high traffic urban pervious area SLU fields
  • Miscellaneous minor error checks and bug fixes.

  • Miscellaneous Bug Fixes
  • Updated Help File with all revisions and new information
  • Added error checks

  • Program structure converted from text file format to database format
  • Added drag and drop user interface
  • Added ability to model all control practices in series
  • Added complete routing of particle size distribution through defined network
  • Added complete routing of complex triangular hydrograph through defined network
  • Added numerous additional source area instances
  • Added Land Use and Control Practice grids to summarize data in modeled network
  • Modified Output Summary and Output tabs to report data from revised network
  • Created WinSLAMM Files folder in root C: directory to allow program to access parameter files as needed
  • Added Media Data form and Evapotranspiration to Biofilter control device
  • Added Filter Strips
  • Added Ultra Urban Highway Source Areas
  • Removed Cisterns from Biofilter control device and made it its own control device
  • Updated Wet Detention Pond, Grass Swale and Porous Pavement forms.
  • Added ability for user to choose time step
  • Added ability to rename all network instances
  • Added ability to import v9.4.0 model files
  • Added standard land use files into a standard land use database. Standard land use files can now be accessed by right-clicking on a land use icon.
  • Added ability for user to set default parameter files to the parameter files listed in the current model
  • Updated Help File with all revisions and new information
  • Added error checks


  • Added the option to route both the outflow particle size distribution and the outflow hydrograph from an outfall wet detention pond to an outfall biofilter for each event.
  • Added the biofilter control practice as an option for the Freeway land use.
  • Routed particle sizes through biofilter engineered soil instead of applying a constant percent reduction value for each engineered soil type.
  • Added the option to modify the biofilter control practice in a group of files at one time.
  • Corrected water balance calculation discrepancies that occurred when modeling biofilters with only engineered soil and/or rock layers and no drain tiles.

Hydrodynamic Devices and Catchbasins

  • Added the Lamella Plate feature option to the hydrodynamic and catchbasin control practices.

Street Cleaning

  • Changes the way the program calculates street cleaning when the user selects a street cleaning frequency (eg, one sweeping per month) while using the Winter Season option. Previously, the program selected April 1st as the first cleaning date, regardless of when the winter season ended. With this change, the program will use the end of winter season date as the date of the first cleaning.

Minor corrections, changes and additional error checks.

Street Cleaning - Modified the programs default street cleaning start date when a cleaning frequency is selected.

  • Grass Swales - Added swale length calculation option to allow users to calculate the swale infiltration volume using the total swale length rather than the swale density times the area served by swales. Adjusted grass swale form input to allow user to set length and average length values independently of swale density.
  • Minor corrections, changes and additional error checks

  • Grass Swales - Corrected an interpolation calculation error for the grass swale infiltration rate between 0.005 and 0.02 inches per hour. If the slope is between 0.005 and 0.0125 ft/ft then the swales will perform better than in v 9.3.1. If the slope is between 0.0125 and 0.02 ft/ft, then the swales will not perform as well as in v 9.3.1. For sets of swales in a community, we found that the overall difference in performance is around 0.1% higher. The performance of a single swale system, of course, will depend upon the slope. If the swale slope is not within the range described above, then the swale performance will not change.
  • Batch Processor - Corrected the grass swale average swale length calculation and fixed a swale file transformation error in the batch processor.

  • Grass Swales - Added grass swale maximum velocity, depth and shear values for each event to the grass swale event detailed output file.
  • Hydrodynamic Devices - Added hydrodynamic devices to Freeway land use Paved Lane and Shoulder Area source areas and corrected hydrodynamic device calculation problems when the device was defined without any inlet flow controls.
  • Minor corrections, changes and additional error checks.

  • Biofilters/Infiltration Devices - Added particulate filtering through the engineered soil.
  • Wet Detention Ponds - Added a scour reduction function to calculate reduced settling in ponds less than three feet deep.
  • Grass Swales - Added particulate filtering in grass drainage systems.Also added more flexibility by allowing users to model different swale systems based upon land use.
  • Hydrodynamic Devices - This control practice is now available as a source area control in addition to the drainage system control.
  • Minor corrections, changes and additional error checks.

Street Cleaning - Modified the programs default street cleaning start date when a cleaning frequency is selected.

  • Minor corrections and additional error checks.
  • Biofilter Modifications - Correct Land Use biofilter problem that incorrectly calculated Fraction of Runoff from Selected Source Areas value. Also changed the way biofilter elevation increments were calculated to remove problems that occurred due to elevation interpolation.

  • Minor corrections and additional error checks.
  • Batch Data File Modifications - This feature, introduced in v 9.2.0, now also includes a site description text editor to allow you to change the site description text for a group of .dat files
  • Outfall Treatment Train Order - The outfall treatment train has been modified to use the 'Other' control practice first, then wet detention ponds, followed by biofilters.

Minor corrections to the porous pavement cost estimation subroutine.

  • Streets - The program includes modified street dirt accumulation and street cleaning functions to reflect newer street dirt research.
  • High Efficiency Street Cleaners - Two types of street cleaners are now available as part of the street cleaning control practice - Mechanical cleaners and Vacuum or Regenerative Air cleaners.
  • Porous Pavement - We have added a routing and storage component to the porous pavement control practice and added a porous pavement cross section graphic to the data input form so that you can more easily visualize the various elements of a porous pavement system. We have also added a porous pavement infiltration degradation function and cleaning function to the program.
  • Rain File Analysis Tool - The program now includes a rain file statistical analysis routine that will help you to analyze a long series of rainfalls to determine which rainfall represents the typical annual rainfall year.
  • Control Over the Detailed Output Options - We have developed a description of the various detailed output option files that the model generates as well as a way to turn these output options on or off.
  • Biofilter Performance Data - We have added a biofilter water balance statistical summary as part of the event-by-event summary. This summary includes the time the water surface is above the top layer of the biofilter and the time that any water remains in the biofilter at all, information that some regulatory agencies require when evaluating biofilters.
  • Stone Weeper Outlet - We have added a stone weeper outlet option for wet detention ponds.
  • Model Version Updates - The model will now automatically update a version 9.0 or later WinSLAMM data file (after the user enters standard update information) to the current version of the model. Older WinSLAMM data files (.DAT files) will, by default, receive a set of parameter files that come with the distribution disk. If you do not want this to occur, you must re-set the default values. To do so, in the Main Menu, go to Tools/Default Model Options and press the 'File Update Options' button. In the .DAT File Update Information window, enter the file names and other variables that you want the program to update older version files to, by default.
  • Batch Data File Modifications - The program will now allow you to add or change selected model parameters in any number of files at one time. This will allow you to analyze a community's runoff volume or pollutant load much more efficiently.
  • Rain Files - We have significantly expanded the number and length of the rain files for the program. There are now over 140 rain files, including many that cover over 40 years of rainfall events. The rain files are available from the web site.
  • Miscellaneous minor upgrades, error checks, bug fixes, and interface changes.