The Source Loading and Management Model (WinSLAMM) is a proprietary stormwater quality model originally developed for the United States Geological Survey (USGS) for the evaluation of non-point pollution in urban areas and pollutant removal at water quality best management practices (BMPs). WinSLAMM uses experimentally derived runoff coefficients to predict runoff and associated pollutant loading from a number of land use types. A unique feature in WinSLAMM is that within defined land use types (e.g., commercial, residential, etc.), the program tracks loading from many different types of source areas (e.g., roofs, parking lots, etc.) and further distinguishes source areas using source area parameters (e.g., is the roof flat or pitched? Does the roof drain to a pervious surface?, etc.). WinSLAMM provides this level of specificity so that unique runoff coefficient and pollutant loading assumptions can be applied to source areas within land use types, allowing for refinement of runoff and pollutant loading results.
WinSLAMM predicts pollutant removal at water quality BMPs based on correlation to experimental results (empirical) as well as modeling the generation and removal of particulates through sedimentation and filtration (physically-based). By modeling hydraulic performance at BMPs and physically tracking the particle size distribution (PSD) through BMPs, WinSLAMM is capable of modeling bypass from BMPs and is capable of predicting performance of BMPs in series (i.e., treatment train). WinSLAMM is a continuous model which can produce results for long term simulations and produce results for individual rainfall events. It should be noted that WinSLAMM simulations are done in batch mode, which means that it is not modeling individual rainfall events in real-time, and accounting for antecedent moisture conditions for pervious runoff, the way that P8 does.
WinSLAMM is a versatile program suitable for total maximum daily load (TMDL) applications ranging from TMDL development, to demonstrating wasteload allocation (WLA) and permit compliance from individual Municipal Separate Storm Sewer Systems (MS4s). Download links for WinSLAMM and related documentation are provided below. As noted above, WinSLAMM is a proprietary model which needs to be licensed or purchased before use.
Note: information provided in the following subsections does not reiterate or re-present information readily available in model documentation files. Instead, guidance provided in this document provides engineers and planners with recommendations for development of model inputs, provides guidance for interpreting and summarizing model results, provides supplementary information not included in model documentation, and provides examples showing how WinSLAMM can be used to demonstrate TMDL compliance.
WinSLAMM is a continuous water quality model capable of summarizing runoff and associated total suspended solids (TSS) and total phosphorus (TP) generation, removal, and outflow loading from individual catchments, individual BMPs and junctions, or as a model wide summary. Additionally, because WinSLAMM models the particulate and pollutant particle-scale distribution through filtration and sedimentation BMPs and models hydraulic performance of BMPs based on watershed loading and BMP dimensions, the model is capable of accurately predicting pollutant removal through BMPs in series as well as predicting runoff and pollutant bypass from undersized undersized BMPs. Due to this flexibility, WinSLAMM is capable of providing accurate pollutant removal estimates regardless of BMP network and subwatershed configuration, and is capable of demonstrating compliance to mass-based Water Quality Based Effluent Limits (WQBELs), concentration-based WQBELS, and areal-loading based WQBELS (e.g., lbs of TSS per acre per year) for both TSS and TP.
The following subsections outline data sources and special consideration related to model inputs, model setup, and model initialization. Note: these subsections do not represent information readily available in Model documentation, but instead highlight data sources (e.g., spatial datasets), special consideration, and important notes for engineers and planners to consider while generating model inputs.
WinSLAMM requires a number of parameter files to generate runoff, particulate loading, and pollutant accumulation and washoff from source areas within each land use type. Each of the required parameters files is described briefly, below:
The Rain File (“.RAN”), which specifies rainfall events, duration, and depth for the modeled period, is regionally specific and should be developed for the study area. The National Oceanic and Atmospheric Administration (NOAA) maintains a searchable database that can be used to search for hourly precipitation data based by city or geographic region (e.g., zip code). Local area airports (e.g., Minneapolis Saint Paul International Airport) are another resource that can be used to develop required precipitation data inputs. As described in model documentation, rainfall input “.RAN” files can be created within the Rainfall File Editor (“Utilities → Parameter Files → Rainfall Files”).
The remaining parameter files (Runoff Coefficient File (“.RSVX”), Particulate Solids Concentration File (“.PSCX”), Source Area PSD and Peak to Average low Ratio File (“.CSV”), Particle Size Parameter Files (“.CPZ”), Street Delivery Files (“.STD”), and Pollutant Probability Distribution Files (“.PPDX”)) contain input values that must be generated from literature values, case studies, or water quality monitoring data. For this reason, default parameter files have been generated for several geographic locations across the United States and are provided in WinSLAMM documentation. Additionally, some state and local agencies have developed state-wide or regional WinSLAMM input parameter files. For this reason, it is recommended that the engineer or designer first determine if local or regional parameter files have been developed before utilizing default regional parameter files provided in the WinSLAMM model files. Documentation shows the six WinSLAMM parameter files regions. The “Great Lakes” region provides coverage for the majority of Minnesota and should be used if local or regional parameter files are not available for the study area.
Job control parameters related to model run time are specified in the “Current File Data” window. Job control parameters are described briefly, below:
Accessed through the “Pollutants” tab on the main WinSLAMM model menu, the “Pollutant Selection” window defines which pollutant(s) included in the Pollutant Probability Distribution (“.PPDX”) file will be modeled and tracked in reported model outputs.
Job control parameters and options related to output generation within WinSLAMM “Program Options” (“Tools → Program Options”) should be reviewed by the engineer or designer before performing model simulation. The “Program Options” window is separated into three tabs. A brief discussion of each of the “Program Options” tab is provided below:
Within WinSLAMM, drainage basins are modeled using one or more “land uses” (e.g., residential, commercial, etc.), which are represented by land use nodes within the model space. Within each land use, area is further delineated into “source areas” (e.g., roofs, sidewalks, etc.). Runoff and pollutant loading from drainage areas is impacted by the land use type, as well as the source area type. A complete list of land use and source area types in provided in the table below. Finally, each source area type is further characterized by a “source area parameters”. For example, the “source area parameter” for a roof source area define whether the roof is pitched or flat, whether the roof drains onto a pervious or impervious surface, etc. Developing this detail of input parameters is possible for small, development-scale study areas, but is not feasible for larger study areas (e.g., municipal-scale study areas). For this reason, WinSLAMM has a number of default “Standard Land Uses” which can be applied to any of the six (6) land use types.
Standard Land Use (SLU) types can be applied to any land use by right-clicking the land use and selecting “Apply Standard Land Use”. There are several SLU types for each land use. For example, the SLUs for commercial land use include “office park”, “strip commercial”, “downtown commercial”, and “shopping center” (see adjourning figure). SLUs simplify land use modeling by applying default source area and source area parameters based on the selected SLU. With and SLU selected, the engineer or designer only needs to assign the acres of area in three different soil texture groups (sand, silt, and clay) and select “Create Land Use and Exit”. It is important for the designer or engineer to review land use assumptions and definitions applied within each modeled SLU. A complete list of SLUs, definitions, and modeling assumptions is provided within model documentation help files (see the “Standard Land Uses and Source Areas” section of the model help file).
WinSLAMM land use and source area types
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|Land Uses||→||Source Areas within Land Uses|
|Other Imperious Areas|
|Other Pervious Areas|
When modeling drainage areas using SLUs in WinSLAMM, user inputs include defining the area in each of the six (6) land use types, assigning appropriate SLU types, and determining the soil textures within each land use type. When possible, land use should be determined using record drawing or parcel data specific to the model area. When site-specific information is not available, land use information can be assigned from regional and national land use databases.
Similarly, soil texture data should be specified using site-specific soil sampling data. When site-specific information is not available, it is recommended that the soil information be determined from publically available spatial soil datasets, such as the NRCS Soil Survey Geographic Database (SSURGO). SSURGO soils data is available for download online through the Web Soil Survey. A summary of soil textures related to hydrologic soil groups (HSGs) and Unified Soil Classifications can be found here.
Structural (e.g., wet ponds, biofiltration, etc.) and non-structural (e.g., street sweeping) water quality BMPs are referred to in WinSLAMM as control practices. Control practices can be applied to the drainage system within the model space, or can be assigned directly to land use source areas. The table below provides a complete list of all control practices included in version 10.4 of WinSLAMM. As can be seen, all twelve (12) control practices can be modeled directly from source areas, but only eight (8) can be modeled within the drainage system model network. Reviewing control practices shows that the BMPs that cannot be modeled in the drainage system network include non-structural BMPs (e.g., street sweeping) and BMPs that are related to a specific source area type (e.g., cisterns typically treat roof areas, porous pavement applies to paved source areas such as parking lots, etc.).
WinSLAMM control practice summary
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|Control Practices||Control practice abbreviation||Source area control practice||Drainage system control practice|
|Other Control Device||(OD)||X||X|
|Wet Detention Pond||(WP)||X||X|
WinSLAMM predicts pollutant removal at water quality BMPs based on correlation to experimental results (empirical) as well as modeling the generation and removal of particulate through sedimentation and filtration (physically based). To model hydraulic performance, filtration, sedimentation, infiltration, and other parameters unique to certain control practices (e.g., cleaning frequency for hydrodynamic separators), WinSLAMM requires many user-input parameters for each control practice (see example of user inputs required for the porous pavement control practice in the adjourning figure). It is critical that the designer or engineer review required inputs as well as removal algorithms and methodology applying to each control practice modeled. A complete description of model parameters for each control practice can be accessed through the help files within WinSLAMM. Additional information describing removal algorithms, referenced research, particle filtration and settling methodology, and processes unique to certain control practices (e.g., pond scour calculations from wet ponds) is provided model documentation. Because WinSLAMM requires control practice dimensions, actively models hydraulic loading and bypass, and tracks the particulate and associated pollutant PSD through control practices, WinSLAMM is capable of modeling bypass from undersized BMPs and treatment through BMPs in series (i.e., treatment trains).
WinSLAMM provides a wide variety of model outputs which can be used by engineers and planners to evaluate and demonstrate WLA compliance. Model outputs can be printed to text files or viewed within the output summary window which launches after completion of a model run. Information that is presented in model output files is controlled by the user-assigned “Output Options” (File → Output Options). It is recommended that Option one (1) “Source Areas by Land Use for Each Rain – Complete Printout” be selected for a majority of reporting purposes related to WLA compliance. A summary of information provided in the output summary and output tabs which launch upon completion of a model run are described, below:
Note: pollutant summary (mass and concentration) is provided in each of the element tabs with the exception of “Control Practices” (only runoff and particulate loading are summarized for control practices). For this reason, if a summary of pollutant reduction is required at a given control practice, junction elements must be placed upstream and downstream of the control practice. Reduction can then be calculated as: [pollutant loading at upstream junction] – [pollutant loading at downstream junction]. For this reason, pollutant load reduction cannot be calculated at control practices applied to source areas.
The table below provides a summary of how to generate common TSS and TP WLA reporting terms. WinSLAMM is a continuous model that provides a variety of detailed model outputs, and can therefore be used for a wide variety of WLA compliance review and reporting purposes. WinSLAMM reports particulate (i.e., TSS) loading and reduction at control practices, but does not report pollutant (e.g., dissolved and particulate phosphorus) loading and reduction. For this reason, pollutant loading and reduction at individual BMPs is required, loading and removal can be calculated by placing “junction” elements upstream and downstream of the control practice. Pollutant reduction can then be calculated outside of WinSLAMM as: [pollutant loading at upstream junction] – [pollutant loading at downstream junction].
Using WinSLAMM to generate load reduction and WLA reporting terms
Link to this table
|Reporting term||Individual BMPs ("Junctions" summary tab)1||Model-Wide summary ("Output summary" tab)|
|Pollutant Loading: TSS, TPP, and TDP|
|Watershed Loading* (lbs)||Junctions → Pollutant Yield (lbs) (US junction)1||Output Summary → Pollutant Yield - No Controls; (A)|
|Pollutant removal (lbs)||Junctions → Pollutant Yield (lbs) (US - DS junction)1||(A) - (B)|
|Outflow Loading** (lbs)||Junctions → Pollutant Yield (lbs) (DS junction)1||Output Summary → Pollutant Yield - With Controls (B)|
|Outflow Loading (ppm; mg/L)||Junctions → Pollutant Concentration (mg/L) (DS junction)1||Output Summary → Concentration - With Controls|
|Watershed and Areal Loading Calculations|
|Contributing Watershed Area* (acres)||"Upstream Drainage Area"2 with Control Practice selected||Output Summary → Total Area Modeled (ac)|
|Watershed pollutant areal loading (pre-treatment) (lbs/acre/year)||[Watershed Loading*] / [Contributing Watershed Area*]|
|Outflow pollutant areal loading (post-treatment) (lbs/acre/year)||[Outflow Loading*] / [Contributing Watershed Area*]|
1WinSLAMM does not summarize pollutant load reduction within the "control practices" summary tab. For this reason, to calculate pollutant load reduction at control practices, "junction" elements must be placed upstream (US) and downstream (DS) of the control practice. Pollutant reduction can then be calculated as: [US junction pollutant load] - [DS junction pollutant load]. Note: using this method, pollutant reduction cannot be calculated for control practices modeled at source areas.
2While in the model space, "Upstream Drainage Area" is displayed at the bottom of the program window for the element selected.
The engineer or designer should perform a thorough review of all model inputs and outputs. Model inputs should be generated using best available datasets, including record drawings, development data, bathymetric surveys, and best-available spatial land use, land cover, and soil databases. Upon model completion, results should be reviewed to ensure control practices were routed correctly and applied correctly to sources areas, subwatersheds were routed correctly, and that pollutant removal and areal loading results are within typical ranges based on BMP and land use type, respectively. A general model result QAQC list for WinSLAMM is provided in the table below. Additionally, a literature review of typical average annual event mean concentration (EMC) values is provided here, and typical TSS and TP removal values for various BMP types is provided here.
General WinSLAMM result QAQC list
Link to this table
|QA/QC review item||Related WinSLAMM output|
|Confirm total watershed area is correct||Output Summary → Total Area Modeled (ac)|
|Confirm area modeled in each land uses is correct||Review land use summary table shown in the main model space window|
|Confirm source areas or applied SLUs are correct for each land use||Review source area modeling assumptions for each land use|
|Confirm correct number of control devices modeled in drainage network and source areas||Review control practice summary table shown in the main model space window|
|Confirm correct parameters modeled in control practices||Review parameters applied in control practice parameter window for all modeled drainage area and source area control practices|
|Confirm land use, junction, and device routing||Review routing in main model space window|
|Confirm precipitation event totals are correct||Review precipitation dates and rainfall depth totals. Can be accessed through a number of output tabs including "Junction → Runoff Volume"|
|Compare source area outflow pollutant concentration to tabled values||Output: Land Uses → Pollutants → Concentration (mg/L)|
|Compare TSS/TP removal rate to typical tabled values||Calculate pollutant removal (%) as described here. Note: must have junction elements upstream and downstream of all control practices|
TSS and TP EMC literature values for TMDL modeling
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|Reference||Average annual EMC (mg/L)|
|Residential (Pitt, 2011; NSQD, 2011/ Region 1)||135||0.4|
|Minnesota Stormwater Manual – Commercial||120-160||0.15-0.35|
|Minnesota Stormwater Manual – Industrial||130-170||0.15-0.35|
|Minnesota Stormwater Manual – Residential||100-170||0.2-0.6|
|Minnesota Stormwater Manual – Freeway/ Transportation||115-155||0.3-0.5|
|Nationally Pooled Urban EMCs (Lin, 2003)||54.5-78.4||0.266-0.315|
Typical BMP TSS and TP removal rates from Minnesota Stormwater Manual
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|BMP type||Typical pollutant removal rate (%)|
|Iron enhanced sand filter||85||77|
|Constructed wet ponds||84||50|