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The Source Loading and Management Model (WinSLAMM) is a proprietary stormwater quality model originally developed for the USGS for the evaluation of non-point pollution in urban areas and pollutant removal at water quality 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 sources 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 scale 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 trains). 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 TMDL applications ranging from TMDL development, to demonstrating Waste Load Allocation (WLA) and permit compliance from individual municipal separate storm 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.

• Model download
• Model purchase
• Model documentation

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.

Applicability to demonstrating WLA 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.