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Storm Water Management Model (SWMM)

SWMM CSO exampleApplication that helps predict the quantity and quality of runoff within urban areas 

EPA's Stormwater Management Model (SWMM) is used for single event or long-term simulations of water runoff quantity and quality in primarily urban areas–although there are also many applications that can be used for drainage systems in non-urban areas. It is used throughout the world for planning, analysis, and design related to stormwater runoff, combined and sanitary sewers, and other drainage systems. SWMM was developed to help support local, state, and national stormwater management objectives to reduce runoff through infiltration and retention, and help to reduce discharges that cause impairment of our Nation’s waterbodies.

SWMM has undergone several major upgrades since it was first developed in 1971, including the addition green infrastructure practices as low impact development (LID) controls. It is widely used to evaluate gray infrastructure stormwater control strategies, such as pipes and storm drains, and is a useful tool for creating cost effective green/gray hybrid stormwater control solutions.

  • Modeling Capabilities

    SWMM-CSO exampleSWMM provides an integrated environment for editing study area input data, running hydrologic, hydraulic and water quality simulations, and viewing the results in a variety of formats. These include color-coded drainage area and conveyance system maps, time series graphs and tables, profile plots, and statistical frequency analyses.

    Hydraulic Modeling: SWMM contains a flexible set of hydraulic modeling capabilities used to route runoff and external inflows through the drainage system network of pipes, channels, storage/treatment units and diversion structures. These include the ability to do the following:

    • Handle drainage networks of unlimited size.
    • Use a wide variety of standard closed and open conduit shapes as well as natural channels.
    • Model special elements, such as storage/treatment units, flow dividers, pumps, weirs, and orifices.
    • Apply external flows and water quality inputs from surface runoff, groundwater interflow, rainfall-dependent infiltration/inflow, dry weather sanitary flow, and user-defined inflows.
    • Utilize either kinematic wave or full dynamic wave flow routing methods.
    • Model various flow regimes, such as backwater, surcharging, reverse flow, and surface ponding. apply user-defined dynamic control rules to simulate the operation of pumps, orifice openings, and weir crest levels.
    • Percolation of infiltrated water into groundwater layers.
    • Interflow between groundwater and the drainage system.
    • Nonlinear reservoir routing of overland flow. Runoff reduction via LID controls.

    Accounting for Hydrologic Processes. SWMM accounts for various hydrologic processes that produce runoff from urban areas, which include the following:

    • Runoff reduction via green infrastructure practices.
    • Time-varying rainfall (precipitation) and evaporation of standing surface water.
    • Snow accumulation and melting.
    • Rainfall interception from depression storage.
    • Infiltration of rainfall into unsaturated soil layers.
    • Percolation of infiltrated water into groundwater layers Interflow between groundwater and the drainage system.
    • Nonlinear reservoir routing of overland flow.

    Spatial variability in all of these processes is achieved by dividing a study area into a collection of smaller, homogeneous sub-catchment areas. Each of the areas contains its own fraction of pervious and impervious sub-areas. Overland flow can be routed between sub-areas, between sub-catchments, or between entry points of a drainage system.

    Pollutant Load Estimation: SWMM can estimate the production of pollutant loads associated with stormwater runoff. The following processes can be modeled for any number of user-defined water quality constituents:

    • Dry-weather pollutant buildup over different land uses.
    • Pollutant wash-off from specific land uses during storm events.
    • Direct contribution of rainfall deposition. Reduction in dry-weather buildup due to street cleaning.
    • Reduction in wash-off load due to best management practices (BMPs).
    • Entry of dry weather sanitary flows and user-specified external inflows at any point in the drainage system.
    • Routing of water quality constituents through the drainage system.
    • Reduction in constituent concentration through treatment in storage units or by natural processes in pipes and channels.

    Add-in Tool for Climate Projections: SWMM includes a software utility that allows future climate change projections to be incorporated into modeling. The SWMM Climate Adjustment Tool (SWMM-CAT) provides a set of location-specific adjustments derived from World Climate Research Programme global climate change models. SWMM-CAT accepts monthly adjustment factors for climate-related variables that could represent the potential impact of future climate changes.

  • Green Infrastructure as LID Controls

    SWMM allows engineers and planners to represent combinations of green infrastructure practices as LID controls to determine their effectiveness in managing runoff. Although some of these practices can also provide significant pollutant reduction benefits, at this time, SWMM only models the reduction in runoff mass load resulting from the reduction in runoff flow volume. SWMM can explicitly model eight different generic green infrastructure practices: 

    SWMM rain gardenRain Gardens. A depressed area, planted with grasses, flowers, and other plants, that collects rain water from a roof, driveway, or street and allows it to infiltrate into the ground. More complex rain gardens are often referred to as bioretention cells.


     

    Bioretention cellBioretention Cells (or Bioswales). Depressions containing vegetation grown in an engineered soil mixture placed above a gravel drainage bed that provide storage, infiltration, and evaporation of both direct rainfall and runoff captured from surrounding areas.


     

    Vegetated swaleVegetative Swales. Channels or depressed areas with sloping sides covered with grass and other vegetation. They slow down the conveyance of collected runoff and allow it more time to infiltrate the native soil beneath it.


     

    Infiltration trenchInfiltration Trenches. Narrow ditches filled with gravel that intercept runoff from upslope impervious areas. They provide storage volume and additional time for captured runoff to infiltrate the native soil below. 


     

    green roofGreen Roofs. A variation of a bioretention cell, green roofs have a soil layer laying atop a special drainage mat material that conveys excess percolated rainfall off of the roof. They contain vegetation that enable rainfall infiltration and evapotranspiration of stored water.


     

    Rooftop downspout disconnectionRooftop (Downspout) Disconnection. This practice allows rooftop rainwater to discharge to pervious landscaped areas and lawns instead of directly into storm drains. It can be used to store stormwater and/or allow stormwater to infiltrate into the soil.


     

    CisternRain Barrels or Cisterns (Rainwater Harvesting). Containers that collect roof runoff during storm events and can either release or re-use the rainwater during dry periods. Cisterns may be located above or below ground and have a greater storage capacity than a rain barrel.


     

    Permeable PavementContinuous Permeable Pavement Systems. Excavated areas that allow rainfall to immediately pass through the pavement into the gravel storage layer below where it can infiltrate at natural rates into the site's native soil. In block paver systems, rainfall is captured in the open spaces between the blocks and conveyed to the storage zone and native soil below.

  • Applications

    Urban wet weather flow cycleTypical applications of SWMM:

    • Designing and sizing of drainage system components for flood control.
    • Sizing detention facilities and their appurtenances for flood control and water quality protection.
    • Mapping flood plains of natural channel systems – SWMM 5 is a FEMA-approved model for National Flood Insurance Program studies.
    • Designing control strategies for minimizing combined sewer overflows.
    • Evaluating the impact of inflow and infiltration on sanitary sewer overflows.
    • Generating nonpoint source pollutant loadings for waste load allocation.
    • Controlling site runoff using green infrastructure practices as low LID controls.
    • Evaluating the effectiveness of best management practices and low impact development for reducing wet weather pollutant loadings.

Related EPA Resources

Support

  • Technical Contact: For comments or questions, Contact Us about SWMM.
  • Listserv: A SWMM users’ listserv was established by the University of Guelph. This listserv allows subscribers to ask questions and exchange information. To subscribe, send an email message to listserv@listserv.uoguelph.ca with the words subscribe swmm-users in the subject line and your name in the body of the email.