SWMM (Storm Water Management Model)

SWMM, the Storm Water Management Model, is software from the US Environmental Protection Agency (US EPA) for urban drainage. It is free and open source and is used to plan, analyse and design storm sewers, combined sewers, sanitary sewers and other drainage in cities. It can run a single storm or many years of rainfall. SWMM has two main parts. The runoff part treats each subcatchment as a small reservoir. Rain fills it, infiltration and evaporation drain it, and when depth passes the depression storage, water flows off at a rate set by Manning's equation using the subcatchment width, slope and roughness. Infiltration can use Horton, Green-Ampt or curve number methods. The routing part carries this runoff through pipes, channels, storage units, pumps, weirs and outfalls. It offers steady flow, kinematic wave or dynamic wave routing. Dynamic wave solves the full Saint-Venant equations and can show backwater, surcharge and reverse flow. SWMM also models pollutant build-up and wash-off and low impact development (LID) controls such as rain gardens, permeable paving and green roofs. Inputs are rainfall time series, subcatchment area, imperviousness, width and slope, and the drainage network with pipe sizes, inverts and roughness. Outputs are flows and depths in every link and node, flooding and overflow volumes, and pollutant loads over time. For example, a city can model a neighbourhood that floods, run a design storm, find which pipes surcharge, then test a detention tank or green roofs to see how much they cut peak flow and flooding. SWMM is free, widely used and accepted for design work. Its limits: it is one-dimensional, so water that leaves the pipes is treated simply as ponding at nodes rather than flowing across streets. It needs calibration to measured flows, and large dynamic wave models can be unstable without careful time steps.