SWMM — The Complete Guide: Free Urban Drainage and Sewer Modelling
SWMM (Storm Water Management Model) was developed by the US Environmental Protection Agency, and it is the most widely used free model for urban drainage and sewer systems. Its engine sits inside several well-known commercial programs, and its method — a runoff model plus a hydraulic model of the pipe network — suits any city, provided you enter local rainfall, soils and impervious areas.
SWMM
US Environmental Protection Agency
Urban drainage model — runoff, sewers, detention and infiltration, including runoff quality.
Get it from the vendorWhat SWMM calculates
- Runoff — subcatchments with area, slope, percent impervious and an infiltration method (Horton, Green-Ampt or Curve Number).
- Hydraulics — pipes, channels, manholes, pumps and valves, with Kinematic Wave or Dynamic Wave routing that also captures backwater and surcharge.
- Detention and infiltration — detention ponds, and low impact development (LID) measures such as green roofs, infiltration trenches and permeable pavement.
- Runoff quality — build-up and wash-off of pollutants from urban surfaces.
How to build a model
1. Divide the area into subcatchments and set area, imperviousness and slope for each. 2. Build the pipe and manhole network with levels and diameters. 3. Enter design rainfall — a synthetic storm or a real record. 4. Run and check manhole flooding, pipe fill and velocities. For preparing subcatchment and network data, QGIS is convenient.
QGIS
QGIS.org
Open-source GIS for spatial analysis, layer work and map production.
Get it from the vendorWhat's missing
The interface is old and runs on Windows only. There is no 2D street flooding model — for that you couple with HEC-RAS or use commercial software. There is no automatic pipe sizing, and the model is sensitive to rainfall and imperviousness data — calibrate against measurements when you can.
HEC-RAS
US Army Corps of Engineers
1D/2D hydraulic model for rivers, bridges and culverts, with floodplain mapping. The world standard.
Get it from the vendorWho it suits
Drainage, infrastructure and water engineers, neighbourhood and master-plan designers, local authorities, and civil and environmental engineering students.
Companion tools
EPANET for pressurised water networks, HEC-HMS for the hydrology of natural catchments, and HEC-RAS for rivers and flood maps.
EPANET
US Environmental Protection Agency
Water distribution network model — flows, pressures and water quality. The engine behind most commercial tools.
Get it from the vendor
HEC-HMS
US Army Corps of Engineers
Rainfall-runoff hydrological model — infiltration, unit hydrograph and routing. Pairs with HEC-RAS.
Get it from the vendorBottom line
For urban drainage design and checking existing systems, SWMM is the basic professional tool — free, recognised and well documented.