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The United States Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) ^{[1][2][3][4][5][6] [7]}is a dynamic rainfall-runoff-subsurface runoff simulation model used for single-event to long-term (continuous) simulation of the surface runoff/subsurface runoff quantity and quality from primarily urban/suburban areas. The runoff or hydrology component of SWMM operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads after simulation evaporation and infiltration losses from the drainage basin. The routing or hydraulics portion of SWMM transports this runoff and possible associated water quality constituents through a system of closed pipes, open channels, storage/treatment devices, pumps, orifices, weirs and regulators. SWMM tracks the quantity and quality of runoff generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period composed of multiple fixed or variable time steps. The water quality constituents can be simulated from the subcatchments through a hydraulic network with optional first order decay and linked pollutant removal, Best Management Practice (BMP) and Low Impact Development (LID) removal and treatment can be simulated at selected storage nodes. SWMM is one of the hydrology transport models which the EPA and other agencies have applied widely.

[edit] History

SWMM was first developed between 1969-1971 and has undergone several major upgrades since then. The major upgrades were: (1) Version 2 in 1975, (2) Version 3 in 1981 and (3) Version 4 in 1988. The current SWMM edition, Version 5, is a complete re-write of the previous Fortran release in the programming language C, and it can be run under Windows XP, Windows Vista and Windows 7.

EPA SWMM 5 provides an integrated graphical environment for editing watershed input data, running hydrologic, hydraulic, real time control and water quality simulations, and viewing the results in a variety of graphical formats. These include color-coded thematic drainage area maps, time series graphs and tables, profile plots, scatter plots and statistical frequency analyses.

This latest re-write of EPA SWMM was produced by the Water Supply and Water Resources Division of the U.S. Environmental Protection Agency's National Risk Management Research Laboratory with assistance from the consulting firm of CDM Inc under a Cooperative Research and Development Agreement (CRADA).

The update history of SWMM 5 from the original SWMM 5.0.001 to the current version SWMM 5.0.018 can be found at the EPA SWMM 5 Downloads in the file epaswmm5_updates.txt.

[edit] Model parameters

Model parameters for subcatchments are surface roughness, flow path length; for Infiltration: Horton: max/min rates and decay constant; Green-Ampt: hydraulic conductivity, initial moisture deficit and suction head; Curve Number: NRCS (SCS) Curve number; All: time for saturated soil to fully drain; for Conduits: Manning’s roughness; for Water Quality: buildup/washoff function coefficients, first order decay coefficients, removal equations. A study area can be divided into any number of individual subcatchments, each of which drains to a single point. Study areas can range in size from a small portion of a single lots up to thousands of acres. SWMM uses hourly or more frequent rainfall data as input and can be run for single events or in continuous fashion for any number of years.

[edit] Capabilities

SWMM 5 accounts for various hydrologic processes that produce surface and subsurface runoff from urban areas. These include:

Time-varying rainfall for an unlimited number of raingages for both design and continuous hyetographs evaporation of standing surface water on watersheds and surface ponds snowfall accumulation, plowing and melting rainfall interception from depression storage in both impervious and pervious areas infiltration of rainfall into unsaturated soil layers percolation of infiltrated water into groundwater layers interflow between groundwater and pipes and ditches nonlinear reservoir routing of watershed overland flow.

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

SWMM also 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:

handle drainage networks of unlimited size use a wide variety of standard closed and open conduit shapes as well as natural or irregular channels model special elements such as storage/treatment units, outlets, 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 steady, kinematic wave or full dynamic wave flow routing methods model various flow regimes, such as backwater, surcharging, pressure, reverse flow, and surface ponding apply user-defined dynamic control rules to simulate the operation of pumps, orifice openings, and weir crest levels

In addition to modeling the generation and transport of runoff flows, SWMM can also estimate the production of pollutant loads associated with this 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 washoff from specific land uses during storm events direct contribution of wet and dry rainfall deposition reduction in dry-weather buildup due to street cleaning reduction in washoff load due to BMP's and LID's 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

[edit] Integrated solution

One of the great advances in SWMM 5 was the integration of Urban/Suburban Subsurface Hydrology with the Hydraulic computations of the drainage network. This advance is a tremendous improvement over the separate Subsurface Hydrologic and Hydraulic computations of the previous versions of SWMM because it allows the modeler to conceptually model the same interactions that occur physically in the real open channel/shallow aquifer environment. The SWMM 5 numerical engine calculates the surface runoff, subsurface hydrology and assigns the current climate data at either the wet or dry hydrologic time step. The hydraulic calculations for the links, nodes, control rules and boundary conditions of the network are then computed at either a fixed or variable time step within the hydrologic time step by using interpolation routines and the simulated hydrologic starting and ending values.

Figure 1. SWMM 5's QA/QC Master Example Network. This one network includes examples 1 through 7 from the SWMM 3 and SWMM 4 Manuals

An example of this integration was the collection of the disparate SWMM 4 link types in the Runoff, Transport and Extran Blocks to one unified group of closed conduit and open channel link types in SWMM 5 and a collection of Node types.

[edit] SWMM 3,4 to 5 converter

The SWMM 3 and SWMM 4 converter can convert up to two files from the earlier SWMM 3 and 4 versions at one time to SWMM 5. Typically you would convert a Runoff and Transport file to SWMM 5 or a Runoff and Extran File to SWMM 5. If you have a combination of a SWMM 4 Runoff, Transport and Extran network then you will have to convert it in pieces and copy and past the two data sets together to make one SWMM 5 data set. The x,y coordinate file is only necessary if you do not have existing x, y coordinates on the D1 line of the SWMM 4 Extran input data[ set. You can use the command File=>Define Ini File to define the location of the ini file. The ini file will save your conversion project input data files and directories.

[edit] SWMM5 objects

The SWMM 5.0.017 main objects are: rain gages, watersheds, nodes, links, pollutants, landuses, time patterns, curves, time series, controls, transects, aquifers, unit hydrographs, snowmelt and shapes. Other related objects are the types of Nodes and the Link Shapes. The purpose of the objects is to simulate the major components of the hydrologic cycle, the hydraulic components of the drainage, sewer or stormwater network and the buildup/washoff functions that allow the simulation of water quality constituents.

A watershed simulation starts with a precipitation time history.

[edit] See also

• DSSAM Model
• Hydrology
• Infiltration
• Hydraulics
• Surface runoff
• Drainage Basin
• Precipitation (meteorology)
• Antecedent moisture
• Evapotranspiration
• EPANET

[edit] References

[edit] External links

• EPA SWMM 5.0.018 page
• SWMM 3,4 to 5 Conversion Tools
• SWMM 2000+