Introduction

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SWMM Abstract

The United States Environmental Protection Agencies (USEPA's) Storm Water Management Model (SWMM) is a comprehensive computer model for analysis of quantity and quality problems associated with urban runoff. Both single-event and continuous simulation can be performed on catchments having storm sewers, or combined sewers and natural drainage, for prediction of flows, stages and pollutant concentrations. Extran Block solves complete dynamic flow routing equations (St. Venant equations) for accurate simulation of backwater, looped connections, surcharging, and pressure flow.

Modeler can simulate all aspects of the urban hydrologic and quality cycles, including rainfall, snowmelt, surface and subsurface runoff, flow routing through drainage network, storage and treatment. Statistical analyses can be performed on long-term precipitation data and on output from continuous simulation. SWMM can be used for planning and design. Planning mode is used for an overall assessment of urban runoff problem or proposed abatement options.

Introduction

The Storm Water Management Model (SWMM) was originally developed for the EPA between 1969 and 1971 (2) and was the first comprehensive model of its type for urban runoff analysis. Maintenance and improvements to SWMM led to Version 2 in 1975, Version 3 in 1981 and now Version 4 (3,4). Version 4.3 of SWMM (November 1993) is the latest edition of this comprehensive computer model for analysis of quantity and quality problems associated with urban runoff.

Both single-event and continuous simulation may be performed on catchments having storm sewers, or combined sewers and natural drainage, for prediction of flows, stages and pollutant concentrations. The Extran Block (4) solves the complete dynamic flow routing equations (St. Venant equations) for accurate simulation of backwater, looped connections, surcharging, and pressure flow. Using SWMM, the modeler can simulate all aspects of the urban hydrologic and quality cycles, including rainfall, snowmelt, surface and subsurface runoff, flow routing through the drainage network, storage and treatment. Statistical analyses may be performed on long-term precipitation data and on output from continuous simulation.

The model may be used for both planning and design. The planning mode is used for an overall assessment of the urban runoff problem and proposed abatement options. This mode is typified by continuous simulation for several years using long-term precipitation data. Catchment schematization is usually "coarse" in keeping with the planning level of analysis. The Statistics Block may be used for frequency analysis of the long term time series of hydrographs and pollutographs and for identification of individual hydrologic events that may be of special interest. A design-level, event simulation also may be run using a detailed catchment schematization and shorter time steps for precipitation input. The Rain Block is used for processing of hourly and 15-minute precipitation time series for input to continuous simulation. This block also incorporates the statistical analysis procedures of the EPA SYNOP model (5,6) for characterization of storm events. The Statistics Block may alternatively be used for statistical analysis of the precipitation time series.

Although the historical basis of the model was for analysis of urban runoff quality problems, the model often is used just for hydrologic and hydraulic analysis. The Extran Block has proven especially valuable for sophisticated hydraulic analysis of urban drainage networks. An option for both the Extran (St. Venant equations) and Transport (kinematic wave equations) Blocks is use of natural channel cross-section data in the same form as required by the Hydrologic Engineering Center's HEC2 program. In this manner, Extran can perform a dynamic backwater analysis.

The model is designed for use by engineers and scientists experienced in urban hydrological and water quality processes. Although the two user's manuals explain most computational algorithms, an engineering background is necessary to appreciate most methods being used and to verify that the model results are reasonable.

SWMM Version 4 is microcomputer based (DOS-compatible), although the Fortran code may be compiled on any machine. Execution times are on the order of a few seconds to several minutes for most jobs on a 386/486 machine. However, simulation of large areas with many subcatchments and/or channels for many time steps can require several hours on a microcomputer. The user is well advised to use the most powerful microcomputer available.

Data Requirements

For hydrologic simulation in the Runoff Block, data requirements include area, imperviousness, slope, roughness, width (a shape factor), depression storage, and infiltration parameters for either the Horton or Green-Ampt equations for up to 100 subcatchments. (Number of subcatchments, pipes, etc. is variable depending on the compilation). The program is driven by precipitation for up to ten gages (distributed spatially), and evaporation. Additional data are required if simulation of snowmelt, subsurface drainage, and infiltration/inflow options are employed. The subsurface drainage option is especially useful in locations where true overland flow rarely occurs because of flat, sandy soils.

Flow routing can be performed in the Runoff, Transport and Extran Blocks, in increasing order of sophistication. Input data include shape and dimensions of closed conduits and open channels, slope, roughness; and for Extran, invert and ground surface elevations. Additional information is required for the description of weirs, orifices, pumps and storage, if simulated. Extran can also simulate dynamic boundary conditions, e.g., tides. Storage-indication routing may be performed in the Transport and Storage/Treatment Blocks, with appropriate data on volume vs. outflow.

Quality processes are initiated in the Runoff Block and include options for constant concentration, regression of load vs. flow, and buildup washoff, with the latter requiring the most data (7). Additional options include street cleaning, erosion, and quality contributions from precipitation, catchbasins, adsorption, and base flow. EPA Nationwide Urban Runoff Program (8) data are often used as starting values for quality computations. Quality routing in subsequent blocks (except for Extran) requires few additional data, except for the Storage/Treatment Block in which several removal processes can be simulated.

Depending upon the simulation objective, input data requirements can be minimal to extensive. For simulation of a complete drainage network data collection can be accomplished within a few days, but reducing the data for input to the model may take up to 3 person-weeks for a large area (e.g., greater than about 2000 acres). For an Extran simulation of sewer hydraulics, expensive and time consuming field verification of sewer invert elevations often is required. However, most data reduction, such as tabulation of elevations, lengths and dimensions, is straightforward. Blocks may be used in series or individually. For example, the user may generate inlet hydrographs by means other than SWMM and then use only the SWMM flow routing options. SWMM interfacing requirements are clearly defined. E.g., output may be directed to the EPA WASP receiving water model (9).

Calibration data consist of measured hydrographs and pollutographs for use in establishing values of input parameters for which a priori estimates are insufficient. It is often possible to obtain good agreement between predicted and measured hydrographs with little calibration effort. This is not true for quality simulation for which calibration data are essential to obtain credible simulations of pollutographs. Without such measured concentrations and loads, SWMM quality simulation is at best only suited for relative comparisons between control strategies and should not be relied upon for prediction of absolute magnitudes of concentrations and loads (10,7). No firm numbers can be given for the required amount of calibration and verification events, but six of each should provide a robust calibration and verification.

Output

Basic SWMM output consists of hydrographs and pollutographs (concentration vs. time) at any desired location in the drainage system. Depths and velocities are also available as are summary statistics on surcharging, volumes, continuity and other quantity parameters. Additional quality output includes loads, source identification, continuity, residuals (e.g., sludge), and other parameters. The Statistics Block may be used to separate hydrographs and pollutographs into storm events and then compute statistics on parameters such as volume, duration, intensity, interevent time, load, average concentration, and peak concentration. A hydraulic design routine in the Transport Block will resize conduits to pass peak flows. Either metric or U.S. customary units may be used.

Most output is tabular. Microcomputer graphics are accessed through exports to spreadsheets or other graphics packages and through third party software for pre- and post-processing. The latter includes options for dynamic plots of the hydraulic grade line produced by the Extran Block. Linkages have also been prepared to geographic information systems (11,12).

Assumptions and Limitations

The two user's manuals clearly explain almost all computational assumptions of the model, several of which have been mentioned in preceding paragraphs. The model performs best in urbanized areas with impervious drainage, although it has been widely used elsewhere. Quantity simulations are enhanced by the calibration/verification process, but can be expected to resemble measured data fairly accurately if good information is known about area, imperviousness and rainfall. Quality simulations must be calibrated in order to be credible in terms of absolute magnitudes (10,7).

Technical limitations include lack of subsurface quality routing (a constant concentration is used), no interaction of quality processes (apart from adsorption), difficulty in simulation of wetlands quality processes (except as can be represented as storage processes), and a weak scour deposition routine in the Transport Block. The biggest impediment to model usage is the user interface, with its lack of menus and graphical output. The model is still run in a batch mode (the user constructs an input file with an editor), unless third-party software is used for pre- and post-processing.

Application History

SWMM has an impressive longevity. It has been used in scores of U.S. cities as well as extensively in Canada, Europe, Australia and elsewhere (13). A large body of literature on theory and case studies is available, partly documented in a bibliography of SWMM-related publications (14) and elsewhere (7,13). The model has been used for very complex hydraulic analysis for combined sewer overflow mitigation as well as for many stormwater management planning studies and pollution abatement projects, and there are many instances of successful calibration and verification. Because of its public domain status, extensive feedback has been received from users on needed corrections and enhancements, and the model is continuously updated through interaction with CEAM.

References

1. U.S. EPA. 1992. CEAM Systems Development Life Cycle Methodology (SDLCM) Statement of Policy, Standards, and Guidelines - Version 1.00. U.S. EPA, Athens, GA, 30605.

2. Metcalf and Eddy, Inc., University of Florida, and Water Resources Engineers, Inc. 1971. Storm Water Management Model, Vol. I. Final Report, 11024DOC07/71 (NTIS PB-203289), U.S. EPA, Washington, DC, 20460.

3. Huber, W.C. and R.E. Dickinson. 1988. Storm Water Management Model, Version 4, User's Manual. EPA/600/3-88/001a (NTIS PB88-236641/AS), U.S. EPA, Athens, GA, 30605.

4. Roesner, L.A., Aldrich, J.A. and R.E. Dickinson. 1988. Storm Water Management Model, Version 4, User's Manual: Extran Addendum. EPA/600/3-88/001b (NTIS PB88-236658/AS), U.S. EPA, Athens, GA, 30605.

5. U.S. EPA. Areawide Assessment Procedures Manual, Three Volumes. 1976 et seq. EPA-600/9-76-014, U.S. EPA, Cincinnati, OH, 45268.

6. Woodward-Clyde Consultants. 1989. Synoptic Analysis of Selected Rainfall Gages Throughout the United States. Report to U.S. EPA, Woodward-Clyde Consultants, Oakland, CA.

7. Donigian, A.S., Jr. and W.C. Huber. 1991. Modeling of Nonpoint Source Water Quality in Urban and Non-Urban Areas. EPA/600/3-91/039, U.S. EPA, Athens, GA, 30605.

8. U.S. EPA. 1983. Results of the Nationwide Urban Runoff Program, Volume I. Final Report, NTIS PB84-185552, U.S. EPA, Washington, DC, 20460.

9. Martin, J.L. 1993. Modification of the Storm Water Management Model's (SWMM's) Transport Submodel for Creation of a Hydrodynamic Linkage to the Water Analysis Simulation Program (WASP). Report to Camp, Dresser and McKee, Inc. by AScI Corp., Athens, GA, 30605.

10. Huber, W.C. 1986. Deterministic Modeling of Urban Runoff Quality. In: H.C.Torno et. al. (eds.) Urban Runoff Pollution, Proceedings of the NATO Advanced Research Workshop on Urban Runoff Pollution, Montpellier, France. Springer-Verlag, New York, Series G: Ecological Sciences, 10:167-242.

11. Huber, W.C., Zollo, A.F., Tarbox, T.W. and J.P. Heaney. 1991. Integration of the SWMM Runoff Block with ARC/INFO and AutoCAD: A Case Study. Final Report to Foster-Wheeler Enviresponse, Inc. and U.S. EPA, Edison, NJ, Contract VN1-320-420000, from Dept. of Environmental Engineering Sciences, University of Florida, Gainesville.

12. Curtis, T.G., and W.C. Huber. 1993. SWMM AML - An ARC/INFO Processor for the Storm Water Management Model (SWMM). Proc. 1993 Runoff Quantity and Quality Modeling Conference, Reno, NV, (NTIS, in press), U.S. EPA, Athens, GA, 30605.

13. Huber, W.C. 1992. Experience with the U.S. EPA SWMM Model for Analysis and Solution of Urban Drainage Problems. Proceedings, Inundaciones Y Redes De Drenaje Urbano, J. Dolz, M. Gomez, and J.P. Martin, eds., Colegio de Ingenieros de Caminos, Canales Y Puertos, Universitat Politecnica de Catalunya, Barcelona, Spain, p.199-220.

14. Huber, W.C., Heaney, J.P. and B.A. Cunningham. 1985. Storm Water Management Model (SWMM) Bibliography. EPA/600/3-85/077 (NTIS PB86-136041/AS), U.S. EPA, Athens, GA, September 1985.