United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA/600/S3-88/001 Aug. 1988
c/EPA Project Summary
Storm Water Management
Model User's Manual,
Version 4
Wayne C. Huber, Robert E. Dickinson, Larry A. Roesner, and John A. Aldrich
Version 4 of the U.S. Environ-
mental Protection Agency's (EPA)
Storm Water Management Model
(SWMM) 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, combined sewers and
natural drainage, for prediction of
flows, stages and pollutant
concentrations anywhere in the
system. The EXTRAN Block solves
the complete dynamic flow routing
equations (the St Venant equations)
for accurate simulation of backwater,
looped connections, surcharging,
and pressure flow. Using the total
SWMM package 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.
Version 4 is primarily micro-
computer-based, although the
Fortran code also may be compiled
for use on mainframe computers.
Detailed descriptions are provided
for all blocks (program modules).
Simulation of hydrologic and quality
processes is performed by the
Runoff, Transport, EXTRAN, and
Storage/Treatment Blocks. Overall
program operation is monitored by
the Executive Block, the Statistics
Block is used for statistical analysis
of output time series, the Graph
Block for line-printer graphics of
hydrographs and pollutographs, the
Combine Block for combining output
files, and the Rain and Temp Blocks
for input of time series of
precipitation, temperatures, wind
speeds and evaporation. The Rain
Block also incorporates statistical
evaluation of long-term precipitation
data formerly available only in the
EPA SYNOP model. Detailed
information is provided on the
theoretical background of the model
algorithms as well as on model input
data and model use.
This Project Summary was
developed by ERA'S Environmental
Research Laboratory, Athens, GA, to
announce key findings of the research
project that are fully documented in a
two-part report (see Project Report
ordering information at back).
Introduction
Urban runoff quantity and quality have
long posed problems for cities that have
for many years assumed the
responsibility of controlling stormwater
flooding and treatment of point sources of
wastewater (e.g., municipal sewage).
Within the past two decades, the severe
pollution potential of urban nonpoint
sources (principally combined sewer
overflows and stormwater discharges)
has been recognized, and federal, state
and local legislation has been enacted for
management and control. Massive
studies and data collection efforts -
notably the EPA Nationwide Urban
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Runoff Program of the early 1980s --
have led to a much better understanding
of the problems of urban runoff and of
methods available for its control. The
advent of modern, high-speed
computers in the 1960s and 70s led to
the development of sophisticated tools
for analysis of both the quantity and
quality of urban runoff. The recent
microcomputer "revolution" of the 1980s
now makes these tools and data bases
readily accessible to all engineers and
scientists.
The EPA's SWMM was originally
developed between 1969 and 1971 and
was the first comprehensive model of its
type for urban runoff analysis, although it
has certainly not remained the only one.
Maintenance and improvements to
SWMM led to Version 2 in 1975, Version
3 in 1981 and now Version 4
The model may be used in both
planning and design modes. 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 (e.g., hourly) input of precipitation
data. (Temperature data also are
required if snowmelt is simulated.) The
catchment schematization is usually
"coarse" in keeping with the planning
and overall assessment level of analysis.
The Statistics Block may be used for
frequency analysis of the long-term
output time series of hydrographs and
poliutographs (plots of concentration vs.
time) and for identification of individual
hydrologic events that may be of special
interest for detailed design or other
purposes. A design-level, event
simulation also may be run using a
detailed catchment schematization and
shorter time steps for any desired
precipitation input.
Although the historical basis of the
model was for analysis of urban runoff
quality problems, the model often is used
for pure drainage, hydrologic and
hydraulic analysis. The EXTRAN Block
has proven especially valuable for
sophisticated hydraulic analysis of urban
drainage networks.
Overall SWMM Description
Computational Blocks
An overview of the model structure
appears in Figure 1. tn simplest terms,
the program is constructed in the form of
four blocks (program modules):
1. The input sources. The Runoff Block
generates surface and subsurface
Service
Blocks
Statistics
Block
Graph
Block
Combine
Block
Rain
Block
Temp
Block
Executive
Block
Computational
Blocks
Runoff Block
Transport Block
EXTRAN Block
Storage/ Treatment
Block
Figure 1. Relationship Among SWMM Blocks. Executive Block manipulates interface file
and other off-line files. All blocks may receive off-line input (e.g., tapes, disks)
and user line input (e.g., terminal, cards, etc.).
runoff based on arbitrary rainfall
(and/or snowmelt) hyetographs,
antecedent conditions, and
hydrologic characteristics of the
catchment. Dry-weather flow and
infiltration into the sewer system
may be generated using the
Transport Block. Surface runoff
quality is generated based on
conceptual buildup-washoff
relationships and/or rating curve
relationships and by other optional
mechanisms.
2. The central cores. The Runoff,
Transport and EXTRAN (Extended
Transport) Blocks route flows and
pollutants through the sewer or
drainage system. (Pollutant routing is
not performed by EXTRAN.) The
Transport and EXTRAN Blocks also
may be used with natural cross
sections, input in the format used by
the Corps of Engineers, Hydrologic
Engineering Center, HEC-2 model.
The EXTRAN Block will solve the
complete St. Venant equations for
gradually varied, unsteady flow, and
thus can produce very sophisticated
simulations of flows and heads in the
drainage network. In-system
storage (e.g., detention ponds) may
be simulated in both the Transport
and EXTRAN Blocks. ™
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. The correctional devices. The
Storage/Treatment Block performs
storage-index flow routing and
simulates removal processes by
removal functions or sedimentation
theory. Elementary cost compu-
tations also may performed.
4. The effect (receiving waters).
SWMM does not include a receiving
water model, but a linkage to the
EPA WASP and DYNHYD models is
provided.
Simulation output is in the form of time
series of hydrographs, heads (from the
EXTRAN Block) and pollutographs at any
desired location in the system. The
Statistics Block also may be used to
perform an event separation and
frequency analysis of any time series,
including input precipitation.
Quality constituents for simulation may
be arbitrarily chosen for any of the
blocks (except for EXTRAN, which does
no quality simulation), although the
different blocks have different constraints
on the number and type of constituents
that may be modeled.
Service Blocks
n addition to the four computational
u.ocks described above, six service
blocks are used:
Executive Block. This block creates
off-line files and monitors the sequential
execution of all blocks. An "interface file"
is used for transmission of output time
series from one block for use as input to
another.
Graph Block. Line-printer plots of
hydrographs, pollutographs and certain
other time series may be obtained using
this block.
Combine Block. This block allows the
manipulation of multiple interface files in
order to aggregate results of multiple
previous runs for input into subsequent
blocks. In this manner large, complex
drainage systems may be partitioned for
simulation in smaller segments. An ASCII
version of the interface file also may be
created for ready access by other
microcomputer software.
Rain and Temp Blocks. Continuous
simulation relies upon precipitation input
using long-term data available on
magnetic tapes from the National
Climatic Data Center in Asheville, NC (or
from the Atmospheric Environment
Service in Canada). Temperature, wind
ed and evaporation data also may
.-nuired for some applications (e.g.,
snowmelt). The Rain and Temp Blocks
process such long-term input data for
use by SWMM. The Rain Block also may
be used to perform an event separation
and statistical analysis of rainfall data in
the manner of the EPA SYNOP
("Synoptic Precipitation") model.
Statistics Block. This block has the
capability to evaluate the time series
output from a continuous (or single
event) simulation, separate output into
discrete storm events, rank the events
according to almost any desired criterion
(e.g., peak or average runoff rate,
pollutant load, etc.), assign empirical
frequencies and return periods to runoff
and pollutant parameters, tabulate and
graph the results, and calculate statistical
moments. Output from this block thus
may be used to identify key events for
further study and for many other
screening and analytical purposes.
User Requirements
Personnel
The model is designed for use by
engineers and scientists experienced in
urban hydrological and water quality
processes. Although the 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.
Computer Facilities
SWMM Version 4 is principally
microcomputer based, although the
Fortran code may be compiled on any
machine. The largest of the individual
blocks is about 500 K bytes. Depending
upon the memory capacity of the
machine, an overlay procedure may or
may not be necessary for compilation
and linking of the whole model.
The compiled EPA version of the
model requires an IBM PC, AT or
compatible microcomputer with 640 K
RAM. A math co-processor and hard
disk are recommended. Execution times
are on the order of a few to several
minutes for most jobs. Simulation of
large areas with many subcatchments
and/or channels for many time steps,
however, can require several hours on a
microcomputer.
Data Requirements
Input Data
Depending upon the simulation
objective, input data requirements can be
minimal to extensive. For simulation of a
complete drainage network, data
collection from various municipal and
other offices within a city is possible to
accomplish 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.
On an optimistic note, however, most
data reduction—tabulation of slopes,
lengths and diameters — is
straightforward. SWMM is flexible enough
to allow different modeling approaches to
the same area. A specific, individual
modeling decision upstream in the
catchment may have little effect on the
predicted results at the outfall.
Furthermore, many problems lend
themselves to a very low level of detail,
especially for quality predictions. In such
cases, input data requirements are
greatly reduced. Input (and output) to all
blocks can be in either U.S. customary or
metric units.
Calibration and Verification
Data
Calibration data are measured
hydrographs and pollutographs for use in
establishing values of input parameters
for which a priori estimates are
insufficient. For example, for simulation of
surface runoff quantity, imperviousness
often is used to calibrate hydrograph
volumes, and subcatchment width (a
shape parameter) often is used to
calibrate hydrograph peaks. In many
cases, it is possible to obtain good
agreement between pedicted 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.
Verification data are provided in the
form of additional measured hydrographs
and pollutographs so that the parameter
estimates made during the calibration
phase can be checked. 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.
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Changes for Version 4
Not all users will require Version 4,
however, because in most respect the
computations are identical to Version 3.
Significant modifications ar listed below.
1. Input/output has been enhanced. All
input is free-format with line (data
group) identifiers. The line identifiers
are now a requirement because the
program uses them as the only
means of separating one data group
from another. Program-generated
error messages make it easier to
locate problems caused by improper
entry of data. Input strings of up to
230 characters are allowed in
SWMM 4. Strict column sequencing
of input data is still possible as long
as at least one space separates the
fields.
Comment lines are allowed in this
version of SWMM. A comment line
begins with an asterisk in the first
column. A template for full screen editing
is included as an example for each block
of SWMM. The templates include brief
comments about each input field.
2. Errors have been corrected for all
blocks as best they are known.
3. EXTRAN is available in a metric
format and uses data group
identifiers. Additional features
include: a "hot start" capability
(restart from end of previous run);
natural channel cross sections, with
cross-sections input as in HEC-2
(also available in the Transport
Block); minor improvements to
surcharge and flow routing routines;
pump rating curves; and automatic
adjustment of small pipe lengths.
4. SWMM output may be linked to the
DYNHYD4 (water quantity) and
WASP4 (water quality) programs for
receiving water quality simulation.
(These models are supported by the
EPA Center for Exposure
Assessment Modeling, Athens, GA.)
Runoff, Transport, Storage/
Treatment, and EXTRAN interface
files can be read by both DYNHYD4
and WASP4. DYNHYD4 reads only
the flows from the interface file.
WASP4 reads water quality loading
rates from Runoff, Transport, and
Storage/Treatment. A model of an
estuary, therefore, can include
Runoff to generate surface pollutant
loadings, Transport or EXTRAN for
detailed simulation of surface routing
network, DYNHYD4 for simulating a
link-node estuary model, and
WASP4 for simulating the water
quality of the estuary under the
stress of the Runoff or Transport
pollutant loadings.
5. The microcomputer version permits
greater manipulation of interface files
and other scratch and I/O files. The
Combine Block may he used to
convert any interface file to
formatted (ASCII/text) files capable
of being read by programs such as
Lotus 1-2-3 or other software. All
interface files can be permanently
saved and retrieved. Users can
input their own interface files.
6. A subsurface routing package
(quantity only) has been added to
the Runoff Block. A separate
accounting is made for the
unsaturated and saturated zones,
and the water table elevation can
fluctuate. Baseflow to Runoff
channel/pipes may be generated
from the saturated zone.
7. The Runoff Block (through access to
the Rain Block) will read the new
National Weather Service format for
precipitation tapes. In general,
continuous simulation is easier, with
several options for input of
precipitation data and other time
series. User-defined input time
series also may be used. Continuous
simulation is capable of using up to
ten rain gages.
Instead of processing continuous
meteorological data in the Runoff Block,
two new blocks have been added--
Rain and Temp. These include the
capabilities of the former Subroutine
CTRAIN in Runoff with additional
statistical analysis similar to the SYNOP
program used for EPA area-wide
assessment procedures. It also is
possible to process rainfall data with
SWMM Statistics Block.
8. Numerical methods have been
improved in the Runoff Block. A
variation of the extrapolation method
is used to couple the equations for
nonlinear reservoir conditions,
evaporation, infiltration, and
groundwater flow. Subroutine Gutter
no longer has convergence
problems. There is no distinction any
more between single event and
continuous simulation, eliminating
parameter ICRAIN. Runoff uses a
wet, dry and intermediate (wet/dry)
time step defined by the user, thus
decreasing the time required for
continuous simulation.
9. This version of SWMM uses more
Fortran primitives. There is one
subroutine to read interface files, one
subroutine to write interface files,
one clock subroutine, one file-
opening routine, etc., for all blocks.
The common functions of all blocks
are exactly the same.
10. This version can be made more
modular than the EPA Version 3 for
mcrocomputer. It is possible to run
files containing only the blocks of
interest, saving the interface file for
use by the next block. This permits
file compression for ease of
distribution and much faster
execution times.
11. The Graph Block is no longer limited
to 200 data points. An unlimited
number of points for both measured
and predicted graphs can be plotted
on the line printer. The Graph Bloc!
plots loadographs (mass/time versu
time) or pollutographs (concentration
versus time). Enough information is
provided about the interface file
(containing the hydrograph and
pollutograph time series) to permit
the use of better graphic capabilities
of microcomputers.
12. The user has more control over
printout in this version of SWMM.
Most printout can be bypassed at the
user's discretion. Error messages are
summarized at the end of a run
instead of being printed every time
step.
13. Microcomputer users will see the
current time or time step, as well as
other program messages, printed on
the screen during the simulation.
When Should SWMM Be Used?
SWMM is a large, relatively
sophisticated hydrologic, hydraulic and
water quality simulation program. It is not
appropriate for all applications or for all
personnel, and alternative hydrologic
models exist. A large body of literature
on theory and case studies is available
for SWMM. A bibliography of SWMM(
related literature is available
(EPA/600/3-85/077).
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SWMM is certainly formidable both in
terms of its size and capabilities. Who,
then, should use SWMM and for what
purposes? Some criteria for usage are
given below:
1. The engineer must be
knowledgeable of the modeling
techniques (e.g., non-linear
reservoirs, kinematic waves, St.
Venant equations, buildup-washoff
equations). An appreciation for how
physical processes may be
simulated in a Fortran program is a
necessity. As a corollary, the
engineer is assumed to be familiar
with the problem to be solved and
with customary techniques for
handling it. A clear problem
definition is a prerequisite to any
solution methodology.
2. By virtue of the problem size (e.g.,
sewer system with hundreds of
pipes) or complexity (e.g., hydraulic
controls, backwater), a sophisticated
model must be used. It may be
borne in mind, however, that if
calibration/verification data are
available, SWMM also may be used
as a very simple "black box" model
with minimal input data, at the
expense of computer overhead to
manage the program size and off-
line files.
3. Quality must be simulated. Although
other models also simulate quality,
SWMM is perhaps the most flexible.
Of course, SWMM often is applied
just to quantity problems.
Users more familiar with an adequate
alternative methodology are probably
better advised to remain with that
methodology. In the final analysis, the
engineer/analyst is responsible for the
decisions made using any technique of
analysis; the technique or model is only
a tool that must be clearly understood by
those using it.
The Stormwater and Water
Quality Model Users Group
This group began as the EPA SWMM
User Group and has functioned since
1973 as a forum for discussing all
aspects of Stormwater quantity and
quality modeling. The SWMM program
has benefited greatly from user
feedback, and the User Group has been
i particularly useful means for
disseminating information on SWMM and
other models. The group is open to all
interested modelers. Semiannual
meetings are held in the United States
and Canada. Information about the group
is published in the newsletter of the
Center for Exposure Assessment
Modeling, which appears periodically.
Further information about the Users
Group and the newsletter can be
obtained from Mr. Thomas 0. Barnwell,
Jr., Center for Exposure Assessment
Modeling, Environmental Research
Laboratory, USEPA, College Station
Road, Athens, Georgia 30613 (telephone:
(404) 546-3210).
SWMM Availability
The program (Fortran source code and
executable code for microcomputers) is
available from the EPA Center for
Exposure Assessment Modeling, (CEAM)
listed above. Documentation is available
from the National Technical Information
Service (NTIS — see last page) and
from the report authors. Limited support
is also available from the University of
Florida. Future updates and
improvements will be made through the
newsletter of the CEAM.
Disclaimer
Every attempt has been made to
ensure that the SWMM program
performs as represented in the
documentation, but as with all large
computer models, some lingering bugs
will persist. The use of SWMM and
interpretation of its output must remain
the sole responsibility of the user.
Neither EPA nor the model authors can
assume responsibility for model use or
decisions based on model use.
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Wayne C. Huber and Robert E. Dickinson are with the University of Florida.
Gainesville, FL 32611; Larry A. Roesner and John A. Aldrich are with Camp,
Dresser and McKee, Inc., Maitland, FL 32751 and Annandale, VA 22003,
respectively.
Thomas O. Barnwell, Jr., is the EPA Project Officer (see below).
The complete report consists of two parts, entitled "Storm Water Management
Model, Version 4--,"
"Part A- Users Manual," (Order No. PB 88-236 641/AS; Cost: $44.95)
"Part B: EXTRAN Addendum," (Order No. PB 88-236 658/AS; Cost: $19.95)
The above reports will be available only from: (cost subject to change)
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
College Station Road
Athens, GA 30613
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-88/001
OOOQ3Z9 PS
0 S EIIVIR PROTECTION flGENCT
REGION 5 LIBRARY
230 S OEftRSORM STREET
CHICAGO IL 60604
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