United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-109a & b Sept. 1984
&ER& Project Summary
Storm Water Management
Model User's Manual
Version
Wayne C. Huber, James P. Heaney, Stephan J. Nix, Robert E. Dickinson, and
Donald J. Polmann
A description is given of the third
update of a user's manual for the U.S.
Environmental Protection Agency (EPA)
Storm Water Management Model
(SWMM). The manual should be used
with Addendum I to run the Extran block
(detailed hydrologic flow routing). The
SWMM is a comprehensive mathemati-
cal model developed for both continu-
ous and single event simulation of
urban runoff quantity and quality in
storm and combined sewer systems. All
aspects of the urban hydrologic and
quality cycles are simulated, including
surface runoff, transport through the
drainage network, storage and
treatment, and receiving water effects.
(The latter component is currently
under revision by EPA.)
Detailed descriptions are provided in
the User's Manual for all blocks except
the Receiving Water Block. Blocks
include Runoff, Transport, Storage/
Treatment, Combine, Statistics, and
Graph. The latter three are service
blocks, and the first three are the
principal computation blocks. In
addition, extensive documentation of
new procedures is provided in the text
and in several appendices.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Urban runoff quantity and quality have
long posed problems for cities who have
for many years assumed the responsi-
bility of controlling stormwater flooding
and treating 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 control
legislation has been enacted. The advent
of modern, high-speed computers has led
to the development of complex,
sophisticated tools for analyzing both the
quantity and quality of nonpoint pollution.
The EPA's SWMM was developed
between 1969 and 1971 and was one of
the first such models; it is by no means
the only one, however. Since its original
development, it has been continually
maintained and updated, and it is perhaps
the best known and most widely used of
the several available urban runoff
quantity and quality models.
In its original form, the SWMM was
strictly a design model oriented toward
the detailed simulation of a single storm
event; it used relatively short time steps
and included as much catchment and
drainage detail as necessary. The model
is now routinely used in a planning
context as well as 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 (e.g., hourly)
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time steps and minimum detail in the
catchment scheme. The SWMM follows
the Corps of Engineers STORM model in
this capability for urban hydrology.
Continuous simulation may also be
used to identify hydrologic events that
may be of special interest for design or
other purposes. For example, event
magnitudes (such as flow volumes or
peaks and pollutant loads) of a desired
frequency or return period can be
obtained from the Statistics Block of the
SWMM. These events may then be
analyzed in detail if desired.
Before a commitment is made to any
large computer program, screening
models may be used to provide a first
estimate of the magnitude of urban runoff
quantity and quality problems. Such
models require no computers.
Overall SWMM Description
Overview
An overview of the model structure
appears in Figure 1. In simplest terms, the
program is constructed in the form of
blocks, as follows:
1. The input sources. The Runoff Block
generates surface runoff based on
arbitrary rainfall hyetographs, ante-
cedent conditions, land use,
topography, etc. Dry-weather flow
and infiltration into the sewer
system may be optionally generated
using the Transport Block.
2. The central core. The Transport and
Extended Transport Blocks combine
and route the inputs through the
drainage system. The user's manual
and documentation forthe Extended
Transport Block has been prepared
as an addendum to the Version III
User's Manual and is available as a
separate document. (Roesner, L.A.,
Shubinski, R.P., and Aldrich, J.A.,
"Storm Water Management Model
User's Manual Version III: Adden-
dum I, EXTRAN," EPA-600/2-84-
109b, U.S. Environmental Protec-
tion Agency, Cincinnati, Ohio.
3. The correctional devices. The
Storage/Treatment Block charac-
terizes the effects of control devices
on flow and quality. The simulation
is conceptual and may be adapted to
most wet- and dry-weather control
devices. Elementary cost
computations are also made.
4. The effect (receiving waters). The
Receiving Block routes hydrographs
and pollutographs through the re-
ceiving waters, which may consist
of a stream, river, lake, estuary, or
bay. This block is currently under-
going extensive revisions at the EPA
Environmental Research Labora-
tory in Athens, Georgia, and is not
presently included in the SWMM
Version III.
Since the program objectives are
directed toward analyzing both detailed
time and spatial effects and also gross
effects such as total pounds of pollutant
discharged in a given storm, it is
considered essential to work with both
continuous output (magnitude versus
time), referred to as hydrographs and
pollutographs, and with daily, monthly,
annual, and total simulation summaries
.Data Card
'Input, Typical
Output
Files
(for continuous simulation). Such
summaries may be augmented using the
frequency analysis of the Statistics Block.
Quality constituents for simulation
may be arbitrarily chosen for any of the
blocks, though the different blocks have
different constraints on the number and
type of constituents that may be modeled.
The Extended Transport Block is the only
block that does not simulate water
quality.
As indicated in Figure 1, the Transport,
Extended Transport and Storage/Treat-
ment Blocks may all use input and provide
output to any block, including
themselves. The Runoff Block uses input
from no other block, and the Receiving
Block provides output for no other block.
Service Blocks
In addition to the Runoff, Transport,
Extended Transport, Storage/Treatment,
and Statistics Blocks mentioned above,
the Executive and Combine Blocks are
included as service blocks. The Executive
Block organizes the sequence of blocks to
be executed, manages off-line files, and
permits input of measured data for
graphing. The Combine Block collects
output from previous runs (stored on off-
line files) into one file for input to a
subsequent run. In this manner, large,
complex drainage systems may be parti-
tioned into smaller segments for simula-
tion.
User Requirements
Computer Facilities
A large, high-speed computer is gener-
ally required to operate the SWMM. The
largest of the blocks (Runoff, with the
Executive Block) requires about 90,000
Output to f'., rt
Other I Data U
Programs Any Output |
Control |
and
Service
Blocks
Blocks
1 Combine
Block
I
tational ^
r
<-
L_
Executive Block
Requires No \
Output File
from Other
Blocks
r
Runoff
Block
"6
/Way Require '
Output File 1
from |
Another i
jfi/ocA
Block
-d?
May Require
Output File
from
Another
f Block ,
fi/ocA
-d?
Ma/ Require i
Output File |
from I
Another •
fB/oc* j ,
Storage/
Treatment
Block
— 1
May Require
Output File
from
Another
,Block
Receiving
Water
Block
e
L
Graph
Block
Statistic
Block
foata
Output File
Created.
| Data \J Typical \ Data I
Figure 1. An overview of the SWMM structure.
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words of storage with overlay procedures.
Examples of machines on which the
program has been successfully run include
the IBM 370 and 3033,' Amdahl 470,
UNIVAC 1108, CDC 6600, CYBER, VAX
11 /780, Prime 550, and Burroughs 1900.
Execution time and use of off-line storage
tend to increase on minicomputers. Typi-
cal execution times on an IBM 3033
range from 5 to 20 CPU seconds, with
costs of $10 to $20. The most time-
consuming blocks tend to be the Runoff
Block for a continuous simulation of
several years and the Extran Block when
it requires a time step of only a few
seconds.
Data Requirements
The data requirements for the SWMM
may be extensive. 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 is time-consuming
and may take up to 3 man-weeks for a
large area (e.g., greater than 2000 acres).
On an optimistic note, however, most of
the data reduction is straightforward—for
example, tabulation of slopes, lengths,
and diameters of the sewer system. The
SWMM is flexible enough to allow
different modeling approaches to the
same area. A specific, individual model-
ing 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.
Metric Units
Use of metric units for input and output
(I/O) of data and results is now allowed in
the Runoff, Transport, and Storage/
Treatment Blocks as an alternative to U.S.
customary units. (Metric I/O to the Extran
and Receive Blocks may be added in the
future.) For the most part, the metric units
are used strictly for I/O; all internal
quantity calculations are still performed
in feet-seconds units.
Calibration and Verification
The SWMM is designed as a determin-
istic model in that the physics of the
processes are simulated sufficiently well
to produce accurate results with minimal
calibration if all input parameters are
accurate. This concept fails in practice
because the input data and the numerical
'Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
methods are not accurate enough for
many real applications. Furthermore,
many computational procedures within
the model are based on limited data
themselves, especially surface quality
predictions.
Asa result, it is essential that local cali-
bration and verification data be available
at specific application sites to lend
credibility to the predictions of any urban
runoff model. These data are usually in
the form of measured flows and concen-
trations at outfalls or combined sewer
overflow locations. Although hydro-
graphs may be predicted fairly accurately
with only a moderate calibration effort,
pollutographs cannot. Thus measured
quality data are crucial to establish proper
magnitudes of quality predictions.
An Example of Output
The SWMM output is varied andean be
voluminous at the user's option. Output
may include echoes of all input data,
time-step listings of hydrographs and
pollutographs, quantity and quality sum-
maries (including continuity checks),
frequency analysis of output, and line
printer plots of predicted and measured
hydrographs and pollutographs. Such
plots are often the most useful form of
output from single-event simulations.
The Stormwater and Water
Quality Model Users Group
The group was originally the EPA
SWMM Users 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 Users Group has been a
particularly useful means for dissemina-
ting information on SWMM and other
models. The group is open to all
interested modelers. Semiannual meet-
ings are held in the United States and
Canada, and the group also publishes a
periodic newsletter. Further information
can be obtained from Mr. Thomas 0,
Barnwell, Center for Water Quality Model-
ing, Environmental Research Laboratory,
USEPA, College Station Road, Athens,
Georgia 30613 (Telephone: 404-546-
3175).
SWMM Availability
The program and documentation are
available from the National Technical
Information Service (NTIS) (see last page),
the EPA Center for Water Quality
Modeling mentioned in the preceding
section, and the report authors. Future
updates and improvements will be
announced through the newsletter of the
Stormwater and Water Quality Model
Users Group.
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 the
SWMM and interpretation of its output
must remain the responsibility of the
user. Neither EPA nor the model authors
can assume responsibility for decisions
made on the basis of the model.
The full report was submitted in fulfill-
ment of Cooperative Agreement No. CR-
805664 by the University of Florida under
the sponsorship of the U.S. Environment-
al Protection Agency.
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Wayne C, Huber, James P. Heaney. and Robert E. Dickinson are with University of
Florida, Gainesville. FL 32611; Stephen J. Nix is with Syracuse University,
Syracuse, NY 13210; and Donald J. Polmann is with Law Engineering and
Testing Company, Tampa. FL 33623.
Douglas C. Ammon is the EPA Project Officer (see below).
The complete report consists of two parts:
"Storm Water Management Model User's Manual, Version III," (Order No. PB
84-198 423; Cost: $38.50. subject to change).
"Storm Water Management Model User's Manual, Version III: Addendum I
EXTRAN," (authored by Larry A. Roesner, Robert P. Shubinski, and John A.
Aldrich who are with Camp Dresser & McKee, Inc., Amadale, VA 22003; Order
No. PB 84-198 431; Cost: $20.50, subject to change).
The above reports will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
-fi U S GOVERNMENT PRINTING OFFICE. 1984 —759-015/7800
United States
Environmental Protection
Agency
Center for Environmental Rdsearctv
Information j 7 ,".
Cincinnati OH 45268 ' " >•
V.
Official Business
Penalty for Private Use $300
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