United States
Environmental Protection
Agency
y
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-223 Oct. 1981
Project Summary
Areawide Stormwater
Pollution Analysis with the
Macroscopic Planning
(ABMAC) Model
Yoram J. Litwin, John A. Lager, and William G. Smith
A simplified, continuous, storm-
water management model (ABMAC)
and user's manual have been devel-
oped to provide an inexpensive,
flexible tool for preliminary assess-
ment of stormwater management
strategy over large developed and
undeveloped areas. The model, in-
tended for use by local engineers and
planners, is adaptable for mini-com-
puter applications.
The model is designed to simulate
runoff, quality, storage, and treatment
while incorporating time and probabil-
ity into the analysis. It allows for
substantial flexibility in terms of input
data requirements and output formats.
All simulated quantities vary on a daily
basis. The model can simulate non-
point source pollution from a maxi-
mum of six land use categories. Six
quality constituents are simulated
including one user specified consti-
tuent.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory. Cincin-
nati. 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
ABMAC is a special-purpose, contin-
uous simulation model employing
simplified techniques for representing
prototype behavior. The input data
requirements are minimal; the areal
and temporal coverage is broad; and the
model is both flexible and easily applied.
The model was specifically developed to
provide a relatively inexpensive, rea-
sonably accurate, and flexible tool for
planning and preliminary sizing of
stormwater facilities over broad areas.
The model was applied and tested in the
San Francisco Bay area from 1976 to
1978, with additional testing from 1978
to 1980. Its acronym stems from
Association of Bay Area Governments
(ABAC) Macroscopic Planning Model
(MAC). ABMAC can be used effectively
in the planning phase of nonpoint
source pollution control studies. Typical
problems for which ABMAC can be
applied include:
• expected changes in pollutant
loadings from urbanization
• long-range pollutant loadings to
water bodies under existing condi-
tions
• general impact of land use changes
on nonpoint pollution
• evaluation of best management
practices and their relative impact
on water quality.
The rational method is used to relate
runoff rate to drainage area, intensity of
rainfall, and perviousness of the surface.
The runoff computations are coupled
with load computations for several
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constituents and land uses. Continuous
calculations for daily time intervals
provide simulation results that are
useful for establishing size-effectiveness
relationships. These easily understand-
able relationships may lead, in some
cases, to the simplification of the
decision-making process in the areawide
planning activities.
A conceptual representation of the
ABMAC model operation is shown in
Figure 1. Rainfall, the driving variable of
the model, is converted into runoff and
thence to an ultimate discharge to
receiving waters through a storage-
•*f
Rainfall
(given)
V_<- -^=^
,',/>t/tt //x // /'/////
//'/// /1 /.'//// //////
treatment balance. This conversion is
based on the gross runoff coefficient (K-
factor) and on quality coefficients
associated with various water quality
parameters. The resulting runoff is
stored in a specific storage volume. If
treatment is specified, actual treatment
facilities and interceptors are simulated.
When the runoff volume exceeds the
specified storage capacity and its
treatment rate, an overflow or discharge
to receiving water occurs.
This is a simple concept. The benefits
of its computer representation are
manifested when a number of water-
' V'/''//''////''//
"'//'//'S^'//'"/'
/////////////7////////'
1 i
Subarea I
Characteristics r ^
(User Specified) I Runoff ^
I (Model
Computed]
Inflow from Separate Subarea
[Lateral Inflow] (Model Computed
from Previous Simulation)
Maximum
Storage
Capacity
(User
Specified)
Pollutant Removals
and Consumptive Use
Storage
(Utilized)
\
Treatment
(User Specified)
Overflow
'[Lateral Outflow]
(Model Computed)
Effluent Discharge
[Lateral Outflow]
(Model Computed)
Receiving Waters
Note: Simulation repeated for each subarea (computational unit).
Figure 1. Schematic representation of ABMAC concept.
2
sheds with different growth character-
istics, land use, and management
practices are being evaluated for a long
sequence of rainfall data.
Data Requirements
The ABMAC model was developed to
provide a relatively inexpensive and
reasonably accurate planning tool with
flexibility in data requirements. The
establishment of a firm data base is,
nevertheless, an important step in the
modeling process. Five categories of
data are needed including system
schematic information, gross runoff
coefficient, rainfall data, hydraulic data,
and quality data.
The system schematic delineates the
configuration of the modeled area, the
characteristics of the individual sub-
areas, and the interrelationship between
subareas. The important characteristics
of a drainage area that must be collected
are: total surface area, percent of the
area that is impervious, general soil
characteristics as they relate to runoff,
area devoted to various land uses,
population density of the residential
areas, and flow conveyance facilities
and reservoir data.
In general, it is desirable to distinguish j
within each drainage area, three broad1
land use categories: natural and protected,
nonurbanized but developable, and
existing urban. For each of these
general land uses, information on the
area, population or population density,
and percent imperviousness is needed
for up to six specific land use categories.
Typical categories include single family
residential, multiple family residential,
commercial, industrial, open space, and
agricultural.
The gross runoff coefficient, defined
as total runoff divided by total rainfall, is
needed to each of the broad land use
categories selected. This coefficient
should be estimated by comparing rain
gage data with monitored runoff data. If
data over a full water year are not avail-
able, the rain gage data should be
compared with runoff monitored during
corresponding storms. In general, at
least 10 storms are needed to provide
adequate representation.
Long-term rainfall data can be ob-
tained from the National Weather
Service (NWS), the best source, either
on tape files or through published daily
summaries. Because the area! variabil-
ity of rainfall for the region in question
must be considered, local rainfall data
sources may be needed to supplement
the NWS data base.
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Information on hydraulic structures
within the broad land use categories
may be also needed, e.g., (1) data on any
reservoirs to determine the change in
storage or change in discharge resulting
from overflows when reservoir is full,
and (2) data on major flow conveyance
facilities, which may affect lateral
inflows to downstream subareas.
Concentration values for runoff
quality constituents can be selected by
either adopting data from national
sources or by local monitoring. The
latter is obviously preferable.
Summary
ABMAC Model represents a tool for
estimating and projecting pollutant
loadings under varying growth plans
and mitigation strategies. The applica-
tion and testing of the model in the San
Francisco Bay area (from 1976 to 1978
and with additional testing from 1978 to
1980) indicated satisfactory arrange-
ment between recorded and simulated
records (within 40%).
The model allows for considerable
flexibility in terms of input data require-
ments and output formats. ABMAC is
also flexible in terms of computer
system applications and has been
tested on a Sperry Univac V76 Mini
Computer* and an IBM Model 3033.
The model offers the option to test all
input data for reliability and consist-
ency—an important feature in large
model applications where preparation
of input data is very tedious and prone to
mistakes.
The model can be used effectively in
conjunction with more sophisticated
models of stormwater runoff and
receiving waters. In such applications,
ABMAC looks at broad areas and long
periods of record, whereas the more
sophisticated models, which can be
both site and storm specific, can be best
applied to selected, relatively small
watersheds.
The project report contains a complete
description of the model, case study
applications, model testing, and user
instructions. The model code is available
through the EPA Project Officer.
The full report was submitted in
fulfillment of Grant No. R-806357 by
the Association of Bay Area Gover-
ments, under the sponsorship of the
U.S. Environmental Protection Agency.
Yoram J. Litwin is with RAMLIT Associates, Berkeley, CA 94705; John A. Lager
and William G. Smith are with Metcalf & Eddy, Inc., Palo Alto, CA 94303.
Douglas C. Ammon is the EPA Project Officer (see below).
The complete report, entitled "Areawide Stormwater Pollution Analysis with the
Macroscopic Planning (ABMAC) Model." (Order No. PB 82-107 947; Cost:
$15.50, subject to change) 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
•ft US GOVERNMENT PRINTING OFFICE 1981—559-017/7400
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
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