United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-106 Feb. 1984
&EPA Project Summary
Stormwater Hydrological
Characteristics of Porous and
Conventional Paving Systems
Gary F. Goforth, Elvidio V. Diniz, and J. Brent Rauhut
When a watershed is physically
altered as the result of urban develop-
ment, local stormwater hydrology and
water resources are affected. Using
porous pavement in parking lots and
other places where stormwater deten-
tion is feasible is one way to lessen the
harmful aspects of urban runoff. A
study of both porous and conventional
pavement systems in Austin, Texas,
was undertaken. The objectives of the
study were to: (1) review past experience
with porous pavements, (2) develop an
aggregate-asphalt mix design and
construction specifications for a porous
asphalt pavement system and con-
struct a parking lot, (3) evaluate porous
and nonporous pavements, (4) develop
a design methodology for porous
pavement stormwater storage systems.
The report, upon which this summary
is based, includes details of precon-
struction planning, construction, and
post construction testing. Each of the 5
pavements studied was instrumented to
sample for climatic, hydraulic, and
water quality parameters. Hydrographs
of pavement discharge were compared
with simulated hydrographs resulting
from a revised version of PORPAV, a
computer program that models the
stormwater hydraulics of a porous
pavement facility. The results of the
comparison indicate the capabilities of
PORPAV and its potential application
to similar future porous pavement
studies. The hydraulic relationships
incorporated into PORPAV were used
to develop a method to aid engineers
and developers in designing porous
pavement systems. Incorporating such
items as the hydraulic relationships of
rainfall intensity, pavement and base
permeability, and soil infiltration rates
makes the method versatile enough to
apply it to various design objectives.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, Ohio,
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
Impervious urban areas such as roofs,
streets, and parking lots reduce infiltration
capacity of urban watersheds and produce
a corresponding increase in runoff rates
and volumes. Stormwater runoff from
developed areas has been recognized as a
source of contaiminant loading to surface
and ground water resources. Impervious
areas generally have limited assimilative
properties and in some cases tend to yield
contaminants that are not amenable to
control and removal using standard
maintenance procedures. A porous pave-
ment facility is an innovative solution to
the problem of stormwater drainage and
detention from parking and other low
traffic areas in the urban landscape. A
schematic cross section of a typical
porous pavement facility is presented in
Figure 1. This type of pavement can use
the natural infiltration capacity of the soil
to absorb rainfall and local runoff after
accumulation in a porous base consisting
of sand or large-diameter, open-graded
gravel. If infiltration into the soil is
undesirable or not practical, lateral
drainage to a sump or channel can be
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Porous Pavement
Surface Layer
L Length of Pavement
W Width of Pavement
D Depth of Base Layer
Sb Slope of Base Layer
Rainfall Intensity
Infiltration Rate
Depth of Water in Base Layer
Qs Surface Discharge
Qb Collection Dram Discharge
Figure 1. Cross section of typical porous pavement facility
Collection Drain
provided. This type of pavement can be
designed to minimize changes in the
runoff characteristics of a watershed
during and after development.
Approach
An extensive monitoring program was
initiated in the City of Austin, Texas, to
document the hydraulic and pollutant
transport characteristics of several
porous and conventional pavement
facilities. The monitoring network of five
parking lots represented a variety of
porous pavement surfaces (porous asphalt,
lattice block and gravel trench) as well as
a conventional asphalt and conventional
concrete lot.
Because the parking lots were small and,
when it rained, the runoff was rapid,
sampling had to be done quickly. This
coupled with the lack of rainfall during the
study period prompted the decision to use
simulated rainfall. With sprinkler-induced
"storms," the intensity, duration, and
timing of the rainfall was controlled.
Impact-type sprinklers, furnished by the
City of Austin Parks and Recreation
Department, were used during the test
with fire hydrants supplying the water.
The number of sprinklers was varied for
each simulated storm, and care was
taken to provide uniform coverage of the
lots. The gravel trench lot was too large
for sprinkler coverage so large water
trucks provided by the City of Austin were
used. Different storm (or rainfall) intensi-
ties were obtained by varying the number
of trucks used, trips made, and number of
trucks releasing water at one time.
Estimates of the runoff were obtained
from water level measurements at a 90-
degree V-notch weir at each lot except for
the gravel trench lot, which incorporated
a collection basin with an outflow pipe.
Sample collection and handling and
analytical techniques conformed to
recommended EPA or American Public
Health Association methodology. Labora-
tory analyses were conducted by the
Guadalupe-Blanco River Authority, Seg-
um, Texas. To determine the potential for
ground water contaimination from trace
organic substances in the discharge from
the porous asphalt and lattice block lots,
samples were analyzed for volatile and
semivolatile priority pollutants. Laboratory
analyses were conducted according to
EPA methodology.
Being able to predict the hydraulic
characteristics of stormwater runoff is a
valuable tool for assessing control
strategies. Stormwater hydraulic char-
acteristics of the porous and nonporous
pavement study sites were evaluated
using a revised version of the computer
model PORPAV. The data collected
during this study and the resulting model
calibration and verification effort provide
an insight into the capabilities of PORPAV
as a model of the stormwater hydraulics
of a porous pavement facility and its
potential application to future studies
where similar pavement projects are
desired.
Results
The review and evaluation of the
porous and conventional pavements
resulted in development of
• an aggregate-asphalt mix design for
the porous asphalt surface
• design specifications for porous
pavement systems, and
• a tentative set of construction
specifications.
Constructing the porous asphalt parking
lot provided valuable experience in
preconstruction planning, installing the
aggregate reservoir base course, and
placing the porous asphalt surface
course. In addition, the results of indirect
tensile strength testing, in situ perme-
ability, and a visual inspection of the
parking surface after 18 months of
vehicle use should aid future porous
asphalt construction. The extensive
stormwater monitoring surveys docu-
mented the hydraulic and pollutant
transport characteristics of the two
paving systems.
The gradations for the stone base, the
stone topping course, and the porous
asphalt that have been developed in the
past and used on this project with some
modifications are quite adequate. The
recommended reservoir base course
consists of aggregates with a maximum
size of 2.5 inches (6.3 cm) and a minimum
size of 1 5 inches (3.8 cm), which should
provide a void space of 40 percent of its
volume for water retention. Two inches of
gravel top course over the base course is
recommended to provide a better surface
for applying the porous asphalt surface
course. The recommended gradation of
the aggregate for the top course of gravel
is Vs-inch (1.6-cm) maximum and %-inch
(0.9-cm) minimum to provide an essen-
tially uniform aggregate of approximately
1/2-inch (1.3-cm) diameter A 2.5-inch
(6.3-cm) depth of the porous asphalt
surface course (5.5 to 6.0 percent
asphalt) with the following specifications
is recommended:
S/eve Size
'/2" (1 3 cm)
Va" (0.9 cm)
#4
#8
#16
#200
Percent Passing
100
90-100
35-60
15-32
2-15
2-5
The type of compaction for the porous
asphalt pavement was less important
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than compacting the surface at a temper-
ature near 180°F (82.2°C). It was
apparent from this project that conven-
tional mixture temperatures in the order
of 260° to 280° F (126.7° to 137.8°C)
should be used for laydown; however,
compaction should always be delayed
until this type of mixture has cooled down
to near 180 °F (82.2 °C). A reasonable
surface can be had by a variety of
compaction methods as long as the
mixture is not too hot when compacted.
Several rollers were utilized to yield a
satisfactory surface, including an 8-ton
(7.3-metric ton) pneumatic roller, a 1 -ton
(0 9-metric ton) pneumatic roller, and a 1 -
ton (0 9-metric ton) flat-wheel tandem
roller.
The tensile strengths of the open-
graded, porous, hot-mix asphalt concrete
(HMAC) cores were lower than for those
conventional, dense-graded HMAC cores.
Tensile strengths for asphalt concrete
mixes are considerably affected by the
temperature of the mix. The strengths at
lower temperatures are still relatively
high, however, and those at the more
critical higher temperatures did not vary
greately from those for dense-graded
mixes
The permeabilities of the porous asphalt
surface after 18 months of use ranged
from 152 in./hr (386 cm/hr) to 5290
in./hr (13,437 cm/hr) with an average
rate of 1766 in./hr (4486 cm/hr).
Permeabilities notably lower than the
average rate occurred where the asphalt
was rolled at a temperature higher than
180°F(82.2°C).
Based on the Austin experience,
porous asphalt construction costs are
comparable to costs for conventional
Table 1 Hydraulic Summary of Stormwater Surveys
asphalt construction: a porous asphalt lot
incorporating a storage reservoir can be
constructed (including engineering,
inspection, and testing) for about $10 per
square yard ($12 per square meter). In
Austin, standard practice in the design
and construction of conventional parking
lots is to incorporate stormwater detention
into the parking lot area with a 6-inch (15-
cm) curb and restricted outlets. Construc-
tion cost for this type of conventional
system is about $8.50-$8.75 per square
yard ($ 10.00-$ 10.50 per square meter). If
engineering, inspection, and testing are
added to this (assuming 17 percent of the
construction costs), the total cost is again
about $10 per square yard ($12 per
square meter). Should the site specifics
(i.e., topography, size, slope, etc.) neces-
sitate grading or an offsite detention
structure, the conventional system cost
would, of course, be higher.
The hydraulic results of the runoff
surveys are summarized in Table 1. The
runoff-to-rainfall ratios greater than
unity resulted from measurement error.
Because the porous asphalt and gravel
trench lots were hydraulically open, their
runoff ratios do not reflect the potential
stormwater storage of the facility. A rela-
tionship between 7-day antecedent rain-
fall and the runoff ratio was not discern-
ible. Detention times were calculated as
the time difference between the inflow
and discharge center of mass. The deten-
tion times at the porous surface facilities
were characteristically longer than at the
impervious lots. Areas of hard-packed
sand at the lattice block lot contributed to
anomalously rapid detention times. Pond-
ing in surface depressions on the con-
crete lot resulted in longer duration times
than expected at that facility.
Stormwater hydraulics for each pave-
ment type were simulated with the
revised PORPAV. PORPAV was calibrated
for each lot with the use of one set of
observed runoff data. The calibrated
coefficients were held constant during
the simulation of the remaining events
for model verification. Calibration was
initialized by varying values of the
estimated parameters to reproduce the
observed runoff volume. Generally, to do
this, the volume of surface storage for the
impervious lots and the base storage (the
product of depth and porosity) for the
pervious lots was adjusted. The estimates
of slope and the roughness coefficient
were varied to reproduce the observed
peak runoff rate. For the porous asphalt
and gravel trench lots, the coefficient of
permeability for the base layer was varied
to reproduce the observed peak base
discharge rate. Overall, there were not
enough data sets to definitively assess
the ability of PORPAV to simulate the
hydraulic response to each lot. Generally,
however, observed hydrographs were
reasonably simulated after calibration
with a prior set of data.
Design Methodology
The greater volumes of stormwater
runoff that result from paved areas often
degrade the quality of the receiving
water. Some municipalities require that
an initial volume of stormwater runoff be
retained to remove accumulated pollu-
tants. The design criteria of stormwater
detention facilities reflect these concerns.
The porous pavement design methodology
was developed to be flexible enough to
satisfy a variety of design criteria.
The design methodology consists of a
series of curves that depict the hydraulic
Pavement Type
Porous Aspha/t§
Lattice Block Lot
Gravel Trench
Asphalt
Concrete
Event
Date
03/22/82
04/O5/82
06/01/82
03/02/82
03/11/82
03/18/82
03/03/82
03/19/82
04/04/82
06/03/81
05/11/82
03/03/81
06/O3/81
1O/07/81
No of
Sprinklers*
8
9
8
4
6
8
3
4
4
tt
8
tt
#
#
Total
Inflow
(ml
094
050
1 53
1 06
1 08
1 08
064
064
064
034
021
085
0.57
045
Duration
(mini
60
62
55
75
60
34
94
70
59
46
10
120
33
90
Average
Intensity
(in/hrl
094
048
1 67
085
1 O8
1 90
0.41
056
065
0.44
1.26
043
1 04
0.30
Peak
Discharge
(cfsl
0.269
0.253
0237
0034
0078
0 113
0440
0.58O
1 667
0.84
0.223
020
0 10
0.07
Time to
Peak
(mini
58
54
53
55
40
24
60
66
55
53
7
58
30
3O
Total
Discharge
(ml
0.58
0.64
056
0 19
0.39
0.25
049
041
049
040
O 15
046
0.28
0.17
Runoff
Ratio*
(in/inl
073
1 28
037
0.18
036
0.23
076
064
077
1 18
071
0.55
048
0.38
Detention
Time
/mini
42
42
42
11
12
11
29
24
19
1
5
18
14
17
7-day
Antecedent
Rainfall*
002
0.09
000
4.03
0.00
0.03
4.03
0.03
0 12
248
099
053
2.48
3.71
* Values for the gravel trench are the number of water trucks used
'Runoff to rainfall ratio Subsurface runoff (underflow) measurements are used in the porous asphalt and gravel trench lots Surface runoff measurements are used for
the lattice block, conventional asphalt and conventional concrete lots
^.Precipitation amounts recorded at the Austin Airport within the indicated number of preceding days.
^Discharge results influenced by infiltration lines along trenches
tDenotes natural precipitation event The remainder were sprinkler-induced events
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characteristics of porous pavement
facilities under the influence of various
rainfall events and site specific factors.
A range of magnitudes of descriptive
physical properties for the pavement
system are represented in the multiple
curves. The design algorithm was devel-
oped to facilitate the design of a porous
pavement system without the necessity
of computer simulation, and hence,
incorporates analytical simplifications. In
general, the design procedures can be
employed for any porous pavement
system that the model PORPAV can
analyze. Appropriate physical and hy-
draulic characteristics are
• The pavement is a single or set of
single uniformly sloping surfaces.
• The surface is underlain by a base
layer of uniform media. This layer
may or may not be of uniform
thickness and is usually separated
from the surface by a permeable
filter course.
• Discharge from the base layer can
be completely restricted, or occur via
infiltration to the underlying soil, or
exit horizontally through seepage to
adjacent soils or through a set of
collection drain pipes located within
the base.
• If a collection drain is present, the
base layer cannot contain baffles or
other mechanisms that restrict the
lateral movement of the water
within the base However, a multiple
drain pipe system with these controls
may be analyzed as individual units
• Impermeable seals may or may not
be placed along the boundary to
prevent leakage to the adjacent soil.
A small computer program, PAVDES,
was developed to execute the methodo-
logy Persons with a microcomputer or
larger facilities can use PAVDES for its
convenience and greater accuracy in
place of the design curves.
Although the PAVDES design method-
ology incorporates the flow-governing
equations used m PORPAV, the two
procedures provide distinctly separate
functions PORPAV is a computer simula-
tion program that models detailed mtra-
event hydraulic characteristics of both
pervious and impervious pavement
facilities PORPAV can be used alone as a
pavement design tool through iterative
executions in a trial and error technique,
i.e., alternative values of the facility's
physical characteristics can be modeled
for the same inflow condition, and the
resulting hydraulic responses can be
compared. The iterative process will
continue until specified criteria, e.g.,
discharge rates, are achieved. As a more
direct, albeit less detailed, solution, the
design methodology was developed to
yield the optimal depth of the pavement
storage facility with a single application.
Given the known characteristics of the
lot, contributing area, average §torm
intensity, and limiting discharge rates or
volumes, the design methodology will
determine the design depth of the base
layer and estimate the resulting discharge
hydrograph. If a greater degree of detail is
desired, PROPAV can then be used to
simulate the hydraulic response of the
designed pavement facility under a
variety of storm conditions. Confidence in
the analytical procedures used in the
design methodology stems from the
success PORPAV has demonstrated in
simulating hydraulics for a variety of
pavement facilities.
The full report was submitted in partial
fulfillment of Grant No. R806338-01-2
with the City of Austin, Texas, under the
sponsorship of the U.S. Environmental
Protection Agency.
4
Gary F. Goforth is with Espey, Huston & Associates, Inc., Austin, TX 78767;
Elvidio V. Dinizis with Resource Technology, Inc., Albuquerque, NM87110;
andJ. Brent Rauhut is with Brent Rauhut Engineering, Inc., Austin, TX 78758.
John N. English is the EPA Project Officer (see below).
The complete report, entitled "Stormwater Hydrological Characteristics of Porous
and Conventional Paving Systems," (Order No. PB 84-123 728; Cost: $25.00.
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
frUS GOVERNMENT PRINTING OFFICE 1984-759-015/7300
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