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
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