Most of the 'paving over' in developed areas is due to common roads and parking lots, which play a
major role in transporting increased stormwater runoff and contaminant loads to receiving waters.
Alternative paving materials can be used to locally infiltrate rainwater and reduce the runoff
leaving a site. This can help to decrease downstream flooding, the frequency of combined sewer
overflow (CSO) events, and the thermal pollution of sensitive waters. Use of these materials can
also eliminate problems with standing water, provide for groundwater recharge, control erosion of
streambeds and riverbanks, facilitate pollutant removal, and provide for a more aesthetically
pleasing site. The effective imperviousness of any given project is reduced while land use is
maximized. Alternative pavers can even eliminate the requirement for underground sewer pipes and
conventional stormwater retention / detention systems. The drainage of paved areas and traffic
surfaces by means of permeable systems is an important building block within an overall Low Impact
Development scheme that seeks to achieve a stormwater management system close to natural conditions.
Some current studies on the effectiveness of permeable pavers for reducing Total Suspended Solid
(TSS), nutrient, metal and thermal loadings are being conducted in Florida, Toronto, and
Washington State.
The parking lot of the Florida Aquarium in Tampa, which serves 700,000 visitors annually, has been
innovatively designed as a research and demonstration project for the use of permeable pavers as
part of a treatment train approach, comparing three paving surfaces in conjunction with
swales.1 First-year results found that load removal efficiencies for metals (copper,
iron, lead, manganese and zinc) ranged from 23 to 59% for asphalt pavement with a swale; 62 to 84%
for cement pavement with a swale; and 75 to 92% for porous concrete with a swale. In general,
metals were measured at much higher concentrations in the basins paved with asphalt than those
paved with cement products. The porous system with a swale also achieved 91% removal efficiency for
total suspended solids, higher than the other two paving systems.
Studies at the University of Guelph in Canada have also observed greater pollutant loads from
asphalt surfaces than from concrete or permeable pavers. There, a research team led by Professor
William James has been performing field and laboratory tests since 1993 on the influence of
permeable pavers on runoff pollutant levels and thermal characteristics. They have found that a
permeable paver made up of interlocking concrete blocks can significantly reduce the surface runoff
loads of such contaminants as nitrite, nitrate, phosphate, phosphorus, metals, BOD, and
ammonium.2 In addition, during a lab simulation, the permeable pavers were found to
reduce surface runoff temperatures by 2 to 4 degrees Celsius compared to the runoff from asphalt
paving. Since the permeable pavers also increase infiltration, the total heat content of runoff
leaving a site is reduced substantially.3
Finally, surface and subsurface runoff samples
are being collected by the Center for Urban Water Resources
Management in Washington State from a test parking area, which
contains five different surface materials.4
Constructed in 1996, the King County employee parking lot contains
nine stalls, of which one is traditional asphalt, and the others are
four pairs of alternative permeable pavement surfaces: gravel-filled
interlocking concrete blocks, soil and grass-filled interlocking
concrete blocks, gravel-filled plastic cell networks, and soil and
grass-filled plastic cell networks.
The project's primary goal is to determine the long-term water quality benefits of these systems
under real world usage. A system of pipes, gutters and gauges collect and enable the measurement
of the volume and chemistry of both the surface runoff and the subsurface infiltrate. A
comprehensive water quality analysis is being conducted over the winter of 2001/2002. Preliminary
results indicate that the subsurface runoff is consistently cleaner than the surface runoff;
statistical analyses and reports will be produced in future months (Derek Booth, Feb. 2002,
personal communication).
For more specialized users, continuing research at Coventry University in England has been looking
at applying nutrients to permeable pavers in order to support a microbial population that can
serve as an in-situ bioreactor for oil degradation in highway and parking lot
runoff.5 Studies have demonstrated the potential to maintain microbial activity for
over 12 months from one application of a slow-release fertilizer, with warnings given about
ensuring the effective use of the nutrients so that high effluent levels will not cause
eutrophication in receiving waters.
Most of the above studies have also examined the influence of permeable pavers on runoff volume,
tending to show a marked reduction in the surface runoff that leaves a permeable paver site due to
increased infiltration. In the University of Guelph experiments, field sites with permeable
interlocking concrete pavers demonstrated a 90% reduction in runoff
volume.3
The treatment train studies at the Florida Aquarium showed that, in general, the use of swales
reduced runoff volume but that paving type also played a major role in runoff reduction, with
permeable pavers being the most effective. The figure below demonstrates this fact as well as the
caveat that the use of swales and permeable pavers has the most influence on runoff during small
storms.6 For high intensity rainfalls or when soil conditions are saturated, runoff is
not reduced as substantially. Note the different scales on the two graphs; the first is for a
rain event that produced just over 0.5 inch of rain in about 75 minutes, while the second is for
an event producing almost 2.5 inches in under 2 hours and occurring less than 24 hours after four
preceding days with rain.
The studies by the Center for Urban Water Resources Management in Washington State have looked for
similar differences in the hydrologic response of pavers based on storm intensity or if the storm
followed a long dry period versus a period of abundant rain. To date, however, results show a
general absence of surface runoff from the permeable pavers regardless of conditions: "it all just
infiltrates, all the time" (Derek Booth, personal communication). The figure below,
representing a typical observation during the study's first year, compares surface runoff produced
from traditional (asphalt) and permeable (Turfstone) pavements.7 The Turfstone permeable
paver is a 60% impervious surface made up of soil and grass-filled interlocking concrete blocks.
The measured surface runoff from the Turfstone is less than 1 percent of the total rainfall and is
probably a result of observed leaks in the covering over the collection system. All other
permeable pavement systems showed equivalent results. The asphalt paving, however, responds quickly
to the rainfall, with most of the rain that hits the surface running off.
It is likely that results are different from those in Florida due to differences in the two
regions' rainfall regimes. The Washington rain event had a maximum rainfall intensity that was
under 0.2 in/hr; this was typical of the storms recorded. In comparison, the heavier rain event
presented in the Florida graph had a maximum rainfall intensity of 1.5 in/hr. Rain events in
Washington State are generally of a lower intensity and longer duration than those measured in
Florida, where the rainfall, particularly in the summer, is dominated by short and more intense
convective events.
1 Rushton, B.T., 2001: Low-impact parking lot design reduces runoff and pollutant
loads. Journal of Water Resources Planning and Management, (May/June), 172-179.
2 James, W., ed., 1997: Advances in Modeling the Management of Stormwater Impacts
Volume 5. Proceedings of the Stormwater and Water Quality Management Modeling Conference,
Toronto, Ontario, February 22-23, 1996, 520 pp.
3 James, W., 2002: Green roads: Research into Permeable Pavers. Stormwater,
(March/April), 48-50.
4 Booth, D.B., J. Leavitt and K. Peterson, 1996: The University of Washington
Permeable Pavement Demonstration Project. Background and First-Year Results, available online
at http://dept.washington.edu/cuwrm/
under Research.
5 Pratt, C.J., A.P. Newman and P.C. Bond, 1999: Mineral oil bio-degradation within a
permeable pavement: long term observations. Wat. Sci. Tech.,
39 (2), 103-109.
6 Southwest Florida Water Management District, 2001: Florida Aquarium Parking
Lot - A Treatment Train Approach to Stormwater Management. Final Report for FDEP Contract No.
WM 662, Brooksville, Florida, 220 pp.
7 Booth, D.B. and J. Leavitt, 1999: Field evaluation of permeable paver systems for
improved stormwater management. Journal of the American Planning Association,
65(3),
314-325.Return to top
ment in Washington State from a test parking area, which
contains five different surface materials.
