As
the use of porous pavements grows, designers and agencies all over North America
are learning for the first time this new approach to stormwater management.
People like me have been asked to speak to them hundreds of times in the last
five years, in workshops, webinars, consulting sessions, and agency testimonies
and reviews. The questions that are raised from all the diverse groups have a
lot in common.
Since
2005, I have saved 230 files of porous pavement questions conveyed in e-mails,
telephone calls, and conference question-and-answer sessions. This article
summarizes the questions that I have received most commonly over the years. My
answers to them are based on 12 years of research and experience in the field,
including surveying research reports, interviews with national experts, and
firsthand observations in the field. There is a huge amount of knowledge about
porous pavements now, and it is continuing to grow rapidly. The questions
reported here are what people most frequently say they need to
know.
Q:
Is there a recognized measure, or index, of permeability for paving
materials?
A:
Pervious concrete and permeable pavers that are properly designed, installed,
and maintained have surface infiltration rates of 140+ in/hr. An example of
research suggesting this is “Study on the Surface Infiltration Rate of Permeable
Pavements,” accessible through the North Carolina State University Web site
listed at the end of this article.
Q:
What is the runoff coefficient?
A:
Almost the only runoff coefficient that has ever been measured on properly built
porous pavements is zero: There is no runoff, because the surface permeability
is so high. But surface runoff coefficient does not take into account the
limited capacity of the pavement’s base reservoir: In a long, intense storm, the
base could become saturated and overflow, either across the surface or through a
perforated drainage pipe if one is provided. At that point, the pavement would
in effect be generating runoff. So it would be prudent to use some positive
number—not zero—for the runoff coefficient. An example would be to set the
runoff coefficient equal to that of the local jurisdiction’s “predevelopment”
condition, which might be forest, meadow, or grass. To assign a coefficient
larger than predevelopment would be arbitrary. A predevelopment grass surface
generates some runoff during large storms, so it provides a valid analogy for
porous pavement hydrology.
Q:
How much credit should be given for the pavement as a “pervious”
surface?
A:
Correctly designed, installed, and maintained pervious pavements have surface
infiltration rates higher than that of almost any natural soil, and several
times greater than the maximum possible rainfall intensity anywhere in the
country—in other words, greater than anything that is already called “pervious.”
So a surface of this type must be given complete credit for “100% perviousness,”
as would a meadow or forest. Giving it any credit less than 100% pervious would
fly in the face of scientific evidence.
Q:
What is involved in maintaining the pavement “right”?
A:
If you are in a municipality where sand or cinders are spread on the roads for
winter traction, then vacuuming will be necessary at least once per year: in the
spring, following snowmelt. The key word is vacuuming, with or without
simultaneous washing, to lift material out and restore the open, permeable
pores. Any washing or sweeping without simultaneous vacuuming would just drive
sediment farther down into the pores. In areas where there is no sanding or
other routine source of sediment, no special maintenance is needed except when
something happens such as construction vehicles tracking sediment onto the
surface; then the sediment can be removed by vacuuming.
Q:
How long will the system last before it becomes a “non-permeable”
surface?
A:
Just make sure that your pavement is selected, designed, installed, and
maintained correctly. If you are duly careful with all these steps, then the
installation should be permeable indefinitely.
Q:
Pretreatment using a filter strip or vegetated swale is required,
right?
A:
Absolutely not: Don’t do that! Any upstream soil, even soil that is grassed or
mulched, can erode and generate pavement-clogging sediment sometime. Adding a
grass strip or forebay would just add more erodible upstream soil. Wherever
earth drains down toward a pavement edge, a swale should be added to divert
runoff and sediment away from the pavement. It is okay to drain impervious roofs
or pavements directly onto a porous pavement, because those surfaces don’t
produce sediment the way soil does.
 |
Photo: Bruce K. Ferguson
Permeable pavers in Minnesota correctly
installed with highly permeable single-sized
aggregate in the joints |
Q:
Should porous pavement be avoided where trees are present? Should overhanging
trees be removed?
A:
The only thing overhanging trees do to porous pavements is deposit their annual
drop of organic debris. The debris decomposes to a minute fraction of the volume
it started with. Vacuuming might be called for after a number of years, to
reopen the pavement’s pores. Trees are immensely helpful for water resource
management, counteracting the urban heat island, shading urban open spaces, and
absorbing carbon, and they should not be discouraged.
Q:
What is recommended when you have a large chemical spill or hazardous material
spill?
A:
The same as if a spill occurred anywhere else in your city: Immediate and
complete cleanup is the legal responsibility of the industry that spilled it, at
their expense. No pavement or drainage anywhere is designed for this
contingency, outside of the grounds of the industry that produces the
chemicals.
Q:
What are the risks associated with hydrocarbon (oil)
contamination?
A:
Letting oil into a porous pavement’s voids is the whole idea in water-quality
improvement. In the pavement, naturally occurring microorganisms biodegrade
hydrocarbons before they migrate to the bottom of the pavement. The constituents
go off as carbon dioxide and water vapor, and very little else; the hydrocarbons
cease to exist as water-quality pollutants. An example of the research
suggesting this, accessible on several Web archives, is C. Pratt’s 1999 paper,
“Mineral Oil Bio-Degradation Within a Permeable Pavement: Long Term
Observations.”
Q:
What’s the use of porous pavement on a clay soil, or where there is a shallow
water table, and water cannot be absorbed into or treated in the soil? Is a
subdrain necessary to ensure good performance? Can a porous pavement work
here?
A:
On clay soils, permeable pavements do not make the 100-year storm disappear; a
perforated drainage pipe is ordinarily required to discharge excess water. But
most of the water-quality benefit of any permeable pavement occurs within the
pavement structure, without regard to the underlying soil; the soil is only a
redundant “backup” system. Porous pavements on clay soils
do:
- Reduce runoff coefficient and impervious cover
-
Detain peak flows
- Treat water quality
- Recharge aquifers by gradual infiltration
of rainwater from small, frequent, year-round storms
Q:
What is the cost difference between standard and porous pavements in the same
situation?
A:
Pervious concrete costs approximately 20% more than conventional impervious
concrete, because of its high cement content and specialized quality control.
Permeable pavers cost about the same as pervious concrete. When you use these
materials intelligently in a site plan to absorb and treat stormwater, and the
municipality gives you credit for their stormwater functions, then the use of
porous paving ordinarily reduces total development cost by reducing or
eliminating the need for additional stormwater facilities.
Q:
How does the use of pervious concrete affect the pavement life in cold climates?
A:
Properly installed pervious concrete is free from freeze-thaw issues as long as
the surface concrete layer drains freely down into an open-graded aggregate
base, thence rapidly into the soil or a perforated drainage pipe. The material’s
durability is ensured by adequate strength, which comes from proper
installation; further help comes from air entrainment and reinforcement with
polymer fibers.
Q:
Salt used for deicing... does it clog the paving?
A:
Deicing salt does not clog porous pavements. The whole idea of deicing agents is
that they dissolve readily in snow and water, lowering the water’s thawing
temperature. The dissolved salt flushes through with meltwater and does not
accumulate. Ongoing research at the University of New Hampshire suggests that
many porous pavements require less salting than impervious pavements, because
the thawed meltwater drains so readily away through the pores.
Q:
Do you use traditional trench backfill material under porous pavement, or do you
use open-graded material instead?
A:
The base material must be open-graded (single-sized) aggregate such as ASTM No.
57, so it can store and convey water.
Q:
Are there standard specs (DOT type) for pervious
concrete?
A:
The American Concrete Institute has adopted Specification 522.1, Pervious
Concrete. In addition, the National Ready Mixed Concrete Association has a
specialty certification program to help identify qualified pervious concrete
installers. It is vital that industry standards such as these be
followed—failures have occurred where established standards have been
ignored.
 |
Photo: Bruce K. Ferguson
A vital step in correctly installing pervious concrete is covering it
quickly with plastic sheets and keeping it covered for seven days. |
Q:
We need options for cost, appearance, etc.
A:
A material that deserves to be used more is permeable pavers, also known as
open-jointed block or PICP (permeable interlocking concrete pavement). These are
manufactured units with openings in the joints where single-sized aggregate
gives the pavement its permeability. Pavers manufactured to ASTM standards (as
almost all of them are) are extremely strong and durable units. It is rather
easy to install them correctly—just stick to the long-established guidelines of
the Interlocking Concrete Pavement Institute. This is not the same construction
as bricks on sand! Firmly specify that only single-sized aggregate must be used
for the base, setting bed, and joint fill.
Q:
For what parts of the country are porous pavement available, in terms of
freeze-thaw, etc.? What site conditions, such as soil type, limit its
use?
A:
Properly selected, designed, constructed, and maintained porous pavements work
wherever they are located. Improperly selected, designed, constructed, and
maintained ones do not.
Q:
How widespread is this usage? How much (and how rapidly) is it changing? What is
the future for widespread adoption?
A:
Porous pavements are still a small proportion of all the paving being done in
the world, but they are growing at an exponential rate. Developers and suppliers
are ready to install these new materials; their motivation is to meet today’s
environmental requirements in economical ways. The potential future application
of porous paving is vast.
Q:
What can my municipality do to encourage the use of porous
pavements?
A:
Make sure your municipality is not an unnecessary impediment. When a developer
proposes porous paving, give it credit for what it can do to satisfy your
stormwater requirements: It reduces impervious cover; lowers the runoff
coefficient; and absorbs, detains, and treats stormwater.
Q:
Where might I find research reports? I need data.
A:
Watch Web sites such as the following for broad new information, links to
detailed sources, and continuing updates: