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Q: Why do we
need porous pavements?
A: There is an increasing concern about the environmental
impacts of urbanization. Impervious coverage such as
rooftops and roadways are replacing wooded natural areas.
Impervious cover prevents infiltration and creates excess
runoff from storms by not allowing rainfall to come in
contact with the natural soil. It is this infiltration that
replenishes the groundwater which in turn provides baseflow
for streams. Impervious systems like gutters and storm
sewers channel rain water directly to streams and rivers.
While paved areas decrease baseflow, they increase flood
flows causing erosion and sedimentation downstream.
Pollutant loading is also increased leading to a degradation
of water quality and aquatic habitat. Best Management
Practices (BMPs) like pervious concrete in conjunction with
infiltration beds are techniques that can be both
aesthetically pleasing and cost effective with the purpose
of protecting and improving water resources.
Q: How do
porous pavements work?
A: Porous pavements, such as pervious concrete and porous
asphalt, have functionality and workability
similar to that of regular concrete and asphalt. However, the
porous mix lacks fine particles found
in regular mixes. This creates a significant amount of
void space which allows water to flow relatively
unobstructed through the pavement. Once through the
pavement, the water is flows into the infiltration bed
beneath, which is filled with rock, producing a 40% void
space for storage of the runoff until it is infiltrated into
the soil.
Q: Who
benefits from porous pavements?
A: Porous pavements and infiltration beds benefit everyone
in the entire watershed by increasing the amount of runoff
entering back into the ground water table and thus the
amount of clean water available for human consumption. It
replenishes the ground water table while at the same time
decreasing the amount of runoff entering streams during
large storm events, virtually eliminating all potential for
downstream flooding events.
Q: How do
pervious concrete and porous asphalt differ?
A: The obvious difference is that concrete uses stone and
sand aggregates, while asphalt is a petroleum based product.
However this study looks to identify further differences in
durability, ability to improve water quality, and sustained
ability to infiltrates stormwater. Check out our
reports to see some of these differences.
Q: Is the
groundwater table at risk of pollution from the infiltrating
stormwater?
A: Pollution is possible if an infiltration BMP is not
placed in the proper area. Siting of this BMPs is critical
to their success. These systems should not be used in areas
where high-levels of contaminants are present. The Villanova Pervious Concrete / Porous Asphalt
Comparison Site is a good
example of a clean site; the major source of pollution is
from the automobiles that park in the parking lot. Stormwater is naturally cleaned as it moves through the soil
stratum, so small amounts of metals and nutrients can be
cleaned out and retained in the soil just as if they were
infiltrated through pervious grass or wooded areas. Parking
lots, driveways, and walkways are ideal locations for these infiltration systems.
Q: Is it better to infiltrate contaminants rather than
putting them in the river?
A: Yes.
Organisms and processes in the soil break down many of the
contaminants.
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