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Influence
of Common Landscaping and Grading Practices in the Creation of
Impervious Surfaces
Impervious
surfaces are mainly constructed surfaces – parking
lots, roads, building, sidewalks or other created surfaces that
cannot be easily penetrated by water. Although imperviousness is usually associated with constructed
surfaces, it is often the consequence of vehicular traffic and
clearing/grading of unconstructed or native soils.
While
imperviousness due to constructed surfaces is
widely recognized,
the impact of clearing and grading or operating vehicles on
native and unconstructed soils is remains largely unrecognized.
Various
studies confirm that created soil imperviousness (i.e. soil
compaction) is not
a minor issue. In fact research demonstrates that
construction activities can create soil imperviousness equivalent
to that of
concrete. Findings also indicate that soil imperviousness
increases significantly as a result of post development activities
in locations such
as athletic fields, park areas, pathways, and unpaved parking
lots
that experience extensive foot and/or vehicle traffic.
When soils are compacted by post-development
impacts, we see decreased
water absorption and increased surface water runoff.
Compounds such as fertilizers,
pesticides, herbicides, gases, oils and other pollutants accumulate
on these “created” impervious surfaces during dry weather
conditions and then form a concentrated first “flush” to
water bodies following a storm event. Some studies tell us
that this problem is so widespread in urbanized landscapes that
impervious
surfaces
are the second highest source of pollutant concentration; only
piped sources were found to be higher.
Many
stormwater management designs erroneously assume that unconstructed
or native soil surfaces are mainly composed of
uncompacted highly permeable soils, leading to serious underestimations
of stormwater and nutrient runoff.
Recent
research
suggests
that
increased surface runoff from the effects of increased soil compaction
should be more accurately reflected in nutrient and stormwater
runoff calculations.
The
stormwater pollution problem is comprised of two components:
the increased volume and velocity of surface runoff and higher
concentrations of pollutants in the runoff. Both are associated
with the large scale conversion of undeveloped/agricultural lands
to residential/industrial lands. According to the Conservation
Design
for Stormwater Management
Manual (1997), the impacts of high density development in combination
with the creation of avoidable impervious surfaces through
intense land grading
practices will adversely impact water quality indicators through:
- Stream
bank erosion.
- Stream
undercutting.
- Stream
flattening.
- Stream
channel straightening.
- Increased
volume of runoff, sedimentation, and erosion.
- Greater
stream runoff velocity during storm events.
- Reduced
stream base flow.
- Reduced
watertable recharge.
- Decreased
aquatic biological diversity.
- Increased
pollutant discharges.
Research
consistently demonstrates that once a threshold of imperviousness
is crossed in a given watershed, water quality and/or stream
habitat
cannot be maintained at the predevelopment level. The consensus
among many independent researchers is that watershed imperviousness
should not exceed 10% in an environmentally sensitive watershed
(Schueler, 1994).
According
to the Center
of Watershed Protection (2003), however,
watersheds containing less than 10 percent impervious cover are
more susceptible to impacts associated with other factors, including
percent forest cover, riparian continuity, soils, historical land
use, and a variety of other stressors. Therefore, impervious
cover as an indicator of stream water quality is most reliable
beyond
the 10 percent threshold, presumably reflecting the stronger influence
of stormwater runoff on stream water quality indicators.
It is
apparent that impervious cover is linked with the quality and
quantity of water. Impervious cover is also a strong indicator
of watershed health and shows how even a small amount of imperviousness
can have disproportionate impacts on stream quality.
References
Center for Watershed
Protection (CWP). 2003. “Impacts of
Impervious Cover on Aquatic Ecosystems.” Ellicott City, MD.
Delaware
Department of Natural Resources and Environmental Control and the Brandywine
Conservancy. 1997. Conservation Design for Stormwater
Management: A Design Approach to Reduce Stormwater Impacts from
Land Development and Achieve Multiple Objectives Related to Land
Use. Dover: DNREC.
Schueler, T. 2000. “The Compaction of Urban Soils.” Watershed
Protection Techniques
3(3): 661-665.
Schueler, T. 1994. “The Importance of Imperviouness.” Watershed
Protection Techniques 2(4): 100-111.
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