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What are post-construction stormwater BMPs and why are they
important? |
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Developed watersheds contain a
higher percentage of impervious surfaces (e.g. roads, rooftops,
and parking lots) and increased drainage density from the
connectivity of stormwater conveyance systems. Instead of
infiltrating into the ground, precipitation runs off these
surfaces as concentrated flow to nearby streams at a much faster
rate, carrying nonpoint source pollutants such as metals, oils,
and sediment. Reduced infiltration rates translate into an
increase in magnitude of stormwater runoff that can potentially
cause flooding downstream. Post-construction stormwater best
management practices (BMPs) can be used in commercial settings
or other developed areas to mitigate flooding and reduce water
quality impacts associated with stormwater runoff. Stormwater
BMPs such as infiltration areas, constructed wetlands and
vegetated buffers can substantially improve water quality, as
can the use of pervious paving techniques. These systems and the
benefits they provide in treating stormwater runoff are
discussed briefly below. |
A high concentration of impervious surfaces (roads, rooftops,
parking lots, etc.) can lead to increased flooding if stormwater
BMPs are not installed
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Bioretention areas like the one
shown above are one type of vegetated swale. |
Vegetated Swales and Buffer Strips |
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Vegetated swales are areas where
stormwater accumulates. The movement of water is slowed and
allowed to be more readily absorbed into the ground. Most
swales are planted with grasses or other low-lying plants that
can withstand periodic flooding. Swales are slight
depressions that are generally sloped and directed toward areas
that can more easily withstand large influxes of water.
Buffer strips are usually established between large impervious
areas such as roads and receiving water bodies; they treat
sheets of water that can accumulate during storm events.
Depending on the size and slope of the area and the vegetation
they contain, their effectiveness can vary, but they do help to
control erosion and sedimentation.
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Infiltration trenches and basins |
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Infiltration trenches and basins
store large volumes of water from storm events and slowly
release them back into the ground or surface water. In most
cases, trenches are the most practical approach, as they can be
easily constructed in most places where stormwater accumulates.
Building an infiltration trench simply involves digging a trench
along the area where water exits an impervious area, and filling
it with gravel. The gravel allows the water to be absorbed more
slowly and evenly along the ground.
Infiltration basins are much more
complicated and are not practical for all sites, but they are
extremely efficient at filtering out impurities that accumulate
in water as it travels across large impervious surfaces. Basins
involve digging a shallow impoundment where stormwater
accumulates, allowing the water to gradually be absorbed into
the ground. As the water seeps back into the groundwater, the
soil it passes through is very effective at filtering out
pollutants. While infiltration basins can be an effective tool
for managing stormwater, their success is highly dependent on
soil type and other site-specific factors. Careful planning and
consultation are critical. |
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Pervious Paving Systems |
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Paved areas that use pervious
materials such as gravel, interlocking pavers, and pervious
concrete allow stormwater to be evenly absorbed into the ground.
Interlocking pavers where vegetation is permitted to grow
between pavers are especially effective at evenly distributing
rainwater across the paved surface. Roads and driveways paved
with gravel can be effective at absorbing and evenly
distributing rainwater, especially in flat areas. Gravel is
less stable than asphalt or interlocking pavers, so it is
critical that the roads are well maintained and new gravel is
laid down periodically. Before constructing a gravel road,
careful attention must also be paid to the slope and natural
drainage patterns of the site. Interlocking pavers, pervious
concrete, and other similar systems commonly have a sub-base
layer that can store water and slowly release it to allow
infiltration and/or runoff. These systems are also effective
pollutant removal devices. |
One type of porous paving system (shown above) involves filling
a synthetic grid with clean gravel.
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Stormwater Wetlands and Ponds |
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Constructing or maintaining
existing ponds and wetlands in areas where stormwater
accumulates is a very effective way to manage stormwater. These
natural or landscaped areas not only reduce the velocity of the
water running off the watershed, they can often mitigate
flooding because they reduce the magnitude of the peak flow,
thereby reducing the “flashiness” of the watershed. These
stormwater devices also filter out impurities that may be
present. Nutrient uptake by plants, along with the filtering
effect of soils can significantly reduce the amount of nonpoint
source pollutants entering the groundwater and nearby streams. |
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Wetlands are very effective at removing pollutants and reducing
flooding. They can also be very attractive. |
If ponds or wetlands already exist
on the property, they may be useful in treating stormwater
runoff. In such cases, water exiting impervious areas may be
directed towards these areas, with care taken to ensure that the
volume of water entering the pond or wetland can be absorbed
without substantial flooding. In order to maintain the
long-term viability of the wetland, a healthy biological
community should be maintained by controlling invasive species
and ensuring that water entering the system is relatively free
of chemical pollutants. Some types of wetland systems are
sensitive to the natural hydroperiod, or length and frequency of
inundation; these systems may not be good candidates for
stormwater treatment as their modification in this fashion can
result in the modification of the plants and animals that
inhabit the area. |
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Constructing a pond or wetland is more complicated, but if done
properly, this can be just as effective a tool as a
natural system. The design of a
pond or wetland is highly site-specific, and it is important to
gain as much information as possible about the soil type of the
area. A true constructed wetland must have a connection with
the groundwater table or have a confining soil layer. The type
of vegetation that is planted will depend on the slope, soil
type, and topography of the constructed wetland, along with
other physical factors. Careful research and consultation with
a professional is highly recommended. |
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What are the
advantages of installing post-construction stormwater BMPs? |
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In addition to the water quality
benefits discussed above, post-construction stormwater BMPs can
also be incorporated into a landscape design as an amenity.
Many BMPs like vegetated swales, buffer strips, and infiltration
trenches are inexpensive and easy to implement given limited
space and other constraints. Finally, installation of BMPs
provides educational opportunities to increase awareness of
water quality improvement strategies and green initiatives. |
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Are there
disadvantages associated with installing post-construction
stormwater BMPs? |
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Post-construction stormwater BMPs
are most effectively implemented in the context of a watershed
management plan or stormwater master plan. Often, random
implementation of stormwater BMPs does not provide significant
water quality benefit to justify the cost involved. Routine
maintenance is required for proper function of many stormwater
BMPs, adding to the total implementation cost.
Resources for more information and technical assistance about
post-construction stormwater BMPs:
North
Carolina State University, Stormwater Engineering Group
http://www.bae.ncsu.edu/stormwater/
North
Carolina Department of Environment and Natural Resources
Business
Stormwater and Runoff Pollution
http://www.ncstormwater.org/pages/businesspage.html
North
Carolina Division of Water Quality, Stormwater Unit
http://h2o.enr.state.nc.us/su/Manuals_Factsheets.htm
North
Carolina Division of Water Quality, Stormwater Unit
Sample Deed
Restrictions for Stormwater BMPs
http://h2o.enr.state.nc.us/su/bmp_forms.htm
North
Carolina State University
An
Evaluation of Cost and Benefits of Structural Stormwater Best
Management Practices in North Carolina:
http://www.neuse.ncsu.edu/Stormwater_BMP_Factsheet.pdf
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