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What
are construction stormwater BMPs? |
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Clearing, grading, and other
construction activities remove vegetation and compact the soil
increasing both runoff and erosion. Excessive runoff erodes
deep gullies, attacks channels, and has the potential to cause
off-site erosion, sedimentation, and flooding problems.
Construction stormwater best management practices (BMPs) are
actions taken before, during and shortly after construction that
control erosion and sedimentation and protect water quality.
Effective erosion and sedimentation control can be achieved by
careful attention to the following principles:
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Protect the land surface from erosion.
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Manage runoff and keep velocities low.
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Capture sediment near the source.
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Integrate sediment control with the construction schedule.
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Inspect and maintain the erosion and sediment control
system.
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Sedimentation from land
disturbing activities where appropriate BMPs have not been
employed is the largest threat to water quality in the Hiwassee.
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Why is it important to install construction stormwater BMPs? |
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The primary benefits associated
with installing construction stormwater BMPs are protection of
water quality and compliance with the law. The North Carolina
Sedimentation Pollution Control Act requires that soil disturbed
during construction remain on the construction site. It is
illegal for soil to wash onto neighboring properties, into
public roads, or into waterways, regardless of how big your
construction site is. Availability of a wide variety of BMPs to
treat diverse development scenarios has made implementation more
cost effective for developers and many types of BMPs can
continue to function as
post-construction stormwater BMPs
or simply attractive landscape features.
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What are examples of some construction stormwater BMPs? |
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The most important construction
stormwater BMPs are planning actions, rather than techniques or
structures that are installed. These are the most important
initial considerations when contemplating construction:
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Schedule construction activities to minimize the duration of
exposure.
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Limit the area disturbed.
Grade no larger an area than
necessary at any one time and leave an undisturbed buffer.
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Divert runoff away from exposed slopes.
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Stabilize and maintain
construction roads, parking areas, and the construction
entrance. Use the designated routes to limit the
development of ruts and erosion.
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Stabilize channels immediately
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Protect graded
surfaces with temporary
vegetation and mulch whenever work is interrupted for an
extended period.
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A variety of methods exist to
construct temporary or permanent structures that will control
erosion and sedimentation during construction activities. These
include building barriers such as check dams and silt fences
that will capture soil on sloped areas and prevent sedimentation
into streams below. Diversion structures can also be used to
direct runoff and soil away from the watershed. These often
involve directing soil and water toward an area that can absorb
it, such as a dry vegetated area or a holding pond. The types
of structures that are used to effectively control erosion vary
widely depending on the site and the resources available.
Careful research and consultation is highly recommended before
employing any of these techniques. |
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Basic structural construction stormwater BMPs include:
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Construction entrance/exit stabilization
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Temporary sediment traps/filters
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Riparian buffers and
filter strips
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Storm drain inlet and outlet protection
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Temporary sediment basins (must have baffles and skimmer to
comply
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Sediment barriers (typically silt fence)
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Sediment is controlled most easily
and effectively near the source. Several sediment traps or
barriers located at the edge of a graded area are more effective
and less hazardous than a single large sediment basin near the
site boundary. Sediment traps, basins, and barriers, such as
sediment fences, operate by reducing runoff velocity to allow
deposition---not by filtering. Filtering runoff with fabric or
gravel is not effective because filters clog too rapidly to
remove much sediment. Therefore, the practices should be
designed to reduce flow velocity and form shallow pools for
settling. The contractor can make sediment control more
effective by providing access to traps and barriers for
maintenance. Anticipate where sediment will accumulate behind
sediment fences, and plan to provide access for cleanout and
maintenance. Traps and barriers must be inspected and cleaned
frequently. |
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Although structural BMPs are
useful and often necessary for controlling erosion during
construction, vegetative cover is the principal method for
stabilizing disturbed sites. It is relatively inexpensive,
tends to be self-healing, and is often the only practical,
long-term solution to site stabilization and erosion control. A
variety of vegetative practices can be used to control potential
erosion problems. Dry straw, hay or wood waste (chips or
sawdust) can be broadly applied to the disturbed area to reduce
erosion and runoff velocities. For enhanced results, these can
be anchored in some way with materials such as erosion control
matting. A more permanent solution to controlling erosion in
disturbed areas is to sow temporary and/or permanent seed, the
roots of which will hold soil in place, thereby minimizing
erosion and soil loss. Transplanting existing vegetation that
is being disturbed, when possible, can enhance these benefits. |
Check dams made from mulch
socks (top) and temporary sediment traps (bottom) are two
structural barriers to sediment movement. |
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