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The Bioretention
treatment cell is a recent innovation in stormwater management,
this system removes pollutants, attenuates peak flow, and
reduces flow volume through evapotranspiration and infiltration.
Biological treatment occurs through the uptake of pollutants
by vegetation and soil microorganisms. Physical and chemical
treatment, which occur on and within the soil media, includes
sedimentation, filtration, and adsorption with organic matter
and mineral complexes.
Water quality treatment performance of
bioretention cells can be effective for most common stormwater
contaminants, however, the treatment's hydraulic efficiency
and tendency to fail by clogging may be problematic.
The bioretention treatment strategy has
led to the creation of a new, holistic development philosophy
known as Low Impact Development (LID). LID strategies focus
on broader hydrologic and water quality issues. The hydrologic
context of LID is reducing runoff from impervious surfaces
in the first place, and should that be unavoidable, accommodating
the runoff and giving it the chance to infiltrate. Bioretention
areas can be designed to mimic the pre-existing hydrologic
conditions by treating and infiltrating the associated volume
of runoff. For the most effective integration of this type
of treatment an analysis of the overall site design should
be made and bioretention facilities integrated throughout
in soils that accommodate infiltration. However, depending
upon the specific case, bioretention areas can be substituted
for other stormwater BMPs with minimal site configuration
changes.
Bioretention treatment areas are
designed to minimize maintenance. Early care may be required
to establish vegetation however, once established, bioretention
areas do not require intense maintenance efforts. The primary
source of failure for bioretention cells is clogging. Early
designs contained specifications for bioretention soil mix
(BSM) with silt/clay content as high as five percent, and
called for geotextile filter fabrics to be installed between
the BSM and crushed stone sub-drains. Newer designs eliminate,
or severely limit silt/clay content of the BSM (<1%) and
substitute a graded sub-base replacing the need for geofabric.
This reduces clogging and maintains high water quality treatment
efficiency.
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