What Is Low Impact Development (LID)?
Types of Best Management
Practices (BMP)
Retention Ponds
Retention ponds are the most widely used BMP. They focus
on remediating the quantity of water flowing off of impervious surfaces
by collecting the first flush (first 1") of run-off, allowing
a slower release into a local water body. This does help hydrologically
by reducing the effects of large quantities of run-off entering
at a fast pace, like bank erosion, but there are short comings to
this BMP.
Pooled water can become a breeding ground for nuisance insects and
is a danger for children who might drown in them if they are not
fenced off in residential areas. The other shortfall is that there
is no treatment of water quality.
Bioretention
The type of LID shown to the right is called a Rain Garden,
or Bioretention cell. The Stormwater pools in the engineered soil
fill and then percolates into the soil or is absorbed by the vegetation,
rather than running straight into the storm drain. This allows
an opportunity for the run-off to be treated by the Bioretention
cell. Many of these are being put into practice in place of retention
cells to address waster quality and quantity from impervious surface
run-off.
Green
Roofs
Green Roofs treat runoff from another type of impervious
surface; roof tops. They can be constructed on roof tops up to
a 45 degree slant and have multiple benefits besides the asthetics
of having a garden on your roof. The soil and vegetation help
to regulate temperature. The BMP absorbs the worst of the heat
during the day to keep buildings cool and acts as a heat retainer
at night to keep heat from escaping. They also retain rainwater
that falls on roof tops, holding the precipitation until it evaporates
or treating it through percolation before it leaves the roof.
Permeable
Pavement
Just as there are many variations of BMP and LID, there
are different kinds of permeable pavement, but the main idea is
consistent; they aim to make traditionally impervious surfaces
permeable. The picture shown is an example of concrete pavers
that are layed with a stone underdrain similat to the bioretention
cell, and with soil planted with grass seed filled into the permeable
space in the pavers. An EPA study found that "The use of
permeable pavement systems dramatically reduced surface runoff
volume and attenuated the peak discharge".
Where
is LID being used?