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As
part of their commitment to the community, Main Line Health wanted to
incorporate some practical, sustainable design solutions into the
expansion of the Bryn Mawr Hospital campus. Where possible, locally
produced, recycled and energy-efficient materials are specified for the
buildings and surrounding site areas. One of the first project areas to
be completed, a parking lot opposite the hospital’s main lobby, features
rain gardens planted in a central landscape island, tree trenches, and a
subsurface infiltration bed.
A portion of the rain water falling onto the parking lot surface is
captured subtly and evenly as it sheet flows across the depressed curb
on the uphill side of the rain garden areas. River rock borders help to
slow the water velocity as it enters the planted area. A raised curb on
the downhill side of the island contains the water within a six-inch
deep depressed area that is filled with predominantly native plants. The
plants were selected for their water-absorbing capabilities and
tolerance to the harsh climatic conditions found in a parking lot. They
do not require supplemental watering or fertilizer.
Click here to view a planting list. The soil in
the rain garden is a blend of sand, topsoil and organic matter, rich in
microscopic biota. As the water percolates through the soil layers, the
plants’ fibrous roots and the soil microorganisms break down, utilize
and absorb silt, hydrocarbons and other pollutants. The cleansed water
that is not absorbed by the plants, through evapotranspiration, is
temporarily stored beneath the soil layer within the voids of a layer of
angular gravel in the retention/recharge zone. As the water seeps back
into the ground, it recharges the underground aquifer, making water
available to plants and streams further downhill from the hospital site.
If the water rises too quickly for the rain garden soil to absorb, it
flows into a yard inlet, which is connected to the subsurface
infiltration bed by a perforated pipe. The inlet grate is fitted with a
filter insert that prevents silt and mulch from entering and potentially
clogging the system. The rain garden provides four seasons of interest,
as well as food and habitat for birds and small mammals, while it also
helps to protect and define another sustainable site feature – the
bicycle parking area.
In order
to support street trees in the urban environment of a paved sidewalk
area, a tree trench system was incorporated to increase the soil area
available for gas and water exchange in the root zone. A secondary
benefit provided by this system is its ability to contain, filter and
absorb runoff from the sidewalk pavements. The first six-foot width of
the sidewalk area behind the curb is comprised of dry-laid brick pavers.
Within this area, a 3-foot deep by 6-foot wide area of structural soil
(angular gravel, soil and water-absorbing polymer) runs between the tree
pit openings and extends beyond them a minimum of 6 feet. This porous
system captures some of the rain water sheeting off the adjacent 6-foot
wide concrete sidewalk, which completes the right-of-way width. A
perforated pipe within an aggregate-filled trench underneath the tree
trench connects all of the tree trenches to the main infiltration bed.
This ensures positive drainage within the tree root zones and provides
some additional stormwater storage and infiltration along the system.
The tree pit openings are planted with ground cover plants and protected
with a decorative steel fence which also serves as the edge restraint
for the brick pavers.
Runoff
from 80% of all site areas, and 88% of impervious surfaces, are captured
and treated by the subsurface infiltration basin. The basin has the
capacity to recharge almost 35,000 cubic feet of water. The system was
designed to recharge the volume of runoff for a 25-year storm event, as
well as control peak rates of runoff for all storms up to and including
the 100-year event. The basin uses 42-inch perforated pipe in a filter
fabric wrapped bed of uniformly graded aggregate. The system design is
based upon an exfiltration rate that is 50% of the lowest measured
percolation rate observed during testing, effectively providing a 2.0
infiltration safety factor. For flows in excess of system design,
infiltration bed overflow is connected to Lower Merion Township’s storm
sewer system. All inlets capturing runoff to the basin are fitted with
Flo-gard+Plus™ filter inserts that prevent trash, small stones and silt
from entering the infiltration bed, and improve the water quality. These
are cleaned manually during regular system inspections.
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