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80
Boston Neck Road, North Kingstown,
RI 02852
Demonstration
Rain Garden
Planning, Design and Installation Details
For
regular updates and a list of plants used,
click here. View our brochure about
the garden (click here for print version.)
Installed May 2, 2005
A
rain garden is a natural or dug shallow depression designed
to capture and soak up stormwater runoff from your roof
or other impervious areas around your home like driveways,
walkways, and even compacted lawn areas.
It is planted with suitable trees, shrubs,
flowers, and other plants that can tolerate temporary pooling
of water and dry periods.
The following are planning, design and
installation details for the North Kingstown Town Hall Demonstration
Rain Garden. There are several methods to plan and size a
rain garden. Various methods often consider the contributing
runoff area, the soil type and slope of the area, and the
amount of runoff you wish to control, among other important
factors. For more information
about designing a rain garden and a suggested plant
list, refer to our fact sheet, Rain
Gardens: Enhancing your home landscape and protecting
water quality.
For information on research results, visit
the The University of Connecticut, Haddam
research/demonstration rain garden web site.
March 2005
Prior to installing the rain garden, roof runoff from the southwest
corner downspout of the town hall entered a 4 inch, underground
plastic pipe which discharged directly onto the surface of
Reynolds street.
After discharging onto Reynolds Street (above) the runoff traveled
towards the front of the photo and eventually made its way
across Reynolds street into a nearby culvert storm drain and
Narragansett Bay.
This garden is located on the Town Hall’s gently sloping
front lawn near the corner of Reynolds Street and Boston Neck
Road. The garden is not near a septic system, well, underground
utilities or within ten feet of the building’s foundation.
Furthermore, if the rain garden overflows during a heavy rain
event, it would slowly spill onto the lawn grass and eventually
work its way to the corner of Reynolds Street and Boston Neck
Road, which is where it originally traveled. The existing storm
drain culvert can handle the overflow that might occur. When
siting a rain garden, it is important to consider what happens
if it overflows. The overflow should not cause new problems,
like soil erosion or flooding.
In order to accommodate frequent public
access and pedestrian traffic, it was decided that the runoff
from the roof gutter downspout would be directed
into the garden through an underground 4-inch
plastic
pipe,
which
would join to the existing underground pipe that discharges
onto Reynolds Street.
In April 2005, a site visit was made to flag the approximate
perimeter of the garden. A soil test was collected and
a couple of test holes were dug to confirm the soil type
to be a well
drained, sandy loam with no compaction problems.
Soils can become
compacted due to heavy foot or equipment/vehicle traffic,
and from recent site development/disturbance. Compacted
soils
can prevent the
rain
garden
from soaking up the water within 4
to 6 hours.
May 2, 2005 installation
At approximately 10’ by 16’ (160 square feet)
and six inches deep, this garden is sized to handle the
first one inch of runoff from the roof gutter downspout
and surrounding lawn area that drains naturally into the
garden. The first one inch of runoff typically contains
the greatest amount of pollutants.
A hand push sod cutter (provided by Taylor Rental Center)
was used to remove the grass and root system from the surface
area
of the new garden. Another option is to place black plastic
over the area a few weeks before installing the garden
to kill the grass and make hand digging easier.
Sizing a rain garden:
This
garden was sized using a straight calculation of the total
surface area contributing one inch of runoff to the garden.
The roof gutter downspout drains
one quarter
of the
original town hall building's roof area (approximately 644
square feet; 28' by 23'). It is estimated that an additional
322 square feet of lawn area naturally drains into the rain
garden for a total drainage area of about 944
square feet. One inch of runoff on 944
square feet (944 sq. ft. X 1"/12" per foot)
equals 80.5 cubic feet of water.
A 6-inch design depth was selected. 80.5
cubic feet divided by 0.5' depth = 160 square feet of area
needed. 10 feet by 16 feet allows for a nice crescent
design. Remember, even if your final dimensions are a
bit smaller than those needed to handle the first one inch
of
runoff, whatever volume of runoff you can retain is better
than nothing.
Once the sod cutter removed the grass and loosened up the
root zone, the garden was hand dug to about 9 inches deep
in the
middle. This allows for a 6 inch storage depth after 3 inches
of mulch is applied. The soil removed in the digging was
used to create a berm on the lower side of the garden. A
laser level was used to check the grade, ensuring a level,
uniform bottom had been dug. The
rain garden fact sheet page links
to a rain garden manual which provides information on how
to set stakes and use string and a hand level to accomplish
the same task. Ideally, you want to balance the cut and fill
needed by taking the natural slope in to consideration when
sizing the depth and area of the garden.
Underground drain
While the garden was being dug, crew members located the existing
underground pipe that carried the roof runoff to Reynolds Street.
A Y connection was created to divert the runoff to a new underground
pipe (installed perpendicular to the original pipe) that would
outlet at the surface into the bottom of the rain garden. This
Y connection provides a safety overflow. If the new underground
pipe were to clogg or a "back-up" of water occurred, the
water would push back through the Y and travel back down the
original
pipe onto Reynolds Street. A section of original underground
pipe was cut to install this Y connection. A clean-out access
was installed to allow for a plumber's snake if necessary.
A trench was dug by hand to install the new section of underground
pipe (approximately 50 feet provided by Wickford Lumber Co.).
A string and marker paint was used to locate a straight line
and the sod cutter was used to remove two strips of lawn grass
along this line.
David Renzi sets the bottom of the pipe outlet.
The pipe sections were
then
laid
from the Y connection to that established outlet elevation
at a certain slope or grade to provide
adequate,
uniform
flow.
The
pipe
should
have at
least 6 inches
of soil
covering
the
top.
In cases where heavy equipment accesses an area, the grade
of PVC pipe used and depth of soil cover would need to be greater.
The laser level is used to check the slope or grade of the
pipe.
The pipe was back-filled and covered with the natural soil
after testing all connections for leaks, and the area was re-sodded.
Water was flushed through the pipe with a hose and allowed
to pool for observation.
Planting and mulching the garden
Compost was mixed with the natural soil in the planting
holes, and spread over the entire garden before mulching.
Compost provided by David Renzi, Out in Front Horticulture
Mulch provided by the Town of North Kingstown Department of
Public Works; plants provided by Holly Ridge Nursery, Little
Tree Farm and Gardens, Morningstar Nurseries, Rose Shack,
Schartner Farms, The Farmer's Daughter and Wildwood Nurseries.
For
regular updates and a list of plants used, click here
Field stone
Field stone was placed beneath the
pipe outlet and extends into the garden a few feet to slow
the runoff
down
and protect against
soil erosion. Crushed stone can also be used for this purpose.
Additional stones were placed in the garden for design interest. The garden will
be monitored during and after storm events for signs of erosion or overflow.
The pipe contains an end cap with many small holes that allow the water to flow
and prevent debris and wildlife access.
This demonstration rain garden was made possible through
generous donations by local businesses. Most notably, David
Renzi, owner
of Out
in Front Horticulture and a URI Master Gardener, provided
assistance with the planning and design of the garden. David
donated his services and crew to install the garden with
assistance from additional URI Master Gardener volunteers.
Special thanks to the following local businesses, organizations and individuals:
David Renzi and crew, Out in Front
Horticulture
Town of North Kingstown
Holly Ridge Nursery
Little Tree Farm and Gardens
Morningstar Nurseries
Rose Shack
Schartner Farms
Taylor Rental Center
The Farmer’s Daughter
Wickford Lumber Co
Wildwood Nurseries
URI Master Gardener volunteers installed and will monitor and maintain the
garden for the next two years:
David Renzi, Valerie Harvey, Dori Gerhardt, Joy Gerstenblatt, Evelyn Quinn,
and Hilary Sowa
Special thanks to Michael E. Dietz,
Department of Natural Resources Management and Engineering,
University of Connecticut for technical review and assistance.
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