Sustainable Landscaping
healthy lawn care
rain gardens



80 Boston Neck Road, North Kingstown, RI 02852

Demonstration Rain Garden
Planning, Design and Installation Details

Installed May 2, 2005

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 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.

Before / planning

Design & installation

For more information about designing a rain garden and a suggested plant list, visit our rain garden page - general information and additional resources.

For more information about this demonstration rain garden including the plant list, photo galleries and color brochure, click here.

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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.

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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 - general information and additional resources 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. Click here for more information about the rain garden plants.

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, monitored and maintained the garden for two years following installation:
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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