Tree Filter Systems for Effective Urban Stormwater Management

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An urban street in Boston just got a little greener as the Mayor of Boston, Thomas Menino, cut the ceremonial ribbon to unveil a sustainable stormwater management project at the German International School in Boston. In addition to more familiar low impact development (LID) features, such as pervious pavers and a rain garden, the project incorporated an innovative tree filter system which combines a common street tree with stormwater collection and remediation.

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Figure 1: Boston Mayor Thomas Menino presiding over dedication of Green Street redevelopment project.

Green Street Systems, LLC was contracted by the Charles River Watershed Association (CRWA) in partnership with Harvard University, and the Massachusetts Department of Conservation and Recreation to design and install a tree filter system to capture and treat stormwater runoff emanating from the combined 1 acre site parking lot and 30,000 square foot building. The purpose of this system technology is to replicate unpaved, predevelopment conditions by encouraging direct infiltration of stormwater runoff close to its source, instead of allowing it to drain into the City's municipal stormwater infrastructure that ultimately discharges into the Charles River.

The CRWA, one of the country's oldest watershed organizations, was formed in 1965 in response to public concern about the declining condition of the Charles River. The approximately 80-mile-long Charles River wends its way through 22 cities and towns prior to reaching the Atlantic Ocean at Boston. Since its earliest days of advocacy, CRWA has figured prominently in major clean-up and watershed protection efforts, working with government officials and citizen groups.

In 2008 a pollutant study by the EPA determined phosphorous levels in the Charles River were far above allowable limits, contributing to excessive algae blooms. On a per acre basis, urbanized commercial and industrial properties are the largest source of phosphorus pollution in the Charles River watershed. One of the best ways to reduce phosphorous, as well as other nonpoint source pollutant loading, is to reduce the amount of stormwater runoff that is flushed into the river by redirecting it back into the ground via direct infiltration through natural systems such as tree filter systems. These technologies are practical and cost effective, resulting in significant reductions in pollution while providing the additional benefits of cooling parking lots in summer and restoring groundwater levels.

Engineered Natural Systems

Tree filter systems rely on the principals of filtration and bioretention. Bioretention is a naturally occurring, primarily aerobic process in which fine sediments and contaminants are removed from the stormwater runoff while relying on a plant/soil/microbe complex to remove pollutants through a variety of physical, chemical, and biological processes.

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Figure 2: Completed tree filter system with 90 foot long "conveyance" trench.

In July 2010, construction commenced at the site and included the installation of the tree filter system. An approximately 90-foot-long narrow "conveyance" trench was excavated along the edge of the paved school parking lot, immediately fronting the heavily trafficked street that borders the property. The purpose of the two-foot deep, gravel-filled trench was to collect stormwater runoff from both the up-gradient parking lot and the roof of the building and direct it via two perforated 6-inch-diameter pipes connected to the adjoining tree filter system. Unlike a typical trench drain system, which is designed to exclude sands and sediment from entering and potentially clogging the drainage pipes, the trench drain system for the Boston project was intentionally designed to collect this material and transfer it to the tree filter system.

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Figure 3: Interior of tree filter system ready for tree planting. In foreground, "pretreatment" sump with weep holes; bypass/overflow pipe also shown.

The tree filter system includes a 4-foot by 6-foot precast concrete frame fully capable of supporting traffic loading. Collected stormwater and entrained sand/sediment flows by gravity through the perforated pipes and enters the tree filter system via openings on the side of the concrete frame. It immediately flows into an approximately 24-inch-deep "pretreatment" sump located within the interior of the concrete frame.

Quantities of heavier sands and sediment fall out of solution and are retained in the sump. As water continues to enter and fill the sump, it rises and ultimately spills over the sump wall on the opposite side of entry, which is lower in height. The water then flows into the main portion of the tree filter system which includes an approximately three-inch layer of mulch overlying a 30-inch layer of an engineered organic media with rapid infiltration capacity and water/soluble pollutant-holding capability to support healthy tree growth and allow for chemical and biological properties to occur.

Finer sediments in solution, which are not captured in the sump, are primarily trapped and held within the pore spaces of both the mulch and media layers, and ultimately become part of these aggregate layers. Water flows downward through both these layers and then collects in an open-bottomed gravel collection chamber. The vertical walls of the chamber are lined to prevent the intrusion of surrounding native soils from clogging the chamber.

The now "cleansed" or remediated stormwater ultimately infiltrates to the groundwater zone. Following the stormwater event, any residual water within the aforementioned pretreatment sump drains out via several small weep holes, thereby preventing a favorable habitat for mosquitoes and other vectors.

The Hardest Working Tree on the Street

Unlike closed concrete box systems, the Green Street system has a unique open design that not only allows for direct vertical infiltration of cleansed stormwater to replenish the native groundwater but also has side openings to allow for unrestricted root growth to support healthy tree growth. This open frame design prevents what is commonly referred to as the "tree coffin" effect, where a confined root system can "strangle" the tree due to the inability of the tree's root system to grow outward, as in a normal unconfined natural setting.

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Figure 4: Alternative design allows for curbside stormwater entry. Prefabricated stormwater chambers spaced between the trees provide additional storage and infiltration.

For curbed streets, or commercial parking lots, a curbside inlet, or frame and grate assembly, is incorporated in the system. Prefabricated stormwater chambers can also be incorporated in the design for additional water capture and infiltration, particularly along a narrow street sidewalk.

As with any stormwater management device, regular maintenance is required to maintain system functionality. Maintenance of the tree filter system is similar to that of a conventional stormwater catch basin, comprising the regular removal of collected debris and sediment from the pretreatment sump by way of a vactoring system or other standard removal device.

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Figure 5: Mayor Menino gets an up close and personal introduction to the hardest working tree on the street.

The Boston project accomplished the goal of capturing, transferring, remediating, and infiltrating rainwater runoff collected from a "curbless" asphalt parking lot. After personally viewing the Green Street Systems tree filter, and hearing the many benefits provided by the "hardest working tree on the street," Mayor Menino responded by saying: "I love this stuff."

About the Author:

Paul Iorio is a Principal and Senior Project Engineer with Green Street Systems, LLC. The company designs and installs engineered natural systems. He may be reached at paul@greenstreetsystems.com. For more information, please visit www.greenstreetsystems.com

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