by Paul Iorio
I recently brought to completion a large parking lot project that illustrates the applications as well as adaptability of tree filter systems in an urban environment. Tree filter systems integrate common street trees with stormwater collection to achieve a viable and sustainable alternative to a traditional “end of pipe” system, while still meeting stormwater management and remediation goals. Tree filter systems utilize the principal of “bioretention” – the natural process by which chemicals and sediments are removed from stormwater runoff prior to subsurface infiltration. Many state regulatory agencies and municipalities now strongly encourage low impact development (LID) techniques such as tree filter systems because they replicate pre-development conditions and reduce the negative impact of land development and surface paving.
How Tree Filter Systems Work
Many of us are familiar with rain gardens, a bioretention stormwater practice that incorporates primarily shrubs and herbaceous perennials within a prepared bed or constructed low lying swale or basin. Rain gardens typically require a surface “footprint” of greater than 100 square feet to provide effective collection and treatment of stormwater runoff emanating from a catchment area of greater than 1,000 square feet. Tree filter systems are unique in that much of the collection and treatment practice occurs beneath the surface allowing for a smaller surface footprint. The actual collection or entry point is typically a concrete structure with a catch basin or gutter opening integrated with the street curbing.
Tree filter systems rely on physical, biological, and chemical remediation functions such as surface sediment containment, soil microbiological breakdown, and chemical transformation processes. These processes act in concert to treat and “cleanse” stormwater contaminated with pollutants such as oil/grease, phosphorous, nitrogen, bacteria, and heavy metals, prior to subsurface infiltration to groundwater and/or the aquifer. Fortunately, trees and plant systems in general have the ability to partition, assimilate, and/or process the low level concentrations of contaminants found in the typical stormwater drain. It would be simplistic to suggest that contaminated stormwater is the chemical equivalent of fertilizer to a tree; however, many of the soluble (and insoluble) contaminants in stormwater are listed ingredients on the back of bags of tree/shrub fertilizer.
Benefits of Tree Filter Systems
These systems provide greater remedial efficacy than most “conventional” stormwater management practices. Tree filter systems are particularly well suited to urban retrofit situations where stormwater conveyance is inefficient or ineffective due to aged or faulty existing subsurface infrastructure systems. Of equal importance, tree filter systems play a role in providing a conduit for groundwater/aquifer recharge by allowing rain water to infiltrate the impervious surface that “clads” a majority of our urban environment.
In addition to providing effective stormwater control, tree filter systems support a valuable urban resource: trees. We are all aware of the importance of the addition of trees in the urban landscape in providing a sanctuary from the sun’s intense rays and promoting cooling of heat energy by shading and by evapotranspiration, while also enhancing air quality. Since tree filter systems require a prepared media that is highly porous yet has the ability to retain moisture, it may represent the perfect balance of oxygen and water that is vital to healthy tree growth (as well as survival), particularly in the urban environment. This favorable environment is in stark contrast to its (tree only) counterpart which may be planted in oxygen depleted hardpan soils or consolidated urban fill. Of additional advantage is the extended rainwater catchment area that services a tree filter system as opposed to a typical sidewalk planting or landscape bed. While a quick passing summer shower or microburst may only percolate the top one inch of pervious surface surrounding a standard tree planting, the additional or exaggerated flow that services a tree filter system would offer greater irrigation, an advantage in periods of drought.
MassDOT Headquarters Parking Lot, Boston MA
In the summer of 2013, StormTree LLC was contracted to provide tree filter systems for a parking lot reconstruction project at the offices of the Massachusetts Department of Transportation (Mass DOT) on Kneeland Street in Boston, MA. Working in conjunction with the BSC Group, a Boston based engineering firm, two tree filter systems were designed to be the primary stormwater management control for the newly constructed parking lot. The R. Zoppo Corporation, Stoughton, MA, provided the excavation and installation services. The parking lot layout included an approximately 150’ (long) by 11’ (wide) island that separated the parking lot into two sections. A six foot wide concrete sidewalk extended the length of the island; the remaining five foot wide portion was a landscape corridor.
Although the typical tree filter system maintains a single point of stormwater entry at curbside, due to the island configuration, this particular application required entry points on opposite sides of the island to collect surface runoff from the approximately one acre parking lot. Additionally, the tree filter system would have to be fully integrated with the sidewalk, providing a seamless walkway.
An 11’ X 5’ X 5’ (length/width/height) open sided and open bottomed concrete structure was designed to serve as the tree filter system. Both structures were cast in the yard of Wachusett Precast in Sterling, MA, and delivered to the site. A collection sump was configured on the front end of each side of the structure to collect sediment, trash, and debris as it entered the opening of the structure, reducing accumulation within the interior of the filter system surface.
A specific media “recipe” was chosen and blended to provide the necessary balance of rapid water infiltration. The mix provided sufficient organic matter to retain contaminants/nutrients and was also bioavailable for microbial activity and tree root uptake and irrigation. Beneath the approximately three feet deep media filter layer was a two feet deep layer of stone that served to collect the filtered stormwater prior to infiltration through the existing soils.
A six inch diameter solid plastic pipe was installed within the structure, beginning slightly above the base of the media filter layer and running vertically through this layer. The solid pipe connected to a perforated horizontal pipe installed within the stone layer beneath the tree filter system. This horizontal pipe system extended the length of the island. The purpose of this installation was twofold:
- When the rate of stormwater flow entering the system exceeded the rate of infiltration, the accumulating water would enter the open top of the vertical pipe and be transferred to the gravel layer and subsurface soils.
- Accumulated water within the gravel layer beneath the tree filter system would be evacuated, reducing the possibility of the tree’s roots being waterlogged or existing in a stagnant condition.
It is important to reiterate that the concrete structure is open on two sides and the bottom to allow for unrestricted root growth and prevent water collecting and potentially “drowning” the tree’s roots.
The majority of the top surface of the concrete structure was open to allow for the installation and growth of the tree, and allow access to the containment sumps and the media filter bed surface for cleanout and ease of maintenance. An ADA (American Disability Act) compliant fiberglass grate was used to surround the portion of the structure that contained the tree and a cutout allowed for tree trunk. Fiberglass grating was chosen since the material was easy to cut with a hand saw, allowing for easy enlargement of the opening as the tree trunk expands in trunk width. Although the fiberglass grating maintained sufficient structural integrity to support foot traffic (500 lbs/ft2), a steel angle bar was incorporated within the concrete structure to provide additional support. The open portion of the concrete structure in line with the sidewalk was enclosed with a steel grate capable of supporting vehicular traffic.
While many small- to medium-sized trees would be suitable for the system, a 3” caliper, 18 foot tall, Acer Saccharum “Columnare” (Columnar Sugar Maple) was installed in each system to complete the project. Since the top of the media filter bed is at an elevation lower than the road surface, the tree is in effect “submerged” by approximately one foot, similar to a tree well installation. This feature provides the unintended benefit of eliminating the possibility of sidewalk upheaval as the natural root flare is well below existing or finished grade.
With proper system maintenance on an annual basis to include the cleanout of the sumps and surface media bed, as well as annual tree inspection, the tree filter system should continue as a sustainable stormwater management facility, with the healthiest tree on the block for years to come.
About the Author
Paul Iorio is a consultant with StormTree, LLC, a Rhode Island-based design and engineering firm specializing in the integration of plant systems with stormwater management in commercial and residential applications throughout the northeast. Over the past 18 years, his work has involved soil and water pollutant removal systems incorporating principals of bioretention and “natural enhancement” such as biological (bioremediation) and plant (phytoremediation) in achieving cleanup goals and providing low impact development solutions. Previously, Paul was an independent landscape design consultant and contractor, and he holds undergraduate and graduate degrees in plant, soil and water sciences, and environmental engineering. He may be contacted at firstname.lastname@example.org or through his website: www.storm-tree.com.