By Paul Iorio:
StormTree provides sustainable stormwater management systems that integrate trees with stormwater runoff collection and pollutant removal. Our open design systems allow for unrestricted tree root growth, promoting healthy tree growth and development.
In green infrastructure applications, when trees are expected to provide aesthetic appeal, function as a stormwater management solution and provide pollutant removal, a supportive subsurface environment is necessary for both tree and roots to acclimate, survive and sustain vigorous growth.
Whether the project is a complete streets rebirth, or a less ambitious sidewalk renewal, oftentimes, the tree installation is an afterthought or — worse yet — a punch list item. These construction practices all require heavy earth moving machinery causing permanent damage to the soil structure due to compaction. Soil compaction severely impedes and restricts a newly planted tree’s ability to acclimate and develop a healthy root system critical to initiate strong vigorous growth, which enables a developing tree to withstand the harsh realities of its impervious environment.
StormTree was contracted by the city of Big Rapids, Mich., to provide tree filter systems for a recent downtown revitalization project, which included sidewalk upgrades. Project funding was provided by a “Go Green” grant initiative that included the desire to capture stormwater to reduce contaminant loading and to provide additional water to newly planted trees.
The StormTree system combines a precast concrete frame with an engineered soil media designed to collect and treat stormwater runoff and provide a healthy tree growing environment. The StormTree frame is open sided with a filter media area allowing for unrestricted tree root growth.
For the Big Rapids project, a 5-foot-by-7-foot-by-4-foot precast concrete frame with an interior pretreatment sump design was selected to integrate with the newly installed sidewalks and curbing. Once the concrete frame was installed, the media was then backfilled into the interior of the frame.
Because StormTree systems are open design — open bottom and open sided — there are several advantages: (1) the units typically weigh less than 10,000 pounds so a mini excavator — no crane is required — is typically used to install the system providing ease of installation and cost management; (2) the system and components can be adjusted on site in the event of field changes or unforeseen obstacles; and (3) the engineered media bed can extend beyond the concrete footprint providing more treatment area without more concrete.
Let the rains begin
Stormwater runoff enters the unit through the throat and collects in an interior sump. Sand, sediment and trash associated with the water is segregated and retained within the sump. The water rises and flows over a baffle wall that separates the sump from the media chamber. The stormwater collects in the chamber and infiltrates through the media where physical, chemical and biological processes take place, which allow nutrients such as nitrogen, phosphorous and metals to affix to media particles becoming immobilized and/or are made available for tree uptake and as a food source for soil microbes.
Infiltrating water continues downwards and outwards and enters the stone base beneath the media layer. This stone base doubles as support for the concrete structure while also serving as a collection basin for “treated” stormwater. A perforated underdrain pipe within the stone layer collects and conveys this water to a downgradient facility, such as a catch basin, manhole or “daylighted” to an above ground swale or retention area. This underdrain pipe is connected to a vertical overflow/bypass pipe within the media chamber that is designed to evacuate and discharge excess water collecting within the system during intense or prolonged rain events. Due to StormTree’s unique open design, the combined collection and discharge piping can be positioned at varying elevations and directions providing unrestricted locating versus a “closed box” system.
Although not designed specifically for this project, the overflow/bypass pipe within the system could also be laterally connected to other trees within the streetscape to provide additional irrigation, with the tree filter system essentially serving as a collection and distribution conduit.
It starts at the roots
Of the several street tree options, Callery pears were selected for the Big Rapids systems. Although traditional cast-iron or steel tree grates are appropriate, American Disability Act compliant, fiberglass grates were selected for this project. These fiberglass grates fully support pedestrian loading and can easily be cut on site, which allows for selecting a specific location for the tree and opening. The flexibility in locating the tree position is a benefit should there be setback requirements or passerby clearance issues. The fiberglass grating covers both the tree filter media area and the interior pretreatment sump.
The tree root growth is naturally suppressed below sidewalk elevation, reducing or eliminating the possibility of pavement upheaval. This “tree well” characteristic allows for greater irrigation due to an enhanced collection area. The tree filter system’s ability to capture and collect additional surface runoff from the surrounding impervious surface allows it to better withstand periods of intense heat and drought or minimal rainfall events.
StormTree’s open design also allowed for electrical wiring to be easily installed in the Big Rapids systems to provide supplemental, aesthetic lighting for the trees. Supplemental irrigation can also be conveniently connected to the systems, particularly in regions where limited rainfall occurs. These amenities are much desired in streetscape and plaza applications with restaurants, entertainment districts and public gatherings.
The city’s department of public works is performing yearly maintenance of the tree filter systems which includes removing the grating and using vactor equipment to remove accumulated sand, sediment, and debris from the sumps.
The Big Rapids tree filter systems have been evaluated after the fall 2014 installation, with positive performance results. The pretreatment sumps within the tree filters have effectively contained quantities of sand and debris. The trees have exhibited greater caliper dimension and overall growth compared to standard sidewalk trees of similar height planted within the same timeframe. This is why we say that StormTree is the “Hardest Working Tree on the Street™.”