Technical Description:
(22) Filterra® Bioretention Systems
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As a part of Montgomery County Maryland's effort to protect local waters, the Glenmont Forest neighborhood was selected for a Green Street project. The residential subdivision was built in the 1940s-1950s, long before modern stormwater control measures were in place. The project aimed to reduce stormwater pollution flowing into Joseph's Branch, a tributary to Rock Creek. The project involved installing 53 small-scale stormwater practices within the county-owned right-of-way to capture water from roadways and sidewalks during storms and help.
Four types of stormwater BMPs were constructed in existing roadside facilities to collect as much first-flush runoff as possible. Three of the BMPs are traditional land-based systems facilities, including bioretention, rain gardens, and downspout rain barrels. The Filterra bioretention systems, often referred to as "Tree Boxes," were the only manufactured/proprietary products specified. All of the BMPs used needed to be approved for 1" water quality control by the state of Maryland to be considered Green Infrastructure.
Filterra has been approved as a Green Infrastructure BMP in Maryland since 2013 and has ample precedent for use in this project. Filterra's small surface footprint and rapid installation (long-term construction in a dense and thriving residential area is not ideal) made it an obvious choice. Also, Filterra's drainage-area to facility-surface-area ratio of 220:1 made it an economically favorable choice, as the project's goal was to capture and treat as much impervious drainage as possible at the least impact and cost. That same ratio for bioretention and rain gardens is 50:1.
Twenty-two Filterra standard offline units (precast concrete box with tree grate) in sizes from 6'x6' through 12'x6' were specified. All units were installed offline and upstream of existing storm drain inlets installed over sixty years ago. Contech provided sizing and facility-site evaluation support to RK&K during the several-year-long design process and provided onsite support to Ecotone for the delivery and installation of the units.
The existing neighborhood was very crowded in terms of lot sizes, the width of roads and sidewalks, and the narrow infrastructure for subsurface utilities. As a result, the space available to retrofit surface stormwater BMPs was extremely limited. Filterra's small surface footprint was ideal for this type of application.
The primary challenge with installation was delivery vehicle access, given narrow roads and street parking. Cranes could not be used due to clearance issues from overhead utilities. The solution was to use a large mobile forklift to offload each unit from the truck, drive it to its final location, and drop it in place with a forklift.
All twenty-two systems were installed and activated (planted) within a few weeks to meet the project schedule.
Technical Description:
(22) Filterra® Bioretention Systems
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