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Thinking Outside the BOX: Filterra® Boxless ™ Design in Residential Applications

By Dan Jamison, PE, Group Manager, Koontz-Bryant, P.C.

As residential design engineers, we’ve all come across situations in subdivisions where, due to the site conditions, fitting a BMP onto the site without losing a buildable lot can be a challenge. When the BMP selected is a wet pond, it can be even trickier to keep the size down when detention is added on top of the water quality requirements. And as the impending stormwater regulations changes are only a few months away, coming up with innovative solutions to CBPA compliance and stormwater runoff quantity control while still maximizing the number of lots on a site will be crucial for making our clients’ projects viable.

That is where the Filterra Bioretention Systems® Boxless ™ design comes in. Introduced last year, the Boxless design operates the same way as Filterra’s® long proven box designs and employs their same media to meet water quality requirements. In contrast to the traditional “tree in a box” design, the Boxless ™ design allows for larger treatment areas and inflow options tailored to your site. Curb inlets are not necessarily required, but can be used.

At Koontz-Bryant, we worked with the staff at Filterra® on a trial basis to take a traditional wet pond design and used the Boxless™ design concept to meet our site’s requirements. Our project problem: to provide adequate water quality per phase in order to bring the entire phased subdivision into CBPA compliance by project build out, and to provide 2 year post-development to pre-development detention. Buffers against adjacent subdivisions, wetlands, and a tight layout were some of the constraints associated with the site.
Our solution: place Filterra® media in a contained area in the bottom of a standard wet pond and grade the remainder of the “pond” to provide the necessary detention. To avoid overburdening the Filterra® media, we installed a system of pipes to disperse the inflow runoff evenly across the treatment area.
The result: a dry pond with the increased pollutant removal efficiency that the Filterra® media provides, but with a smaller footprint than a traditional wet pond. It still provides adequate storage to provide 2-year detention. As runoff producing events occur, water will pool on top of the Filterra® media, then drain through and out with a underdrain system. Larger storms will be able to bypass the system using a standard riser and barrel configuration.

This design is currently under review at Chesterfield County for compliance with all regulations. Once constructed, this “pond” will be planted and will become an attractive amenity. An added advantage of this design is that any wet pond can be retrofit for higher removal efficiency after it is already constructed.

Let Koontz-Bryant, P.C. work with you on your next residential project to maximize your buildable area and turn your BMP into an attractive feature. Contact Dan Jamison at 804-200-1903 or via email.