Green Infrastructure Stormwater BMPs

Editor’s note: This article first appeared in the May-2016 issue of Stormwater. 

In 2014, EPA began a project seeking design and construction of two green infrastructure stormwater best management practice (BMP) projects on Cape Cod in Massachusetts to serve as a model for promoting public awareness and building public support for problems and challenges associated with stormwater. The primary objectives were to design and construct two green infrastructure stormwater BMPs to treat nitrogen, and to monitor both BMPs for a period of years to evaluate stormwater flows and water-quality parameters.

Locations
To seek interested communities, EPA initiated a preliminary site selection process that targeted eight southern Cape Cod communities with stormwater draining to Nantucket Sound. Each community was responsible for identifying potential sites on publicly owned property that would be suitable for BMP construction. Ideal sites would be suitably large enough for BMP construction, be located close to waterbodies along the south coast of Cape Cod, have an existing municipal separate storm sewer system (MS4) draining a large watershed in close proximity, and have public visibility and space for informational signs and/or kiosks. Communities must also be willing to operate and maintain the BMPs, and provide in-kind services to help the project succeed. Additional consideration would be given to proposed locations draining to impaired waterbodies or those with an approved total maximum daily load (TMDL) for nitrogen.

EPA then performed a detailed review on all proposed sites to determine the best candidate locations. Based on the above criteria, two potential locations were selected, one in Barnstable and a second in Chatham. The sites are as follows:

1. Barnstable: Gateway Marina/Hyannis Inner Harbor park area located near the intersection South Street and Pleasant Street. This location abuts an existing drainage line that discharges to the Hyannis Inner Harbor, which is listed on the Massachusetts Final Year 2012 Integrated List of Waters as a Category 5 Impaired Waterbody (Segment ID: MA96-82) for fecal coliform and total nitrogen.
2. Chatham: An undeveloped parcel located at the intersection of Oyster Pond Furlong and Absegami Run. This location abuts an existing drainage line that discharges to Oyster Pond, which is listed on the Massachusetts Final Year 2012 Integrated List of Waters as a Category 4a Impaired Waterbody (Segment ID: MA96-45) with TMDLs prepared for both fecal coliform (EPA ID# 36772) and total nitrogen (EPA ID #36219).

Once selected communities and sites were finalized, EPA selected a design team consisting of two consultants to provide a turnkey solution for each location. Selected firms consisted of the following:

• WaterVision LLC of Acton, MA, was responsible for overall project implementation, budgeting, educationand outreach, and sampling program design.
• Comprehensive Environmental Inc. (CEI) of Marlborough, MA, was responsible for project design, BMP construction, and development of an operation and maintenance plan for each location.

After a kickoff meeting with EPA and separate meetings with each community, the design team began to design and size BMPs at each location. Using a combination of information provided by each town and desktop analysis of the surrounding area, CEI assessed the estimated total watershed size, land usage and impervious area, and general layout of the contributing drainage system. Both sites are in urban settings with considerably impervious watersheds. The Barnstable location had an estimated 6.9-acre watershed, of which approximately 3.5 acres was classified as impervious (51%), while Chatham had an even larger estimated 16.9-acre watershed with approximately 5.7 acres of impervious area (34%).

Site Constraints
Potential BMP storage is controlled by several factors, primarily available space for construction and depth to groundwater, which serve to limit potential BMP space in three dimensions. The entire parcel at the Barnstable location totaled approximately 0.35 acre; however, this location is an existing park area with a brick walkway serving to connect an overflow parking area with the ferry terminal. Stakeholders wished to preserve the walkway and public access portion of this area, thereby restricting available surface space to approximately 0.15 acre.

The Chatham location is much larger, totaling several acres in size. However, an existing drainage ditch traverses the middle of the site. This ditch also has wetlands associated with it, and thus is regulated under the Massachusetts Wetland Protection Act as well as local bylaw requirements. As such, available space is restricted to approximately 0.29 acre of surface space, nearly all of which is located within regulated wetland buffer areas. Based on the location of each site (close to the harbor and close to wetlands), groundwater depth was expected to be shallow. To determine actual depth to groundwater, CEI performed onsite soil borings at each location, which indicated that groundwater was very shallow, at approximately 1 to 2 feet below the existing ground surface. An important requirement for gravel wetland systems is to avoid mixing between stormwater for treatment and in-situ groundwater. Due to the shallow groundwater system, the system will have to be lined with an impermeable barrier to prevent mixing.

BMP Choice
Because of the highly visible location within the park at Barnstable and the public demonstration and outreach component of the project, BMPs had to be both functional to treat stormwater and aesthetically pleasing. BMPs such as a gravel wetland are highly functional, but less ­aesthetically pleasing, whereas a bioretention rain garden is visually appealing, but generally is not capable of treating the required volume of stormwater from the large contributing watersheds associated with this project. Therefore, a hybrid bioretention and gravel wetland system was chosen for implementation at each site, combining the aesthetic component of a bioretention system with the storage and treatment capacity of a gravel wetland. The BMP would generally consist of an aerobic surface bioretention system capable of ponding a large volume of stormwater and an anaerobic subsurface gravel wetland cell. This aerobic-to-anaerobic treatment train is critical to the removal of nitrogen.

These volumes above translate to approximately 30% of the WQv. As the WQv is equivalent to the 1-inch storm event, the design storm volume at each location is approximately equal to the 0.3-inch storm event.

Design and Construction
Construction at both sites entailed installation of a new deep-sump manhole within the existing stormwater drain line to provide pretreatment by allowing sediment particles to settle out, helping to keep the BMP free of sediment and making maintenance easier. This structure was also equipped with a diversion wall, effectively making it serve as an inlet control structure. In its finished configuration, small storms up to 0.3 inch, as well as the beginning part of larger storms, are designed to encounter the diversion wall where stormwater is directed into the BMP via a new pipe. This structure is also designed so that larger storms exceeding the design capacity of the BMP will flow over the diversion wall and continue through the existing drainage pipes, similar to existing conditions.

Upon leaving the inlet structure, stormwater directed into the BMP will pass through a dedicated sampling manhole, which will be equipped with monitoring equipment at a later date, before flowing into the aerobic portion of the BMP. Here, it is exposed to aerobic bacteria for aerobic oxidation and nitrogen conversion. To adequately store the stormwater volume equivalent to a 0.3-inch storm, stormwater is allowed to pond up to a depth of approximately 1.6 feet in Barnstable and 2.4 feet in Chatham.

The bulk of the surface aerobic portion is lined to maximize the flow path through the internal storage reservoir. A small portion is left unlined as part of an infiltration zone to connect the aerobic zone to the anaerobic zone. This area, comprising highly permeable sand and gravel supplemented with organic material, allows stormwater to quickly percolate into the underlying anaerobic gravel cell.

Additionally, this area was seeded with an initial carbon source to assist with the denitrification process and speed anaerobic bacteria development to create anaerobic conditions in the internal storage reservoir. A perforated riser pipe was also incorporated into both BMPs to provide additional capacity should the infiltration zone become plugged. Once in the anaerobic cell, bacteria provide denitrification as previously outlined.

Stormwater migrates through the internal storage reservoir to an outlet control structure equipped with 1-inch-diameter orifices, designed to slowly release stormwater over a 24-hour to 48-hour period to maximize flow path through the anaerobic zone for optimal denitrification. Outlet orifices are located at the top of the internal storage reservoir, thereby keeping this zone fully saturated at all times and in anaerobic conditions. Once released from the BMP, stormwater will flow through a second dedicated sampling manhole, to be equipped with monitoring equipment at a later date. Overflow from the Barnstable BMP flows back into the existing drainage line where it flows into Hyannis Inner Harbor, similar to existing conditions, while the Chatham BMP outlets stormwater into the wetland complex at the rear of the site. Both BMPs are also equipped with overflow/bypass manholes that will collect and safely bypass stormwater that exceeds the capacity of the BMP.

Additional design elements included a perimeter drain around both BMPs to prevent mixing of groundwater with stormwater. A maintenance road was also constructed at the Chatham location to provide maintenance access. Maintenance requirements are anticipated to be relatively minor. Communities will need to periodically monitor the deep-sump inlet control structure and clean out sediment if needed. Periodic landscaping will also be required to trim excess vegetation growth. Each community will be provided with an operation and maintenance plan to document future required action.

Both BMPs are designed as a plug-flow system, such that stormwater will remain within the internal storage reservoir until the next storm occurs. This future storm will exert a hydraulic head on stormwater from the first storm within the subsurface gravel wetland cell, and thus push it out of the overflow structure to make room for the new storm.

Landscaping
Landscaping was an important feature at both BMP sites, both to provide nutrient uptake through the root systems and for aesthetic reasons. Because of the Barnstable BMP’s location within the park area, the BMP had to incorporate plantings to suitably accentuate the beauty of the bioretention area. For this reason, the Cape Cod Commission designed a planting plan consisting of several hundred planting plugs. Native species were chosen to be particularly drought, flood, and salt tolerant. The Chatham BMP was planted with grass seed within the basin itself; however, the side slopes were planted with native shrubs to provide slope stabilization, wildlife habitat, and project beautification.

Future Monitoring
As a final project component, both BMPs will be equipped with sampling equipment at a later date, designed to analyze stormwater flows and water-quality parameters of stormwater influent, effluent, and stormwater bypasses of each system. Federal and state environmental agencies will monitor both sites to evaluate water-quality parameter removal efficiencies realized by installing these BMPs. Sampling is expected to commence approximately one year after systems are brought online to allow bacteria to “seed” within each BMP, and will likely continue for at least two years.