Jones M.

Leo W.

Licata A.

McLaughlin J.

Mehrotra S.

Stein J.

English

I-Revues

NOVATECH 2013 
1 
Performance of Green Infrastructure Stormwater 
Controls in New York City 
La performance des infrastructures vertes de gestion des 
eaux pluviales dans la ville de New York 
 
Angela Licata*, John McLaughlin**, Julie Stein***, Sandeep 
Mehrotra, PE****, William Leo, PE*****, Matthew Jones, PE, 
PhD****** 
 
* New York City Department of Environmental Protection, 59-17 Junction 
Boulevard, 11th Floor, Flushing, NY, USA  angelal@dep.nyc.gov 
** New York City Department of Environmental Protection, 59-17 Junction 
Boulevard, 11th Floor, Flushing, NY, USA  johnm@dep.nyc.gov 
*** New York City Department of Environmental Protection, 59-17 Junction 
Boulevard, 11th Floor, Flushing, NY, USA  julies@dep.nyc.gov 
**** Hazen and Sawyer, PC, 498 Seventh Avenue, 11th Floor, New York, NY, 
USA  smehrotra@hazenandsawyer.com 
***** HDR-HydroQual, 1200 MacArthur Boulevard, Mahwah, NJ, USA 
William.Leo@hdrinc.com 
****** Hazen and Sawyer, PC, 4011 WestChase Boulevard, Suite 500, Raleigh, 
NC, USA  mjones@hazenandsawyer.com 
 
RÉSUMÉ 
Dans le cadre des mesures visant à répondre aux préoccupations concernant la capacité des égouts 
et les rejets unitaires de temps de pluie, le Département de la Protection de l’Environnement de la 
Ville de New York a entrepris des actions pour tester des infrastructures vertes à New York. Plus de 
20 pilotes de suivi des sources d’eaux pluviales ont été construits dans des chemins, terre-pleins 
centraux, parcs, logements sociaux et sur les toits. Les activités de mesure quantitative et qualitative 
sur ces installations ont permis d’évaluer la logistique de mise en application et les performances de 
ces contrôles au sein de l’environnement ultra-urbain de la ville de New York. L’équipement de 
mesure à distance installé sur ces sites pilotes a fourni des informations sur les taux et volumes 
entrants et sortants, ainsi qu’un aperçu des taux d’abaissement et autres aspects de la fonctionnalité 
des infrastructures vertes. Ces évaluations ont démontré que les infrastructures vertes présentent des 
avantages considérables en matière de détention et rétention de ruissellement lorsqu’elles sont 
installées dans un scénario de modernisation. Dans certains cas, les contrôles des eaux pluviales ont 
entièrement retenu le ruissellement entrant, en éliminant efficacement l’impact des surfaces 
imperméables tributaires sur le système d’assainissement en aval. 
ABSTRACT 
As part of efforts to address sewer capacity concerns and combined sewer overflows, the New York 
City Department of Environmental Protection has undertaken efforts to pilot green infrastructure 
throughout New York City. More than 20 stormwater source control pilots have been constructed 
within rights-of-way, roadway medians, parks, public housing facilities, and rooftops. Quantitative and 
qualitative monitoring activities at these facilities have evaluated the logistics of implementation and 
performance of these controls within the ultra-urban environment of New York City. Remote monitoring 
equipment installed across these pilot sites has provided information on inflow and outflow rates and 
volumes, as well as insight into drawdown rates and other aspects of green infrastructure functionality. 
These evaluations have demonstrated that green infrastructure can provide substantial runoff 
detention and retention benefits when installed in a retrofit scenario. In some cases, stormwater 
controls have fully retained incoming runoff, effectively eliminating the impact of tributary impervious 
surfaces on the downstream sewer system. 
KEYWORDS 
Bioretention, Combined sewer overflow, Infiltration, Maintenance, Monitoring, Stormwater source 
control, Sustainability, Urban 
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1 GREEN INFRASTRUCTURE IN NEW YORK CITY 
Within New York City, more than 450 combined sewer overflow (CSO) outfalls handle excessive wet 
weather sewer flows from the largest city in the United States. To address this challenge, the New 
York City Department of Environmental Protection (DEP) has supplemented ongoing grey 
infrastructure approaches to CSO control with green infrastructure, through the PlaNYC initiative. By 
utilizing natural processes for improved stormwater management, these controls are expected to 
reduce the rate and volume of runoff discharged to the combined sewer system, while also providing 
numerous benefits that aren’t typically associated with grey infrastructure controls. At the foundation of 
this initiative is a pilot program, supporting the City’s adaptive management approach by examining 
the implementation and performance of green infrastructure controls across representative sites 
throughout the City. Examination of implementation logistics, as well as qualitative and quantitative 
monitoring analyses, is expected to inform future green infrastructure implementation, promoting more 
effective stormwater management and CSO reduction. 
As outlined in the 2010 NYC Green Infrastructure Plan, New York City is planning to manage up to 2.5 
cm of rainfall from 10% of impervious surfaces within combined sewer areas. While various green 
infrastructure controls have been effectively implemented in other areas throughout the world, there 
are some unique opportunities and challenges within New York City, allowing a pilot program to 
provide valuable insight. 
2 OVERVIEW OF STORMWATER PILOT PROGRAM 
The green infrastructure pilot program included the construction of more than twenty source control 
retrofits within high priority areas for CSO management in the boroughs of Brooklyn, Queens, and the 
Bronx (Figure 1). These pilots included bioretention, enhanced tree pits, street-side infiltration swales, 
constructed wetlands, blue roofs, green roofs, permeable pavement, and subsurface detention and 
infiltration systems. Pilot implementation focused on public properties expected to reflect the wide 
range of potential green infrastructure sites throughout the City, which included parks, roadway 
medians, street-side sidewalks, public housing facilities, parking lots, and rooftops. 
 
Figure 1: Overview of stormwater pilot locations 
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There were several aspects of the pilot program that were intended to provide information in support of 
future green infrastructure efforts. First, planning and design efforts focused on developing green 
infrastructure source controls that could be realistically utilized within New York City and contribute 
towards CSO reduction. These efforts have resulted in the modification and adaptation of general 
green infrastructure designs utilized in other areas, as well as the implementation of relatively unique 
stormwater source controls.  Evaluations during the construction of these controls provided valuable 
information regarding the logistics of construction activities, including unique aspects of working in an 
ultra-urban environment. Finally, a comprehensive post-construction pilot monitoring program has 
provided critical quantitative and qualitative data regarding the performance of these green 
infrastructure controls. 
Remote monitoring equipment was the predominant mechanism for evaluating stormwater pilot 
performance. Quantitative monitoring focused on the effect of stormwater pilots on runoff rates and 
volumes, as well as evaluating elements such as storage volumes and drawdown rates. Because CSO 
reduction is a major objective of green infrastructure implementation, improved knowledge regarding 
these aspects of stormwater source control performance were expected to inform future planning and 
modelling efforts. Various flumes and weir configurations, in conjunction with pressure transducer 
water level loggers, were utilized to measure inflow and outflow from stormwater controls (Figure 2). 
Additional monitoring equipment such as piezometers and surface water level loggers supported 
general assessments of pilot functionality. Flow and stage monitoring data were typically collected at a 
5-minute interval. Tipping bucket rain gauges, and in some cases, more comprehensive weather 
stations, were installed in the vicinity of pilot stormwater controls. 
 
Figure 2: Inlet weir box installed within a bioretention curb cut 
To support quantitative evaluations, on-site testing and calibration activities were conducted for most 
monitoring installations. Generally, metered flow from a hydrant was discharged immediately upstream 
of the pilot flow monitoring device. Measured hydrant flow was compared to the depth recorded by the 
installed remote monitoring equipment in order to develop an on-site calibrated rating curve for each 
flow measurement location. In some instances, these on-site rating curves differed substantially from 
those reported in the literature or resulting from lab calibration efforts, illustrating the sensitivity of 
these measurements to on-site conditions.  In addition to improved calibration, hydrant testing yielded 
valuable information regarding general pilot functionality by effectively simulating a storm event. These 
simulated storms provided insight into both monitoring equipment and stormwater pilot maintenance 
needs. 
In addition to water quantity evaluations, a basic water quality monitoring program was developed for 
the stormwater pilots. This water quality monitoring program was intended to compare observed water 
quality at the NYC stormwater pilots to evaluations conducted for other stormwater controls reported in 
the literature. Of particular interest was whether pollutant loadings from these ultra-urban watersheds 
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differed substantially from those reported in other areas. Another objective of the water quality 
monitoring program was to identify potential long term maintenance needs, which may be caused by 
the build up of sediments or certain contaminants. Water quality sampling was conducted primarily 
through the use of self-sealing first flush sampling bottles and discrete grab sampling. In some cases, 
water was collected from infiltration wells and soil samples were analyzed to gain a broader 
perspective on pilot performance. Analyses focused primarily on gasoline and diesel hydrocarbons, 
total suspended solids, nutrients, and metals. 
Beyond quantitative assessments, qualitative evaluations of the stormwater pilots are providing further 
insight into the functionality of these systems and the unique aspects of their implementation in an 
ultra-urban area like New York City. Assessments of the frequency and nature of maintenance 
requirements, success of vegetative elements, and public perception were among several aspects of 
qualitative evaluations. 
3 MONITORING RESULTS AND DISCUSSION 
Stormwater pilot monitoring began during the spring of 2011 and expanded as construction was 
completed at additional pilot sites. In general, monitoring activities were planned for a 2-year duration 
at each of the stormwater pilots. Preliminary results through the end of 2011 for several stormwater 
pilots are presented herein. 
3.1 North and South Conduit Avenues Bioretention 
A pair of hydraulically connected bioretention areas was constructed within a wide grassed median in 
Queens. Runoff from surrounding streets and sidewalks, totalling approximately 7,600 m², was 
diverted to the bioretention areas through a variety of mechanisms, including several curb cut 
configurations, retrofits of existing catch basins, and installation of new catch basins. The bioretention 
areas themselves cover an 800 m² area. Each cell incorporates 15 cm of surface storage, 60 cm of a 
sandy engineered soil media, and a 30 cm stone drainage layer. A perforated underdrain pipe 
hydraulically connects the subsurface layers of the bioretention areas, while a vegetated channel 
connects the surface storage layers after the depth exceeds 15 cm. An overflow grate within the 
eastern bioretention area conveys excess surface flow and underdrain flow to the sewer system. A 
stop log structure was installed within a manhole immediately downstream of the bioretention, 
effectively forcing the bioretention subsurface layers to be saturated before water is discharged from 
the system. 
 
Figure 3: Overview of roadway median bioretention areas at North and South Conduit Avenues 
Monitoring activities have demonstrated that the bioretention areas are effective at retaining storm 
flows, particularly for smaller events (Figure 4). The facility has typically drained within 24-48 hours 
after a storm, providing storage capacity for future storm events. In addition to the bioretention areas 
themselves, vegetative conveyance swales at this site have enhanced stormwater retention, in some 
NOVATECH 2013 
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cases infiltrating all runoff from small storms before flows reach the bioretention soil. Bypass losses at 
curb cuts, estimated around 30%, have limited the overall effectiveness of the system in capturing all 
runoff from adjacent impervious areas. 
 
Figure 4: Proportion of runoff volume retained by roadway median bioretention areas 
3.2 Bronx River Houses Bioretention 
A number of green infrastructure source controls were constructed at a public housing facility located 
in the Bronx, including five bioretention areas surrounding the central community center. These 
bioretention areas were constructed within existing grassed areas and manage runoff from sidewalks 
throughout the facility. Curb cut retrofits with flagstone sumps were implemented to divert runoff from 
those sidewalks into the bioretention areas. Ratios of contributing impervious area to bioretention area 
varied from 6:1 to 20:1. These bioretention areas contained approximately 20 cm of surface storage 
capacity, 30 cm of bioretention soil, and 20 cm of a stone drainage layer surrounding an underdrain. 
 
Figure 5: Bronx River Houses bioretention during a storm event 
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During most smaller storm events, these bioretention areas have retained nearly 100% of the runoff 
they receive (Figure 6). Detailed monitoring evaluations indicate that captured runoff is being 
temporarily detained at the surface and seeping into the soil underlying the bioretention soil and stone 
drainage layer. In many cases, the surface of the bioretention was drained before the storm had 
ended. Similar to the North and South Conduit Avenues bioretention, bypass losses around 30% have 
been observed at the curb cut retrofit locations. 
 
Figure 6: Proportion of runoff volume retained by one of the Bronx River Houses bioretention areas 
3.3 Metropolitan Avenue Blue Roof 
Like many ultra-urban areas, rooftops in New York City represent a substantial portion of total 
impervious surfaces. A blue roof pilot was implemented on top of a storage building in Brooklyn in 
order to evaluate several mechanisms of locally detaining rooftop runoff. These mechanisms consisted 
of a flow restriction installed at the existing roof drain, a series of concentric check dams upstream of 
the existing roof drain, and a series of plastic trays with restrictive orifices and drainage layers (Figure 
7). Each of these blue roof configurations, along with an unmodified experimental control, were 
evaluated on separate quadrants of the roof. Specifically, the modified roof drain consisted of an 
orifice restriction in conjunction with an overflow weir, which was installed over the existing roof drain. 
The check dam blue roof was comprised of 2.5 cm angle aluminium with a series of small orifice 
perforations across its length, and stone immediately upstream. The blue roof tray system consisted of 
plastic trays, approximately 60 cm square and 10 cm deep. A series of orifices were perforated in the 
bottom of the trays to allow flow to exit onto the existing roof. A geotextile was installed at the bottom 
of the trays to control the drainage rate, and stone was added within the trays for ballast. 
NOVATECH 2013 
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Figure 7: Blue roof check dams (left) and trays (right) 
Monitoring evaluations have shown that each blue roof type has provided some level of runoff 
detention, although the tray and check dams have provided a higher degree of detention more 
consistently. The tray system in particular reduced the peak runoff rate from the rooftop by 80% or 
more for most storm events. It is expected that limited detention storage around the modified roof 
drain, due to the slope of the roof, has impacted the performance of that blue roof control.  Although 
designed primarily for detention, evaluations have demonstrated that each of the blue roof types is 
also providing runoff retention benefits, primarily through depressional storage. The check dam and 
tray systems have provided more consistent retention benefits than the other treatments, with the tray 
system consistently retaining the greatest proportion of rooftop runoff (Figure 8). The median volume 
retention provided by the tray system for storms smaller than 2.5 cm was 69%, compared with 26% on 
the unmodified experimental control. 
 
Figure 8: Proportion of runoff volume retained by the blue roof tray system 
3.4 Comparison of Pilot Performance 
With different design objectives and constraints, a comprehensive comparison of performance 
between pilot sites presents some challenges; however, it is possible to draw some generalized 
conclusions. Both the Bronx River Houses bioretention and North and South Conduit Avenues 
bioretention utilize similar designs, but are implemented at different scales. Both of these systems 
were similarly effective in retaining storm flows. Comparisons across individual bioretention areas at 
Bronx River Houses suggests that performance may be influenced by the size of the tributary drainage 
area in relation to the bioretention footprint; however, data did not support a narrowly defined 
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correlation.  In addition to tributary drainage area, the characteristics of underlying in-situ soil likely had 
a substantial influence on bioretention pilot performance. Although designed for runoff detention, the 
blue roof systems provided a moderate level of volume retention. Overall retention performance was 
inferior to the bioretention systems, which was expected due to the lack of infiltration as a viable 
retention mechanism for a blue roof. Still, monitoring results indicate that retention benefits should be 
considered when evaluating the impact of a blue roof system. 
4 CONCLUSION 
Monitoring activities at pilot sites throughout New York City have demonstrated green infrastructure 
controls are providing valuable stormwater management benefits. While detention and retention 
characteristics have varied with each pilot type and location, the performance of these pilots has 
generally met or exceeded their designed objective. In addition to the detention and retention 
performance summaries provided herein, detailed evaluations into the effect of these controls on 
runoff characteristics are improving the understanding of green infrastructure and its expected impact 
on the combined sewer system in New York City. With this enhanced understanding of on-the-ground 
performance, it is expected that green infrastructure can be effectively utilized as a stormwater 
management strategy to address combined sewer overflows within the largest city in the United 
States. 


La performance des infrastructures vertes de gestion des eaux pluviales dans la ville de New York

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