1. ENGR 59910: Introduction to GIS
Final Project
CSO REDUCTION IN THE GOWANUS CANAL WATERSHED
THROUGH INSTALLATION OF GREEN INFRASTRUCTURE
Benjamin Hamm
December 17, 2014
Professor Michael Piasecki
The City College of New York
138th Street & Convent Avenue
New York, New York 10031

2. Table of Contents
Table of Contents..........................................................................................................................................1
1.0 Abstract...................................................................................................................................................2
2.0 Introduction ............................................................................................................................................2
3.0 Methods..................................................................................................................................................5
4.0 Results.....................................................................................................................................................9
5.0 Conclusions ...........................................................................................................................................10
Bibliography ................................................................................................................................................12
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3. 1.0 Abstract
This paper analyzes the impermeable surfaces in a high priority combined sewer outfall area in
Brooklyn, specifically the neighborhoods surrounding the Gowanus Canal in the Red Hook and Owls
Creek Sewersheds, and estimates the reduction in peak discharge and total runoff by incremental
investment and installation of green infrastructure. The Gowanus Canal has been identified by the New
York City Department of Environmental Protection as a “high priority” Combined Sewer Overflow (CSO)
outfall area and is a United States Environmental Protection Agency Superfund site. New York City is
currently investing in gray infrastructure improvements to this site, but these improvements are not
sufficient to reduce peak discharge and stormwater runoff. Green infrastructure investment and
installation, including bioswales, rain gardens, subsurface detention systems, and blue and green roofs,
will undoubtedly have a significant positive impact on the watershed and aid in CSO reduction, but
determining to what degree and cost is the next critical step. The DEP is currently in the midst of Phase I
of its Green Infrastructure Plan and has installed many bioswales throughout Brooklyn, Queens, and the
Bronx. Because roofs constitute nearly half of the impermeable surface area in the Gowanus Canal
watershed, a close examination of green roof performance in this area is a logical next step. This paper
analyzes the performance of green roofs on one residential city block in the Gowanus Canal watershed,
specifically by estimating the peak discharge and total runoff during a 5 year return period rainfall event.
The impermeable surface area data is provided through the NYC OpenData website, specifically the
2009 “Roadbed” dataset created by the New York City Department of Information Technology and
Telecommunications. The rational method was used to estimate peak discharge and the SCS method
was used to determine total runoff from the study area. Estimates for both values were determined at
incremental green roof values, ranging from the present state of 0% to 35% of the roof surface area.
Ultimately, it was determined that a 25% increase in green roofs on a typical residential block in the
Gowanus Canal Watershed would lead to the DEP’s stated goal of capturing the first inch of
precipitation, and a 35% increase in green roofs would lead to a 5.86% reduction in peak discharge,
hardly a significant increase to merit the significant cost of green roof installation and upkeep.
2.0 Introduction
In the past decade the City of New York invested billions of dollars in the sustainable design
movement and intends to pour billions more into this cause. From bike lanes to blue roofs, former
mayor, Michael Bloomberg, and the current mayor, Bill de Blasio, have placed a high priority on
sustainable development and reducing the city’s negative impact on the local environment. One of the
more notable campaigns put forth by Mayor Bloomberg and his administration is the NYC Green
Infrastructure Program1
, a multiagency effort led by the New York City Department of Environmental
Protection tasked with constructing and maintaining a variety of sustainable green infrastructure
projects such as green roofs, rain gardens, and Right-of-Way Bioswales on-city owned property. More
specifically, the NYC Green Infrastructure Program allocates and oversees $5 million of grant funding
1
City of New York (2012). “NYC Green Infrastructure Program.” NYC Department of Environmental Protection,
<http://www.nyc.gov/html/dep/html/stormwater/using_green_infra_to_manage_stormwater.shtml>
(October 30, 2014).
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4. designated for the design and construction of green infrastructure on private property, including blue
and green roofs and subsurface capture and detention systems.
While there are many benefits to green infrastructure, including aesthetics and reducing air
temperature, the impetus for this program is singular: prevent stormwater runoff from entering New
York’s combined sewer system, thereby reducing the number of Combined Sewer Overflows (CSO) and
improving the local water quality. While the Green Infrastructure Plan includes upgrades to the grey
infrastructure of the combined sewer system, the core of the program is the goal of capturing rainfall
from 10% of impervious surfaces through green infrastructure. More specifically, the objective is to
capture the first inch of rainfall on 10% of the impervious areas in combined sewer watersheds through
detention or infiltration techniques over the next 20 years. By preventing one inch of precipitation from
becoming runoff in 10% of the impervious areas, the DEP estimates that CSOs will be reduced by
approximately 1.5 billion gallons per year. In order to meet this demand, the DEP intends to capture
1.5% impervious area by 2015, an additional 2.5% by 2020, and additional 3% by 2025, and the
remaining 3% by 2030.
CSOs are a leading cause of pollution in rivers, lakes, and estuaries in the United States, and in
New York City in particular. According to a United States Environmental Protection Agency report, 828
nationally permitted combined sewer systems release approximately 3.2 million m3
, or 850 billion
gallons, of untreated sewage to surface water bodies from approximately 9,300 CSO discharge points.2
In New York City, CSOs are the largest single source of pathogens in the New York Harbor.3
As little as a
tenth of an inch of rain per hour, or 0.40 inches a day, can cause a CSO in New York City. CSO events
occur about once per week in New York City, with some outfalls experiencing up to 70 CSOs in a year.
On average, New York City experiences 460 annual CSOs, leading to more than 27 billion gallons of raw
sewage and polluted stormwater entering New York Harbor. Weekly polluted discharge is
approximately 500 million gallons.4
More than a liberal, feel-good cause, green infrastructure is increasingly becoming part of the
city’s public record. The DEP’s Stormwater Performance Standard was amended in 2012 to reflect the
city’s desire to reduce the demand on the combined sewer system and lower the amount of CSOs.
Chapter 31, Title 15 of the Rules of the City of New York was modified to reflect a stricter standard for
stormwater release rate, lowering it from a maximum of 2.5 cfs to 0.25 cfs for new developments, a
tenfold increase.5
While this new standard can be routinely regulated for new construction, retrofitting
massive connections to the city’s sewage system is not a realistic or feasible goal. However, through the
Green Infrastructure grant program, the DEP is actively encouraging private citizens to use public money
to invest in their neighborhood.
2
USEPA, 2004. Report to Congress: Impacts and Control of CSOs and SSOs. EPA 833-R-04-001. August 26, 2004. <
http://nepis.epa.gov/Exe/ZyPDF.cgi/30006O5F.PDF?Dockey=30006O5F.PDF> (November 1, 2014).
3
New York City Department of Environmental Protection (NYCDEP) (2002). “2002 New York Harbor Water Quality
Report.” Page 20.
4
“Combined Sewage Overflows (CSOs)”. (2014). Riverkeeper, Inc.
5
City of New York (2012). “Guidelines for the Design and Construction of Stormwater Management Systems.” NYC
Department of Environmental Protection and in consultation with the NYC Department of Buildings. New
York, NY. Page 2.
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5. Recently, the NYC DEP has coordinated a series of grant workshops designed to educate the
public on CSOs, green infrastructure, and how to submit an application for a grant to install green
infrastructure on their private property. The application is an online one, available on the DEP website.
The minimum requirement for approval is to manage 1” volume of stormwater runoff from the
contributing impervious area, but other factors including cost effectiveness, overall design, feasibility,
replicability, and community development influence the approval process. In essence, approved sites
will have all design and construction costs be covered by the grant. By increasing the number of green
roofs, blue roofs, and subsurface detention systems in high priority CSO outfall areas, the stormwater
runoff can be reduced, and in some cases be recycled.
Additionally, the NYC Department of Buildings oversees a one-year green roof property tax
abatement program that was initially enacted in 2008. Property owners can apply for a one year tax
abatement of $4.50 per square foot up to $100,000. This program stipulates that at least 50% of
available roof space must be covered by a green roof, and that at least 80% of the vegetation layer be
covered by drought resistant hardy plants. While the number of property owners taking advantage of
this program is relatively small6
, the program was renewed in 2013, allowing more New Yorkers to take
advantage of the incentive program.
From an academic standpoint, there has been growing interest in green infrastructure
development in the past decade, especially in the New York City area. One study conducted by a team
of researchers at Columbia University, which includes City College of New York civil engineering
professor Reza Khanbilvardi, highlights the many positive impacts of a large green roof in New York City
(ConEdison Learning Center), including increasing local evapotranspiration, reducing heat conduction
from the building below, and most importantly, significantly increasing water retention. The
researchers estimate that if all roof areas in New York City are covered with a 4 inch sedum based green
roof layer, approximately 10-15 billion gallons of annual rainfall would be retained.7
Another team of researchers, also from Columbia University, expanded upon the above analysis
of ConEd Learning Center to include two other green roof sites in New York City: a Columbia University
residential building and the United States Postal Service Morgan general facility building. Using
precipitation records from Central Park for a 40 year period, they modeled the water retention
performance for each roof. They estimated the total rainfall retention to be 45%, 53%, and 58% for the
residential, USPS, and ConEd green roofs respectively.8
Even more pertinent to the scope of this paper, Franco Montalto performed a cost analysis of
low impact developments, including green roofs, to limit CSOs in New York City, specifically, near the
6
Loria, Keith (2011). “Green on Top.” The Cooperator, < http://cooperator.com/articles/2223/1/Green-on-
Top/Page1.html> (October 30, 2014)
7
Gaffin, S. R., Rosenzweig, C., Khanbilvardi, R., Eichenbaum-Pikser, J., Hillel, D., Culligan, P., McGillis, W., and Odlin,
M. (2011). “Stormwater Retention for a Modular Green Roof Using Energy Balance Data.” Columbia
University, Center for Climate Systems Research. New York, NY.
8
Carson, T.B, Marasco, D.E., Culligan, P.J., McGillis, W.R. (2013). “Hydrological Performance of Extensive Green
Roofs in New York City: Observations and Multi-Year Modeling of Three Full-Scale Systems.” Columbia
University. IOP Publishing. New York, NY.
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6. Gowanus Canal.9
He analyzed the impervious surfaces for one CSO outfall, OH-007, and concluded that
up to 90% of the row houses that “serve” this outfall could support a green roof. Montalto compared
the cost effectiveness of three “Low Impact Development” options: green roofs, porous pavement, and
stormwater treatment wetland scheme. Within his study area, 55% of the land use is residential, and
85% of the CSO-shed is impervious, with nearly half (46.75%) of the impervious surfaces being rooftops.
Montalto determined that if all rooftops in the study area were green roofs, CSO events at this outfall
would decrease by 26%. Considering 260,000 m3
is discharged from this outfall annually, which is
approximately one-fourth of all discharge into the Gowanus Canal, a reduction of 26% is a significant
amount.
3.0 Methods
In order to reduce stormwater runoff and CSOs, the baseline discharge needs to be calculated.
The NYCDEP utilizes the Rational Method for determining peak developed site flow, or rate of runoff
from the site. This method uses the total site area, rainfall intensity, and the site’s surface coverage and
computes the total site developed storm flow, QDEV, in cubic feet per second (cfs), the allowable flow,
QALL, and the detention facility maximum release rate, QDRR, also measured in cfs, the required detention
facility volume, VR (in ft3
), and the detention facility maximum storage depth, SD (in feet).10
Once the
developed flow is calculated, it is compared to the allowable flow, and if the developed flow exceeds the
allowable flow, the runoff must be captured and detained in order to comply with DEP’s stormwater
performance standard.11
In this paper, the impervious surfaces of a residential block in the Gowanus Canal Watershed in
Brooklyn, NY were analyzed through ArcMap. See Figure 1 below for maps of the study area and Figure
2 for the CSOs in the Gowanus Canal. A geodatabase was established and included various shapefiles
that were made accessible through the NYC Open Data website. The sewershed boundaries were
digitized, made into its own layer and merged with the surface area data from the “Roadbed” file. Once
the data was imported in ArcMap, the majority of the time and effort in ArcMap was spent on
symbolizing and labeling the various surface areas. All maps presented in this paper were created by
the author.
9
Montalto, F., Behr, C., Alfredo, K., Wolf, M., Arye, M., Walsh, M. (2007). “Rapid assessment of the cost-
effectiveness of low impact development for CSO control.” Landscape & Urban Planning 82 (2007). Pages
117-131.
10
City of New York (2012). “Criteria for Detention Facility Design.” NYC Department of Environmental
Protection, New York, NY.
11
City of New York (2012). “Guidelines for the Design and Construction of Stormwater Management Systems.”
NYC Department of Environmental Protection and in consultation with the NYC Department of Buildings.
New York, NY.
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7. Figure 1 – The Gowanus Canal Sewershed and an overhead view of typical residential blocks within the study area.
Figure 2 – CSO outfalls in the Gowanus Canal and the current green infrastructure in the watershed.
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8. Surface area values for roofs, pervious surfaces, and concrete in the study area was exported
from ArcMap and used in the calculations below. The following coefficients were used to calculate the
weighted surface area coefficient:
C = 0.95 for roof areas
C = 0.85 for pavement & concrete
C = 0.70 for porous asphalt/concrete and permeable pavers
C = 0.70 for a green roof with 4 in. of growing media
C = 0.70 for synthetic turf field
C = 0.65 for gravel parking lot
C = 0.30 for undeveloped areas
C = 0.20 for grass, planted, bio-swales, or landscaped areas
These C values were used in Equation 1 below to calculate the weighted runoff coefficient for each zone:
𝐶𝐶𝑊𝑊 =
1
𝐴𝐴
∑ 𝐴𝐴𝑘𝑘 𝐶𝐶𝑘𝑘
𝑛𝑛
𝑘𝑘=0 (Equation 1)
where
A is the total site area, acres (ac)
k is the index for each onsite surface coverage type
Ak is the area of each surface coverage type, ac
Ck is the runoff coefficient associated with each surface coverage type
Cw is used directly in the calculation of developed site flow.
The CW values for each zone were then tabulated in Matlab.
The developed discharge, QDEV, was calculated by using Equation 2 below:
𝑄𝑄𝐷𝐷𝐷𝐷𝐷𝐷 = 𝐶𝐶𝑤𝑤
𝐴𝐴𝑆𝑆
7,320
(Equation 2)
where
QDEV is the developed peak discharge from the site, measured in cfs
CW is the weighted coefficient for the site
AS is the total site area, measured in ft2
7,320 is 43,560 ft2
/ac divided by the rainfall intensity of 5.95 in/hr for the event
with a five year return period and a six minute time of concentration.
While the rational method is helpful to determine peak discharge from a site, it does not assist
in the determination of runoff capture, which is the primary goal of the DEP’s green infrastructure plan.
In order to estimate runoff from a site, the procedure developed by the Soil Conservation Service (SCS)
was used. The SCS method estimates the runoff from site based on the cumulative precipitation and
runoff curve numbers, which are determined by the surface area of the study area. For the sake of
consistency with the rational method (5 year return period), the precipitation value used for this study
was 1.9 inches over 1 hour. See Figure 3 for the IDF curve used to determine this value.
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9. Figure 3 - IDF curve used to determine precipitation
In order to determine the weighted CN number for the site, the values from Figure 4 were used.
All impermeable surfaces were assigned a CN number of 100. Green roofs were assigned a CN number
of 61, which corresponds to a B-level Hydrologic Soil Group.
Figure 4 - Curve Number Chart used to determined weighted curve number for the study area.
The weighted CN value is then used to determine the soil-moisture storage deficit, which is used
to determine runoff (see Equations 3 & 4 below).
𝑆𝑆 =
1000
𝐶𝐶𝐶𝐶
− 10 (Equation 3)
𝑅𝑅 =
(𝑃𝑃−0.2𝑆𝑆)2
(𝑃𝑃+0.8𝑆𝑆)
(Equation 4)
where
S is the soil-moisture storage deficit (in)
CN is the weighted curve number for the surface area (unitless)
P is cumulative rainfall
R is cumulative runoff
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10. 4.0 Results
A macro-level analysis of the watershed through ArcMap reveals that roofs constitute a majority
of the surface area. See Figures 5 & 6 below for maps that highlight the various types of impermeable
surfaces and roofs sorted by increasing surface area.
Figure 5 - Maps that highlight all impermeable roofs and parking lots in the watershed.
By exporting the shapefile data into a spreadsheet, charts that reveal the breakdown of
impervious surfaces were created. See Figure 6 below.
Figure 6 - A summary of the impervious surfaces in the watershed.
Of greater interest to the research question of this project, increasing the percentage of green
roofs in the study area from 0 to 35% will lead to a 58% increase in captured precipitation during 60
minutes of a 5-year return period storm. In order to meet the DEP 2030 goal of capturing the first inch
Roofs
54%
Parking
Lots
4%
Sidewalks
14%
Roadbeds
28%
Impervious Surfaces in the
Gowanus Sewershed
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11. of rain in this study area, the percent of green roofs in the study area would have to be at least 25%,
which is 15% more than the 10% impervious surface area goal. See Table 1 below for a summary of the
precipitation capture results.
Table 1 - Summary of Precipitation Captured
Percent Green Roofs Weighted CN Runoff (in) Capture (in) Percent Captured
0% 93.70 1.28 0.62 32.7%
5% 92.70 1.20 0.70 37.0%
10% 91.60 1.12 0.78 41.1%
15% 90.60 1.05 0.85 44.9%
20% 89.50 0.98 0.92 48.5%
25% 88.50 0.91 0.99 51.9%
30% 87.40 0.85 1.05 55.1%
35% 86.40 0.80 1.10 58.2%
While decreasing the percentage of impermeable surfaces by installing green roofs will lead to
sizeable increase in captured precipitation in a heavy storm, it will not significantly lower the developed
peak discharge. In a five year return period rainstorm, roughly 1/3 of the precipitation falls in the first
six minutes of the storm, essentially overwhelming the surface area. A 35% increase in the amount of
green roof surface area will result in a 5.86% reduction of peak flow, hardly enough to mitigate the
demand on the combined sewer system or significantly reduce the number of CSOs. See Table 2 below
for a summary of the peak discharge results and Figures 7 & 8 for the plots of Cw values against peak
discharge and peak discharge reduction.
Table 2 - Summary Peak Discharge Results
Percent Green Roofs Weighted Cw Peak Q Percent Reduction
0% 0.799 15.82 0.00%
5% 0.792 15.69 0.84%
10% 0.785 15.55 1.68%
15% 0.778 15.42 2.51%
20% 0.772 15.29 3.35%
25% 0.765 15.16 4.19%
30% 0.758 15.02 5.03%
35% 0.752 14.89 5.86%
5.0 Conclusions
The DEP’s laudable goal of capturing the first inch of rainfall on 10% of the impermeable
surfaces is largely contingent upon significantly increasing the amount of bioswales throughout the city.
The results of this study in many ways confirm that green infrastructure projects other than green roofs
will be more successful in reducing runoff, and at a lower cost. The estimated cost of green roofs is $10
per square foot. A 25% increase in green roofs in the study area is approximately 19,400 square feet,
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12. which would cost at least $194,000, hardly a manageable cost for the DEP for one city block. In fact, this
cost constitutes nearly 4% of the $5 million grant program. Unless the tax abatement program is
ramped significantly, there is not sufficient incentive for private property owners to transition their roofs
to green roofs.
While the benefits of green roofs are innumerable, they are not a realistic solution to the CSO
issue that New York City is currently experiencing. Although bioswales do not provide the same surface
area coverage that blue or green roofs offer, they have a much lower Cw coefficient, between 0.20-0.30.
The difficulty with bioswales in the Gowanus Canal Watershed is finding sidewalks with suitable widths
so that they do not overly interfere with pedestrian traffic. Regardless of the practicality of installing
green roofs on a mass scale, the results of this research will be helpful to city planners and engineers
tasked with identifying suitable sites for green infrastructure.
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13. Bibliography
Carson, T.B, Marasco, D.E., Culligan, P.J., McGillis, W.R. (2013). “Hydrological Performance of Extensive
Green Roofs in New York City: Observations and Multi-Year Modeling of Three Full-Scale
Systems.” Columbia University. IOP Publishing. New York, NY.
City of New York (2012). “Criteria for Detention Facility Design.” NYC Department of Environmental
Protection, New York, NY.
City of New York (2012). “Guidelines for the Design and Construction of Stormwater Management
Systems.” NYC Department of Environmental Protection and in consultation with the NYC
Department of Buildings. New York, NY.
City of New York (2012). “NYC Green Infrastructure Program.” NYC Department of Environmental
Protection,
<http://www.nyc.gov/html/dep/html/stormwater/using_green_infra_to_manage_stormwater.s
html> (October 30, 2014).
“Combined Sewage Overflows (CSOs)”. (2014). Riverkeeper, Inc.
Gaffin, S. R., Rosenzweig, C., Khanbilvardi, R., Eichenbaum-Pikser, J., Hillel, D., Culligan, P., McGillis, W.,
and Odlin, M. (2011). “Stormwater Retention for a Modular Green Roof Using Energy Balance
Data.” Columbia University, Center for Climate Systems Research. New York, NY.
Wilpen Gorr and Kristen Kurland. “GIS Tutorial. Basic Workbook 1.” Esri Press. Redlands, CA. 2013.
Houghtalen, Osman, et al. Fundamentals of Hydraulic Engineering Systems. 4th
edition. Prentice
Hall, 2010.
Loria, Keith (2011). “Green on Top.” The Cooperator, < http://cooperator.com/articles/2223/1/Green-
on-Top/Page1.html> (October 30, 2014)
Montalto, F., Behr, C., Alfredo, K., Wolf, M., Arye, M., Walsh, M. (2007). “Rapid assessment of the cost-
effectiveness of low impact development for CSO control.” Landscape & Urban Planning 82
(2007). Pages 117-131.
New York City Department of Environmental Protection (NYCDEP) (2002). “2002 New York Harbor Water
Quality Report.” Page 20.
NYCDOITT (2009). Roadbed. [Data file and map]. Retrieved from: <https://data.cityofnewyork.us/City-
Government/Roadbed/>
USEPA, 2004. Report to Congress: Impacts and Control of CSOs and SSOs. EPA 833-R-04-001. August 26,
2004. < http://nepis.epa.gov/Exe/ZyPDF.cgi/30006O5F.PDF?Dockey=30006O5F.PDF>
(November 1, 2014).
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