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What is the NYC 2021 Unified Stormwater Rule?

Updated: an hour ago


Clogged Storm Drains Cause Flooding

Here at Stormwater Infrastructure Matters (SWIM) Coalition we talk a lot about NYC's stormwater runoff, how it impacts the health of our local waterways and the many ways that stormwater can be sustainably managed rather than just letting it "go down the drain."


So, we're excited about an upcoming regulation, the NYC 2021 Unified Stormwater Rule, that will help NYC manage stormwater runoff on new and retrofit construction sites citywide.


First, Some Context on Stormwater in NYC:


Because of our dense development patterns in NYC, ~72% of our landmass is impervious. This means that, during wet weather, large volumes of stormwater (rain and snowmelt) runs off most of our buildings, sidewalks, streets and parking lots into to storm drains (like the one pictured above) that convey it into a series of labyrinthian conveyance systems underneath our streets. As the runoff travels to the drains it collects trash and debris and all manner of contaminants.


We have two types of stormwater conveyance systems in NYC (see details on the systems below). Stormwater runoff is a root cause of a whole host of challenges in NYC such as sewage pollution in our waterways, flooding in our streets, erosion of our fragile shorelines, and deterioration of our antiquated sewer infrastructure just to name a few. How we manage our stormwater really matters!


NYC's Stormwater Management Systems and Drainage Areas


NYC has three types of sewer drainage areas and two stormwater management systems:

  • We have a Combined Sewer System (CSS) in ~60 % of the city (the green area on the map to the left);

A Separate Storm Sewer System (MS4) in ~35% of the city (the yellow areas on the map);and


A Direct Drainage area (grey areas on the map) in ~5% of the city





Below are details on NYC's stormwater conveyance systems and the challenges they pose in NYC:


Combined Sewer System

Our Combined Sewer System collects all of the wastewater from NYC's homes and buildings citywide + 60 % of our stormwater runoff and conveys the wastewater and stormwater through the same set of pipes to one of 14 wastewater treatment plants.

During many wet weather events, stormwater runoff (sometimes as little as one tenth of an inch of rain) overwhelms the capacity of our Combined Sewer System (CSS) causing it to discharge ~20billion gallons of raw sewage mixed with polluted stormwater directly into our local waterways every year. Yuk. These discharges are called combined sewer overflows (CSO). Here is a map of all the CSO discharge outfall points along our shorelines in the CSS area.


CSO events have many negative impacts on the ecosystems in and around our waterways and can make humans sick if we come into contact with the water

after a CSO event. CSO pollution is currently the largest ongoing source of pollution in NYC's waterways.

Municipal Separate Storm Sewer System

NYC's Municipal Separate Storm Sewer System (MS4) collects ~35% of the city's polluted stormwater runoff in a separate set of pipes that convey it to outfall points along the shoreline of the MS4 area where it is discharged directly into our waterways without any treatment. Here is an interactive map that shows where all the known MS4 discharge points are located along the shoreline of the MS4 area. Not all of the outfall points have been mapped yet nor has the total volume of stormwater discharges from the MS4 system been confirmed. DEP is currently in the process of mapping all of the impervious areas of the city so that accurate measurements of stormwater volume from this and all the drainage areas can be calculated.


Additionally, sometimes, in the MS4 area of the city, a building's wastewater pipes will inadvertently be connected to the MS4 pipes instead of the Combined Sewer System pipes causing illegal discharges of untreated sewage from the building's toilets, sinks, etc., directly into a nearby waterbody; this is called an illicit discharge. Here is an article about how illicit discharges from the MS4 system negatively impact our local waterways.


NYC does not yet have a firm account of exactly how many illicit discharge scenarios we have around the city, they are difficult to track down. Local waterway stewards are often the most likely folks to detect the discharges with noses or see them exiting an outfall on a dry weather day. We know that NYC DEP is working on a variety of innovative ways to track illegal discharges (drones) and address the improper pipe connections. Citizens can report these scenarios via the 311 system.


Direct Drainage Areas

An additional ~5% of our stormwater runoff flows directly into our waterways from direct or surface drainage areas. These areas are found throughout the city as indicated in the map above in grey. Direct Drainage areas range from places that have septic systems (this is the case on Staten Island), abandoned industrial sites and brownfield sites, municipal sites such as rail yards or, as in the image to the left, sites where construction equipment and materials are stored just to name a few. Stormwater runoff from these sites causes erosion of our fragile shorelines in addition to contributing potentially polluted stormwater in the waterways at the edges of the properties. Environmental watchdog groups monitor these types of sites closely for possible illegal dumping violations.

Needless to say, the waterways that surround our City of Islands and the city's infrastructure carry a heavy burden because our densely developed building practices. The more we build, the more impervious surface we create, and the more impervious surface we create, the more stormwater runoff we have to contend with! Here is a link to a 2019 article about how NYC's "building boom" exacerbates fooding in our streets.


The next decade is crucial in terms of how we address our stormwater and climate change related challenges in NYC. According to the NYC Panel on Climate Change, our region can expect to see a 1 – 8% increase in precipitation by the 2020s, and 4 – 11% increase by the 2050s. More precipitation means more stormwater runoff!


How NYC is Managing Stormwater Runoff Now:


NYC has developed a series of stormwater management plans such as the 2008 Sustainable Stormwater Management Plan, which was a precursor to today's Green Infrastructure (GI) Plan . NYC's GI plan, introduced in 2010, aims to manage the first inch of stormwater runoff on 10 % of NYC's impervious surfaces (within the combined sewer area of the city) with green infrastructure solutions by 2030, this translates into a goal of greening 8,000 acres. We also have new Sustainable Roof Laws that went into effect in 2019 and a Green Roof Tax Abatement program that has recently been updated to incentivize more green roofs on NYC's as yet, fairly untapped, inventory of rooftops. A new DEP grant program to support installation of GI on existing large private properties (50,000 sq ft and above) citywide will go into effect in 2021.


In 2017/18, a Stormwater Management Plan (SWMP) for the city's Municipal Separate Storm Sewer System (MS4) was initiated to develop a series of programs that will help reduce polluted stormwater runoff and the illegal discharges that pollute the receiving waters along the shores of the MS4 section of the city.


The 2021 Unified Stormwater Rule:


When it is introduced in 2021, the Unified Stormwater Rule will be a positive addition to the city's existing stormwater management efforts.


The 2021 Unified Stormwater rule will apply to all new and retrofit construction sites in the Combined Sewer area and the MS4 area of the city. The city-wide aspect of the Unified Stormwater rule is an important step forward for NYC! To date, the stormwater rule only applied in the MS4 area of the city and did not necessarily incentivize GI. Environmental advocates have long called for a more holistic approach to solving our stormwater runoff management challenges. The EPA also recommends an integrated approach to municipal stormwater and wastewater management.


  • The new unified rule will require managers of the construction sites to develop plans for how stormwater runoff will be managed on-site during and after construction so that it is either detained during wet weather events or, better yet, retained completely and ideally reused as a resource. Managing stormwater on site near or where it falls lessens the amount of volume that ends up in our streets, sewer systems, and waterways.


  • The new rule will also require that on-site stormwater infiltration feasibility studies be conducted to determine if green infrastructure solutions can be deployed. DEP will review the studies and work with site managers to ensure that they have explored all avenues for on site stormwater management and will provide them with a design manual to guide them in their installation of the best systems for their site conditions.


There are many aspects of the new rule that still need to be defined as it gets developed in the coming months. Step one for enacting the new rule took place when the City Council and the Mayor introduced a legislative bill, Intro 1851, held a public hearing on the bill in August 2020, and signed it into law last month:


  • Intro 1851 expands the current construction/post construction stormwater management rules for the city's MS4 area into the Combined Sewer System area of the city and authorizes the NYC Department of Environmental Protection (DEP) to begin the rule making process for a new Unified Stormwater Rule. NYC DEP will begin developing the new rule this fall. Here is SWIM's August, 2020 public testimony on Intro 1851.


  • One important aspect of the new rule is that DEP is going to change the lot size of sites that will have to comply. To date, the lot size has been one acre and above which meant not that many sites had to comply with the current rule. It is our understanding that DEP plans to change the lot size to 20,000 square feet and above. While this is an improvement, many stakeholders believe it should be even smaller. See our comments on this and other preliminary aspects of the rule below.


Our Initial Comments for what the new rule should address:


NOTE: these comments were compiled in close collaboration with and guidance from SWIM Coalition member, Gowanus Canal Conservancy (GCC), who has been leading a Net Zero CSO advocacy approach for development sites in the Gowanus neighborhoods for several years now in relation to their Gowanus Lowlands Master Plan for the area. We thank GCC for their leadership on this important work which is applicable to other areas of the city as well.


Below are some of the key concerns waterway advocates across the city have raised regarding the new stormwater rule:


  • The lot size needs to be smaller than 20,000 square feet in order to ensure that a larger number of sites have to comply with the new rule. Construction and development on any size lot can lead to increased CSO events.


  • Lot size should not be the only criteria for the types of lots that have to comply with the stormwater management regulations. We also need to consider the groundwater table (high or low), bedrock challenges, a property's proximity to superfund sites, impaired waterbodies, waterbodies that have CSO Long Term Control Plans underway, and the surrounding density of the construction site.


  • No Net Increase analysis (NNI) should be required for all pollutants and all waterbodies impacted by the construction site. In the current MS4 rule, NNI analysis is only required for “impaired waterbodies.” If a waterbody is not listed as impaired, as it stands right now, the analysis can be skipped entirely and if the analysis cannot be skipped, it is still not required for all pollutants.


  • NNI analysis should be required, and it should use the most stringent analysis, which is the Nitrogen NNI. These standards should also be applied to other pollutants: pathogens, phosphorus (sewage), CSOs, and floatables. It is only by conducting a more thorough analysis that we can understand how a waterbody will truly be impacted, and employ the proper mitigation measures to ensure a Net Zero CSO effect.


  • The new rule needs to include requirements for special conditions when infiltration is infeasible: A test for infiltration feasibility would provide evidence that a site's stormwater management plan (SMP) is effectively going to manage stormwater


  • More green infrastructure alternatives should be allowed as SMP practices. Green infrastructure can help with SMP for sites that have specific or special conditions. Wet swales, permeable sidewalks and swale combos, enhanced tree plantings, street end stormwater terraces, smart stormwater systems with real time controls, and other options should be included in the revised Design Manual.

SWIM Coalition will continue to update our comments as we learn more about the rule this fall from DEP and gather additional input from our coalition members.


DEP will reach out to the public in the fall of 2020 through community workshops in order to get input on the new rule. SWIM will be sure to alert our constituents about participating in the input sessions. This is an important step forward for NYC and we want to ensure that our voices and recommendations are heard!







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SWIM Coalition is sponsored by the NYC Soil & Water Conservation District and Riverkeeper, Inc. Your donations to SWIM's work are tax deductible under the IRS code170(c)3. 


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