• SWIM

What is the NYC 2021 Unified Stormwater Rule?

Updated: Mar 4


Here at Stormwater Infrastructure Matters (SWIM) Coalition we talk a lot about NYC's stormwater runoff, how it negatively impacts the health of our local waterways and the city's stormwater and wastewater infrastructure.


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 hard surfaces such as buildings, sidewalks, streets, and parking lots and overwhelms our sewer systems. 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!


So, we're excited about an upcoming regulation, the NYC 2021 Unified Stormwater Rule, that, in combination with a series of programs underway now and coming to fore in 2021, will help NYC move toward our 2030 water quality improvement goals for our overburdened waterways. DEP will hold a public hearing in early 2021 to share the draft rule for public comment. We will get the word out about the hearing via twitter: @SWIMCoalition and e-blast. Sign up here for our e-blast.

The 2021 Unified Stormwater Rule:


When it is introduced in early 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 citywide.


The city-wide aspect of the Unified Stormwater rule is an important step forward for NYC. Below are the highlights of the new rule:


  • Managers of construction sites will be required to develop plans for how stormwater runoff will be managed on-site during and after construction. Managing stormwater runoff near or where it falls lessens the amount of volume that ends up in our streets, sewer systems, and ultimately, our waterways.


  • The new rule will also require that on-site stormwater infiltration feasibility studies be conducted in an effort to prioritize green infrastructure solutions. DEP will review the studies and work with site managers to ensure that they have explored a series of on site stormwater management starting with GI practices and ranging in retention, detention and re-use solutions.


  • DEP will provide a revised design manual to help guide site managers on the types of solutions they can deploy


There are many aspects of the new rule that still need to be defined as it gets developed in the coming months:


  • 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 (i.e. 10,000 square feet).


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. 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 that 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 city's revised Design Manual.

SWIM Coalition will continue to update our comments as we learn more about the rule from DEP and gather additional input from our coalition members. At the city council hearing in 2020 that authorized DEP to start the rule making process, we submitted public testimony which you can find in our library here. DEP will hold a public hearing in early 2021 to share the draft rule for public comment. We will get the word out about the hearing via twitter: @SWIMCoalition and e-blast.


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!


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