What is the NYC 2021 Unified Stormwater Rule?

Updated: Nov 12, 2020

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, in combination with a series of programs underway now and coming to fore in 2020 and 2021, will help NYC move toward our 2030 water quality improvement goals for our overburdened waterways.

First, a bit of background on where we are right now with our stormwater management 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 hard surfaces such as buildings, sidewalks, streets, and parking lots and doesn't get absorbed back into the ground.

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! 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.

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!

NYC has initiated a series of programs to address the large volumes of runoff that inundate our overburdened infrastructure and local waterways.

In 2005, the City and State entered into an agreement, called a consent order, that frames and guides a series of plans to improve water quality in NYC's waterways by the year 2030.

The water quality improvement plans aim to reduce stormwater pollution from the city's municipal separate storm sewer system (MS4) and combined sewer overflow pollution from our antiquated combined sewer system. See a map of both systems here. Stormwater runoff from our impervious surfaces is the root cause of both pollution sources. Below is a list of the key plans and programs underway or coming to fore:

In 2008, the city issued a Sustainable Stormwater Management Plan, which was a precursor to today's Green Infrastructure (GI) Plan and eleven individual CSO Long Term Control Plans. Both programs are underway now through 2030.

  • The Combined Sewer Overflow Long Term Control Plans aim to use a combination of grey and green infrastructure methods to reduce the ~ 20 billion gallons of CSO pollution that discharge into our waterways each year, making many of them unsafe to touch ~20-30x per year. We are concerned that these plans, once completed, will not meet the 2030 goals. Here is a link to details about these plans and our concerns.

  • NYC's GI plan, has a goal of managing the first inch of stormwater runoff on 10 % of NYC's impervious surfaces (within the combined sewer area of the city) through green infrastructure solutions by 2030. This translates into a goal of greening 8,000 impervious acres. So far, we've greened ~1230 acres, mostly on publicly owned property. Nearly half of the 8000 impervious acres are on privately owned property. Incentive programs for GI on private property are:

DEP's GI Grant program has not yielded as much GI on private property as

anticipated. They recently revamped the program to focus solely on green roofs.

We 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 rooftops.

A new DEP grant program to support installation of GI practices on large

private properties (50,000 sq ft and above) citywide will go into effect in


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 citywide.

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.

  • The new unified rule will require managers of construction sites 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.

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 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 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.

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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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