A municipal separate storm sewer system (MS4) is a publicly-owned conveyance or system of conveyances (including but not limited to streets, ditches, catch basins, curbs, gutters, and storm drains) that is designed or used for collecting or conveying stormwater and that discharges to surface waters of the State. Separate storm sewers carry stormwater runoff directly to local waterbodies and serve approximately 30–40% of New York City.
As stormwater flows over streets and other impervious surfaces, it sweeps up pollutants such as oils, chemicals, pathogens, and sediments. In separate sewer areas, this pollution is carried by stormwater and discharged directly into local waterways. This can have a negative impact on water quality and recreational uses.
MS4 Permit & Stormwater Management Program
The City of New York received its first Municipal Separate Storm Sewer System (MS4) Permit on August 1, 2015, which was renewed in 2022. This permit is required under the Clean Water Act and is issued by New York State Department of Environmental Conservation (DEC). The intent of the MS4 permit is for the City to implement measures to reduce pollution in stormwater runoff. The MS4 Permit significantly expands the City’s previous obligations to reduce pollutants discharging to the MS4. Numerous City agencies have significant responsibilities under the MS4 Permit. The New York City Department of Environmental Protection (DEP) is responsible for coordinating the interagency efforts to meet the City’s MS4 Permit requirements.
The MS4 Permit requires the City to develop a Stormwater Management Program (SWMP), which includes numerous programs designed to reduce pollution in stormwater.
The SWMP Plan describes the ways in which the City will satisfy the requirements of the MS4 Permit by managing stormwater discharges into and from the City’s separate storm sewers. Most chapters of the Plan include a description of any relevant existing City programs; new initiatives and/or program enhancements; and measureable goals for future assessment of the program.
The SWMP Plan was released to the public on April 4, 2018. The public was invited to submit comments on the SWMP Plan from April 4 through May 15, 2018. DEP also held two stakeholder meetings during the public comment period to provide background on the Plan and to accept verbal comments from meeting attendees. On August 1, 2018, DEP submitted NYC’s SWMP Plan to the New York State Department of Environmental Conservation (DEC). On March 14, 2019, DEC approved the SWMP Plan. In the fall of 2022, the City undertook a holistic update of the SWMP to reflect the current status of program implementation and the City’s compliance with the 2022 MS4 Permit. Notable revisions included updating the Illicit Discharge, Detection and Elimination (IDDE) and Pollution Prevention and Good Housekeeping (PPGH) sections to reflect changes to the MS4 Permit.
MS4 Annual Reports
The 2022 MS4 Annual Report includes information on SWMP implementation from January 1 to December 31, 2022.
To view our MS4 deliverables from previous years, visit our Document Portal and search for “MS4.”
View the stormwater permitting requirements for all new and redevelopment sites that discharge to City sewers.
Unified Stormwater Rule
The purpose of the Unified Stormwater Rule (USWR) is to align stormwater-related construction requirements throughout NYC.
Stormwater Control Measures for Municipal Facilities and Operations
The City has an extensive network of municipal facilities and operations that serve New Yorkers and keep vital infrastructure functioning properly. To reduce the potential of these facilities and operations to contribute to stormwater runoff pollution, the City has developed suite of stormwater control measures to reduce stormwater pollution from municipal facilities and operations.
Download the Stormwater Control Measures
Industrial & Commercial Facilities
If you own or operate an industrial or commercial facility in the municipal separate storm sewer system (MS4) area of New York City, your facility may be inspected or have to submit copies of certain paperwork to DEP.
Trash in Waterways
Stormwater runoff can transport trash and debris from urban areas into local waterbodies. Once waterborne, these materials are referred to as floatables. The City relies on key programs to control trash and debris, including street sweeping, catch basin hooding and maintenance, catch basin inspection and cleaning, and booming and netting to catch materials that could potentially discharge via the outfalls. Public education, outreach, involvement, and participation are also important parts of the City’s efforts to control floatables.