NYC Stormwater Management

The New York City Department of Environmental Protection (DEP) has a stormwater performance standard, which is outlined in Chapter 31 of Title 15 of the Rules of the City of New York, Rule Governing House/Site Connection to the Sewer System Standards for Release Rates, that is intended to reduce the adverse effects on the city’s combined sewer system from runoff during rainstorms that exceed what the sewers and related facilities are designed to handle.  This excess runoff can result in sewer overflows, flooding and backups.  In an effort to slow the flow of stormwater into sewers, by requiring greater on-site storage of storm water runoff and slower release rates into the city system.  Below are the relevant requirements for “new development” and “alteration” projects:

New Developments – Projects that require a New Building Permit:
The Stormwater Release Rate must be no more than the greater of 0.25 cfs (cubic feet per second) or 10% of the Allowable Flow or, if the Allowable Flow is less than 0.25 cfs, no more than the Allowable Flow.

Alterations – Any horizontal building enlargement or increase of impervious surfaces:
The Stormwater Release Rate for the altered area must be no more than the stormwater release rate for the entire site, determined in accordance with the requirement above, multiplied by the ratio of the altered area to the total site area. No new points of discharge are permitted.

These requirements generally apply to projects on medium to large size lots, as smaller sites typically do not generate runoff in excess of 0.25 cfs.

The DEP permits different types of stormwater management systems to be utilized to comply with this performance standard.  These include detention systems, which store and slowly release water to the sewer system and retention systems, which dispose of water on site through infiltration to the soil, evapotranspiration and on-site recycling.  Below are six commonly submitted and accepted systems:

Subsurface Systems

  • Storage vaults, or tanks, can be constructed from pre-cast concrete structures, concrete rings, culverts, pipes, vendor-provided products, or cast-in-place concrete. Tanks and vaults can be built with or without a bottom slab. If built without a bottom slab, a vault system can promote infiltration.
  • Gravel beds are excavated areas filled with uniformly-graded gravel. The void space within the gravel is used to detain water. These systems can also promote infiltration.
  • Perforated pipes use a combination of pipe storage and gravel storage to provide detention and promote infiltration.
  • Stormwater chambers are commercially available in a variety of shapes and sizes. These structures detain stormwater within the chamber and gravel surrounding each chamber for structural support. These open-bottom systems also promote infiltration.

Rooftop Systems

  • Blue roofs, also known as controlled flow roof drain systems, provide temporary ponding on a rooftop surface and slowly release the ponded water through roof drains. Blue roofs have weirs at the roof drain inlets to restrict flow.
  • Green roofs consist of a vegetative layer that grows in a specially-designed soil that may sit above a drainage layer. Green roofs detain stormwater in the void space of the soil media and retain stormwater through vegetative uptake and evapotranspiration.

Contact us today for assistance in selecting/designing the appropriate stormwater management system for your project!