New York City developers now have a new factor to consider in project design.  On January 4, 2012, the Department of Environmental Protection (“DEP”) adopted a new stormwater performance standard (the “performance standard”), requiring large and medium-scale development projects in combined sewer areas to incorporate larger detention basins or install relatively expensive green or blue roof systems.  The performance standard also imposes stormwater reduction requirements on certain building alterations that increase impervious surfaces.

The DEP rules amend Chapter 31 of Title 15 of the Rules of the City of New York, which regulates the construction, permitting, and inspection of sewer connections.  For a new development, the performance standard establishes a “stormwater release rate” equal to the greater of 0.25 cubic feet per second (“cfs”) or 10% of the new development’s “allowable flow,” the stormwater flow that can be released into a storm or combined sewer based on existing sewer design criteria.  In no case, however, will the new release rate exceed allowable flow. 

For medium and large sized lots, these changes would effectively require a significant increase in stormwater detention on site.  Lots smaller than 5,000 square feet are not likely to have a release rate that exceeds 0.25 cfs, so they will probably only need to comply with existing sewer availability and connection application process requirements.

Redevelopment projects in combined sewer areas may also need to meet new performance requirements.  If a redevelopment requires an alteration permit from the Department of Buildings (“DOB”) and will increase existing impervious surfaces (including building footprints) by more than 20%, the release rate for the altered area must be proportional to the ratio of the altered area to the total site area.  Moreover, on such projects, no new points of stormwater discharge are permitted.  Proposed redevelopments that increase im­pervious surfaces or building footprints by 20% or less are exempt from the stormwater per­formance standard unless an additional sewer connection is proposed.

In addition to the new performance standard, the DEP and DOB have developed new “Guidelines for the Design and Construction of Stormwater Management Systems” (the “Design Manual”) which the City hopes “will ease the development community’s transition to stricter stormwater release rates when connecting to the City’s combined sewer system.”  The stormwater performance standard and Design Manual are key elements of the broader New York City “Green Infrastructure Plan” unveiled by Mayor Bloomberg on September 28, 2010, which aims to reduce combined sewer overflows (“CSOs”) into the City’s waterways by 40% by 2030.

Finally, because the City expects more stringent federal and state Mu­nicipal Separate Storm Sewer Systems (MS4) requirements to be published within the next year, the City expects to revisit the stormwater performance standard and the Design Manual to add stormwater management requirements in separately sewered areas.  The City will then also re­visit the adequacy of the stormwater management program in combined sewer areas.

 For more information on the new stormwater performance standard, contact Michael Bogin.