Long-Term Creation of Nature Based Integrated Stormwater Management Systems

The NYC Department of Environmental Protection (NYCDEP) developed a Catskill/Delaware watershed protection program that met the requirements for unfiltered water supply systems as determined by the EPA and NYS Department of Health. Under the resulting 2007 Filtration Avoidance Determination (FAD) program DEP installed Stormwater BMPs within the Kensico Reservoir to reduce pollutant loading and improve water quality. Implementing BMPs impacted existing wetland buffers, open water, and submerged aquatic vegetation habitats. A comprehensive review of mitigation opportunities was completed and the former Armonk Bowling Alley in Armonk, NY was selected for the creation of off-site mitigation. The Bowling Alley was privately owned, but it provided an opportunity for permit compliance that supported larger goals of water quality for the Reservoir, including retention of the first flush and reduction of sediment loads. The site doubled the adjoining acreage of two previously completed wetland enhancement projects and will cumulatively improve wetland functions by creating a contiguous headwater complex that filters and treats water prior to entering the reservoir. The Bowling Alley, constructed in the 1960s, ditched a tributary of the reservoir - Bear Gutter Creek. The mitigation design restored the Creek, meandering it through created wetland and forested floodplain. The site is within a system prone to flash flooding and was designed to provide flood mitigation and storage during rain events. Retention of water buffers the downstream stream corridor from erosion, delays the peak flow, and allows suspended sediments to drop outside of the reservoir. Permitting of the 3.13-acre mitigation was challenging as aquatic submerged vegetation impacts were compensated for by wetland and open water creation. A functional assessment validated that the mitigation design compensated for the impacted habitat functions. Modeling confirmed the new mitigation would not alter the hydrology of the adjacent wetland area and no adverse upstream or downstream flooding would occur. Lessons from implementation of previous mitigations were incorporated into the design to improve site success, while construction presented its own lessons on soil management, installation of instream structures, post-construction monitoring and associated adaptive management. The presentation will provide attendees with knowledge of NYCDEP's Filtration Avoidance Determination and how watershed management results in wins for not only water quality, but also preservation of open space, creation of highly functioning habitat complexes, and flood risk reduction. We will discuss many of the lessons learned during the design, permitting, and construction phases; including how regulatory hurdles translated in to design decisions.