Monitoring Performance of Green Infrastructure Source Controls in New York City

Authors:

  • John McLaughlin - NYCDEP
  • William Leo - Hydroqual
  • Sandeep Mehrotra and Matthew Jones - Hazen and Sawyer

Learn more about urban green infrastructure.

As part of the Mayor’s PlaNYC 2030 initiatives, the New York City Department of Environmental Protection (NYCDEP) is piloting green infrastructure source control retrofits throughout the City in order to examine the functionality of these systems and the suitability of expanding their use to better manage combined sewer overflows (CSOs). A comprehensive monitoring program serves as a key element of this pilot effort, providing valuable information on the challenges these systems present, and benefits they can provide within the ultra-urban environment of New York City. The monitoring program was developed to examine issues related to runoff quantity management, influent and effluent water quality, maintenance concerns, and general system functionality.

The pilot monitoring program encompassed more than 20 source control retrofits distributed throughout the City, including bioretention, enhanced tree pits, street-side infiltration swales, constructed wetlands, blue roofs, green roofs, permeable pavement, and subsurface detention and infiltration systems. These retrofits were constructed within parks, medians, street-side sidewalks, public housing facilities, parking lots, and on rooftops. Monitoring was predominantly accomplished through the use of remote monitoring equipment, with special considerations incorporated to minimize opportunities for vandalism and interference with existing uses of these facilities, while optimizing collection of data pertinent to system functionality.

The water quantity component of the monitoring program was developed to examine the effect of the source control retrofits on runoff flow rates and volumes, thereby informing their effect on CSO abatement. Flow monitoring was primarily accomplished through the use of weirs, orifice plates, and flumes in conjunction with pressure transducer water level loggers. Additionally, monitoring activities evaluated infiltration rates, soil moisture content, and surface and subsurface water storage across various pilots. Precipitation measurements and other weather data were also collected to evaluate system performance under a variety of storm characteristics and weather patterns.

The water quality monitoring component was developed to compare the performance of these systems within New York City to evaluations conducted elsewhere, as well as identify maintenance concerns. Water quality sampling designs utilized a combination of first flush sampling bottles, grab samples, and infiltration wells. Constituents examined include diesel and gasoline, nutrients, total suspended solids, total organic carbon, salts, and metals. Collection of soil samples was also incorporated into monitoring activities for infiltration based practices to evaluate pollutant buildup within the soil.

For some stormwater pilots, variable design elements, such as outlet stop gates containing various orifice and weir configurations, were implemented in order to evaluate the effect of these elements on system performance. In addition to water quantity and quality monitoring elements, monitoring plans were developed to gather information on general system performance, including the frequency and nature of maintenance activities, system aesthetics, and public acceptance of the stormwater pilots.

Stormwater pilot monitoring activities began during the spring of 2010 and will examine system performance for a period of 2 years. Results of the monitoring study are expected to better inform future green infrastructure designs within the City and provide valuable information for forthcoming modeling and planning efforts.

To request a copy of the full paper, please contact the author at mjones@hazenandsawyer.com.

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