Hydrological performance of extensive green roofs in New York City observations and multi-year

Hydrological performance of extensive green roofs in New York City observations and multi-year

Hydrological performance of extensive green roofs in New York City: observations and multi-year modeling of three full-scale systems

1 Department of Civil Engineering and Engineering Mechanics, Columbia University, NY, USA

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1. Introduction

Stormwater runoff has become a major environmental issue for many dense urban areas in North America due to its contribution to flooding and pollution of nearby surface waters. In cities that utilize combined sewer systems (CSSs) to manage both stormwater and sewage, these problems are compounded because even relatively small rainfall events, as little as 3 mm in 1 h (Montalto et al 2007 ), can trigger combined sewer overflows (CSOs). CSO events release sediments, nutrients, gasoline and other chemicals from urban surfaces into local waterways, as well as pathogens and organic matter from human waste. This pollution undermines the productivity of urban water bodies by impairing local residents’ ability to swim, fish, and conduct other water-based recreational and commercial activities.

In the United States (US), CSOs are reported to impact 746 communities in 32 states and cause 850 billion gallons of pollution each year (US EPA 2004 ). In New York City (NYC) alone, 433 outfalls release over 20 billion gallons of CSO per year (Mayor’s Office of Long-Term Planning and Sustainability 2008 ). In accordance with the Clean Water Act of 1972, NYC has adopted a stormwater management plan aimed toward reducing runoff volume and peak flow rates to help mitigate local CSO pollution. In NYC’s latest plan, an increased effort is being made to implement decentralized, low-impact development techniques, also called ‘green’ stormwater infrastructure, as an alternative to traditional methods such as underground detention basins. A major goal of NYC’s plan is to capture ten per cent of the first 25 mm of runoff from the impervious surfaces in every NYC sewershed using green strategies, which includes the use of vegetated rooftops (NYC DEP 2010 ).

Vegetated rooftops, known as green roofs, eco-roofs, or living roofs, have become an increasingly popular alternative to impervious roof types. A typical green roof is constructed by placing a drainage course, growing substrate, and vegetation on top of a roof’s waterproof membrane. In some installations, green roofs may also have additional geosynthetic layers for preventing plant root penetration damage, limiting sediment intrusion into the drainage course, and/or water storage. At present, the US does not have national green roof standards and, as a result, the materials, configuration, and installation methods for green roofs can vary widely from site to site.

It is common for green roofs to be classified as either extensive or intensive based on the thickness of the growing substrate layer. Extensive roof substrates are typically 15 cm thick or less and feature short rooting, drought resistant plants, whereas intensive roof substrates are greater than 15 cm thick and may be sowed with deeper rooting plants including shrubs and trees. Generally, extensive green roofs are cheaper, require less maintenance and are lighter than intensive systems. Therefore, extensive systems are implemented more frequently than intensive systems, most especially on existing building stock where rooftop weight limitations come into play. Due to their wider applicability, extensive green roofs are the focus of this study.

Within the extensive green roof classification three major construction types have emerged: vegetated mat, built-in-place, and modular tray systems (Oberndorfer et al 2007 ). Typically, both the vegetated mat and built-in-place systems require a specialized drainage course to prevent ponding and surface flow that would otherwise cause substrate erosion. The two systems differ, however, in how the substrate is installed. In mat construction the growing substrate is bound within a geo-composite used for off-site pre-planting, whereas the growing substrate for a built-in-place system is placed within bordered rooftop regions and landscaped on site. In contrast, the walls of the modular trays already restrict surface runoff, while the base provides corrugated air space for drainage, therefore these systems may be placed directly on a roof’s waterproof membrane. Each construction type imposes a unique set of boundary conditions on the growing substrate layer that affects the drainage behavior of runoff. For example, the mat and built-in-place systems promote lateral runoff movement to varying degrees, whereas the unconnected modular trays generally facilitate vertical percolation. The type of construction may also determine the non-vegetated area required for maintenance and the feasibility of different vegetation types. As a result, the installation method might be a significant factor in overall green roof performance.

While green roofs have been shown to provide a range of environmental benefits compared to typical impervious roofs (Berndtsson et al 2009. Getter et al 2009. Sailor and Hagos 2011. Yang et al 2008 ), their ability to attenuate stormwater runoff is typically the main target of existing incentive programs for their construction. Recently, a number of studies have helped to better understand the role green roofs might play in mitigating CSO pollution and minimizing problems associated with urban runoff in general (Berndtsson 2010 ). These studies report a wide range of hydrologic behavior due to differences in, among other parameters, green roof construction type, growing substrate depth, vegetation type, and areal coverage. Even similar systems may have significant performance variation since the water retention ability of green roofs is heavily influenced by local climate; where the distribution, size, and intensity of rainfall events (Stovin 2010 ), as well as seasonal evapotranspiration rates (Bengtsson et al 2005 ), are thought to play a key role. The role of local climate in green roof water retention ability is important for two reasons: first, since green roof hydrologic performance is impacted by regional conditions, green roof studies are needed across a range of climate zones to fully understand the feasibility of using this technology in an effective stormwater management strategy. Second, the period in which green roof monitoring studies are conducted impacts the reported overall green roof performance. For instance, a study during a period in which large storms were prevalent will result in lower reported green roof rainfall retention rates than a study during which smaller storms were recorded. Consequently, there is a need to develop methods for estimating green roof retention rates over multiple years or rainfall patterns to reduce any bias caused by rainfall distribution within the monitoring period itself.

Hydrological performance of extensive green roofs in New York City observations and multi-year

In the following sections hydrological monitoring data from three full-scale, extensive green roofs in NYC, one of each major construction category, are reported with the intent of: (1) filling a gap in knowledge of the stormwater retention performance of full-scale green roofs in NYC’s climate region; (2) providing a comparative analysis of the performance of the vegetated mat, built-in-place and modular tray roof systems; and (3) presenting a method for estimating green roof retention performance that can account for variations in rainfall distribution patterns not experienced during rooftop monitoring periods.

2. Summary of previous green roof hydrologic monitoring studies

To date, the potential for reduction of runoff volume is the most cited hydrologic performance metric of green roofs. Generally, volume reduction is reported as the per cent of total rainfall captured during a given study period and is usually obtained using a mass balance approach by comparing continuous rainfall and runoff data. Pilot scale studies indicate that rainfall retention between 30 and 86% is possible for extensive systems (Berghage et al  2009. DeCuyper et al  2004. VanWoert et al  2005. Getter et al  2007. DiGiovanni et al  2010. Morgan et al  2012. Nardini et al  2011. Schroll et al  2011. Stovin et al  2012 ). These studies show that green roof retention increases with: thicker growing substrate depths (DeCuyper et al  2004. VanWoert et al  2005 ), lower roof slopes (Getter et al  2007. VanWoert et al  2005 ), and higher evapotranspiration rates (DiGiovanni et al  2010 ). Evapotranspiration rates were found to increase due to a variety of factors, including: greater areal plant coverage (Berghage et al  2009. Morgan et al  2012 ), higher transpiring plants (Nardini et al  2011 ), and warmer weather (Schroll et al  2011 ). In addition, Mentens et al (2003 ) used 32 lysimeter test boxes at 20° and 40° slopes to determine the impact of green roof orientation on evapotranspiration. The results indicate that, in the Northern Hemisphere, south facing sloped roofs have the highest evapotranspiration rates among the four orientations, while north facing have the lowest rates (Mentens et al  2003 ). Finally, Villarreal (2007 ) demonstrated that rainfall retention is also a function of precipitation characteristics, such as intensity and duration. For example, rainfall retention from a 1.5 m 2 extensive green roof test box was lowest when exposed to constant rainfall intensity (20–29%), and higher for variable intensity (34–52%) (Villarreal 2007 ).

All studies referenced in the above paragraph were conducted on a pilot scale, using elevated test boxes or similar modules, with watershed areas between 0.37 and 12 m 2. While these studies, and many others at the pilot scale, have been instrumental in helping to identify and quantify relationships associated with runoff reduction, it is uncertain how accurately they forecast full-scale performance. Typically, the main difference between pilot and full-scale testing is the inclusion of non-vegetated regions in the latter case, which are generally required on most full-scale green roof installations for egress, maintenance, rooftop equipment, or to manage load restrictions. These regions, along with larger drainage watersheds in general (e.g. 300 m 2 or more in this study), significantly alter the behavior of runoff and, consequently, green roof stormwater volume retention capability.

Hydrologic studies on full-scale green roof systems, those conducted on an entire watershed or partitioned sections of an occupiable building’s rooftop, are summarized in table 1. The range of rainfall retention in these studies is 12–74%, generally lower than those reported in pilot tests. This is likely due to non-vegetated sections and irrigation requirements for many full-scale systems. For instance, in Spolek (2008 ) the monitored green roofs were irrigated during the summer months, significantly reducing retention capability. With the exception of Gregoire and Clausen (2011 ), which evaluated a modular tray green roof, all other studies in table 1 were conducted on built-in-place systems; highlighting the need for additional research using different construction types. Further, full-scale studies with monitored drainage areas of 300 m 2 or more, as presented in this study, are limited. The literature summary provided in table 1 does not include studies reported in the German language, for which the authors were unable to identify key parameters specified in table 1. For a detailed review of studies reported in German see Mentens et al (2006 ).

Table 1.   Summary of studies on the hydrologic performance of full-scale green roofs. Columns from left to right identify the author(s) and year of publication, geographic location, dated range of data collection, size of monitored (M.) drainage area, number of individual events observed, depth of the growing substrate, and reported per cent of rainfall captured during the monitoring period for each study. ‘N’ is used for fields where information was unavailable.

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