Vegetated Roofs: Performance and Policy in Athens, Ga

Urban land area in the United States is projected to increase to 8.1% of total land area by the year 2050. These human-dominated environments create conditions that degrade both terrestrial and aquatic ecosystems. If cities are to reduce their environmental impact, innovative practices must be developed that replace ecosystem services lost during the urbanization process. This study evaluated the performance and feasibility of using vegetated or green roof systems for urban ecosystem remediation. The stormwater retention performance of a thin-layer green roof was evaluated using an experimental field test plot. Average stormwater retention was found to be slightly under 78% of rainfall from storm events over the course of one year. The additional stormwater storage created on the rooftop allowed for a curve number of 86 to be developed for the green roof. This curve number was then used in a modeling analysis of Tanyard Branch watershed, a highly urbanized watershed in Athens, Georgia. Spatial analysis demonstrated how impervious surface cover could be reduced in the watershed by using green roofs. Total impervious area in the downtown commercial zone was reduced 20% when all the roofs were greened. Roof greening also resulted in significant hydrologic changes in the watershed. A benefit-cost analysis (BCA) was also performed for the life cycle of the green roof system. In Tanyard Branch, the net present values of green roofs are greater than traditional roofing although expected changes in technology, energy prices, and market conditions were shown to reduce green roof life cycle costs to below traditional roofing costs. A green roof policy was developed for Athens, GA based on the performance and economic analysis of the experimental green roof. This policy uses private incentives and public demonstration sites to promote green roof infrastructure. A stormwater best management practice specification for green roofs was created that may be included in future versions of the Georgia Stormwater Management manual. Green roofs are shown to be a potentially valuable tool for increased sustainability in highly developed urban areas. INTRODUCTION One specific component of the built environment often overlooked is the use of the rooftop as environmentally beneficial space. Rooftops comprise a large proportion of surface land area particularly in downtown regions of the city as building footprints can occupy entire city blocks. Transforming the rooftop space into an environmental amenity can add value to the building owner and perform ecosystem services in the city. This transformation can be accomplished by applying vegetation and engineering growing media to the roof surface and creating a “green” roof. The rooftop is then able to retain and utilize stormwater for plant growth, reduce building temperatures through shading by the plants and evaporative cooling, and increase urban habitat. The practice of designing and building green roofs is becoming increasingly popular with architects, landscape architects, stormwater managers and ecological design firms in densely developed urban areas. There are two general types of modern green roof systems: intensive and extensive. Intensive systems are characterized by deep (> 6”) growing media, opportunities for a diverse plant palate on the rooftop and high cost and maintenance requirements. Extensive systems are designed to be lightweight and easily retrofitted on existing roof surfaces. They contain thin growing media depths (26”) and can support a limited number of drought-tolerant plants that thrive in the limited water and nutrient conditions. Extensive systems are by far the most common in Germany with over 80% of green roofs being extensive in 2002 (Harzmann, 2002) The objectives of this study were 1) to evaluate the stormwater retention performance of an extensive green roof system, 2) to examine the watershed scale effects of widespread extensive green roof implementation, 3) perform a cost-benefit analysis of extensive green roof systems, and 4) evaluate policy tools that may encourage green roof implementation. The interdisciplinary nature of this study reflects the recognition that urban environments must be studied holistically to include human decisionmaking as an essential biotic component in the structure of the urban ecosystem for remediation to be successful.