Vegetated roofs for stormwater management
Carter, Timothy Lynn
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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 as both peak watershed storm flows and total storm flow volumes were reduced, most notably for small storm events. 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.