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Modeling a Hydrologically Optimal Green Roof Media Mixture
Citation:
Bollman, Mike, G. DeSantis, R. Duchanois, M. Etten-Bohm, D. Olszyk, J. Lambrinos, AND P. Mayer. Modeling a Hydrologically Optimal Green Roof Media Mixture. Ecological Society of America Annual Meeting, Portland, OR, August 06 - 11, 2017.
Impact/Purpose:
Green infrastructure uses plants and growing media to provide environmental, social and economic benefits to urban areas, especially by providing water quantity and quality control. Vegetated “green” roofs are becoming widely accepted as an effective means to collect, store, and gradually release rainwater over time, with the added benefit of decreasing energy costs by acting as a building insulator and increasing the reflection of light from the roof. The water holding and release characteristics of the growing medium strongly influence plant survival, particularly in the Pacific Northwest, where winters are cold and rainy and summers are warm and dry. However, there is a lack of systematic study of those characteristics, and, thus recommendations for green roof design can occur without a sound scientific basis. In this study we studied the water relations of several commonly used green roof growing media, and developed a simple spreadsheet model to optimize hydrologic performance of media mixtures. We gathered data on movement of water (hydraulic conductivity or HC), wet weight, and water held over time for different types of media typically used for Pacific Northwest green roofs: peat moss, perlite, pumice, red cinder, sand, and vermiculite; as well as mixtures of these media. The media data were used to develop a simple spreadsheet model which indicated that mixtures using perlite and/or pumice had the best hydrologic performance. This study provides the EPA with important new recommendations to help cities design green roofs to better help control stormwater runoff, and ameliorate water pollution problems associated with resulting sewage overflows. This study contributes to SSWR 5.02A “Monitoring the performance of green infrastructure stormwater controls in a wide range of communities.”
Description:
Background/Questions/MethodsA key environmental concern in managing urban ecosystems is controlling stormwater runoff to ameliorate pollution problems and sewage overflows. Vegetated green roofs have become an important green infrastructure tool to collect, store, and gradually release rainwater over time, with the added benefit of decreasing energy costs by acting as an insulator and increasing albedo. However, a major constraint to the survival of plants on green roofs is the lack of available water, particularly in the Pacific Northwest, where winters are cold and rainy and summers are warm and dry. The hydrologic attributes of the substrate used as growing medium strongly influence water retention, and, thus, plant survival. In this study we developed a simple spreadsheet model to optimize hydrologic performance of green roof media mixtures using data on the hydraulic conductivity {HC}, wet weight {WW}, and water held {WH} at saturation and after 14 days of drying for individual and mixtures of media constituents (peat moss, perlite, pumice, red cinder, sand, vermiculite) typically used in the Pacific Northwest. In addition, the proportion of processed constituents (perlite, vermiculite) was considered as a selection factor. The results of this study are intended to identify optimal green roof media mixtures for specific applications.Results/ConclusionsWe fixed the amount of organic matter (peat moss) at 20% by volume for media mixtures, so variation in hydro logic performance was driven by the composition of the inorganic fraction. Because perlite was light weight and had a high HC, yet held high amounts of water both when saturated and after 14 days, media mixtures dominated by perlite had the best hydrologic characteristics. Pumice also functioned relatively well, but was heavier. Although vermiculite performed very well in the first wetting, its water retention sharply decreased after undergoing a drying and re-wetting cycle, which resulted in an approximate 50% decrease in volume. Mixtures using perlite and/or pumice best addressed the performance criteria. This study demonstrates the potential to design green roofs with an appropriate media to enhance dry season water availability, while optimizing water release. With vegetation adapted to these media and the local environment, green roofs may be more effectively designed to not only mediate runoff, but also assist in cooling buildings and providing habitat.