Date of Award

8-4-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Cliff Davidson

Keywords

green infrastructure;green roof;low impact development;sustainability;urban stormwater;vegetated roof

Subject Categories

Civil and Environmental Engineering | Civil Engineering | Engineering

Abstract

As the climate continues to change, humanity is increasingly under threat of the associated environmental consequences. Sustainable cities, resilient in the face of changing climate, are needed as a habitat for humanity. Cities are complex and there are many factors that influence their resiliency and health. Some recent efforts have focused on increasing urban green spaces and their associated ecosystem services as a way to address multiple urban issues synergistically. One green infrastructure technology, green roofs, provide the opportunity to add green spaces to cities without losing valuable real estate at street level, while providing multiple ecosystem services not provided by a traditional roof. Despite increasing construction of green roofs within urban centers, the performance of green roofs has not been adequately measured quantitatively. My work aims to enhance our understanding of the thermal and hydrologic performance of a large extensive green roof. Further, to bridge the gap between research and the practice of engineering, this study also considers the performance of commonly applied models in predicting green roof performance. Empirical data are collected at Syracuse, New York’s OnCenter green roof over the course of eight years, including rainfall, runoff, soil moisture, thermal roof properties, and meteorological parameters. As we collectively work to adapt to a changing climate and to design more resilient cities, green roofs are being investigated for their role in the overall energy balance of buildings. In the first part of this work, I determine the thermal properties of the OnCenter green roof using temperature sensors installed during roof construction. Temperature sensors were installed at five stations across the roof to measure temperature at four depths within the roof layers. Heat fluxes range from −5.76 W m−2 to 9.46 W m−2. Negative (downward) heat flux is found during summer and early fall, and positive (upward) heat flux dominates during the heating season. Solar radiation can heat the upper layers of the roof significantly above ambient air temperatures during the summer. Accumulated snow acts as an insulator during the winter months. Thermal resistance, R, is determined during a two-week period with significant snow accumulation, during which time heat flow through the roof reached a quasi-steady state. Thermal resistance for the overall roof is found to average 3.1 m2 K W−1. The largest individual thermal resistance is from the extruded polystyrene insulation layer (R = 2.6 m2 K W−1). Overall, the green roof dampens the temperature or heat flux responses often observed on urban roofs. Vegetation and substrate layers may be used in addition to insulation but are not recommended in lieu of insulation for a Central New York climate. Success in creating resilient cities also relies on the ability of urban areas to function with the hydrologic changes brought on by climate change. Green roof hydrologic performance reported in the literature varies widely – the result of differences in green roof design and climate, as well as limitations to study design and duration. In the second part of this work, I quantify the hydrologic performance of the extensive green roof on the OnCenter over a period of 21 months. Over the monitoring period, the roof retains 56% of the 1062 mm of rainfall recorded. Peak runoff is reduced by an average of 65%. Eleven events exceed 20 mm and are responsible for 38% of the rainfall and 24% of the annual retention. Retention in the summer is lower than that in the fall or spring, as a result of greater rainfall intensity during the period sampled. Soil moisture during winter months remains high, reducing the ability of the roof to retain rainfall volume from new events. Comparison of seasonal data demonstrates the strong influence of rainfall intensity on runoff and the effect of initial soil moisture on event retention. Green roofs are being applied as a modern stormwater management tool at an increasing rate across the globe. To apply this technology, however, practitioners must conform with regulatory requirements, for which two methods dominate: the SCS Curve Number method and the Rational method. Universally accepted model inputs (CN and Cv respectively) do not exist for green roofs, and likely vary based on roof composition and region. In this study, I calibrate CN and Cv using nearly seven years of rainfall-runoff data from the OnCenter green roof. Median event CN and a least-squares estimate both result in a CN of 96. When season is included in the analysis, calculated CN in the winter (CN = 99) exceeds that generally used to model an impervious surface (CN = 98), while the summer has the lowest CN (95). Event Cv ranges from 0 to 0.99 with a median of 0.06, however Cv increases with depth of rainfall. Overall, the values skew towards the higher side of what is reported in the literature and closer to impervious surfaces than natural vegetated surfaces. This pattern may indicate the inappropriateness of the currently accepted methods to fully capture the performance of green roofs and their contribution to urban stormwater management. The results of this study suggest overestimating the hydrologic performance of a green roof via lower or inaccurate curve numbers may have negative consequences in practice. This research makes a valuable contribution to our understanding of green roofs and their performance within the context of the Central New York climate. The contribution of this work, however, extends beyond the region by highlighting future areas of research and capturing the inability of commonly-used hydrologic models to accurately account for the performance of green roofs. The results of this work inform the design and adoption of green roofs by practitioners, and the regulations enacted by policymakers that influence our built environment.

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Open Access

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