published_or_final_version / Environmental Management / Master / Master of Science in Environmental Management

abstract / Geography / Doctoral / Doctor of Philosophy

Urbanization replaces once permeable surfaces with relatively impervious ones, thereby degrading the natural hydrologic cycle. Impervious surfaces intensify stormwater runoff in terms of overall mass and temporal response, especially under torrential rainfalls. Since such runoff could become massive in volume and concentrated in time, they place significant stress on the urban drainage system and increase the risks of combined sewage overflow and flooding, which could introduce a range of deleterious consequences to cities and surrounding natural habitats. In sustainable urban stormwater management like the Low Impact Development, green roof presents an on-site source-reduction measure that mimics the pre-development hydrologic functions of storing and gradually releasing precipitation. Green roof can retain and detain stormwater as well as delay and suppress peak discharge. However, green roof stormwater studies have largely been conducted in non-tropical regions of the world. Since green roof’s quantitative hydrologic performance can be much influenced by local meteorological conditions, the degree to which such findings can be generalized to other climates, such as Hong Kong’s humid subtropical regime, calls for investigation. Moreover, substrate depth has long been regarded as an influential factor in green roof stormwater retention, but two recent studies have provided contradictory results. The objectives of this study are: 1) To evaluate green roof stormwater mitigation performance and potentials in Hong Kong for the first time; 2) To investigate systematically the effect of substrate depth on quantitative hydrologic performance; 3) To identify factors that affect green roof performance; 4) To develop a holistic conceptualization of the various system water storage spaces within a green roof system, for a better understanding of their role in stormwater mitigation. Using small-scale (1.1 m2) raised green roof plots placed on an actual urban rooftop, the effect of 40 mm soil, 40 mm soil + 40 mm rockwool, 80 mm soil, and 80 mm soil + 40 mm rockwool on stormwater mitigation performance relative to control were analyzed. Three core performance indicators (percent retention, peak delay, and peak reduction) were employed to evaluate green roof performance. The results suggest that, while the retention performance of the studied green roofs under Hong Kong’s heavy rainfall regime seems to be less effective, remarkable peak reduction and peak delay were observed even when the green roof system has reached full moisture-storage capacity. Such findings are in line with the proposed Green-roof System Capacity model that green roof serves as an effective buffer that regulates water flow through the system. No statistical significance was found between substrate-depth treatments, despite the higher performance across all three indicators for treatment 80. However, satisfactory peak performance of the 40-mm thin substrate suggests that green roof can be applied even on existing buildings that have limited loading capacity. Pertinent meteorological factors were identified. All in all, extensive green roof remains as an effective and promising alternative mitigation strategy to urban stormwater management in Hong Kong with potential application to other tropical areas. / Li Ka Shing Prize, The Best MPhil Thesis in the Faculties of Architecture, Arts, Business & Economics, Education, Law and Social Sciences (University of Hong Kong), 2012-2013. / published_or_final_version / Geography / Master / Master of Philosophy

Thesis (M.Sc.)--University of Hong Kong, 2008. / Includes bibliographical references (p. 47-51).

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 72-75). Also available on the World Wide Web.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 191-178).

With the growing urbanized population, cities have become a major contributor to global energy consumption and greenhouse gas (GHG) emission. The urbanization processes also cause local climate change through excessive anthropogenic heat emission and modification of the land biophysical properties. The resultant urban heat island (UHI) effects and aggravating human heat stress have become key environmental issues in city management. Cities can be designed to be climate-conscious and energy-efficient not only to contribute to urban sustainability, but also address global climate-change issues at the local level. Green roof presents a feasible strategy for climate-conscious urban design (CCUD). With the notable thermal effects in microclimate amelioration and energy conservation, it has great potential to help cities tackle local and global climate issues. Understanding the status and underlying mechanism of green-roof thermal effects can inform optimal design and management, and provide scientific basis to promote green-roof application. This study formulates a multidisciplinary framework to assess green-roof thermal effects at building, neighborhood and district scales, based on a case study in Hong Kong. Firstly, the building-scale field measurement found that the 484 m^2 experimental extensive green roof can significantly ameliorate rooftop microclimate and cut building energy consumption. The peak surface and air temperature can be reduced by 11℃ and 4.5℃ on sunny summer days, and 7.2℃ and 2.3℃ on cloudy days, leading to an electrical energy saving of 2.80×〖10〗^4 kWh for a summer of air conditioning. The thermal performance was sensitive to background environmental factors. Solar radiation and relative humidity governs the seasonal and diurnal variation of air and surface temperatures reduction. Substrate moisture can significantly regulate the subsurface temperature but has limited effect on evapotranspiration (ET). Based on correlation and scenario analyses, this study has formulated an irrigation scheme which could balance between plant growth, thermal performance and water efficiency to achieve sustainable management of tropical extensive green roofs. Secondly, the neighborhood-scale modeling revealed that greening all roofs in residential communities can extend the cooling effects from the rooftop to the entire neighborhood. Urban design factors such as building height, distance, site coverage and orientation can affect the diurnal, horizontal and vertical pattern of the “cool-islands” created by green roofs. Green roof can also enhance the rooftop thermal comfort by alleviating the intensity and duration of heat stress. The findings suggest that compact cities can green the roof and podium space to provide thermally comfortable and recreational venues for urban residents. Thirdly, the district-scale cost-benefit analysis found that large-scale construction of green-roof infrastructure in Hong Kong can be well justified by its thermal benefits. The extensive green roof has an annual monetary benefit of HK$0.9–1.7 billion, and the intensive, HK$1.4–2.6 billion, in terms of energy saving, CO2 and air pollutant reduction. The life-cycle benefit-cost ratio (BCA) is 3.7–7 for extensive green roof, and 1.4–2.7 for the intensive, indicating the high cost-effectiveness of both types, with the extensive being more economically attractive than the intensive. / published_or_final_version / Geography / Doctoral / Doctor of Philosophy

Owing to the concern about the serious factors influencing global warming and climatic change, the process of sustainable landscape construction as well as ecologically friendly developments needs to be addressed. In particular, the questions to ask is are we nearer to accepting sustainable growth advantages in South Africa , primarily the greater eThekwini Municipal Area? In this research paper an analysis of both a theoretical and practical approach to conventional understanding s relating to development practices and issues that encompass greening of cities and the notion of rooftop gardens will be assessed. In so doing it will investigate the discourse surrounding urban ecology and sustainable landscape developments and how both processes incorporate the topic of rooftop gardening, urban agriculture and people’s attitudes towards nature in the city, which regrettably from a South African perspective has very little comprehensive literature written about it. The research will give clarity and hopefully show that there is sufficient evidence to demonstrate that rooftop gardens form an intricate part of urban ecology. In addition to this that they can provide general environmental, associated aesthetic and health benefits for cities and their inhabitants. Hopefully in culmination this research study will promote a greater insight into rooftop gardens benefits for city management systems. / Thesis (M.T.R.P.)-University of KwaZulu-Natal, Durban, 2009.

Thesis (M.L.A.) -- University of Texas at Arlington, 2009.

Green roofs have become an important urban mitigation technology due to their ability to address multiple environmental issues. One of the most common benefits attributed to green roofs is the reduction in heating and cooling loads in buildings by dissipating heat through evaporation. This study focuses on evaluating the effect that evaporative cooling has on the thermal performance of green roofs. Sponge and floral foam were used as porous media for their ability to retain water inside its body, transport it to the surface, evaporate it at a constant rate and for their different pore sizes. Test trays containing sponge or floral foam saturated with water were tested in a low speed wind tunnel equipped to measure weight, temperature and heat flux. Two types of experiments were conducted: one with evaporation at the surface, and the other with evaporation blocked by an impervious layer. The testing conditions for all tests were kept constant except for the ability of evaporation to happen. Evaporation rate for floral foam was 0.14 kg/m2hr and 0.29 kg/m2hr for sponge. Results of tests with evaporation show a decrease of 45-49% in heat conducted through the roof when compared to the tests without evaporation. For optimal thermal performance of green roofs, a material that enhances water transport and thus evaporation at the surface is necessary with large pores and low field capacity. Surface temperatures on test with evaporation were found to be between 3-7°C lower than those without evaporation. Applying a 2 sample t-test to the data, the relationship between heat flux and evaporation was found to be statistically significant.