Comparative Analysis of Urban Green Roofs and Stormwater Management Practices

Thesis Abstract

Urban environments face escalating flood risk, thermal islands, and biodiversity losses driven by rapid development and inadequate stormwater management. This study addresses the comparative effectiveness of urban green roofs as a nature-based solution against conventional stormwater control measures in mitigating surface runoff, reducing urban heat, and enhancing ecological value. The aim is to evaluate how green roofs perform relative to traditional practices across structural, hydrological, and socio-ecological dimensions in mixed-use cities. Specific objectives are (i) to quantify and compare peak runoff, total runoff, and runoff duration under green roof and conventional drainage configurations; (ii) to assess temperature reduction, humidity modulation, and microclimate benefits afforded by green roofs; (iii) to evaluate biodiversity potential and ecosystem services provided by green roofs versus conventional systems; (iv) to analyze cost-effectiveness, maintenance requirements, and policy implications; and (v) to synthesize urban planning and stakeholder perspectives to inform implementation. The study employs a comparative, mixed-methods design anchored in the theoretical frameworks of Nature-Based Solutions (NBS) and the Urban Resilience theory. A multi-site, cross-sectional approach is used in three mid-sized cities with comparable climatic and socio-economic profiles. Hydrological performance is assessed through controlled experimental plots on public buildings and semi-structured retrofit simulations on private developments, encompassing 18 green-roof installations and 18 conventional roofs. Data collection combines quantitative instrumentation and qualitative inquiry hydrological sensors (tipping-bucket rain gauges, flow meters, soil moisture sensors) monitor 24 months of rainfall events; infrared thermography and in-situ temperature loggers capture thermal behavior; biodiversity indicators (pollinator presence, lichen and moss cover, arthropod sampling) are recorded seasonally. Economic metrics include life-cycle cost analysis and maintenance time-use diaries, while policy and stakeholder data are gathered via key informant interviews (n=40) and structured surveys (n=300 households, tenants, and property managers). Analytical techniques include generalized linear mixed models to compare runoff parameters while controlling for antecedent moisture and roof area, repeated-measures ANOVA for microclimate effects, and a difference-in-differences approach where feasible. Regression-based cost-effectiveness analysis and multi-criteria decision analysis (MCDA) integrate hydrological outcomes with ecological and social benefits. Qualitative data are analyzed through thematic analysis to extract stakeholder perceptions and barriers to adoption, with triangulation to validate findings. Expected findings anticipate that green roofs reduce peak rainfall runoff by 25–45% and total annual runoff by 15–30% relative to conventional systems, with concomitant reductions in roof surface temperatures during hot spells by 2–5°C and improved humidity buffering of adjacent urban canopies. Biodiversity gains are expected to be modest on retrofit systems but substantial where extensive green roofs are implemented, with notable enhancements in pollinator activity and moss diversity in suitable substrates. Economic analysis is likely to show higher upfront costs for green roofs offset by lower long-term maintenance and resilience benefits, yielding favorable cost-effectiveness under scenarios of climate risk and stormwater credit schemes. Stakeholder insights are anticipated to reveal policy fragmentation, knowledge gaps among developers, and demand for standardized performance benchmarks and incentive structures. The study contributes to knowledge by providing a robust, context-specific comparison of green roofs and traditional stormwater systems, integrating hydrological performance with microclimate, biodiversity, and socio-economic dimensions under a unified analytical framework. It advances the empirical basis for NBS adoption in urban retrofit programs and informs policy design, budgeting, and planning guidelines for scalable implementation in similar urban contexts. The main conclusion is that green roofs offer superior multi-functional benefits when coupled with supportive policy instruments, capacity-building, and long-term maintenance planning; however, effectiveness is highly contingent on roof typology, substrate depth, connectivity to the drainage network, and local climate. The recommendations call for adopting standardized performance metrics, aligning incentives with resilience objectives, promoting pilot projects with rigorous monitoring, and embedding green roofs within integrated urban water management strategies to maximize systemic benefits.