Comparative Assessment of Urban Green Roofs in Flood Resilience

Thesis Abstract

Urban flooding poses escalating risks to densely built environments, and green roofs are increasingly promoted as nature-based solutions to reduce runoff, mitigate peak discharge, and enhance urban resilience. This study addresses the gap between theoretical benefits of green roofs and their comparative performance under flood-prone conditions across diverse urban contexts. The aim is to evaluate the flood resilience contributions of urban green roofs through a cross-sectional, mixed-methods approach that contrasts residential, commercial, and public-sector buildings in three mid-sized cities with distinct climates. Specific objectives are (1) to quantify runoff reduction and peak flow attenuation attributable to green roof installations; (2) to compare hydrological performance across roof designs (extensive, semi-intensive, and intensive) and substrate types; (3) to assess co-benefits such as thermal regulation, biodiversity potential, and maintenance costs; (4) to identify governance and policy drivers facilitating widespread adoption; and (5) to develop a transferable framework for optimizing green roof configurations for flood resilience in varying urban morphologies. The methodology adopts a convergent parallel mixed-methods design, integrating quantitative hydrological instrumentation with qualitative stakeholder insights. The population comprises 120 green roofs installed in three cities over the last decade, with a stratified sample of 60 sites (20 per city) representing the main roof typologies. Data collection combines (a) instrumental measurements of rainfall, runoff volume, and substrate moisture using 30-minute interval sensors over a 12-month monitoring period, (b) roof-level data on substrate depth, vegetative cover, drainage layer characteristics, and insulation properties collected via site surveys and engineering records, (c) thermal performance indicators from infrared thermography and energy meters, and (d) semi-structured interviews with 36 property managers, municipal planners, and designers to capture operation, maintenance, and governance aspects. Validity and reliability are ensured through calibration of sensors, cross-validation with building management records, and inter-rater reliability checks for qualitative coding. Data analysis employs multiple regression to quantify the relationship between roof variables and runoff reduction, ANOVA to test differences across roof types and cities, and a hierarchical linear model to control for building footprint and climate. Thematic analysis is applied to interview transcripts to extract governance enablers and barriers. A conceptual model grounded in the Hydrological Green Roof Theory and the Resilience Theory is used to interpret linkages among design features, flood response, and system resilience. Expected findings indicate that extensive green roofs achieve 20–40% reductions in peak discharge for events up to 25-year return periods, with semi-intensive and intensive roofs offering greater attenuation under heavy rainfall but at higher maintenance and loading costs. Substrate depth and drainage efficiency are anticipated to emerge as primary predictors of runoff performance, while vegetation type influences evapotranspiration and thermal regulation co-benefits. The study is likely to reveal context-dependent performance, with larger gains in cities exhibiting higher impervious surface fractions and poorly connected stormwater networks, moderated by policy instruments such as incentives, performance-based metrics, and maintenance regimes. Qualitative results are expected to highlight governance factors—such as developer requirements, retrofit incentives, and post-occupancy evaluation practices—as critical determinants of uptake and sustainability. Contribution to knowledge includes (i) a rigorous comparative assessment framework for flood resilience benefits of urban green roofs across roof typologies and city contexts; (ii) empirically derived thresholds for substrate depth, drainage efficiency, and vegetative coverage that optimize runoff attenuation; (iii) integrated evidence on co-benefits and cost considerations to inform decision-makers; and (iv) a policy-and-practice guidance toolkit that aligns architectural design with municipal flood management objectives. The study concludes that when designed, maintained, and governed within an enabling policy environment, urban green roofs can provide quantifiable flood resilience gains without compromising urban density or energy performance. Recommendations emphasize standardized monitoring protocols, scalable retrofit financing, performance-based incentives, and cross-sector collaboration to mainstream green roofs as a core component of urban flood risk management.