Assessing Urban Green Roofs for Stormwater in Melbourne's Building Sector

Thesis Abstract

Urban stormwater management challenges in Melbourne's rapidly intensifying built environment necessitate robust, nature-based solutions to reduce peak flows, improve water quality, and enhance urban resilience. This study addresses the problem of limited empirical understanding of how extensively implemented green roofs influence stormwater performance in Melbourne's building sector, and how factors such as roof media, slope, irrigation regimes, and building typologies modulate hydrological outcomes. The aim is to assess the runoff attenuation, water quality benefits, and operational constraints of urban green roofs within Melbourne, informing policy and practice for sustainable urban drainage. Specific objectives are to (i) quantify rainfall-runoff response and peak mitigation provided by green roofs across representative Melbourne building typologies; (ii) evaluate the water quality performance, including nutrient and contaminant removal, under different irrigation and substrate configurations; (iii) identify design, maintenance, and regulatory factors shaping system performance and longevity; (iv) develop a transferable performance model linking roof characteristics to stormwater outcomes; and (v) provide evidence-based recommendations for stakeholders, including planners, builders, and facility managers. The methodology adopts a mixed-methods, multiple-case study design aligned with the socio-ecological systems framework and theories of green infrastructure performance and resilience, drawing on the Hydrological response theory and the Urban Metabolism perspective. The population comprises commercial, residential, and institutional buildings in Melbourne equipped with retrofit or new green roofs. A purposive sample of 12 sites, stratified by building type and scale, will be selected, with on-site monitoring of 8 operational roofs plus 4 experimental roofs installed as part of the study to test substrate, depth, and irrigation variations. Data collection will combine continuous hydrological monitoring (per-site data loggers measuring rainfall, soil moisture, runoff volumes, and discharge), water quality sampling (weekly for five months each season, analyzing TSS, TN, TP, NH4-N, NOx, and chlorides), and roof condition assessments. Instruments include calibrated rain gauges, hydrographs, tensiometers, and standardized maintenance checklists. Complementary modelling will employ a distributed hydrological model (SWMM) calibrated to observed runoff and validated against a holdout dataset. The analysis will include descriptive statistics, regression analysis to quantify relationships between roof characteristics and peak flow reduction, ANOVA to compare performance across roof typologies, and multivariate analysis to identify key predictors of water quality improvements. A thematic analysis of stakeholder interviews (n ? 25) with facility managers, designers, and policy makers will elucidate operational constraints, maintenance demand, and regulatory perceptions. A conceptual performance model will be developed to relate roof design variables, maintenance inputs, and external climatic factors to stormwater outcomes. Expected findings indicate that Melbourne green roofs achieve measurable peak flow reductions ranging from 20% to 45%, with greater attenuation on deeper media and for roofs with higher retention volumes under moderate rainfall frequencies. Water quality improvements are anticipated for nutrients and TSS, notably when substrate depth exceeds 100 mm and irrigation is regulated to prevent leaching, although performance may decline during extreme rainfall events without supplementary drainage. The analysis is expected to reveal significant differences across building typologies, with institutional and large commercial roofs delivering more consistent performance due to enhanced maintenance regimes and monitoring. The study will identify design thresholds (media depth, slope, drainage layer characteristics) and maintenance practices that optimise performance, and reveal regulatory and operational barriers limiting full-scale adoption. The study contributes to knowledge by providing an empirical, context-specific assessment of green roof stormwater performance in a temperate urban setting, offering a validated modelling framework and design guidance transferable to similar cities. It advances understanding of how substrate composition, irrigation management, and roof area influence hydrological responses, bridging gaps between engineering performance and policy uptake. The recommendations will inform planning policies, building codes, and maintenance regimes, supporting scalable implementation of green roofs as a climate adaptation strategy. The main conclusion is that well-designed, properly maintained green roofs can substantially mitigate urban stormwater impacts in Melbourne, but achieving consistent performance requires integrated design standards, routine maintenance, and clear regulatory guidance. The study recommends the adoption of standardized performance metrics, mandatory monitoring for retrofit roofs, incentives for deeper substrate designs, and development of a Melbourne-wide green roof guidelines manual to facilitate broader implementation.