The influence of Green Infrastructure on the resilience of urban drainage systems

In light of emerging threats and future uncertainties, increasing resilience in urban drainage systems is vital to minimise failure magnitude and duration under extreme conditions. The introduction of green infrastructure, decentralised strategies that mimic nature, and their integration with grey infrastructure can help fill the gap for resilience-enhancement solutions. This work aims to understand the impact of green infrastructure on urban drainage resilience. Three main knowledge gaps are addressed: (1) understanding the spatial relationship between green infrastructure and resilience, (2) developing standardised and comprehensive resilience assessment tools for evaluating the impact of green infrastructure on resilience, and (3) determining if resilience has been effective over time as a strategy for adaptation. The conceptualisation of green infrastructure is centred around stormwater management, focusing on the benefits of water quantity (effects on sewer flooding) and water quality (effects on combined sewer overflows), and it is limited to three types (bioretention cells, green roofs, and permeable pavements). Green infrastructure and its impact on urban drainage systems are modelled using the Storm Water Management Model (SWMM). The spatial relationship between green infrastructure and resilience performance is examined with a framework that integrates location sensitivity analysis and spatial analysis methods. The framework is applied in Topsham (United Kingdom), where all green infrastructure types were spatially correlated with at least one of the resilience indicators selected. The cluster maps obtained highlight areas of high and low resilience performance, where the finding of conflicting clusters indicates performance trade-offs in green infrastructure placement in specific locations. A quantitative framework is introduced based on the Global Resilience Analysis extension, enabling measuring resilience under continuous rainfall. Long-term continuous simulations comprehensively examine rainfall patterns and characteristics, including extremes and frequency of dry and wet periods. In addition, the proposal of CSO discharge performance metrics and a resilience index allows a more comprehensive analysis of the effects of discharges. This framework was applied in Fehraltorf (Switzerland) for historical rainfall and different climate projections. Green roofs are the most beneficial strategy for this catchment when considering all the CSO performance metrics at individual outfall and system levels. When considering climate change, the adaptation capacity varies significantly depending on the type of green infrastructure, the outfall studied, and the indicators used. Although green infrastructure enhances resilience, its application appears insufficient for climate change impacts under the climate projections analysed. Lastly, recommendations for operationalising a resilience performance-based implementation of green infrastructure are discussed based on the research findings. Overall, green infrastructure positively affects resilience, but its spatial deployment and how its impacts are assessed are critical to this effect. This research provides evidence of green infrastructure's influence on resilience performance and a set of tools for its evaluation which can help to scientifically formulate urban planning and urban drainage management decisions to achieve city resilience