Global Resilience Analysis of Combined Sewer Systems Under Continuous Hydrologic Simulation

There is a need to manage and reduce combined sewer overflow discharges (CSOs) to support the resilience of combined sewer systems (CSS) and minimise their environmental impacts. However, the lack of a standardised approach to resilience analysis presents a challenge for developing CSO reduction strategies and enhancing the resilience of CSS systems. To address this issue, this study proposes a method for quantifying the resilience performance in CSS by expanding the existing Global Resilience Analysis (GRA) to a top-down approach, linking threats (i.e., rainfall) to impact (i.e, CSOs) under continuous and long-term simulation. This approach enables the evaluation of a wide range of rainfall patterns, including extreme events such as excessive precipitation and drought. Eight new metrics are introduced to analyse the impacts of the CSO discharges from a resilience performance perspective. In a real-world case study (Fehraltorf, Switzerland), the performance of three types of GI (bioretention cells, green roofs, and permeable pavements) was analysed over a period of 38 years. GI increased resilience in terms of all the considered indices including total CSO discharge volume; however, significant differences were observed for individual CSO performance metrics and their locations within the system. Overall, the proposed method provides a comprehensive understanding of the consequences of rainfall as a threat to CSS and helps to make informed decisions regarding the implementation of GI to enhance resilience. By offering a standardised approach to resilience analysis, this study has the potential to inform and guide the development of strategies for CSO reduction and the resilience enhancement of CSS