Spatial-economic scenarios to increase resilience to urban flooding.

Due to accelerating climate change and the need for new development to accommodate population growth, adaptation of urban drainage systems has become a pressing issue in cities. Questions arise whether decentralised urban drainage systems are a better alternative to centralised systems, and whether Nature Based Solutions' (NBS) multifunctionality also brings economic benefits. This research aims to develop spatio-economic scenarios to support cities in increasing their resilience to urban flooding with NBS. The novelty of our work lies in the automated routines to assess the potential for decentralised NBS within the existing urban catchment. The identification of locations and dimensioning is based on open, publicly available geospatial data. Moreover, a block-based decentralization potential metric is developed to indicate stormwater mitigation potential in any urban setting. The Ecully catchment, Lyon metropolitan area (France), is presented as a case study to achieve zero combined sewer overflow (CSO) for specific design storm events. This planning workflow enables project cost savings through the most suitable allocation of distributed interventions, with cost functions also incorporating scaling effects. By reducing the number of decentralised NBS sites compared to smaller, wide-distributed interventions up to 34 % of project costs are saved when planning for a 5-year design storm and up to 7 % for a 100-year design storm. When the decentralised NBS scenario is analysed alongside other urban stormwater management practices, the centralised constructed wetland for CSO results to be the most economical solution due to the higher retention capacity and scaling effect, significantly outperforming the grey alternatives.