Steering towards the most adequate performance indicator for an economic and hydrological evaluation of low-impact development practices.

Low-impact development practices, including rain gardens and bioretention systems, can help mitigate combined sewer overflows (CSO) in urban areas. However, selecting the most appropriate economic indicator for evaluating their cost-effectiveness remains a key challenge for decision-makers. A methodology was developed in this study to guide the selection of the most suitable low-impact development implementation scenario for reducing CSOs. Various performance indicators were compared, including net present value, benefit-cost ratio, cost-effectiveness, and benefit per runoff reduction, to identify the optimal scenario. Applied to two case studies in Quebec, Canada, under various implementation scenarios using continuous rainfall data, the methodology revealed that bioretention systems achieved 83-100 % CSO frequency reduction in Laval and 43-58 % in Montreal, rain gardens showed superior benefit-cost ratios (up to 3.43 in Laval, 1.84 in Montreal) and net present values reached 4.47M CAD in Laval and 80.1M CAD in Montreal at 4-5 % implementation rates. The findings also demonstrate that relying solely on the benefit-cost ratio can be misleading, as it overlooks total investment considerations. Instead, a multi-criteria approach integrating hydrological performance and economic indicators is essential for informed decision-making. This study provides a framework for optimizing low-impact development implementation in urban drainage networks, ensuring regulatory compliance and maximizing long-term benefits.