**1. Introduction**

Green infrastructure (GI)—also recognized with the acronyms NBS (Nature-Based Solutions), SUDS (Sustainable Urban Drainage Systems), LID (Low Impact Development), BMP (Best Management Practices), WSUD (Water Sensitive Urban Design) and many others [1]—contributes to improve urban stormwater management and has several other societal benefits like air quality improvements, reduction of heat island effects, aesthetic and recreational values, and others [2]. Socio-economic assessment of GI is an important tool for urban drainage planning and decision making of climate change adaptation strategies [3].

Several studies have presented socio-economic assessments of different climate change adaptation options focusing on direct and indirect benefits derived from flood damage reduction capacity of GI. Velasco et al. [4] presented a cost-benefit analysis where only direct benefits were included in terms of avoided flood damages obtained by different adaptation scenarios in Barcelona: structural measures

(pipe enlargement and stormwater tanks), GI, flood barriers for ground floor doors of businesses and private buildings and early-warning systems. Zhou et al. [3,5] presented a framework and its application to a Danish case study for economic assessment of different climate adaptation options focusing on flood impacts. The economical assessment was based on a cost-benefit analysis (CBA) with direct and indirect benefits derived from flood damage reduction that were monetized using flood models together with damage costs for houses, basements, sewers, roads, lakes and people health and also administrative and traffic delay costs. The damage costs were calculated using unit costs reported from case-specific literature. In these papers, flood adaptation options based on pipe enlargements were compared to stormwater infiltration through GI focusing on flood reduction benefits.

Further studies present socio-economic assessments including additional benefits not only related to direct or indirect flood damages [6]. Löwe et al. [7] presented a cost-benefit analysis (CBA) for comparing different flood adaptation options in Australia. The flood adaptation options consisting of pipe enlargement, flood zoning and rainwater harvesting through GI were compared including flood reduction benefits and also additional benefits derived from reduction of drinking water consumption. Zhou et al. [8] presented an integrated hydrological cost-benefit analysis for comparison of different climate adaptation options such as open urban drainage systems, pipe enlargement and local stormwater infiltration. Here, benefits derived from flood damage reduction were integrated with additional monetized benefits derived from increased property values in the areas where GI was planned and the consequent increase in property taxes. Finally, Cooper et al. [9] presented an integrated costs-benefits analysis of a berm (sea wall) to mitigate the effects of coastal flooding from sea storms. Here, the monetized benefits of the project included: avoided costs derived from building damages, management expenses, fatalities, debris removal, utility and municipal damages; benefits derived from recreational and health value and indirect costs derived from interruption of key transportation and commercial infrastructure located in the area. The recreational and health values were linked to the ecosystem services and health benefits to the surrounding community generated by the planned green areas along the berm.

Further studies underlined the importance of analyzing GI with a multidisciplinary approach. Venkataramanan et al. [10] presented a multidisciplinary literature review focusing on the interaction between human dimensions and socio-ecological-technical systems that are involved with GI in the context of flood risk management. Additionally, Wilkerson et al. [11] analyzed the role of socio-economic factors involved in the planning and management of urban ecosystem services.

The aim of this paper is to present a cost-benefit analysis that includes multiple benefits derived from green infrastructure in the context of urban drainage planning. The novelty of this study is the integration of water quantity and quality and other socio-economic benefits into CBA of GI in the context of urban drainage planning. The application of this analysis to two different case studies can also be considered as novel since the application of CBA is generally used for comparing different adaptation measures within the case study. GI benefits are calculated from direct and indirect flood impacts reduction, water quality related benefits and additional societal benefits. Benefits of flood damage reduction are calculated as avoided direct and indirect flood damage costs to buildings, vehicles, urban infrastructure and indirect costs. Flood damage costs are calculated using coupled 1D urban drainage and 2D surface runoff models together with tailored depth–damage and permeability coefficients functions. Water quality related benefits derived from CSO and waste water treatment cost reduction are calculated using a 1D urban drainage model and costs of wastewater treatment and CSO spills obtained from literature. Finally, additional societal benefits like increased aesthetic value, air quality improvement, habitat provision and reduced urban heat island effect and energy consumption, are calculated based on unit costs from literature. The socio-economic assessment is applied to two different case studies: the Spanish municipalities of Barcelona and Badalona. These two case studies were part of the two European H2020 research projects: BINGO (Bringing Innovation to onGOing water management. www.projectbingo.eu) and RESCCUE (Resilience to Cope with Climate Change in Urban Areas. www.resccue.eu). The aim of presenting two cases is mainly to show that

the methodology can be applied to different cases. Nevertheless, the comparison can also bring new points of view in the discussion of GI in the context of urban drainage planning. The methodology proposed can be considered generally applicable to other cities in the context of green infrastructure and urban drainage planning.
