Valuing the contribution of green roofs to pluvial flood risk mitigation: A cost-benefit analysis

Cities in the 21st century face increasing pressures from population growth, urban sprawl, and emissions, while flood-related challenges exacerbated by climate change strongly intensify their vulnerabilities. Consequently, urban global change is becoming an urgent necessity globally. Nature-based solutions (NBS) have gained increasing attention as valuable sources of ecosystem services, which can also address these multiple societal challenges. Green roofs are widely used for stormwater management and treatment in compact urban environments. However, to date, most research has been conducted regarding green roof costs or flood risk mitigation benefits at a citywide scale; while local administrations need more evidence on the economic viability of green roofs that may increase the willingness to consider these nature-based solutions. Hence, the objective of this study is to develop and apply a spatially explicit assessment of flood risk mitigation impacts (biophysically – water depth), costs and benefits of NBS (economically – implementation costs, avoided damage costs, net present values and benefit-cost ratios) under current (2013) and future (2050; RCP 4.5) climate conditions – with a case study for green roofs in Rapallo (Italy). The spatial biophysical-economic approach integrates the InVEST Urban Flood Risk Mitigation model (spatial resolution: 5 m × 5 m), benefit transfer methods, and geographic information systems into a cost-benefit analysis. Results show that flood risks under current (2013) climate conditions imply significant building damage costs (~6.5 million €/yr for Rapallo), that these costs increase when considering future (2050) climate conditions (by about 7 %), and that NBS (green roofs) implementation can reduce these costs (by almost 90 %). Moreover, green roofs result to be economically viable from a flood mitigation perspective alone when considering Low NBS costs, while flood mitigation benefits contribute to, respectively, 87 % and 63 % of the green roof annual implementation costs when considering Medium and High NBS costs. Finally, results show that the economic viability of green roofs differs across neighbourhoods – hence allowing for the economic prioritization of green roof implementation across neighbourhoods. By quantitively assessing NBS impacts, costs, and benefits at the neighbourhood level, this study supports the decision on the most viable locations for the implementation of NBS for flood risk mitigation – highlighting the need for spatial assessment studies to support urban NBS development strategies.