2.3.1. Costs

The capital (CAPEX) and operation and maintenance (OPEX) costs of the planned green infrastructure are based on unit costs obtained from both literature and local experience. The costs ranges found in provider websites, unpublished documents and literature have generally a large spread. In this cost-benefit analysis (CBA) the costs were derived partly from literature [21] and partly from unpublished documents and internal research projects. The different costs were converted into the same year value using consumer price indices. The CAPEX of extensive green roofs are assumed to be 80 €/m2 and the OPEX 2.33 €/m2/y. Bianchini et al. [22] reported a CAPEX range of 120–152 €/m2 and an OPEX one of 1–12 €/m<sup>2</sup> for extensive green roofs. The CAPEX of bioretention cells are 45 €/m2 plus 2.25 €/m<sup>2</sup> for plant implementation and the OPEX 0.45 €/m3/y. The CAPEX of detention and retention ponds are 100 €/m<sup>2</sup> and the annual OPEX is 1.49% of the CAPEX. The CAPEX of permeable pavements are 49.5 €/m2 and the OPEX 1.375 €/m3/y. The CAPEX of infiltration trenches are assumed to be 185 €/m3 and additional 742 €/m<sup>3</sup> the OPEX 50 €/m3/y. The additional CAPEX of infiltration trenches in this case include the costs of additional manholes, inlets and pipes that need to be constructed since these systems are supposed to be constructed into a public park area where existing drainage connections are limited. The estimated CAPEX of the infiltration trenches proposed in Badalona are similar to the costs paid by the municipality for an executed project. Zhou et al. [8,23] used investment costs of infiltration trenches in the range between 16 and 91 €/m2. Alves et al. [6] estimated annual OPEX as 3% of CAPEX costs.

The lifetime of an infrastructure can vary depending on Its maintenance: the higher the maintenance costs the longer the lifetime [24]. In Badalona extraordinary maintenance was assumed to be carried out every 20 years with a cost equal to the 23% of the CAPEX at each intervention. Similarly, in Barcelona it was assumed every 20 years for bioretention cells and 50 years for green roofs and retention and detention ponds with a cost equal to the 50% of the CAPEX at each intervention.

Residual GI value at the end of the project evaluation period was also considered according to European recommendations for evaluations of investments [25]. This reflects the value of the remaining potential use of GI since its services will be provided further beyond the end of the CBA evaluation period [25]. In this study, it was considered as a negative cost but it could also be considered as a benefit as the choice does not affect the net present value Equation (1) (it only affects the graphical presentation of cost and benefits).
