3.4.2. Technical Detailed Assessment: Risks Modeling

Different climate impacts have been assessed for the two case studies of the project RESCCUE. Although other hazards have been addressed, only floods, both pluvial and fluvial, and CSO spills are considered here for comparative purposes of the two cities. Damages due to floods and CSO spills can be many, and these can be classified as direct or indirect, and in turn tangible or intangible [28]. A variety of damages caused by these hazards have been assessed in these two cities, such as flood damages to properties and vehicles, traffic disruption or damages to the electrical grid. Table 2 summarizes the different hazards and risks assessed for each city.


**Table 2.** Summary of detailed assessments conducted in Barcelona and Bristol.

Various methodologies to assess these risks have been employed in the two case studies. In the Barcelona case study, the following detailed assessments were carried out to evaluate the efficiency of the proposed adaptation scenarios within the pluvial flooding strategy:

#### *Sustainability* **2020**, *12*, 4807


For the CSO spills reduction strategy, two detailed assessments were carried out to understand the impacts of the adaptation scenarios:


In the Bristol case study, the evaluation of adaptation measures for pluvial and fluvial flooding was carried out under the following methodology:

• Damage assessment: flood models were developed evaluating property damage through application of the damage assessment tool developed in EU project CORFU (Collaborative Research on Flood Resilience in Urban Areas) [43] and designing intervention scenarios representing property level protection. Flood models included fluvial and pluvial events at a range of return periods (T20 to T1000 and T10 to T100, respectively). Analysis of peak flood depth mapping for a baseline representing current day conditions and a climate change scenario derived from UKCP09 [19], assuming BAU emissions up to the year 2115 was carried out.

A damage cost assessment per scenario was generated by combining flood depths with building classifications [44] and depth-damage curves [45]. Intervention scenarios designed to protect properties within the 20-year flood outlines, either for all residential buildings in the three worst affected areas (zonal target) or for the same number of worst-impacted properties distributed across the study area (individual target), were compared. Interventions were represented by adapting the damage curve to prevent damages below a 600 mm water-depth threshold, reflective of a likely effective level of protection [46].


These methodologies have been applied for the BAU scenario, in which no adaptation is considered and for a scenario in which measures are implemented. For instance, once a certain flood is modeled (i.e., 10-year return period design storm), the risk for pedestrians, based on a person's stability threshold, is assessed first without measures (BAU). Afterwards, a measure is implemented in the flood model, which yields a lower flood, and thus the risk is re-assessed. The comparison of both model results yields the variation in the high-risk area for pedestrians, considered an effectiveness indicator of risk reduction measures. Similar procedures have been conducted for the hazards and risks assessments listed in Table 2 for the two cities.

#### 3.4.3. Economic Assessment: Environmental Cost-Benefit Analysis

Cost-benefit analysis (CBA) was the method selected for the final comparison of the potential adaptation scenarios. This method allowed to integrate all previous assessment results involving direct or indirect changes in environmental, economic, and societal variables, translated into monetary terms [6]. It provided a comparative overview of the potential effects of the different scenarios in terms understood by all stakeholders.

The net sum of all relevant positive and negative outcomes of a scenario is known as the total economic value (TEV) [6] and is typically divided between use and non-use (passive) value [6]. In this study, the focus was set on use values, which relate to the actual use of the good in question, as for example, the use of green areas in cities for recreation [6]. Whereas, non-use values are those related to their existence, altruistic or bequest value [6]. When possible, market prices were used to

value the changes provided by the measures' implementation (or related goods or services), while benefit transfer was used when direct values were not available. Benefit transfer relies on unit values obtained in previous studies to estimate the value in the study site, adapting them to the characteristics of the new site [50]. Ecosystem services are understood as the direct and indirect contributions of ecosystems to humans [51]. They play an important role, since their principles have been accounted for in the sustainable urban drainage (SUDS) measures [52], in an attempt to demonstrate the benefits of "greening" urban areas. This followed the trend in the urban planning sector of putting emphasis on accounting for the co-benefits provided by nature-based solutions [52–56].

The time horizon selected for the analysis was 2020–2100, aligned with the timespan of the general assessment for the project RESCCUE. Costs included all required investment efforts of each city council, as well as the operating costs for the lifespan of the analysis. Benefits came from two sources. First, the avoided costs were estimated through the difference between the estimated economic damage assessment in the BAU scenario and in each of the alternative scenarios [11]. In addition, monetary values were included to account for the benefits of the improvement in the provision of ecosystem services of SUDS measures. They were adapted using the benefit transfer method [57]. These benefits include the reduction of the heat island effect, which implies reductions on electricity consumption [58,59]; air quality improvements [58]; habitat creation and aesthetic value, related to the increase in willingness to pay for properties with surrounding green areas [59]. Net benefits (Equation (4)) aggregate benefits and costs to determine the TEV or complete impact of the scenarios (*j*) proposed:

$$\text{Net benefit}\_{\text{j}} = \text{Benefits}\_{\text{j}} - \text{Costs}\_{\text{j}}.\tag{4}$$

The net benefits expected through the lifetime of the project (years from t = 1 to T = 80) were discounted to reflect future values in present terms, obtaining the net present value (NPV) (Equation (5)), using the same discount factor as the CEA. Similarly, the results were annualized for comparative reasons, thus presented using the annual equivalent present value (AEPV) (Equations (6) and (7)), for each scenario *j*:

$$NPV\_{\bar{j}} = \sum\_{t=1}^{T} \frac{Benefits\_{\bar{j},t} - Cost\_{\bar{j},t}}{(1+i)^t},\tag{5}$$

$$AEPV\_j = \frac{NPV\_j}{A\_{(t,j)}},\tag{6}$$

$$\mathcal{A}\_{(t,i)} = \frac{1 - \left(1 + i\right)^{-T}}{i},\tag{7}$$

where *A*(*t*,*i*) is the annuity factor of the present value.

#### *3.5. Stage 4: Final Ranking*

The multiple results obtained through the detailed assessment allowed to rank adaptation scenarios under various criteria. In the Barcelona case study, scenarios were prioritized by: (1) area of risk reduction, (2) by avoided damage, (3) by costs, and (4) by net benefit criteria. In the Bristol case study, scenarios were ranked based on total damage.

#### **4. Results: Application in Two European Cities**

The application of the methodology is presented in stages, comparing the two case studies in order to display the different approaches taken.
