*2.3. Step 2: Definition of Objectives, Criteria and Metrics*

In the second step, urban resilience objectives, criteria and metrics were identified. Resilience has to be a tangible concept that cities are able to understand and measure, in order to build robust strategies and prioritize investments. The assessment of the RAF framework considers NBS performance by evaluating the contribution of these solutions at city, service, and infrastructure levels. In this way, it will be possible to identify how, when and where to act first in case of incipient resilience.

The resilience objectives highlight the several NBS contributions to urban resilience. The resilience criteria cover the aspects or points of view that evaluate the achievement of the objectives. The proposed metrics allow a clear assessment of the criteria, supporting the definition of explicit targets and monitoring of results. The use of quantitative and qualitative metrics allows the incorporation and evaluation of objective and subjective information, covering a more comprehensive definition of urban resilience.

The RAF includes metrics that assess performance, cost and risk of the NBS in accordance with the standard EN 752:2008 [38]. In metrics' definition, the related urban resilience capabilities (e.g., to be prepared) are identified. The metrics determination can resort to data from different sources and complexity, allowing the RAF application by cities with different information maturity. The RAF includes metrics with three levels of complexity—based on the existing data in the city (data based), based on a procedure defined for specific metrics (procedure based), or based on results from a mathematical model (model based). The method for metric determination and the specification of the required information was defined in this step.
