**1. Introduction**

Climate change has fostered the need to develop and improve urban resilience by promoting a resilient city's capabilities to absorb disruption, learn from the past, adapt, transform and prepare for the future. Resilience emerged as an interesting concept on cities, often theorized as highly complex adaptive systems [1,2]. Resilience is commonly understood as the capacity of a system to absorb disturbance and re-organize while undergoing change so as to still retain essentially the same function, structure, identity and feedbacks [3]. The concept of resilience emerged in the 1960s from the growing interest in ecology to determine population stability between communities. Resilience emerged as an interesting perspective on cities, often theorized as highly complex adaptive systems [4–6].

In the urban water cycle, the evolution of the drainage systems followed the evolution of the resilience concept, from one single point of view (e.g., economic or social resilience) to a broader and more inclusive definition, encompassing the multiple dimensions of urban resilience. Urban water services are of fundamental importance in the promotion of urban resilience. Urban waters are essential to support nature-based solutions (NBS) functions and to ensure the provision of ecosystem services from NBS, such as air quality improvement or urban heat island mitigation. Evaluating and enhancing resilience in the urban water cycle is a crucial step toward more sustainable urban water management [7].

Historically, the main objectives of urban drainage systems were to ensure efficient management of peak flows and adequate treatment of polluted waters, aiming to ensure public health and prevent flooding. More recently, integrated approaches for urban water management emerged and other key issues were identified for sustainable water management, such as surface and ground water quality, ecological concerns, and recreational uses [8].

The European Commission defines NBS as actions that aim to helps societies address a variety of environmental, social and economic challenges in a sustainable way [9]. In essence, NBS can be defined as living solutions inspired by, continuously supported by and using nature, which are designed to address several societal challenges in a resource-efficient perspective and to provide simultaneously economic and environmental benefits [9]. NBS involve actions for conservation or rehabilitation of natural ecosystems and improvement or creation of natural processes in modified or artificial ecosystems [10]. Some examples of NBS for stormwater management are infiltration basins, green roofs, constructed wetlands or swales with vegetation cover, among others.

In the water sector, the NBS concept can be found in technologies such as sustainable urban drainage systems (SUDSs), which mimic nature to manage stormwater runoff and provide other services to the urban environment. SUDSs are recognized as one of the main NBS techniques to improve urban resilience regarding stormwater management. These techniques can also be found with different designations, such as low impact development (LID), best management practices (BMPs), and green infrastructures. In this sense, an integrated approach of urban water management that incorporates a SUDS as a fundamental component is the Water Sensitive Urban Design, originate and widely applied in Australia. This approach to urban planning and design aims to integrate in the urban design the various disciplines of engineering and environmental sciences associated with the provision of water services [11].

To date, several studies were developed focused on the analysis of the resilience and sustainability enhancement based on SUDS implementation [12,13]. In the UK water sector, SUDSs are increasingly promoted in order to enhance flood resilience in urbanized areas and its application to increase resilience has also been studied [14,15]. For example, an approach adopted to quantify the cost-effectiveness of resilience measures and integrative and adaptable flood management plans was proposed [16].

The European Research and Development Program promotes a large number of projects related to NBS to increase knowledge and to create technical, political and other conditions for cities renaturalization. Improving risk management and resilience, using nature-based solutions, represents one of the mains goals of the EU Research and Innovation agenda [9]. These R&D projects will analyse several objectives and perspectives, as the improvement of regulatory instruments, the increase of the natural capital through NBS or the capacity to obtain a more sustainable and resilient urban ecosystem. In the context of urban resilience, some NBS studies were carried out focusing on some ES enhancement or on specific challenges, such as urban heat island mitigation [17,18], air quality improvement, climate mitigation and adaptation [19,20], and water quality improvement [21], among others.

Public participation is becoming increasingly embedded in the decision-making processes [22]. In this sense, several studies highlight the need to ensure a broader stakeholders' engagement in the development and implementation of assessment tools [23]. The development of the assessment process with stakeholders' collaboration promotes their empowerment and enhance their role in decision-making processes [24]. Regarding to the assessment process, stakeholders help to highlight weaknesses, to prioritize interventions and to identify the assessment tools adequacy to diverse locations. Stakeholders networks on NBS design, planning, and implementation are essential to ensure the transference of successful approaches between countries, communities and case studies [25]. Several available assessment frameworks include the stakeholders' participation in the development and implementation processes [26–28].

Currently, there is a need to analyse the NBS contribution to urban resilience and to develop tools that demonstrate the long term value of these solutions [29]. Several frameworks to assess resilience are being developed, such as the RESCCUE Resilience Assessment Framework (RESCCUE RAF) [26], the Disaster Resilience scorecard for cities [27], the Arup and Rockefeller city resilience framework [28] and the USEPA framework [30], among others.

Based on the review of the available resilience assessment frameworks (RAFs), the relevant *attributes* for resilience assessment were identified [31]. From this analysis, an RAF needs to i) propose a multi-dimension methodology that includes subjective and objective information, allowing us to measure urban resilience in one scale; ii) identify resilience objectives and criteria; iii) use qualitative and quantitative metrics addressing performance, cost and risk; iv) define reference values and a final resilience assessment; and v) identify the urban resilience capabilities associated with the proposed metrics. Additionally, there is a need for the RAF to consider and to allow the assessment of short- and long-term changes [30].

Regarding specifically the NBS for stormwater management and control, some assessment frameworks have already been developed, focusing on assessing NBS effectiveness in the face of climate change [32], supporting the design and impact assessment of the NBS for climate resilience [25] and to specific urban challenges, such as green space management or air quality [33]. Even though the NBS assessment frameworks are not directly focused on urban resilience, they may support a specific assessment framework focused on NBS contribution to urban resilience. To date, a comprehensive assessment approach has not been published to assess the contribution of NBS to urban resilience.

In this sense, among the several projects analysed, common concerns and knowledge gaps relevant to the development of a specific RAF for NBS were identified. Based on this analysis, an appropriate RAF for NBS should assess the following *aspects*: (i) social, environmental, economic, and governance dimensions; (ii) spatial and land use planning at the city level; (iii) service and infrastructure management; (iv) potential capabilities to provide ecosystem services (ES) and to enhance natural capital and biodiversity; (v) impacts on the surrounding area; (vi) infrastructure implementation and design, including adequate monitoring and maintenance processes; (vii) infrastructure performance under normal and stressing condition, considering acute shocks and continuous stresses; and (viii) infrastructure interdependencies with other urban services.

The main objectives of this paper are to present the methodology adopted for the construction of an RAF for NBS, as a specific RAF to evaluate the NBS contribution for urban resilience, and the stakeholders' validation, with focus on the analysis of relevance and applicability of metrics to cities. The main innovative contribution of this work is to propose a multidimensional and comprehensive RAF to assess the NBS contribution for urban resilience, focused on NBS for stormwater management and control. The proposed framework, driven by resilience objectives and assessment criteria, aims to integrate the *attributes* identified for urban resilience assessment and the relevant *aspects* for the NBS evaluation.

### **2. Methodology**
