Urban waters represent a crucial component for the enhancement of urban resilience due to their importance in cities. Nature-based solutions (NBS) have emerged as sustainable solutions to contribute to urban resilience in order to meet the challenges of climate change. In order to promote the use of NBS for increasing urban resilience, tools that demonstrate the value of this type of solutions over the long-term are required. A performance assessment system provides an adequate basis for demonstrating this value, as well as for diagnosing the current city situation, selecting and monitoring the implementation of solutions. Regarding NBS management, some assessment approaches have been published, focusing on assessing the effectiveness of NBS in the face of climate change and supporting their design and impact assessment. Nevertheless, an integrated approach to assess the NBS contribution for urban resilience has not been published. This paper presents a comprehensive resilience assessment framework (RAF) to evaluate the NBS contribution for urban resilience, focused on solutions for stormwater management and control. Furthermore, details on stakeholders’ validation, with focus on the metrics’ relevance and applicability to cities, is also presented.

Assessing the Nature-Based Solutions (NBS) contribution to urban resilience, along with developing tools capable of demonstrating their long-term value, comprises a most needed step forward. Nevertheless, the NBS contribution to urban resilience has been yet slightly explored. Given the urban resilience complexity and multiple NBS capabilities, specific needs for a robust assessment should be investigated. This paper identifies the requirements for a comprehensive assessment of the NBS contribution to urban resilience and analyses the main assessment frameworks focused on resilience and NBS for urban drainage. First, the evolution of the resilience concept and relevant resilience assessment frameworks (RAF) are presented. Secondly, NBS challenges to enhance resilience and NBS assessment proposals are analysed. Thirdly, the attributes for assessing urban resilience and aspects to assess the NBS contribution are analysed. To conclude, a critical analysis of the assessment approaches is presented. Important challenges across the RAF have been identified, especially regarding their feasibility of application. Based on the performed analysis, most RAFs are not feasible for city benchmarking and assessing the resilience evolution over time, neither for assessing comprehensively the NBS contribution. Regarding specifically the NBS assessment proposals, just one focused on urban resilience was developed to date, which allows evaluating their contribution over time, between cities, or between different NBS.

Urban drainage systems face intrinsic constraints related to the deterioration of infrastructure, the interaction between systems, and increasing requirements and stresses that lower the quality of provided services. Furthermore, climate change and the need for the efficient use of resources are providing additional pressures that cannot be addressed solely with “Business-as-usual” solutions. In this paper, the consequences of such problems and limitations on the urban environment have been assessed through the identification of linked major impacts (e.g., urban flooding and pollution events) and societal externalities (e.g., economic losses, health and social issues, and environmental risks). Since Nature-based Solutions (NBS) consider human well-being, socio-economic development, and governance principles, they open new perspectives regarding urban sustainability, quality of life, and climate change adaptation. To highlight their added value to existing Urban Drainage Systems (UDS), the synergies that result from implementing NBS with traditional urban drainage systems were identified and assessed. Based on a comprehensive framework, for both wastewater and stormwater, the relevant opportunities for rethinking UDS and NBS were identified. Most relevant positive effects go beyond the mitigation of existing intrinsic constraints of traditional systems (e.g., dealing with the control of pollutants or stormwater management) since NBS also provide important economic, social, and environmental co-benefits by including water in urban planning and providing greener open spaces. This integrated and complementary solution not only represents a contribution to the sustainable management of urban water, but also enables an increase in the resilience of urban areas and, in particular, water services against climate change and for additional social co-benefits.

Cities face unprecedented demographic, environmental, economic, social, and spatial challenges. In recent years, the implementation of nature-based solutions (NBS) is becoming more relevant in cities to improve urban resilience and to cope with climate change. NBS represent cost effective solutions that simultaneously provide environmental, social, and economic benefits and help build resilience. A comprehensive and multi-dimension Resilience Assessment Framework (RAF) to evaluate the NBS contribution to urban resilience, focused on NBS for stormwater management and control, was developed. This RAF is aligned with the RESCCUE RAF and the main assessment frameworks focused on NBS and urban resilience. This RAF for NBS is driven by the definition of resilience objectives and is able to evaluate short- and long-term changes, considering a comprehensive definition of the urban resilience and addressing the environmental, social, and economic capabilities. Regarding the initial resilience maturity and the available information in the city, three analysis degrees were proposed for the RAF application, namely, the essential, complementary, and comprehensive degrees, for which a pre-defined selection of metrics is proposed. This paper aims to present the application of the RAF essential analysis degree and its extensive validation regarding cities with different resilience maturity and available information. The application to seven cities with different resilience and NBS challenges allowed an in-depth validation of the pre-defined metrics included in the RAF essential analysis. In this sense, the analysis of the resilience maturity of the participating cities is presented, the main challenges and consolidated aspects in the cities are identified, and the cities ready to apply the complementary analysis degree are recognized. To conclude, to validate the essential analysis degree, the assessment of the main requirements of the RAF for NBS are verified, based on the RAF metrics results for the cities. In this light, the main requirements of the RAF for NBS were aggregated in three main categories, namely, NBS aspects, resilience capabilities, and the performance, risk and cost analysis.

The presence of dissolved oxygen (DO) in water flows is an important factor to ensure the aerobic conditions recognised as beneficial to preventing the occurrence of detrimental effects. The incorporation of DO in wastewater flowing in sewer systems is a process widely investigated in order to quantify the effect of continuous reaeration through the air-liquid interface or air entrained due to the presence of singularities such as drops or junctions. The location of sewer drops to enhance air entrainment and subsequently reaeration is an effective practice to promote aerobic conditions in sewers. In the present paper, vertical drops, backdrops and stepped drops were modelled using the computational fluid dynamics (CFD) code FLOW-3D^® to evaluate the air-water flows due to the turbulence induced by the presence of these types of structures. An assessment of the hydraulic variables and an analysis of the air entrainment based on the available experimental studies were carried out. The results of the CFD models for these structures were validated using measurements of discharge, pressure head and water depth obtained in the corresponding physical models. A very good fit was obtained for the hydraulic behaviour. After validation of numerical models, analysis of the air entrainment was carried out.