The evaluation of benefits and co-benefits of nature-based solutions related to flood risk mitigation have been extensively studied and include more traditional assessment models [3,4] and participatory assessment models [7]. The peri-urban Confluence park in Prague represents a good example of how to create harmony among supporting natural processes, economic interests and visitors activities [16]. 

Aesthetics, Mental wellbeing/quality of life, Physical health, Healthier and more attractive lifestyles: Floodable park can improve the aesthetics of urban landscape and when provided with appropriate facilities, they can support more active and healthy lifestyles, attracting people to walk, run, play and meet, supporting their health and wellbeing [8]. 

Species increase, Biodiversity conservation, Pollinators increase: Floodable parks can be natural habitats for birds, insects, and many other animals. 

Enhance stability of the urban infrastructure: Floodable parks can contribute to the resilience of urban infrastructure (road, bridge, etc.) by decreasing flood risk [9]. 

Increased water security, better water management and quality: Nature-based solutions can play an important role in establishing a more efficient and more circular management of water streams in urban settlements. However, the management of surface runoff of floodable parks follows the conventions of urban drainage where the primary focus lies in removing water from the city as quickly as possible [10]. Nevertheless, as presented in a study looking at wetlands, floodable parks may be designed to conserve drinking water quality supplies, thus reducing water treatment volumes, costs and energy consumption [11]. 

Rainwater management and retention 

WWTP: Water reuse and recycling (urban irrigation). Tertiary treatments 

Tree planting (urban forestry/urban trees), parks and (semi) natural urban green areas 

Hard-drainage flood prevention 

Grassed swales and water retention pounds 

Water-retentive pavements: hard drainage pavements; green parking pavements 

Sustainable Urban Drainage (SUD) systems 

Constructed wetlands 

Rain gardens 

Geospatial information (incl. Satellite) 

Policy and regulatory/legal framework: 

Diverse policy and regulatory frameworks can enable implementation of floodable areas. 

At EU level, within the overall framework of the EU Green Deal: 

• The European Biodiversity strategies: cities with a population more than 20,000 are called upon ‘to develop Urban Greening Plans, including measures to create biodiverse and green urban forests, parks and gardens; urban farms; green roofs and walls; tree-lined streets; urban meadows; and urban hedges’. To facilitate this work, the Commission set up an EU Urban Greening Platform with cities and mayors in 2021 under the Green City Accord [17]. 

• The Flood Directive 2007/60 invites all relevant authorities to develop Flood Risk Management Plans (FRMPs). Although FRMPs should take into account the characteristics of the particular catchment area and include the promotion of sustainable land use practices, the Flood Directive does not specify what kind of water retention measures are preferable. However, natural water retention measures are mentioned in the text and can be considered a type of nature-based solution (NBS). These measures have the potential to reduce extreme flow discharge and thereby help to level out extreme rain, flood events, etc. Several public authorities at the local and regional levels have implemented NBS to cope with floods in a sustainable way (e.g., relocating dikes, using floodplain forests). However, the uptake of NBS to mitigate flood risks is low, determined by different factors such as traditions, insufficient awareness of the benefits, lack of experience to scale up solutions, lack of capacity to manage or carry out NBS projects, limited financial resources or lack of evidence on the effectiveness and long-term impacts of NBS as compared to structural measures. The Flood Directive suggests consideration of these measures but does not include any obligations [15]. 

At local level, cities may refer to diverse strategies/norms and laws. Climate adaptation and mitigation plans and strategies developed at local level could set criteria in accordance with possible increased risk of flooding and then boost the development of floodable parks within the city. 

Project governance and implementation modalities:  

• Inclusive and participatory planning and design of floodable parks can boost the use of such areas thus increasing the co-benefits generated in terms of physical health and wellbeing.  

• Stakeholder collaboration is crucial to support and co-fund such type of NBS. For instance, in the case of the Confluence park in Prague, the local municipalities signed the Memorandum of Joint Collaboration for Development and Future Use of the Confluence Area and the Prague Institute of Planning and Development (IPR PRAHA) created a catalogue of more than 30 projects and an investment plan for the area sharing them with all interested parties [16]. 

Technical aspects/infrastructure: 

• Specific expertise would be needed in choosing flood tolerant plants and the amount of ground excavated.  

• Water retention spaces should be lower than the surrounding area with some elevation change [5].  

• Geospatial information can be used to understand and model the effectiveness of the floodable park to be developed. 

Urban form and layout, Climate and geography: 

Land use change is considered one of the main causes of the increasing severity of floods and the waterproofing of the soils, increasing overland flow and reducing infiltration, thereby bypassing the natural storage and attenuation of the subsurface [13]. For this reason, planners should consider existing land use constrains, urban form and layout and possible de-sealing option in critical flood prone areas.  

Funding and financing: 

In the short term, floodable parks have a higher cost of implementation with respect to traditional grey infrastructure, while in the long run, cost effectiveness analysis could support a broader integration of co-benefit evaluation. A research funded by the OPERANDUM project analysed various nature-based solutions (NBS) used for flood prevention, among those floodable parks and flood prone areas. The common benefits considered include prevented damage costs, omitting infrastructures, profit loss to businesses, prevented erosion damage, and prevented agricultural losses, while  life cycle costing approach (LCC) and Return on Investment (ROI) are less explored options [14]. An alternative method for evaluation of NBS is multi-criteria analysis (MCA) that integrates quantitative environmental and economic benefits and qualitative social benefits. This approach may support decision makers when there are multiple and conflicting criteria to be considered [15]. 

Project governance and implementation modalities: 

Implementation of floodable parks can be constrained by factors such as traditions, insufficient awareness of the benefits, lack of experience to scale up solutions, lack of capacity to manage or carry out NBS projects, or lack of evidence on the effectiveness and long-term impacts of NBS as compared to structural measures.  

Literature 

[1] Pires Veról, A. et al. 2019. The urban river restoration index (URRIX) - A supportive tool to assess fluvial environment improvement in urban flood control projects. Journal of Cleaner Production, Volume 239, 1 December 2019, 118058. https://doi.org/10.1016/j.jclepro.2019.118058. 

[2] Bigate Lourenço, J. et al. 2020. Land as a sustainable resource in city planning: The use of open spaces and drainage systems to structure environmental and urban needs. Journal of Cleaner Production, Volume 276, 10 December 2020, 123096. https://doi.org/10.1016/j.jclepro.2020.123096. 

[3] Bertilsson, L. et al. 2019. Urban flood resilience – A multi-criteria index to integrate flood resilience into urban planning. Journal of Hydrology Volume 573, June 2019, Pages 970-982. https://doi.org/10.1016/j.jhydrol.2018.06.052.  

[4] Rosenzweig B. R. et al. 2020. The Value of Urban Flood Modeling. Earth's Future, 9. https://doi.org/10.1029/2020EF001739. 

[5] Sarneel JM, et al. 2019. Alternative transient states and slow plant community responses after changed flooding regimes. Glob Chang Biol. 2019 Jan 14;25(4):1358–67. https://doi.org/10.1111/gcb.14569. 

[6] Valença Pinto, L. 2021. Assessment of NBS Impact on Pluvial Flood Regulation Within Urban Areas: A Case Study in Coimbra, Portugal. In: Ferreira, C.S.S., Kalantari, Z., Hartmann, T., Pereira, P. (eds.) Nature-Based Solutions for Flood Mitigation. The Handbook of Environmental Chemistry, vol 107. Springer, Cham. https://doi.org/10.1007/698_2021_769. 

[7] Pagano et al. 2019. Engaging stakeholders in the assessment of NBS effectiveness in flood risk reduction: A participatory System Dynamics Model for benefits and co-benefits evaluation. Science of The Total Environment, Volume 690, 10 November 2019, Pages 543-555. https://doi.org/10.1016/j.scitotenv.2019.07.059.  

[8] Quyen Le, T. et al., 2019. Flood‐resilient urban parks: Toward a framework. Area. 51. 10.1111/area.12543. https://doi.org/10.1111/area.12543. 

[9] Beceiro P. et al. 2020. The Contribution of NBS to Urban Resilience in Stormwater Management and Control: A Framework with Stakeholder Validation. Sustainability 2020, 12(6), 2537. https://doi.org/10.3390/su12062537. 

[10] Langergraber, G, et al. 2021. Framework for Addressing Circularity Challenges in Cities with Nature-Based Solutions. Water 2021, 13, 2355. https://doi.org/10.3390/w13172355.  

[11] Shutes, R., et al. 2010. Constructed wetlands for flood prevention and water reuse. In: 12th International Conference on Wetland Systems for Water Pollution Control, 4th-8th Oct 2010, Venice. http://library.oapen.org/handle/20.500.12657/30978. 

[12] EEA, European Environmental Agency. 2010. Mapping the Impacts of Natural Hazards and Technological Accidents in Europe: An Overview of the Last Decade. EEA Technical report, 13/2010. https://www.eea.europa.eu/publications/mapping-the-impacts-of-natural.  

[13] Wheater, H. and Evans, E. 2009. Land use, water management and future flood risk, Land Use Policy, Volume 26, Supplement 1, 

2009, Pages S251-S264, https://doi.org/10.1016/j.landusepol.2009.08.019. 

[14] Ruangpan L., et al. 2020. Nature-based solutions for hydro-meteorological risk reduction: a state-of-the-art review of the research area, Nat. Hazards Earth Syst. Sci., 20, 243–270. https://doi.org/10.5194/nhess-20-243-2020.  

[15] Alves, A., et al 2016. A Model-based Framework for Selection and Development of Multi-functional and Adaptive Strategies to Cope with Urban Floods, Procedia Eng., 154(December), 877–884. https://doi.org/10.1016/j.proeng.2016.07.463. 

Websites 

[16] Oppla, CONFLUENCE Project: Creating a Periurban Park in Prague, https://oppla.eu/casestudy/18911. 

[17] European Commission, The Green City Accord, https://environment.ec.europa.eu/topics/urban-environment/green-city-accord_en. 

[18] Tredje Natur, Enghaveparken – Climate park, https://www.tredjenatur.dk/en/portfolio/enghaveparken-climate-park/.  

The main impact of floodable parks is to reduce risk to natural and climate hazards (i.e., Heavy rainfall; Flash / surface flood; River flood; Droughts & water scarcity; Storms).  

The impact is related to the quantity of water that could be effectively stored and retained by the areas in case of extreme weather events or river floods and the reduction of flood velocity in urban areas. The reduction of imperviousness increases water infiltration and water retention capacity of soil. As shown in the Quinta de São Jerónimo study site, for instance, the implemented solution can retain runoff produced by 20-years flood, decreasing the flood peak and flood hazard in downstream urban areas [6].  

Greenhouse gas reduction would depend on the type and density of the vegetation integrated in the floodable park. 

Integrated land use and urban planning  

Land use planning regulation 

Geospatial information 

LCA/LCC 

User engagement 

Projects 

URBAN GreenUP aims at the development, application and replication of renaturing urban plans in a number of European and non-European partner cities to mitigate the effects of climate change, improve air quality and temperature, manage water, as well as to increase the sustainability and liveability of cities through innovative nature-based solutions. The project is implemented in Izmir (Turkey), Liverpool (UK), Valladolid (Spain), Chengdu (China), Ludwigsburg (Germany), Mantova (Italy), Medellin (Colombia) and Quy Nhon (Vietnam), and involves a number of other cities in its activities (H2020, 2017-2022). 

OPERANDUM delivers tools and methods for the demonstration and market uptake of nature-based solutions to reduce hydro-meteorological risks. Site-specific and innovative solutions are co-designed, co-developed, deployed, tested and demonstrated with partners and local stakeholders in open-air laboratories (H2020, 2018-2022). 

NATURVATION seeks to develop understanding of the potential of nature-based solutions in cities. Among other activities, the project created an Urban Nature Atlas, a comprehensive database of nature-based solutions for cities. The atlas is based on a systematic survey of nature-based solution interventions in 100 European cities (H2020 project, 2017-2022). 

Oppla is the EU Repository of Nature-Based Solutions with the purpose of simplifying how we share, obtain and create knowledge to better manage our environment. It is an open platform that is designed for people from science, policy and practice; public, private and voluntary sectors; organisations large and small, as well as individuals.