Better water management, Reduced risk of environmental disasters: Water management avoids floods, e.g. retention trenches and swales are effective to prevent urban flooding, together with drainage systems (UNaLab). 

Increased green space coverage and quality: Water retention solutions provide urban green, which is acknowledged to improve citizens’ wellbeing and the perceived quality of life in the area. [10] (UNaLab). 

Decreased maintenance costs: Fewer damages caused by heavy rain (Rainman). CENTAUR provides an innovative and cost-effective local autonomous sewer flow control system to reduce urban flood risk and to decrease maintenance costs. 

Increased water security: Since heavy rains are not increasing pressure on urban wastewater management (CWC). 

Grey water treatment (including NBS) and reuse 

Rain water retention e.g. on roofs 

Vertical green infrastructure: Green fences, Green noise barriers, vertical mobile gardens 

Shading structures: Green covering shelters, green shady structures 

Allotments, community gardens, floating gardens 

Green filter area 

Urban garden bio-filter 

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

Urban green areas connected to grey infrastructure 

Hard-drainage flood prevention 

Grassed swales and water retention pounds 

Floodable park 

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

Sustainable Urban Drainage (SUD) systems 

Irrigation and maintenance technologies 

Constructed wetlands 

Rain gardens 

Green roofs/green facades 

Pre-conditions 

Technical aspects/infrastructure:  

Implementing urban water management systems and retention solutions calls for certain technical considerations like load factors for pavements or roofs, or dimensioning of retention or water harvesting areas. Technical integration of different systems and parts of the infrastructure requires technical skills.  

Climatic/geographic context:  

Water retention systems are most efficient in mild and warm climates. In cold climates, the frost period prevents partially the retention capacity. This is typical in early winter periods and in early spring periods when the temperature fluctuates a lot. 

Enabling Conditions 

Funding and financing:  

Access to EU/public funding can be crucial to support the initial investment for the new solutions and also to encourage urban green for public procurement (CENTAUR, UNaLab).  

Policy and regulatory/legal framework:  

It is important that public procurement takes into account the urban green aspect and not only the water retention aspect.  

Project governance and implementation modalities:  

Local water utilities should focus on technical aspects as well as their governance, such as in the Stop-IT project, which created material for decision makers, risk officers and modellers, as well as real-time operation and maintenance managers. 

Funding and financing:  

• Urban nature-based water management systems have in some cases slightly higher investment cost and maintenance costs compared to traditional rainwater management systems [11]. 

• A challenge related to funding is that the benefits associated with nature-based systems cannot be capitalized by any organisation or company, which leads to the problem of ownership. Therefore, the new types of innovative partnerships and business models are needed [12]. E.g., public-private-partnerships can be an effective tool for development.  

• Although the benefits of nature-based solutions are well renowned, lack of finance is still an issue [11]. 

Technical aspects/infrastructure:  

Availability of urban space or existing infrastructure can limit implementation. Designing retention solutions and water management systems, especially integrating new and existing structures, requires specialised skills. Technical guidebooks and best practices are available (e.g. in Rainman, UNaLab and CWC). Intensive green roofs require irrigation and space for rooting (although the growing media is relatively thick). Permeable surfaces have limited load on paved area (UNaLab). 

Some solution-specific potential barriers according to [9]: 

• Retention ponds 

• Land take may limit use in high density sites 

• May not be suitable for steep sites, due to requirement for high embankments 

• Infiltration trenches  

• Limited to relatively small catchments 

• Swales 

• Not suitable for steep areas or areas with roadside parking 

• Limits opportunities to use trees for landscaping 

• Risks of blockages in connecting pipe work 

• Infiltration reservoir roadway and Infiltration trenches 

• Need to give careful consideration to crossings 

• Infiltration basin 

• Comprehensive geotechnical investigations required to confirm suitability for infiltration 

• Not appropriate for draining pollution hotspots where high pollution concentrations are possible 

• Requires a large, flat area. 

• Soakaways 

• Not suitable for poor draining soils 

• Not suitable for locations where infiltration water may put structural foundations at risk, or where infiltrating water may adversely affect existing drainage patterns 

• Not appropriate for draining polluted runoff 

• Increased risk of groundwater pollution 

• In extensive green roofs: Limited spread of flora and fauna because of regular maintenance and management in extensive green roofs. [8] 

• Retention ponds [9]: 

• No reduction in runoff volume 

• Anaerobic conditions can occur without regular inflow 

• Colonisation by invasive species could increase maintenance 

• Perceived health & safety risks may result in fencing and isolation of the pond. 

• Infiltration trenches[9]: 

• High clogging potential if there is not an effective pre-treatment  

• Build-up of pollution difficult to see 

• High historic failure rate due to poor maintenance, wrong siting or high debris input 

• Infiltration reservoir roadway and Infiltration trenches [9]: 

• Incorrect planting can cause silt build up 

• Infiltration basin [9]: 

• Potentially high failure rates due to improper siting, poor design and lack of maintenance, especially if appropriate pre-treatment is not incorporated 

• Soakaways [9]: 

• Some uncertainty over long-term performance and possible reduced performance during long wet periods 

• Where the property owner is responsible for operation and maintenance, performance is difficult to guarantee. 

Literature 

[1] Szewrański, S.; Chruściński, J.; Kazak, J.; Świąder, M.; Tokarczyk-Dorociak, K.; Żmuda, R. Pluvial Flood Risk Assessment Tool (PFRA) for Rainwater Management and Adaptation to Climate Change in Newly Urbanised Areas. Water 2018, 10, 386. https://doi.org/10.3390/w10040386. 

[2] Szymon Opania and Paulina Gama Marques, 2021, Analysis of Water Retention Possibilities Based on Programs, Strategies and Selected Projects in Poland, IOP Conf. Ser.: Mater. Sci. Eng. 1203 032103. https://iopscience.iop.org/article/10.1088/1757-899X/1203/3/032103/meta 

[3] Godyń, I.; Grela, A.; Stajno, D.; Tokarska, P. Sustainable Rainwater Management Concept in a Housing Estate with a Financial Feasibility Assessment and Motivational Rainwater Fee System Efficiency Analysis. Water 2020, 12, 151. https://doi.org/10.3390/w12010151.  

[4] Tiantian Dou, Stéphane Troesch, Alain Petitjean, Pálfy Tamás Gábor, Dirk Esser, Wastewater and Rainwater Management in Urban Areas: A Role for Constructed Wetlands, Procedia Environmental Sciences, Volume 37, 2017, Pages 535-541, ISSN 1878-0296. https://doi.org/10.1016/j.proenv.2017.03.036.  

[5] UNaLab, Performance and Impact Monitoring of Nature-Based Solutions, Deliverable, D3.1, 2019.  

[6] CWC City Water Circles, transnational online handbook on circular urban water management and use, Thematic Catalogue 1, Smart assessment tools for potentials’ mapping of urban water use, 2021.  

[7] Sustainable and integrated rainwater management, A philosophy and a toolbox of alternative techniques, Elia Desmot, Officer, Adopta, Panel 3: Design and Best Practices. Accessed via https://www.interreg2seas.eu/nl/wrc . 

[8] UNaLab, Nature Based Solutions – Technical Handbook, Part II, 2019. 

[9] Urban water management, Creating climate-resilient cities, Version 1, July 2022, IWA publications.   

[10] Vincenzo Giannico, Giuseppina Spano, Mario Elia, Marina D’Este, Giovanni Sanesi, Raffaele Lafortezza, Green spaces, quality of life, and citizen perception in European cities, Environmental Research, Volume 196, 2021. https://doi.org/10.1016/j.envres.2021.110922  

[11] Seddon N, Chausson A, Berry P, Girardin CAJ, Smith A, Turner B. 2020 Understanding the value and limits of nature-based solutions to climate change and other global challenges. Phil. Trans. R. Soc. B 375: 20190120. http://dx.doi.org/10.1098/rstb.2019.0120  

[12] UNaLab, Business Models & Financing Strategies, Deliverable, D6.3. 

Websites 

[13] SUSDRAIN community for sustainable drainage, SuDS Components  

[14] WATERMED 4.0, https://www.watermed-project.eu/  

The impact of retention solutions can be measured by the following indicators and metrics [5]: 

• Runoff in relation to rainfall  
  Metric: Run-off in relation to precipitation quantity (mm/%) 

• Infiltration capacity  
  Metric: Infiltration capacity (%; change in precipitation infiltration capacity measured using ring infiltrometer and extrapolated/modelled for full unsealed area) 

Nature based solutions reduce the need for energy-intensive materials and technologies for water retention, e.g., plastic ducts, concrete structures, or other energy and fossil raw materials. They also reduce the need to treat the wastewater and thus contribute to the reduction of CO2 emissions [9]. 

Nature based solutions are also an important tool for carbon sequestration, actively contributing to lowering the CO2 levels in the atmosphere [9]. 

Retention ponds 

• Regulate heavy rain 

Permeable surfaces 

• Water quality protection 

• Storm water management 

• Reduced surface runoff 

• Controlled infiltration 

• Temporary water storage 

• Water filtering 

• e.g. porous asphalt can [8] 

• Reduce storm runoff by 70-90 % 

• Improve water quality (removes 85-95% suspended solids, 65-85% phosphorus, 80-85% nitrogen 

• 30% nitrate, up to 98% metals) 

Green roofs (see more details in rain water retention factsheet) [8] 

• Water retention capacity: 20-160 l/m² 

• Weight 20-680 kg/m2 

Trainings, e.g., solutions, best practices 

Infopoints, e.g., solutions, best practices, contact information of different product and solution providers 

Awareness campaigns, e.g., webinars on best practices, available funding 

City coaching, e.g., monthly webinars on best practices 

Voluntary measures with stakeholders, e.g., reporting the number of retention solutions installed in a certain area 

Public procurement rules favouring nature based rainwater management 

CWC City Water Circles focuses on water saving, water harvesting and grey water recovery through updating the municipal water infrastructure systems. The aim is to create new tools for urban water management and governance (EU Interreg, Central Europe, 2019-2022). 

Rainman aimed to reduce the losses in the natural and built environment caused by heavy rain. The project supported municipalities and regions in coping with the hazards of heavy rain and to mitigate heavy rain risks. The project developed a toolbox, which includes information on heavy rain management and solutions (EU Interreg, Central Europe, 2017-2020). 

CENTAUR Cost-Effective Neural Technique to Alleviate Urban flood Risk provided an innovative and cost-effective local autonomous sewer flow control system to reduce urban flood risk. CENTAUR was developed and tested using computer models and a laboratory test facility at the University of Sheffield. In 2017, a pilot system was installed and tested in Coimbra, Portugal. In 2018, a demonstration site in Toulouse, France was installed and tested (Horizon 2020, 2015-2018). 

Stop-IT focuses on the strategic, tactical and operational protection of critical water infrastructures against physical and cyber threats. The focus is on identification of risks and co-development of an all-hazards risk management framework for the physical and cyber protection of critical water infrastructures (Horizon 2020, 2017-2021). 

UNaLab aims to develop smarter, more inclusive, more resilient and increasingly sustainable cities through the implementation of nature-based solutions such as green roofs, bioswales, and water retention ponds. It focuses on water management systems and on how to monitor and collect water condition information for urban water management platforms. The project includes also cold climate solutions. The project has three front-runner cities, Tampere (Finland), Eindhoven (The Netherlands) and Genova (Italy), and seven follower cities, Başakşehir (Turkey), Cannes (France), Castellón de la Plana (Spain), Prague (Czech Republic) and Stavanger (Norway), Buenos Aires (Argentina) and Hong Kong (Hong Kong) (Horizon 2020, 2017-2022).