Pre Conference Forum


Urban flooding risk reduction: Is nature-based infrastructure a viable solution?

According to United Nations (2015)  estimates, two thirds of the world’s population will be living in urban areas by 2050.  Leaving aside the fact that urban centers are commonly situated around areas of inherent vulnerability against natural hazards (like rivers), the mass population migration to urban environments forces the growth of informal settlements: with land under pressure, these settlements develop in floodplains, coastal zones, steep hills or other areas exposed to natural hazards such as flooding, landslides or sea-level rise.

Along with the huge humanitarian impact that these natural hazards entail, the global economy is also at stake. Sixty percent (60%) of the global gross domestic product (GDP) is produced by only 600 urban centers, according to McKinsey (2011), and this concentration of economic activity in urban areas is expected to grow with years. The increasing exposure of urban populations to natural hazards inflicts greater global economic losses.

To make things worse, climate change will continue for many decades, bringing along an increase in the frequency and severity of climate-related extreme events around the globe, with flooding being the most frequent and damaging of all. Over the past 20 years, 90% of all disasters have been weather-related, and of these, flooding accounts for almost half (UFCOP, 2016). Going only as far back as last summer, extended flooding in China caused the death of 116 people and affected thousands more, entering the top 5 of costlier weather-related events globally for 2017, with $7.5 billion of total economic loss (Aon Benfield, 2017). Beyond such mega-events, however, hundreds of floods around the world each year affect population and increase economic losses.

As the frequency and adverse impacts of flooding grow, new impetus has been given to the development of improved policies and techniques for successful flood risk management. Conventional engineering measures (termed as ‘gray’ or ‘hard’ infrastructure) such as dams, levees or floodwalls, are no longer considered sufficient. Although necessary in many circumstances, grey infrastructure is not adaptive and usually serves as a one-purpose solution, while in some cases it can actually increase flood risk—for example, when disconnecting rivers from their surrounding floodplains (Nature Conservancy, 2014). The loss of ecosystem services (for example the purification of water) or the loss in landscape value come as additional concerns on the socio-environmental impact of grey infrastructure solutions. Last but not least, grey infrastructure comes with a significant cost. In the aftermath of Hurricane Katrina in 2005, New Orleans spent about $14 billion to reinforce its levee system (UFCOP, 2016). Evidently, it’s not always affordable for communities to spend such amounts in order to build – or rebuild – their flood protection systems; especially when it comes to countries of the developing world.

This inequality between efficacy and costs of hard infrastructure against flood risk mitigation has led to the exploration of alternatives, in order to address the matter via a more integrated approach. More sustainable solutions receive attention and appreciation among policymakers (EEA, 2017) and as such, there is an increasing momentum for the use of nature-based measures as part of a resilience-based strategy regarding flood disaster risk mitigation. Nature-based solutions (including among others street side swales, porous pavements, floodplains restoration or re-meandering), commonly termed as green infrastructure, are based on natural processes in order to mitigate the hazard imposed by floods. Green infrastructure (GI), for example, has the ability to reduce the risk of urban flooding by controlling surface runoff.

GI may be used either as an alternative or complementary solution to hard infrastructure measures, with the latter resulting to ‘hybrid’ systems, such as the restoration and preservation of a wetland in a floodplain and relocation of a dike. During the past years, such measures have been taken on board by financially advanced countries, while lately, interest is also rising among developing countries, where urbanization is more intense. Green infrastructure solutions can mitigate flood risk, while being cost-efficient, both in terms of investment and maintenance costs: they can be up to 30% cheaper to construct and 25% less costly over their life span compared to traditional gray infrastructure (Kloss and Calarusse 2006; Garrison and Hobbs 2011). Apart from the evident economic and safety impacts, GI comes also with a range of significant additional social-environmental services. Such measures meet the regulatory requirements of flood protection, biodiversity conservation or water quality improvement, as these are specified by the environmental policies of countries worldwide. A good example of GI is a healthy floodplain ecosystem which not only provides flood prevention but also delivers water filtration and groundwater recharge, as well as recreation opportunities, carbon storage, timber, areas of rich biodiversity and interconnected wildlife refuges (EEA, 2017).

And while global policymakers strive towards the issue of principles and standardized implementation guidance for sustainable and effective GI (NOAA, 2016; EEA, 2017; World Bank, 2017), barriers still exist for its broad implementation. Dissemination of knowledge among stakeholders and awareness of decision-makers regarding the benefits, costs and GI funding opportunities is still lacking. The level of coordination between authorities of upstream and downstream communities of river basins is also identified by the EEA as key inhibiting factor. Solutions are viable, but stakeholders need to collaborate and take the leap forward, investing in the adaptive GI alternative.



European Environment Agency (2017). Promoting cost-efficient flood risk reduction via green infrastructure solutions.

Garrison, N., and Hobbs, K. (2011). Rooftops to Rivers II: Green Strategies for Controlling Stormwater and Combined Sewer Overflows. Natural Resources Defense Council.

Kloss, Ch., and Calarusse C. (2006). Rooftops to Rivers: Green Strategies for Controlling Stormwater and Combined Sewer Overflows. Natural Resources Defense Council.

McKinsey and Company (2011). Urban World: Mapping Economic Power of Cities. McKinsey Global Institute.

Nature Conservancy. 2014. “A Flood of Benefits—Using Green Infrastructure to Reduce Flood Risk.” Nature Conservancy, Arlington, VA.

NOAA (2015). A Guide to Assessing Green Infrastructure Costs and Benefits for Flood Reduction. National Oceanic and Atmospheric Administration: Office for Coastal Management.

United Nations (2015). Issue Brief: Reducing Disaster Risk in Urban Settings, Ministerial Roundtable. World Conference on Disaster Risk Reduction, Sendai, Japan, March 14–18, 2015.

Urban Floods Community of Practice [UFCOP] (2016). The Role of Green Infrastructure Solutions in Urban Flood Risk Management.

World Bank (2017). Implementing nature-based flood protection: Principles and implementation guidance. Washington, DC: World Bank.


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