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The world¡¯s coastal cities are some of the most rapidly growing centers of human activity. Sprawling formal and informal settlements are being built at the expense of the ecosystems that act as a buffer against the forces of floods and hurricanes. These ecosystems, including wetlands, mangroves, dunes and coral reefs, can be effective in reducing disaster risk for vulnerable communities. In  for example, mangroves mitigate the impact of flooding for more than half a million people per year ¨C many of whom are living in poverty ¨C and avert more than $1 billion in damages. Similarly, coastal wetlands on the US East Coast  from Hurricane Sandy by an estimated $625 million.

Until recently, most flood risk management interventions involved conventional engineering measures. These measures are sometimes referred to as ¡°hard¡± engineering or ¡°gray¡± infrastructure. Examples include building embankments, dams, levees, and channels to control flooding. Currently, nature-based solutions are gaining momentum as an approach to manage disaster risk given the wide range of benefits they can provide. Nature-based solutions can be completely ¡°green¡± (i.e. consisting of only ecosystem elements) or ¡°hybrid¡± (i.e. a combination of ecosystem elements and hard engineering approaches). A growing body of evidence demonstrates that such nature-based solutions can  and simultaneously create a broad value chain for other sectors, including tourism, environment and housing, and provide livelihoods for local communities. The capacity of mangroves to absorb carbon is also much higher than terrestrial ecosystems. The combination of economic, social and environmental benefits make green infrastructure an important solution for resilient coastal and urban landscapes.

For example, the restoration and expansion of fringing coral reefs ¨C as is being implemented with World Bank support in  ¨C can be an alternative or addition to the construction of tropical breakwaters for coastal flood management. Coral reefs are often much cheaper than breakwaters, can reduce waves by 92%, and offer large potential for tourism income, livelihoods for local fisheries, as well as important biodiversity benefits. Similarly, the integration of parks and other green spaces in urban development can help reduce peak runoff and , while improving quality of life, , and increasing land value. Beyond flooding, nature-based solutions are effectively applied to address mountain hazards, such as .

Types of nature-based solutions that are being applied by growing flood-prone cities.

ľ¹ÏÓ°Ôº has been piloting nature-based solutions in its disaster risk management operations. Between 2012 and 2019, nearly 70 projects included some use of nature-based solutions, but these have so far been limited to relatively small-scale individual case-studies. As the Bank¡¯s work in making cities more resilient to rising climate and disaster risk gains pace, so does the potential for considering, assessing and costing nature-based solutions as feasible options for better risk management.

As with conventional engineering solutions, the effective application of nature-based solutions requires a comprehensive assessment, implementation and monitoring process. It requires an understanding of the drivers of risk as well as the functioning of the ecosystems that can be protected, expanded or constructed. The design of a balanced risk management strategy using nature-based solutions requires a long-term time horizon and a large spatial scale, which poses challenges for government priorities, budgets and procurement systems.

New guidance has been developed to facilitate the integration of nature-based solutions in the disaster risk management operations of the World Bank and other development agencies ¨C together with partners from Deltares, UNDP and Ecoshape ¨C developed a set of key principles and implementation guidance. The , which was developed by GFDRR, the World Bank, Deltares, UNDP and Ecoshape, outlines key principles and provides implementation advice for technical teams working on projects to reduce disaster risk and build urban resilience. It was jointly funded by the  (PROFOR) and the  (GFDRR), and benefited from inputs and review of major engineering and environmental partners, including the Army Corps of Engineers, IUCN and UNEP. The guidance note is also available in  and .

Alongside the guidance note, an interactive web platform has been developed which hosts an extensive overview of nature-based solution projects of the World Bank and external partners. This platform allows for the exploration, comparison, and analysis of nature-based interventions across various hazards and ecosystems.

The case for inclusion of nature-based solutions in the range of risk management options is clear. While they may not be a competitive or feasible option everywhere, there is compelling evidence of effectiveness and sustainability of these interventions to reduce disaster risks and protect vulnerable people worldwide.

For more information, contact Brenden Jongman (bjongman@worldbank.org) or Simone Balog-Way (sbalogway@worldbank.org)