Nature-Based Solutions for Flood Control

What are nature-based solutions for flooding?

In 2022 the UN Environment Assembly agreed a definition of nature-based solutions: actions to protect, conserve, restore, sustainably use and manage natural or modified ecosystems, so that they address social, economic and environmental challenges while benefiting people and biodiversity.

Applied to flooding, nature-based solutions slow water down, store it, let it infiltrate, reduce the energy of waves and surges, and create room for water where it causes the least damage. In the United Kingdom this approach is known as natural flood management (NFM), or “working with natural processes”.

Examples of nature-based solutions for flooding

• In catchments and upstream areas: woodland on slopes, along rivers and on floodplains; leaky barriers made of wood in small streams; runoff attenuation features that temporarily hold water; and restored floodplains. The Environment Agency lists these among its “working with natural processes” measures.
• Along rivers: giving rivers more room by relocating dikes, lowering floodplains and digging high-water channels. In the Netherlands, the Room for the River programme applied this at 34 locations and was completed in 2019.
• On coasts: saltmarshes, dunes and mangroves that reduce wave energy, combined where needed with engineered defences.
• In cities: green roofs, permeable surfaces, urban wetlands and water squares that store rainwater temporarily, often combined with rainwater harvesting.

The sections below show how these approaches apply on coasts, inland and in cities.

What the evidence says, and its limits

The Environment Agency’s evidence review concludes that working with natural processes typically reduces flood risk for smaller floods in small to medium catchments, complements rather than replaces traditional engineering, almost always brings other environmental benefits, and is still largely backed by modelled rather than observed data.

For large floods and large catchments, nature-based measures alone rarely provide a guaranteed protection level. That is why they are usually combined with engineered measures.

How nature-based and engineered (grey) measures work together

The World Bank and the World Resources Institute show that combining natural systems, such as floodplains and forests, with conventional “grey” infrastructure, such as dikes, reservoirs and pumps, can improve performance and resilience and lower costs compared with either approach alone.

In practice this can mean a restored floodplain that reduces peak flows, combined with a lower dike downstream; a mangrove belt in front of a coastal defence; or retention in the catchment, combined with temporary measures at critical points while permanent works are prepared.

Monitoring and early warning tie these measures together: they tell operators when storage is filling up and when temporary measures need to be deployed.

Sources

• UN Environment Assembly, Resolution 5/5: Nature-based solutions for supporting sustainable development (2022).
• Environment Agency, Working with Natural Processes: evidence summaries (2017).
• Rijkswaterstaat, Room for the River: measures in and around rivers.
• World Bank and World Resources Institute, Integrating Green and Gray: Creating Next Generation Infrastructure (2019).