The time and place for nature-based solutions in flood risk management

Nature-based solutions (NBS) are a key consideration in nearly all flood risk management and other large development projects nowadays. Taking inspiration from and integrating with nature, they can manage, reduce and remove flood risk from communities and developments.

They take many forms, from natural flood management (NFM) measures such as leaky dams or peatland restoration to sustainable urban drainage systems (SUDS) such as rain gardens or green roofs. A scheme needs targeting to a specific development or need, not all will be suitable for all locations, but with such a wide array of different solutions, an option can be found to meet a range of goals in different ways for different places.

How natural flood management makes a difference

Fathom has built datasets and hydraulic models that cover the globe, with detail on topography, river networks, rainfall, river flows and climate change. By combining this data with their expertise in large-scale rapid modelling and working closely with partners specializing in nature-based solutions, they have been able to test benefits that could be gained through the implementation over large regions and catchments. In particular, they have studied land use in urban areas, assessing which nature-based solutions would be most suitable by analysing the impact on hydrology within the catchment, such as slowing runoff, increasing filtration and increasing storage. These changes were then compared against the baseline to provide the impact of nature-based solutions to flooding – and the subsequent impact on the population, property and damage values. 

One study in particular involved exploring the value of urban greening for Dar es Salaam. With rapid urbanization expected to take place in cities across the African continent in the coming years, coupled with rising temperatures, the project was commissioned to help future-proof the city against current and future flood and climate risks. In particular, the aim of the research was to quantify the benefits of implementing nature-based solutions in an urban landscape. The project involved mapping the local terrain, projecting future urbanization patterns and running simulations of current flooding versus flooding with NBS in situ. The assessment showed that NBS can reduce flood extents significantly, and they work well at reducing flooding in regular, low return period events, which research has shown cause some of the highest damage costs over long periods. As well as material costs, this regular, nuisance flooding can have some of the highest impacts on people’s lives and well-being, and resolving flooding at this level should be considered some of the most essential to achieve. Interventions such as rain gardens and tree planting proved beneficial not just to reduce flooding but also as methods of reducing the urban heat island effect – when an urban area becomes significantly warmer than rural areas. 

Challenges in nature-based flood management

The results of this assessment in East Africa clearly show the value of NBS interventions. However, these and similar studies have also shown that NBS is not a cure-all against flooding in all locations. NBS can generally only provide modest volumes of flood storage or reductions in flow. This means that extreme events can often overwhelm solutions, and continue to cause damaging flooding. The Environment Agency advises that nature-based solutions are a method to reduce flood risk and need to be considered within a whole catchment approach to effectively reduce flood risk to communities. With a changing climate, we are expecting to see an increase in extreme rainfall events, and often an increase in extreme river flows; these extreme events are already the ones that NBS often only provides limited benefit, and with an increased intensity of extreme events questions could be asked if NBS is the right solution. 

Whilst a lot of focus (quite rightly) is on climate change, and the increases it will bring to flood risk, population and development growth in existing flood plains will also be one of the biggest drivers in flood vulnerability going forward. Where growth is happening in these areas already at risk from extreme flood events, NBS is only likely to provide a reduction in the risk, and the vulnerability will remain high, or continue to grow with other growth.

Wider considerations

NBS should not be considered for its flood risk management properties alone. It nearly always brings additional benefits, such as wildlife habitat restoration, water quality improvements, urban realm improvements or carbon storage. As well as all being cornerstones of sustainable development, these additional benefits can also open additional lines of funding and support that may enable an NBS scheme to be achieved where the development of traditional flood risk management schemes has failed.

However, alongside this comes new challenges such as different land take, building and maintenance requirements compared to traditional solutions, and working to find the correct legal, regulatory and funding frameworks to implement them. The benefits provided are often measured in different ways, and development frameworks may not always reflect this.

NBS in flood risk assessment

As a community of water and environmental managers, we are at the forefront of flood risk management and sustainable development. NBS may be seen in a positive light, but we must remember that they are not the solution to all problems but are a key tool that we can use to help us meet goals in both of these areas. It is important that they are considered within a wider assessment, looking at how a catchment is expected to change, with climate change, development, NBS and other flood risk management solutions. This evidence is essential for paving the way for the development of both individual schemes, as well as the framework and regulation to support the next generation of flood risk management that is needed in a changing world.