Flood management: more concrete or more nature?

Emma Wren says nature can help us reduce flooding and improve the climate resilience of our communities.

Global economic losses due to flooding totalled US$82bn in 2021, and floods accounted for 31% of losses from natural catastrophes. The latest report from insurers Swiss RE also found that flooding in Europe in July last year was the costliest natural disaster on record in the region, and that across the world in 2021 there were more than 50 severe flood events. The record-breaking floods that hit eastern Australia in February and March this year engulfed more than 20,000 homes and businesses in Queensland, and damaged more than 5000 homes in New South Wales. According to the Australian Climate Council, the total value of insurance claims from these events will exceed those of previous floods.

Climate change is expected to cause more devastating floods. Is the answer to build more and higher traditional flood defences? Or should we be looking for alternatives? What if nature was part of the solution?

More than a flood defence

Natural flood management (NFM) works with natural processes to reduce flood risk, often across a catchment area.

NFM is essentially focused on protecting functioning natural environments, recovering those that are degraded and replicating them where they have been lost. Protecting the natural distribution and circulation of water or its hydrologic processes includes ensuring soil can support water infiltration, retention and movement, and a river is free to meander and spill onto a floodplain during periods of high flows. Restoring good hydrological processes involves things like rehabilitating upland peatland to reduce runoff and mimicking what occurs naturally, such as creating offline water storage areas and installing leaky barriers in rivers – these are woody dams and mirror naturally occurring obstructions, such as fallen trees and dams built by wildlife.

These measures will reduce flood risk downstream as well as facilitate groundwater recharge and provide long-term water storage for use during periods of drought. However, what sets NFM apart from traditional engineered flood defences, such as walls and weirs, is its capacity to produce long lasting benefits beyond flood risk management. These are called co-benefits and include improved water quality and soil health, and habitat compensation and carbon sequestration – helping to address the climate and ecological emergency facing the world. And, where these measures are also designed to encourage people to be active and to explore nature, they support physical and mental wellbeing.

Delivering the right solutions

In England, the Environment Agency is an advocate for NFM, and at Mott MacDonald we’re working with the Agency and Leeds City Council to introduce NFM across 700km2 of the River Aire catchment in Yorkshire to reduce the risk of flooding in the city and the surrounding area. Our designs for swales, bunds, leaky barriers and ponds were installed at a local farm to test their effectiveness, and these measures significantly slowed the flow of water locally during storms Ciara and Dennis in 2020. We’ve now developed a systematic, repeatable and auditable process for managing the development and delivery of NFM solutions to be used at hundreds of sites across the catchment. This includes:

• a suite of innovative geographic information system (GIS) tools to provide a consistent approach
• a risk-based leaky barrier tool to help pinpoint the best locations for installation
• a Woodland Carbon Code tool to estimate carbon sequestration from the creation of woodland

We’re also working with Natural Resources Wales to install 12 leaky barriers and 5000m2 of riparian planting in the upper Nant Barrog catchment to help protect the community in Llanfair Talhaiarn in Conwy, which has a history of flooding.

These projects are not isolated examples and NFM is gaining in popularity, but as flood risk intensifies in many places, we need to accelerate the pace of progress to complement or substitute for hard engineered defences.

Our new NFM manual for UK construction industry body CIRIA aims to speed up the delivery of more NFM projects. It sets out the key stages in the delivery process – from initiating a scheme and understanding the catchment and the interests of local people to selecting, designing and constructing the interventions, as well as monitoring and managing their performance – and the role of regulators, landowners, utilities and local authorities, communities and landowners. Each organisation has important strengths to bring to a catchment-wide project, but none is currently structured to deliver NFM from cradle to grave, so it is essential they work together to maximise funding options and ensure the outcomes work for everyone.

The manual also includes details of specific NFM interventions, checklists, top tips, and advice on funding as well as how to assess environmental opportunities and constraints.

Including a proportionate assessment of the multiple value of NFM measures and the resulting co-benefits, is recommended and would enable decision-makers to properly compare the costs and total benefits of all flood risk management measures – both grey and green. Often the value of the co-benefits will exceed the flood risk benefits, making a more compelling business case for NFM. At Eddleston Water on Scottish Borders, the annual monetised flood risk benefit from the introduction of various NFM features over a 70km2 catchment is estimated to be £32,000, whereas the value of the co-benefits is £141,000 a year. This is spread across amenity, biodiversity and ecology, carbon sequestration, education, flows in watercourse, water quality and pollution.

The success of NFM projects rests on early engagement, to gather support from the community and landowners as well as flood risk authorities. Support from these stakeholders will provide a platform to deliver NFM and inspire confidence in the project.

The natural alternative

Engineered flood defences lock us into a cycle of building our way out of trouble, which is unsustainable. NFM seeks to break this cycle by rehabilitating the water catchment to delay and reduce the flood volumes forming in the headwaters.

NFM should be seen as an increasingly important piece of the jigsaw to make our towns and cities more resilient to flooding. It needs to go hand in hand with engineered solutions and measures to help educate people about the risks. There is no need to choose one over the other. It’s about taking a catchment- and system-wide approach – and selecting the right interventions in the right places.