20 MAY 2022


Mott MacDonald’s Emma Wren introduces the new CIRIA manual on natural flood management, an essential guide for climate resilience.

Global economic losses due to flooding totalled US$82bn in 2021, and floods accounted for 31% of losses from natural catastrophes. 

In the UK, a report published last year from ABI and Flood Re found that flooding is the ‘greatest natural disaster risk.’ 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 5,000 homes in New South Wales. 

To be climate resilient, we need more investment in flood defences. But if we are to meet the climate and ecological emergencies, we can’t just keep pouring more concrete.

That’s why CIRIA appointed us to author the first start-to-finish route map for natural flood management (NFM) projects. 

CIRIA’s Natural Flood Management Manual (C802) is designed to move the dial on NFM and offer a practical guide to move the industry from ‘why’ to ‘how.’ 

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.

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.  

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.

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.

Click here to download the new CIRIA Natural Flood Management Manual (C802).

Emma Wren is natural flood management lead at Mott MacDonald and lead author of the new CIRIA manual on natural flood management.


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