What are nature-based solutions?
Traditional water meadows on the River Stour in Suffolk – an example of how nature-based solutions have long been used for flood protection (credit: iStock/Laurence Berger)
The term ‘nature-based solutions’ (NBS) is used to describe actions or measures that are inspired by, supported by or copied from nature.
They can be used to help address a variety of environmental, social and economic challenges, including flood protection, temperature regulation, cleaning of air and water, treating wastewater and the provision of energy.
NBS involve protecting, managing and enhancing existing natural solutions, but can also be artificial solutions that mimic natural processes.
NBS often result in additional co-benefits for health, the economy, society and the environment, such as stimulating economic growth and improving health and wellbeing by providing amenity space in urban areas; enhancing biodiversity; storing carbon; and improving resilience.
Using NBS in flood risk management
Enhanced mangrove forests in Thailand help in defending coastal areas from flooding (credit: iStock/MariMarkina)
NBS are commonly used throughout the world to help mitigate the risk of flooding. Examples include:
- Installing sustainable drainage systems (SuDS) in new housing developments to manage surface water run-off
- Protecting and enhancing mangrove forests to provide natural coastal defences
- Improving the quality of soil on farmland to slow down and store rainwater run-off
In this context, the solutions are sometimes referred to as natural flood management (NFM) or natural water retention measures (NWRM).
In its Natural Flood Management Handbook, the Scottish Environmental Protection Agency (SEPA) defines NFM as “techniques that aim to work with natural hydrological and morphological processes, features and characteristics to manage the sources and pathways of flood waters.
"These techniques include the restoration, enhancement and alteration of natural features and characteristics, but exclude traditional flood defence engineering that works against or disrupts these natural processes.”
For inland areas, NFM interventions achieve this in one of two ways:
Reducing the volume of floodwater at peak flow by increasing temporary water storage and infiltration in the landscape.
Examples include using ponds that fill up when watercourse levels rise; restoring floodplains so that excess water in river channels can spill out into the floodplain; or increasing the permeability and storage capacity of soil so that more water infiltrates, instead of running rapidly into river channels.
Slowing and dispersing water flows in the landscape.
This can be achieved by increasing the ‘roughness’ of the ground surface so that rainwater arrives at the watercourse over a longer period, reducing the river’s peak flow. Flows in watercourses can also be slowed using in-river features, such as leaky dams, which can have the added benefit of altering the timing of peak flows of different tributaries so that they do not arrive simultaneously.
Estuarine and coastal NFM solutions focus on reducing wave and tidal energy in front of defences or providing natural defences to help enhance the standard of protection provided by other flood risk assets.
Common natural flood management solutions
The type of NFM solution that is selected depends on the intended outcome and the landscape in which the intervention is taking place.
Broadly speaking, solutions can be divided into four categories:
River and floodplain management
This category includes measures such as river restoration and floodplain restoration, as well as creating temporary storage using in-water solutions – for example, leaky or woody barriers and offline areas of floodplain that are adapted to retain water in a managed way.
Woodland management
Woodland has many flood mitigation properties, including catching and evaporating rainwater into the atmosphere, storing water, promoting soil permeability and increasing surface roughness. It can also generate major co-benefits, such as improving air quality, providing amenity space, increasing carbon storage and enhancing biodiversity.
Run-off management
A key element of flood prevention is to reduce the volume of water that reaches watercourses – both temporarily (during high rainfall) and permanently. Run-off management techniques include managing land and soil to improve infiltration and reduce the speed of water as it flows across a landscape. Run-off can also be managed by creating artificial ponds and swales to temporarily hold water during peak flows and to catch sediment, preventing its entry into watercourses.
Coast and estuary management
The ICE's CPD explainer on Shoreline Management Plans explains the options available to coastal engineers for managing coastal flooding, including ‘managed retreat’, in which land is allowed to flood in one area to enable other areas to be protected. Natural protection methods include managing and restoring natural features such as mudflats, salt marshes and sand dunes, as well as beach nourishment, in which material is periodically added to beaches.
Why use NBS for flooding?
In the right circumstances, natural flood management schemes and interventions can have many advantages over traditional ‘hard’ infrastructure solutions, including:
Cost
Targeted NFM measures can cost less than built infrastructure. Their relatively low capital cost may mean an NFM solution can be used to protect an area or community when the cost of traditional flood defences cannot be justified.
Working with nature
NFM solutions work with natural hydrological processes and landscapes.
Co-benefits
NFM has the potential to bring many co-benefits, including social capital, biodiversity enhancement, improved water quality and carbon storage.
Climate adaptation
NFM solutions may be more adaptable to changing climate effects than hard structures, as they can be more quickly altered, enhanced or removed if conditions change.
Enhancing infrastructure resilience
NFM can enhance the level of protection provided by existing downstream flood defence infrastructure. NFM can also be used alongside traditional engineering methods – for example, to help reduce the required height of flood walls or embankments, or to extend their life.
One issue that can make NFM measures more difficult to implement is land ownership. The greatest benefit is usually achieved by implementing a catchment-wide network of interconnected measures, rather than individual, disparate features.
However, this would inevitably involve installing, managing and maintaining flood management features on land that is owned or operated by many different people or organisations. Some may be asked to retain water or make changes to their land to prevent flooding elsewhere, and so might perceive that they are shouldering a maintenance burden for something from which they won’t receive a direct benefit.
That is why the Working with Natural Processes: Evidence Directory says it is vital that engineers engage with all stakeholders who might be affected, identify experts in the community and work with them to establish common ownership of problems and solutions.
It is also a reason why engineers should understand all of the potential co-benefits that can arise from NFM measures – for instance, farmers asked to realign a river, reconnect a floodplain or take out land drains to benefit communities downstream may see other benefits on their own land, such as improved soil quality or increased biodiversity.
What legislation and standards are in place?
In Scotland, there is a framework for delivering a more sustainable approach to flood risk management under the Flood Risk Management (Scotland) Act 2009. This requires the Scottish Environmental Protection Agency to work with local and other responsible authorities to identify the most sustainable actions to manage flood risk, including NFM.
There is no equivalent legislation in England; however, between 2017 and 2021, the UK government invested £15 million into a programme of 60 NFM pilot projects across England.
Data collected from these pilots is being used to update the Working with Natural Processes: Evidence Directory, a new version of which is due to be published in late 2023. This will help engineers to choose the best NFM options and quantify their benefits.
Design guidance is already available and can be found in the Natural Flood Management Manual, published by the Construction Industry Research and Information Association (CIRIA), which gives detailed information on inland NFM measures.
Both documents set out an important hierarchy for engineers considering flood management projects:
Protect
Retain and protect features of the existing landscape that are already providing protection against flooding.
Restore
Restore or enhance natural features or mechanisms that will reinstate natural hydrological processes.
Mimic
If the existing landscape no longer contains features that can be retained or restored, design new solutions that emulate or mimic natural hydrological processes – for example, creating ponds or swales.
All flood risk management projects should use a catchment-based approach, in which the existing hydrological and hydraulic processes in the catchment are fully understood before any design work or option choosing begins.
Engineers must first understand how much water flows within the catchment, its sources and how it reaches the area they want to protect. Then they can assess the capacity of the existing flow path to contain flood water, and start to model potential NFM solutions to achieve the desired flow reductions.
NFM measures – such as leaky barriers, bunds, ponds, hedgerow planting and floodplain reconnection – can all be designed and assessed using the same range of hydrological and hydraulic methods used for hard-engineered structures. The Natural Flood Management Manual explains in detail the hydrological and hydraulic considerations for each solution and how to approach them.
Three things to remember:
- Ensure NFM is a key part of a portfolio of measures used to manage flood risk.
- Think what was there in the first place – this will help you to select the right suite of measures and install them in the right place to maximise their benefits.
- Sometimes the best solution is no solution – for example, natural recovery and assisted natural recovery can be used to restore rivers, floodplains and coasts.
Case study: Grassmoor Country Park
Grassmoor Country Park natural flood management scheme
Project partners: Derbyshire County Council; Friends of Grassmoor Country Park
Project funders: National Heritage Lottery Fund and Green Recovery Challenge Fund;
A natural flood management scheme has recently been completed at Grassmoor Country Park in Chesterfield, Derbyshire, with the aim of slowing the flow of water running off areas of the park into Calow Brook during heavy rainfall.
The project involved re-landscaping areas of the park into features such as scrapes, embankments and ponds that can temporarily hold back water so that it drains into the brook more slowly.
Some of these features will be dry for most of the year and will fill only during heavy rainfall, draining over a few days following the end of a period of rain. Others will remain wet to increase permanent wetland habitat.
More information on the project can be found here.
Grassmoor flood management scheme
Source: Penny Anderson Associates
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