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Flooding by design

Emma Wren, Principal hydrologist

Around the world in recent years unprecedented rainfall has triggered extensive flooding, putting lives in peril, damaging property, and impacting local and regional economic output.

Communities in flood-risk areas want more to be done to protect their homes and businesses, but building more defences and higher walls is not always practical or cost-effective.

We can help alleviate flood risks more sustainably if we work with natural processes to manage river catchments. One approach is natural flood management (NFM), defined by the UK’s Environment Agency as “taking action to manage fluvial and coastal flood and coastal erosion risk by protecting, restoring and emulating the natural regulation function of catchments, rivers, floodplains and the coast”.

In other words, flooding by design, rather than by default.

NFM has relatively low operating and maintenance costs compared to traditional hard engineering schemes; it also provides an opportunity to make greater use of the planet’s natural capital.

If landowners and farmers can be persuaded to allow their fields to be flooded, it will create room upstream for rivers to expand naturally during periods of high flows in order to help protect communities downstream.

Another form of NFM is planting trees and creating new woodlands to aid run-off attenuation. Turning pasture to woodland through extensive tree planting could, over time, slow downstream flows and improve flood protection. In towns and cities, we should fully exploit available green spaces to reduce run-off and allow water to soak into the ground, releasing it slowly to reduce flood risk or storing it for use.

Environmental benefits

The wider environmental benefits of such schemes include improved water quality and greater biodiversity in rivers and wetlands, creation/preservation of natural habitats, carbon sequestration, and enhanced landscapes and public realm spaces, which combine to create better places to live. But for farmers the main attraction will probably be soil conservation.

This underlines how important it is to remember that different stakeholders have different priorities. All parties should be consulted, and selling the concept of working with natural processes will mean demonstrating – in clear, jargon-free language – how they and their communities will benefit.

Community engagement will also depend on finding ways to incentivise landowners and rural businesses to provide a flood-risk mitigation service. And incentives or compensation should cater for tenants as well as landowners.

A new regulatory framework could open up alternative sources of funding, which will help to enact change and promote more co-operative ways of managing our river catchments. Innovative mechanisms such as partnership funding, where costs and risks are shared in a transparent way, will aid local engagement and accountability.

Empowering communities

Hydraulic and hydrological modelling and geographic information system (GIS) tools are used to determine the optimum location for interventions such as non-floodplain wetlands, overland sediment traps and storage ponds.

With this level of analysis, we can not only identify the most appropriate types of intervention for each catchment, we can empower communities by helping them to build a robust business case to secure public funding.

We can assess the effectiveness and economic viability of different NFM features by evaluating the indicative reduction in losses as a result of implementing them. The flood protection NFM provides may be harder to measure compared to concrete defences, but we can still say, for example, that installation of storage ponds to trap flood water across a catchment will serve to reduce the flood risk downstream – and if enough storage is added, then we can alleviate the risk of flooding downstream for a flood event of a given size.

A recent study in Lincolnshire, England, has shown this approach will create significant capacity in managing run-off: it could realistically achieve a 25% reduction for the 100-year return period peak flood flow, and much bigger reductions for more frequent flood events.

Working with natural processes will not be the best option in every scenario but it can be used in conjunction with conventional flood management methods. Even if the direct benefit (ie the standard of flood protection) cost ratio is unity, the wider societal and environmental benefits can justify taking the natural approach.

Implementing schemes that will deliver these benefits will require collaboration among community groups and landowners. Where stakeholders have co-operated on projects to build up the resilience of communities prior to a flood event, these communities and their economies have recovered more quickly.

We can no longer look at assets – whether natural or man-made – in isolation. Joined-up thinking is a requisite to a holistic, catchment-wide approach to making our infrastructure more resilient to major flooding events.

Emma Wren

Principal hydrologist

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