Natural Flood Defences

In this blog, Emilie Denyer, an environmental consultant at Groundsure, discusses flood management and natural flood defences. If you have any questions about this blog, you can contact the residential consultancy team at residential@groundsure.com.


Of the 28 million homes in the UK, over 5 million are at risk of flooding from coastal, river, and surface water flooding. [1] The Environment Agency, which is responsible for just under half of all flood defences in the country, helps mitigate and manage the risks from flooding by maintaining and building flood defences throughout the UK.  But what is the best approach to prevent flooding? Many people know about river dams and sea walls, but what about other techniques being used throughout the UK?

There are numerous different ways that floods are prevented. These can be put into 2 categories:  ‘hard’ and ‘soft’ flood engineering techniques. [2] Hard engineering techniques use measures to prevent flooding by a means of built, permanent structures, these are more costly to build and maintain compared to soft engineering techniques. For example, in February 2014 storms brought down a sea wall in Dawlish, Devon, leaving the railway swinging in mid-air over the water, subsequently this cost the economy £1.2bn. [3] Soft engineering techniques take a more holistic approach and work with nature to prevent flooding in an area.

The principle behind natural flood management is to ‘slow the flow’. This is when a river is at flood peak, and water is restricted to reduce the flood peak downstream, increasing the time available to prepare for floods.

There are 3 ways this is achieved:

  1. Increasing soil infiltration
  2. Storing water (for example in ponds or ditches)
  3. Slowing water for example by planting trees or constructing leaky dams. [4]

Leaky dams are a holistic approach that mimic nature – when logs fall across the river channel, they then start to gather together and hold back the flow. Leaky dams are dug into the bank to ensure they don’t move during high flows. This can be seen effectively in Pickering, North Yorkshire. The leaky dam scheme, where more than 260 woody dams were built, slowed river peak flow by 15-20%, saving Pickering’s museum and several homes.[5] According to Defra’s Final project report this scheme cost £247,000. [6]

Another way to stop the flow is from ‘offline ponds’. Ponds are dug next to river edges and field margins and act as a storage pond for water that has been diverted from the river flow. This has been successfully implemented in the village of Belford, Northumberland to protect homes that have suffered repeated flooding. Multiple soft engineering techniques have been used here which have  not only reduced flood risk downstream, but also increased biodiversity, created habitats, and prevented loss of topsoil for farmland. [7]

Natural flood management is not a replacement for hard engineering techniques, but it can be used in conjunction to help with long-term flood planning. Soft engineering techniques are cheaper to maintain, which can only act to help the Environment Agency with future flood protection when the current government is looking to cut costs.

The budget for spending on new flood defences has been agreed until 2021 [8], however this prevents the Environment Agency from taking a long term approach on maintaining existing flood protection. [9] More priority needs to be put on maintaining defences already in place. If there is a limited and unknown budget for this it can mean that new investment in defences will be needed sooner than expected. Landowners taking responsibility and making sure the issues are more clearly understood can help maintenance. Another approach is to invest in natural defences. Since 2004, 154 natural measures have been introduced, but these are only seen on smaller rivers. While natural defence measures are proving to be effective and are the cheaper option compared to hard engineering techniques, they cannot be relied upon as a complete alternative to investing large amounts of money in more permanent engineered structures, particularly along larger river channels, so a combination of the two seems to be most the effective approach to take.


Groundsure produces various reports around flood risk – click here to take a look. Also, Groundsure Avista, the most comprehensive residential search report available, includes this as one of its seven searches.


References

[1] http://www.rics.org/uk/knowledge/consumer-guides/guide-to-flooding/

[2] https://geographyas.info/rivers/flood-management/

[3] http://www.bbc.co.uk/news/uk-england-devon-37639243

[4] https://www.shropshirewildlifetrust.org.uk/slow-the-flow

[5] http://www.bbc.co.uk/news/uk-england-york-north-yorkshire-36029197

[6] https://www.forestry.gov.uk/pdf/stfap_final_report_Apr2011.pdf/%24file/stfap_final_report_Apr2011.pdf

[7] https://research.ncl.ac.uk/proactive/belford/papers/Runoff_Attenuation_Features_Handbook_final.pdf

[8] https://www.theguardian.com/environment/2017/apr/04/flood-defence-plans-heavily-favour-london-and-south-east

[9] https://www.theguardian.com/environment/2015/mar/25/one-in-six-uk-homes-risk-from-flooding-mps-report

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