What are flood
defences?

Flooding, hurricanes and their subsequent rainfall, as well as other severe weather events, caused an estimated USD 344 billion in global economic losses and USD 132 billion in global insured losses in 2017 (AON, 2017). Flooding and related weather events have been the costliest natural hazards (economic and insured) almost every year for the last ten years (Aon, 2018a; 2018b). In the UK alone, 1 in 6 homes were at risk of flooding in 2016 (Groundsure, 2017).

With a growing population and limited space around the globe, a larger number of houses are being built on floodplains. This introduces increased risk of inundation and flood defences are becoming more necessary to protect against this risk. In this blog, we take an introductory look at what defences are and how JBA accounts for them.

What do we mean by flood defences?

Flood defences are systems put in place to reduce, or ideally prevent, damage by flood water. Flood defences are by no means a new concept. Examples of ancient flood defences can be found in China’s Yellow River basins, with levee systems that are approximately 2900 years old (Edward, 2014), or the raised embankments protecting Roman London in England, AD 50-60 (English Heritage, 2011). Flood defences constructed in 16th century England under Henry VIII’s reign can also still be seen and protect parts of London today (English Heritage, 2011).

Modern day flood defences come in many different forms. Firstly, they are available for different hazards; river, surface water flooding or coastal storm surge. Secondly, the terms ‘hard’ and ‘soft’ engineering are often used to characterise the systems; ‘hard’ referring to more direct structural methods and ‘soft’ referring to indirect defences such as basin management or afforestation.

Likewise, man-made defences can be fixed, demountable or temporary (Table 1). Flood defence schemes which are fixed are more commonly larger schemes protecting large numbers of properties; demountable and temporary defences can be either larger schemes protecting several properties or methods used at individual property level by the homeowner. The most common examples of defences are summarised in Table 2.

Table 1:  Explanation of fixed, demountable and temporary defences

Table 2:  Overview of common defence types

Engineered defences usually have a standard of protection (SoP), which is the return period of a flood event against which the defence should be effective. For example, a flood defence could have a SoP of 100 years, meaning it should be effective against a 1 in 100-year flood event. Failure can occur either through the water exceeding this return period or due to weakness in a defence because of degradation over time.

According to a study, degradation prior to a major event contributes largely to defence failures (DEFRA and EA, 2007). Installing the most appropriate defence type and design, and maintaining those defences, is critical to reduce the likelihood of failure. Fragility curves are used during the design of a defence to conceptualise the likelihood of failure depending on the force exerted by the water (Casciati and Faravelli, 1991 in DEFRA and EA, 2007). However, fragility and probability of failure are very difficult to quantify as they require information on the maintenance and status of a defence, which is very difficult to undertake and sustain on a nationwide scale.

What can JBA provide to help manage flood risk?

At JBA, we use the best available information to identify both the location and SoP of flood defences. This information goes into the creation of our Defended Areas dataset, with both flood maps and Defended Areas available globally. Having the Defended Areas as a separate dataset to our flood maps allows our clients to use the data to make informed business decisions in line with their individual risk appetite, by incorporating all, some or none of the defences in their risk estimation.

If you would like to know more about any of the datasets JBA has to offer, get in touch.

References

Aon Benfield, 2017, 'Weather, Climate & Catastrophe insight', Accessed 06 November 2018.

Aon Benfield, 2018a, 'Catastrophe Insight: Global Economic Losses', Accessed 06 November 2018.

Aon Benfield, 2018b, 'Catastrophe Insight: Global Economic Losses', Accessed 06 November 2018.

Groundsure, 2017, 'Understanding Flood Risk for property purchasers', Accessed 06 November 2018.

Edward, O., 2014, 'Yellow river flood defences dated. Geographical', Accessed 06 November 2018.

English Heritage, 2011, 'Roman and Medieval Sea and River Flood Defences', Accessed 06 November 2018.

DEFRA and EA, 2007, 'Performance and Reliability of Flood and Coastal Defences, R&D Technical Report FD2318/TR1', Accessed 06 November 2018.

Casciati and Faravelli, 1991, in DEFRA and EA 2007: https://books.google.co.uk/books/about/Fragility_Analysis_of_Complex_Structural.html?id=gUp5QgAACAAJ&redir_esc=y.

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