Stepping into the breach: understanding the impacts of defence breach in a UK context&#160;
Authors: Burns Douglas , Oldham Phil 
Published: March 2022

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&lt;p&gt;&lt;span&gt;Flood defences help mitigate the impacts of flooding on&amp;#160;people,&amp;#160;properties,&amp;#160;and infrastructure. In&amp;#160;JBA&amp;#8217;s catastrophe flood model for the UK,&amp;#160;defences are included&amp;#160;to help estimate flood losses for re/insurers.&amp;#160;It is important that these defences operate properly,&amp;#160;although&amp;#160;there is some uncertainty whether defences&amp;#160;will&amp;#160;continue to offer protection to properties if their condition&amp;#160;deteriorates&amp;#160;or&amp;#160;a&amp;#160;flood&amp;#160;event&amp;#160;reaches a&amp;#160;loading&amp;#160;greater&amp;#160;than&amp;#160;a defence was&amp;#160;designed to&amp;#160;mitigate&amp;#160;against.&amp;#160;The&amp;#160;breach&amp;#160;of a&amp;#160;defence&amp;#160;drastically increases&amp;#160;losses to properties&amp;#160;local to the point of&amp;#160;breach, and&amp;#160;representing this in a catastrophe model can help re/insurers&amp;#160;better&amp;#160;understand the impact defences&amp;#160;can have on losses.&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;The Environment Agency publish&amp;#160;fragility curve&amp;#160;data&amp;#160;for different types of&amp;#160;defence&amp;#160;structures, condition grades, and&amp;#160;breach&amp;#160;mechanisms&amp;#160;which estimate the probability of&amp;#160;a&amp;#160;flood&amp;#160;defence&amp;#160;failing.&amp;#160;Using these&amp;#160;data&amp;#160;in&amp;#160;the&amp;#160;catastrophe flood model as part of a probabilistic&amp;#160;defence&amp;#160;breach&amp;#160;algorithm&amp;#160;shows&amp;#160;that&amp;#160;the&amp;#160;uncalibrated&amp;#160;fragility curves&amp;#160;vastly over-estimate the likelihood of&amp;#160;defences breaching in the UK, where the condition of defences is generally&amp;#160;good&amp;#160;and investment is strong.&amp;#160;Calibration of the&amp;#160;fragility curves is required to accurately reflect the&amp;#160;observed rate of&amp;#160;breaches&amp;#160;given&amp;#160;the&amp;#160;maintenance regime in the UK.&amp;#160;Catastrophe models which have not been calibrated to match observed&amp;#160;breach&amp;#160;rates&amp;#160;could&amp;#160;over-estimate loss,&amp;#160;this&amp;#160;finding is&amp;#160;supported by&amp;#160;previous research&amp;#160;funded by the JBA Trust&amp;#160;which&amp;#160;concluded the&amp;#160;breach&amp;#160;rate was being overstated&amp;#160;in the&amp;#160;N&lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt;FRA.&amp;#160;We&amp;#160;investigate&amp;#160;the effect of defence breach in&amp;#160;our&amp;#160;catastrophe model.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;JBA has&amp;#160;developed&amp;#160;an&amp;#160;ultra-flexible approach to catastrophe modelling,&amp;#160;which enables &amp;#8220;what if&amp;#8221; type questions&amp;#160;to be&amp;#160;easily answered. Models&amp;#160;are&amp;#160;configured, built and executed, all&amp;#160;at run-time. The modularity inherent in&amp;#160;our&amp;#160;catastrophe&amp;#160;model&amp;#160;means&amp;#160;that&amp;#160;the development and integration of a probabilistic defence&amp;#160;breach&amp;#160;algorithm is quick and straightforward. The parameterisation&amp;#160;of the model makes&amp;#160;it&amp;#160;simple&amp;#160;to select from interchangeable methods, swap input data sources and to set&amp;#160;the&amp;#160;value of&amp;#160;variables. This paradigm is ideally suited for&amp;#160;calibrating&amp;#160;defence&amp;#160;breach&amp;#160;parameters and&amp;#160;for&amp;#160;running different defence&amp;#160;breach&amp;#160;scenarios effectively.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;By comparing losses&amp;#160;modelled with&amp;#160;uncalibrated and calibrated&amp;#160;defence breach probabilities, we&amp;#160;quantify the potential over-estimation of loss when using&amp;#160;the&amp;#160;input fragility curve data&amp;#160;in its&amp;#160;published form.&amp;#160;In the UK we have relative wealth of information to work with, however defence fragility and breach rates around the world are not well understood and emphasis should be put on&amp;#160;the benefits of&amp;#160;collecting data to help accurately quantify the&amp;#160;effect&amp;#160;of proper maintenance and&amp;#160;to&amp;#160;achieve accurate loss estimates.&amp;#160;&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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