Fragility curves

Even if a fixed standard of protection is used as a design condition, in reality flood defences may not always perform to the standard, as a result of deteriorating structural condition caused by repeated loading, settlement, wave action or scour.

Detailed geotechnical analysis is needed to understand fully the structural condition of the defence and predict its performance under flood conditions. This level of analysis is not always practical because data collection and setting up a suitable geotechnical model may be difficult or expensive. Detailed modelling is not usually feasible for large-scale risk models.

A defence is more likely to fail when it is subjected to extreme hydraulic loading. The most obvious definition of 'loading' is the water level on the exterior of the defence (although other variables such as velocity could be considered). Different failure modes can be defined, which might include simple over-topping of the defence when the water level is higher then the defence crest level. Uncertainty about the performance of a particular flood defence asset can then be expressed as a conditional probability distribution for the failure of the defence, given the hydraulic loading (Fig. 16.2). This conditional distribution is called a fragility curve. Different fragility

Wate-

Increasing water level (hydraulic load)

level

Increasing water level (hydraulic load)

level

Increasing defence failure probability

Fig. 16.2 Channel cross-section with flood bank showing a relationship between the return period (RP) of the hydraulic load on the defence, represented by the water level, and the probability of the defence failing. This relationship is called a 'fragility curve'.

Increasing defence failure probability y

Fig. 16.2 Channel cross-section with flood bank showing a relationship between the return period (RP) of the hydraulic load on the defence, represented by the water level, and the probability of the defence failing. This relationship is called a 'fragility curve'.

curves can be defined for different types of defence, classes of structural condition, ages or maintenance regimes.

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