Residual Risk (as part of the flood risk assessment)


In spite of protection measures on planning and operational level risk cannot be banned totaly. This remaining risk is called residual risk and describes the amount of  risk after structural or non-structural flood management measures have been applied.

For example:

  • Residual risks of flooding are those which arise after the construction of flood defences or control systems, when the defences or systems are subjected to unquantifiable or extreme events. The events may exceed the design parameters and cause overtopping of the defences or they may subject the defences to large forces which cause structural failure and subsequent collapse of sections of the defences.
  • Other types of residual risk events which can lead to flooding include breakdown or failure of pumps and water control structures and major blockages within drainage systems.

By their very nature, residual risks have a low probability of occurrence.
However, the consequences arising from a breach of tidal or fluvial flood defences can be very significant and, in some instances, dangerous to life.


Different levels of risk

The residual risk cannot be allocated or exactly quantified but it has to be borne by the potentially affected parts of a community. Therefore experts are talking about an acceptable and an unacceptable level of risk. The aim of each flood management measure is to reduce risk to an acceptable level. People should be aware that structural flood management measures do not exclude all kind of risk. Reasons for this circumstand may have a technical or a financial background.






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Flood Risk Management






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