(Un)acceptable risk (as part of the flood risk assessment)


A risk concerns the likelihood of an event that an individual does not wish to occur and, therefore, any risk will be unwelcome. However, it is neither practically nor economically feasible to eliminate all the risk the most suitable approach is to manage the risk best.

For this purpose, risks or concerns can be banded into three distinct levels:

  • acceptable risk - a risk which is regarded as low
  • tolerable risk - a risk which is not regarded as low, but is considered worth taking (there is a willingness to live with the risk). To tolerate a risk means that we do not regard it as negligible, or something we might ignore, but rather as something we need to keep under review, and reduce still further if and as we can.
  • unacceptable risk - a risk which can not be justified

In practice, the majority of risks fall in the tolerable region, in which case they should be managed so that they are as low as reasonably practicable.


How to evaluate acceptable risk?

There are four methods, commonly used for determining acceptable risk:

  • Risk aversion involves the maximum reduction of risk possible with little or no comparison with other risks or with benefits. (Example: National ambient air standards of the Clean Air Act).
  • Risk balancing means a procedure in which a minimum of the costs is sought from the cost-risk relationship. Alternatively, reliability balancing means a procedure in which a maximum of the net benefit is sought from the benefit-reliability relationship.
  • Cost-effectiveness analysis seeks for the maximum reduction of risk per unit cost.
  • Cost-benefit balancing seeks for the maximum net benefit.

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Flood Risk Management






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