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Chances and risks

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The basis of this article is especially written for the Coastal Wiki by the main author referred to at the bottom of this page.

This article explains the differences between chances and risks. The phenomenon of coastal erosion is used as an example.

Chances and consequences

Some (coastal) events cause damage to properties; e.g. erosion of dunes during a severe storm surge might cause damage if close to the brink of the dunes houses or hotels have been built. Given the position of e.g. a house on top of the dunes, there exists a certain chance [per year] that the house will be destroyed during a storm surge. This destruction might be called consequence [expressed in money and/or losses of life in some serious cases]. Chances as well as consequences might vary from case to case.

Intuitively one might understand that an event with a rather large chance, but with small consequences could be more or less similar and comparable with an event with a rather small chance, but with large consequences.

Risk and return period

The notion risk takes the similarity as was mentioned above into account, viz.:

       risk = chance x consequence

The unit of risk is in simplified form: [amount of money per year].

In management issues the notion risk is a far better parameter than e.g. chance. The outcome of a risk calculation might serve as a first rough estimate of the money one has to save on a yearly basis (and put that money on a safe bank!), in order to be able to pay the costs of rebuilding of the property involved from time to time. (Average period between two rebuilding operations is equal to the return period [unit: year]:

      return period = 1/chance


Consider a house (value: € 500,000) on top of the dunes at a position that the chance of destruction is 1/500 per year. The risk for the owner of that house is then ≈ € 1000 per year. The owner of the house has to make a quite personal choice whether a nice sea view is worth about € 3 per day!

If the same house, assuming having the same value (but that is quite questionable, since sea view houses are much more expensive than comparable non-sea view houses), is situated at a (more landward) position with a chance of destruction of 1/5000 per year the risk is only € 100 per year.

See also


  • Benjamin J.R. and Cornell C.A. (1970) Probability, statistics and decision for civil engineers.
  • Cooke R., Mendel M., Vrijling H. (1997) Engineering Probabilistic Design and Maintenance for Flood Protection.
  • Winckel P.R., Vrijling J.K. and van de Graaff J. Developing a building policy for the erosion zone: Solutions to some key (Dutch) questions. Coastal Engineering. An International Journal for Coastal, Harbour and Offshore Engineers. Volume 55, No. 1, Januari 2008.
The main author of this article is Jan van de Graaff
Please note that others may also have edited the contents of this article.