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PPS 15: Planning and Flood Risk
Annex B: How Flood Risk is Estimated

B19 The probabilities of any site being flooded lay between virtually zero and near certainty. Even in areas generally free from flooding, local conditions and exceptional rainfall may lead to a flood event. It is therefore appropriate that the susceptibility of land to flooding is a material planning consideration.
B20 Flood risk may be simply explained as the combination of the statistical probability of an event occurring and the scale of the potential consequences.


B21 The likelihood of a flood event happening is usually expressed in terms of its predicted frequency of return. For example, a flooding event may be referred to as a 1 in 100 year event or, as having a 1% probability of being equalled or exceeded in any one year. Similarly, a 1 in 200 year event may be expressed as having a 0.5% probability of being equalled or exceeded in any one year.
B22 It is however important to recognise that apparently low probability floods, considered over a long period of time have a significant likelihood of happening. For example, a 1-in-100 year flood has a 25 % chance of occurring at least once in a 30-year period (a typical mortgage duration) and a 50% probability of happening at least once in a 70-year period (a typical human lifetime).
B23 While the probability of a flood event of a particular magnitude must be understood in these terms it should also be remembered, as recent experience of flood events across the UK has confirmed, low probability flood events can occur within a few years or even months of each other.

Potential Consequences

B24 The consequences of a flood event will depend on the vulnerability of the area to flooding and on the resultant economic, social and physical effects of that flooding. Among the factors which may influence the estimation of the consequences are:
  • the characteristics of the area susceptible to flooding (including the amount of built development present);
  • the depth, likely flow rate and duration of flood;
  • the extent and standard of existing flood defences;
  • the effects of climate change;
  • the likelihood of effects on areas such as public open space, private amenity space and natural habitats;
  • the nature of any development proposed (including its projected lifespan);
  • the effect of flood on access, including by emergency services;
  • the extent to which the development, its materials and construction is designed to be flood resistant; and
  • the extent of the allowance made for freeboard.
B25 The estimation of flood risk is therefore dependant on a range of often discrete circumstances of which the probability of an event happening will be only one component. For example, differing depths of flood flow or duration of event may result in impacts of varying scale and severity. Flood risk estimation therefore, cannot be regarded as a precise forecast, but rather a best estimate influenced by the interaction of a range of variables that will include the potential impact of climate change.
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