- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 01 Apr 2019
A 100-year event (or 100-year return period event) is an event that occurs (or is exceeded) on average once in every one hundred years (such as a storm, flood or rainfall event). This can also be expressed as 1 in 100 or 1:100.
For example, a 100-year storm is a storm that is likely to occur once every hundred years.
A ‘return period’ is an estimate of the likelihood of a particular event occurring. The return period of a rainfall event is the average time between events of at least a given magnitude.
Events can also be described by their ‘probability of occurrence’ (or frequency of occurrence) which is expressed as a percentage, or as 1 divided by the return period. The probability of occurrence of a 100-year event is 1%.
 Related articles on Designing Buildings Wiki
Featured articles and news
How can it benefit the built environment?
The benefits of early contractor involvement.
Why it is so important for health and wellbeing.
A highly effective method of managing supply chains.
How it can benefit construction.
Free guide to commissioning for site managers published by NHBC and BSRIA.
Resolving quickly to minimise delay and costs.
Tackling domestic abuse.
Disallowed costs vs. defined costs. Which is which?
Coping with the loss of local authority conservation services.
Remedial works could save the NHS £95 million a year.