What does a 1 in a 100-year hazard event mean?

Answer

There are a lot of misconceptions about this. Firstly, it does not mean the event will occur exactly once every 100 years, or that it will not happen again for another 100 years. The correct definition is that a 100-year event is an event of a size that will be equalled or exceeded on average once every 100 years. This is calculated by meteorologists based on statistical approaches using historical data. Using flood as an example, this is typically a probability distribution based on historical river levels and rainfall data for the past 100 years. The probability of a 1 in 100-year physical risk event happening for any year is 1%.