What we mean by the likelihood or probability of flooding
The probability or likelihood of flooding is described as the chance that a location will flood in any one year.
If a location has a 1.3% chance of flooding each year, this can also be expressed as having:
- a 1 in 75 chance of flooding in that location in any year
- odds of 74 to 1 against a location being flooded in any year
However, this doesn't mean that if a location floods one year, it will definitely not flood for the next 74 years. Nor, if it has not flooded for 74 years, will it necessarily flood this year.
When we describe the chance of flooding, we give it one of three descriptions or put it into one of three categories:
- Significant: the chance of flooding in any year is greater than 1.3% (1 in 75)
- Moderate: the chance of flooding in any year is 1.3% (1 in 75) or less, but greater than 0.5% (1 in 200)
- Low: the chance of flooding in any year is 0.5% (1 in 200) or less
The lower the percentage then the less chance there is of flooding in any one year; the higher the percentage then the more chance there is of flooding in any one year.
An example of chance is:
If you pull one ball out of a bag containing 200 balls of which one is red and the rest are green you have a 1 chance in 200 of the ball you’ve chosen being red. That chance is less than the chance of getting a red ball from a bag containing 75 balls where again one ball is red and the rest green (in this case the chance is 1 in 75). In the case of flooding, the fact a flood has happened doesn’t change the chance of it happening again, so to mimic that situation the ball that had been pulled out would be put back in the bag before the next one is pulled out so the chance remains unchanged.
Similarly if the chance of flooding is 200 to 1 (or 1 chance in 200) then there is less chance of flooding than somewhere believed to have a chance of flooding of 75 to 1 (or 1 chance in 75).