What is 1 in 100 year Standard of Protection (SoP)?

The likelihood that a certain level of flooding will occur is described as 'flood risk' or the 'chance' that a location will flood in any one year. 

This risk can be expressed as a 1 in 100 (1%) chance of flooding in any one year, or a 1 in 200 (0.5%) chance, and so on. 

The chance is related to the scale of the flooding. In any one year a large (1 in 200) flood event has less chance of happening than a smaller 1 in 100 or 1 in 50 flood event.  

It is important to understand that a 1 in 100 chance of flooding does not mean that a flood will only happen once every 100 years. The chance remains the same every year. 

Whilst the probability of flooding may seem quite low in any one year the effects have proven devastating in the past where the required level of flood protection has not been in place.

Jargon Buster
1in100 Year Definition 1in100 Year Definition