Such a flood has a *return period* of 100 years and is often known as the *100 year flood*. The calculation of flood return periods is flawed , to say the least, and I know from experience how poor they can be. Without going into the details (although I can of anyone /really/ wants to know), the main problems are:
- most rivers have maybe 20 or 30 years of flow data so making calculations about flows which may never have occurred in the dataset is dubious.
- calculations are very sensitive to the length of data available
- the calculation assumes that not only would climate not change during the 100 years (if calculating the 100 year flood), but also that climate did not change during the period of the dataset. Even without *global warming* this is a false premise.
- Any flow with a return period of more than about 1.5 years will be out of bank in most UK rivers. Such flows are often estimated rather than directly measured anyway.
I would be very wary of making decisions such as buying a house based on calculations such as these.
David