Method
6 min read
What a 1-in-100 flood means after 2020
Ines Carvalho
A 1-in-100 flood is a probability, not a calendar. On the Port-Aven quay, the same water level now comes back about every twelve years.
A probability, not a calendar
When a council hears “1-in-100”, it often hears “once a century”. It means something else: a one in a hundred chance of that flood in any given year. The chance does not reset after a flood, and over the life of a sea wall it adds up.
The record moved
Return periods are calculated from past records, and the records keep changing. On the Port-Aven quay, the +2.0 m level was a 1-in-100 event in 1950. With the floods of 2010, 2018 and 2024 now in the record, the same level comes back about once every twelve years.
Nothing about the quay changed. The sea did. A defence drawn on the 1950 numbers is still standing; it is simply answering the wrong question.
What we do with it
We replay 120 years of storms on a 2 m terrain model of the site, with the recent floods in the record, and design for the event the model shows rather than the one in the old drawings. At Port-Aven, that is a 1-in-100 flood on today’s sea: modelled peak depth at the quay went from 1.12 m before the works to 0.71 m after.
When we present a design to a council, every return period comes with the date of the record behind it. A return period without a date is a number nobody can check.