A town is told its new defences will protect against a thousand-year flood. Two years later the town floods. The defences are called a failure and the engineers are blamed.
The phrase is the problem. A thousand-year flood does not mean a flood that happens once every thousand years. It means a flood with a one-in-a-thousand chance of occurring in any given year. Those are different statements, and only the second one is true.
The difference matters in two ways. First, a one-in-a-thousand event can happen twice in a decade without anything being wrong, in the same way that a coin can land heads twice in a row. Second, over a long period the chance accumulates. A house standing for seventy years faces roughly a seven per cent chance of seeing that flood at least once, which is low but is nothing like the impression the phrase gives.
There is a third difficulty that is more serious. The probability is estimated from past records, and in many places the past is no longer a good guide. If rainfall patterns are changing, a flood calculated as one-in-a-thousand from a hundred years of data may now be one-in-two-hundred, and the label on the defences will not have been updated.
Several agencies have begun replacing the phrase with a plain annual percentage, for a simple reason: a one-in-a-thousand chance per year is harder to misread than a thousand-year flood.
Vocabulary in this text
- protect /pɹəˈtɛkt/ — to keep someone or something safe
- flood /ˈflʌd/ — a large amount of water covering land
- decade /dɛˈkeɪd/ — a period of ten years
- coin /ˈkɔɪn/ — a piece of metal money
- row /ɹˈoʊ/ — a line of things side by side
- accumulate /əˈkjumjəˌleɪt/ — to increase in amount over time
- difficulty /ˈdɪfəkəlti/ — a problem, or the state of being hard
- serious /ˈsɪɹiəs/ — important and not to be joked about
- probability /pɹˌɑbəˈbɪləti/ — how likely something is
- estimate /ˈɛstəmət/ — to say approximately how much or how many