Having worked in the pump industry for nearly 51 years, I can tell you that the "1 in 100 year" flood is the greatest load of rubbish ever fed to the public. I have lived through 5 or 6 of them.
Some people, however, seemed to believe it. Years ago, I had a quotation request from a Consulting Engineer for flood mitigation pumps for the basement of an apartment block. The specified flow rates were so high that I rang him to check. His reply was that he was designing for the "1 in 100" year flood. I asked him just where he intended to pump such an amount of water and he said rather angrily "Storm water drains, of course". As gently as I could, I enquired of him whether it was just possible that we would be having a flood because the storm water drains were already overflowing. Try as I might, I could not make him understand this and he became very abusive. I wonder what he's doing now.