One of the biggest floods in the UK was the Boscastle flood in 2004 which killed a lot of people. On the day it happened it was a sunny day that turned to a stormy day, But because it was sunny the ground was dry and when the storm came all the water started to flood and then that causes the terrible flood that happened. A total of £20k - £40k of damage was needed to repair the damage done by the flood. Many disabled people were in desperate need of help because they could not afford their medical bills.
Over 60 mm of rainfall (typically a month's rainfall) fell in two hours.
The ground was already saturated due to the previous two weeks of above average rainfall.
The drainage basin has many steep slopes, and has areas of impermeable slate causing rapid surface run-off.