Most civilizations were established and grew around sources of fresh water, which means that there would have been villages and cities on or near rivers and lakes. It's not unusual for rivers to have seasonal flooding, with the occasional large-scale flood that can destroy crops and buildings and take enough lives that they become part of the local lore.
One "evidence" that is used for the validity of the global flood is that there are many flood stories around the world that purport to there having been a worldwide flood. But the fact that flooding would have been a common human experience, and the way our myths and legends develop, make it inevitable that stories about "great floods" would be so widespread across so many different cultures.