The Biblical flood coincides approximately with the ending of the last ice age. If it occurred suddenly and catastrophically instead of gradually, it might have supersaturated the atmosphere with water as the sea levels rose, causing world wide raining and overflooding. Just one theory.
Even if the flooding was thousands of years earlier, there would likely be some oral history of its occurrence. How that might change with time is up for debate.
I knew it! Global Warming®