Here's a few ideas about a possible explanation for an ancient flood. I think the Bible is a written record of oral traditions passed down for what might have been thousands of years. We're dealing with ancient peoples who had zero knowledge of any science. To them, the forces of nature were magical, supernatural events. Its not their fault they didn't know what we know today. They simply didn't know, so they described events as best they could within their own cultures.
The Sumarian's had a similar account of a great flood in their records, very similar to later Biblical accounts. So I assume they might refer to the same event. Sumeria was the first major civilization and developed the first written language, lirerally thousands of years before the bible was written.
The last Ice Age ended about 12,000 years ago. As the glaciers melted, oceans levels would have risen. Low lying lands would have been flooded, quickly or slowly, depending on how fast the ice melted and flowed to the oceans. The Sumerians or their predesessors were alive and well during this period of glacial melting and ocean levels rising. They were probably to far south to be aware of glaciers, but they would have been aware of rising ocean levels as lands increasingly were flooded.
The rest of the story about Noah, etc.,blah blah gave the story the much needed human element. What good is story telling without a good story? The basic fact of the end of the last Ice Age needed jazzing up, and with a few thousand years and a host of story tellers, no wonder the Bible story is what it is.
Bottom line: the Bible story of the flood may have been an ancient account of the end of the last Ice Age.