Why? How do you determine what is literal in the bible and what is not literal?
When you walk into a library, do you assume that every book in that library is the same kind of book? Do you assume that The Hobbit is the same kind of book as a biography on Alexander the Great? Of course you don't, you easily recognize that different kinds of books are read differently, if for no other reason than there are different kinds of literary genre.
The Bible isn't a book. The Bible is a library. It contains many books, many different sorts of books. Some books are instruction (Torah), some books are history, some books are poetry and wisdom literature, some books are apocalypses, some books are prophetic, some books are personal correspondence, some books are proclamations.
The idea that the Bible should be treated as monolithic makes zero sense. It's not. And historically was never read that way.
Genesis is one of the five books of the Torah. The Torah refers to God's instructions which He gave to the Jewish people. Genesis is therefore first and foremost Torah, instruction, the narrative is in a sense subservient to that purpose.
For example, it's not an accident that the story of Sodom and Gomorrah occurs immediately after the story of Abraham's Hospitality, and that both stories involve angelic visitors. The stories serve as stark contrasts to one another. Abraham's hospitality serves as the blueprint of how one should treat strangers; the story of Sodom is the complete antithesis--the Sodomites (that is, the inhabitants of Sodom) treatment of strangers is horrific, attempting to commit sexual violence against the visitors. Note that the story has nothing to do with sexuality--in spite of how some modern people interpret it--the story has to do with not only inhospitality, but a violent inhospitality. These are stories about the treatment of strangers, one demonstrates the right way--to welcome, to feed, to show compassion; while the other demonstrates the wrong way--to be violent and horrible.
Genesis is Torah, instruction. The stories serve that point.
Genesis, as a narrative, also serves a purpose of giving us a kind of prologue to the Exodus. The key story of Israelite identity is the exodus from Egypt and the establishing of the covenant with them by God; Genesis establishes a prologue to this story, by presenting a mytho-historical account that moves from creation to the call of Abraham and finally to the Israelite presence in Egypt.
The point of the flood story isn't to say, "Golly, God sure was mad and then He destroyed everything with a lot of water", rather the story is somewhat subversive. The problem of evil in this world can't be resolved by destroying everything and starting over, which is on the surface what the story describes. God sees the wickedness of man, regrets that He made man, then sets to destroy everything and start over again with "righteous" Noah and his family. But what actually happens? Has anything actually been fixed, has anything been resolved by the flood? Well no, because immediately as soon as the flood waters recede, and Noah and his family start living on the dry land again then crap hits the fan. Noah gets drunk and passes out naked in his tent, his son Ham sees him in this state, and then Noah gets angry and curses Ham's son Canaan. It's just more of the same human nonsense.
The world can't be fixed by destroying everything and starting over. To fix the world, to repair the world (Tikkun Olam), requires fixing people from the inside out, by God working with and through people. So God calls a man by the name of Abram, later taking the name Abraham, and says that he will have a son, and that through this lineage Abraham would be the father of many nations, and would be a blessing to the world. And so the story unfolds, through Isaac, and Jacob, and then to Moses, and to David and Solomon, and all the prophets. And, for Christians, that story's ultimate point and purpose is Jesus.
I don't read the story of the flood as literal, because I don't think that's the point. I think that the literary context, the narrative point happening, et al point to the story as communicating a deeper point about God's intentions with the world--as the One who will bring healing to the world through the covenant made with Abraham.
-CryptoLutheran