Allegory. In our traditional form of writing, allegory, metaphors, parables, etc. are commonplace. Typically, they are written as if they actually had happened or were about ready to happen.
Even in the early church there was discussion over Jesus' use of parables as to whether they were real events or narratives with symbolic values, and the consensus was they they were the latter. "Revelations" is full of these forms of symbolism.
The importance of the Flood narrative isn't whether it actually happened, but what are the teachings found within the narrative, and those can be rather easily pulled out if one reads them carefully.
Allegory...well that raises then a few controversies that you would have to reconcile.
1/
Genesis 6:13 After that God said to Noah: " I have decided to put an end to all flesh, because the earth is full of violence on account of them, so I am bringing them to ruin together with the earth.
Genesis 7:4 For in just seven days, I will make it rain on the earth for 40 days and 40 nights, and I will wipe from the surface of the ground every living thing that I have made.
Was God just venting or making a threat he had no intention in carrying out?
Can God lie?
2/ we are told that Noah spent perhaps 50 years building a huge floating vessel of approximately 40,000 cubic metres for the survival of his family and a few animals. Why would this be recorded in such detail if it were mere allegory?
3/ we have the eye witness account of Jesus, who personally witnessed the event.
Matthew 24:37-39 For just as the days of Noah were, so the presence of the Son of man will be. 38 For as they were in those days before the Flood, eating and drinking, men marrying and women being given in marriage, until the day that Noah entered into the ark, 39 and they took no note
until the Flood came and swept them all away, so the presence of the Son of man will be.
Now if this was all mere allegory, Jesus words would have no real meaning or relevance, but if his words are true, and he claims to be the beginning of Gods creation, and thus has seen all of human history, how can you reconcile this account with it being just an allegory?