A five-letter synonym for “flood” could be “deluge”.
“Deluge” implies a great flood or an overwhelming downpour. For example, “The storm brought a deluge that inundated the town.” It conveys a sense of a large amount of water coming all at once, similar to “flood”. It can be used in both literal and figurative contexts. When used figuratively, it might describe a large number of things coming in a rush, like a deluge of emails or a deluge of questions.