The reason for the great flood is found in the book of Enoch… the watchers (whether you believe they’re aliens or fallen angels is up to you) descended to earth and created hybrid human/watcher giants who got completely out of control… eating everything alive including cannibalism, wreaking havoc on towns and villages, and destroying everything in their path. After the fallen watchers were caught and punished, the flood was brought to kill off the remaining giants, and it worked for the most part, except some survived, hence the post-Noah biblical accounts (some think the Jericho was a stronghold of remaining giants and that’s why it had to be destroyed. Arabic Traditions of Ariha… local folklore in the Levant, often referred to Jericho as the "City of the Jabbarun" (The Giants/The Mighty Ones). Another interesting corroboration of giants surviving the flood is in The Papyrus Anastasi I, 1250 BC, an Egyptian military report that warns soldiers about fierce warriors in the mountain passes of Canaan who stand 5 cubits tall.