I assume you mean the flood where Noah and his family survived in the ark.

Here is how I understand it from a biblical perspective.
God created man, man fell. Angels left their habitation(fallen angels) and mated with daughters of Adam. This brought in to being Giants or Men of old and they were of a corrupt blood line. Noah and his family was still of a pure blood line. God wiped out the half angel half man species.
After the flood another influx came in. Israel was tasked with destroying them. (Goliath for example and many other tribes)