Noah wasn't the only passenger in the Ark, and according to Enoch there was a man, son of a granddaughter or something, who was born of nephilim descent, with bright red hair, and the Head of Days tells Enoch that it was a mistake - A MISTAKE... yes, the angelic host made a mistake with the consist of the Ark's human passengers

and i'm pretty sure that Noah's boat wasn't the only survivor, there was a few others here and there, and almost certainly according to genetic analysis, some tens of thousands of humans survived the event around 12000 years ago, there was earlier ones, one around 75000 years ago necked the human gene pool down to 5000 individuals, and of course nephilim genetics means nothing, it was the knowledge that gave them power, so i'm pretty sure some of them also survived

if you haven't read the various assemblages of text that are the history given by Enoch, i highly recommend you get your hands on a copy... it makes it ABSOLUTELY CLEAR that angels were humans with white skin and usually blonde or red hair, taller, bigger heads, and these "giants" were F1 hybrids, the direct children of these unlawful couplings referred to in Genesis

the genetics of these humans is partly why also the early characters in the bible, mostly genesis, had such long lifespans, but as time passed these genetics for ~1200 year lifespans faded