Quick blog post for a common question. If you want deeper explanations, I will link resources below. In my opinion, the most comprehensive exploration of the giant narrative in Scripture is T.J. Steadman's book/podcast, Answers to Giant Questions.
The question presumes that the nephilim are a result of the union of the spiritual sons of God and daughters of men and they are connected to the giant clans of the conquest, etc. There are, of course, other ideas about the nephilim that are disconnected from this question (for instance, that the nephilim are actually unrelated to the sons of God directly).
How were there Nephilim/giants post-flood?
Option 1: The sons of God went into the daughters of men both before the flood as well as after the flood. This means the incident occurred multiple times.
Option 1a: The incident happened multiple times, but not necessarily in the same way each time. There could, for instance, have been an initial incursion which caused the proliferation of depravity in man that led to the flood, while after the flood there was some sort of restriction that prevented them from doing the exact same thing, so they changed tactics. (The nephilim before and after the flood would not be exactly the same, but would be related in that the spiritual sons of God would be involved.) See options 5, 6 and 7.
Option 2: Noah was a nephilim or had "giant DNA."
Option 2a: A member of Noah's family had "giant DNA."
Option 3: There was a stowaway on the ark.
Option 4: The flood was regional, not global, so some survived.
Option 5: The giants after the flood weren't really "related" directly, except in size and/or concept (literary or metaphorical connection for the enemies of God/terrorizing or "scary" people groups).
Option 6: Ritualistic temple prostitution between the sole "image of the god" (the king) and a temple prostitute with the possibility of a pagan deity "indwelling" the king.
Option 7: Necromancy or other rituals.
Where do I land on the question?
(I wasn't going to include this part, but I realize it would be annoying to be sent to a page and not be able to tell where the author lands.)
I think we get too propositional with it and want the text of the Bible to be something it's not (a modern, chronological, history), so I take it as the ancient person intended: as their history (which wasn't written using an "objective" stance and so might include things such as repurposing old stories or writing polemics) and as a narrative design thread that spreads across the text. The question we are asking may not have even been in their minds. The "giant DNA" or "stowaway" options feel like they do damage to the intent of the story and the character of Noah and his family. I have no problem with a local/regional flood, but I don't think we "need" that when we've got spiritual entities who are involved with historical man, and a local flood would definitely not have been in the mind of the original audience.
The Bible's narrative arc from Genesis to Joshua to King David to Jesus to Revelation seems to give credence to the idea that rebellious elohim have been at work in many different ways, that those ways got "less incursion-y" over time through the Old Testament til Jesus came in victory, and since Jesus their control has weakened but will not be fully defeated til the eschaton. See episode 089 - Nephilim: Before and Also Afterward, with T.J. Steadman. Any answer we give should center on the gospel--its purpose and its results.
Resources
The Unseen Realm by Michael S. Heiser (local flood or multiple incursions)
Answers to Giant Questions by T.J. Steadman (multiple incursions/different methods)
The Lord of Spirits podcast (ritual)
