flood vs Noah's flood

flood and Noah's flood both are nouns.

Word	Noun	Adjective	Verb	Adverb
flood	Yes	No	Yes	No
Noah's flood	Yes	No	No	No

As nouns, Noah's flood is a hyponym of flood; that is, Noah's flood is a word with a more specific, narrower meaning than flood:

flood: the rising of a body of water and its overflowing onto normally dry land
Noah's flood: (Biblical) the great deluge that is said in the Book of Genesis to have occurred in the time of Noah; it was brought by God upon the earth because of the wickedness of human beings

Other hyponyms of flood include debacle, flash flood, flashflood, Noachian deluge, Noah and the Flood, the Flood.

flood (noun)	Noah's flood (noun)
the act of flooding; filling to overflowing	(Biblical) the great deluge that is said in the Book of Genesis to have occurred in the time of Noah; it was brought by God upon the earth because of the wickedness of human beings
a large flow
light that is a source of artificial illumination having a broad beam; used in photography
the occurrence of incoming water (between a low tide and the following high tide)
the rising of a body of water and its overflowing onto normally dry land
an overwhelming number or amount

flood (verb)	Noah's flood (verb)
cover with liquid, usually water
become filled to overflowing
fill quickly beyond capacity; as with a liquid
supply with an excess of