flash flood vs inundation

flash flood and inundation both are nouns.

Word	Noun	Adjective	Verb	Adverb
flash flood	Yes	No	No	No
inundation	Yes	No	No	No

As nouns, inundation is a hypernym of flash flood; that is, inundation is a word with a broader meaning than flash flood:

flash flood: a sudden local flood of great volume and short duration
inundation: the rising of a body of water and its overflowing onto normally dry land

Other hypernyms of flash flood include alluvion, deluge, flood.

flash flood (noun)	inundation (noun)
a sudden local flood of great volume and short duration	the rising of a body of water and its overflowing onto normally dry land
	an overwhelming number or amount