flashflood vs inundation

flashflood and inundation both are nouns.

Word	Noun	Adjective	Verb	Adverb
flashflood	Yes	No	No	No
inundation	Yes	No	No	No

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

flashflood: 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 flashflood include alluvion, deluge, flood.

flashflood (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