flash flood vs alluvion

flash flood and alluvion both are nouns.

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

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

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

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

flash flood (noun)	alluvion (noun)
a sudden local flood of great volume and short duration	clay or silt or gravel carried by rushing streams and deposited where the stream slows down
	the rising of a body of water and its overflowing onto normally dry land
	gradual formation of new land, by recession of the sea or deposit of sediment