alluvion vs flash flood

alluvion and flash flood both are nouns.

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

As nouns, flash flood is a hyponym of alluvion; that is, flash flood is a word with a more specific, narrower meaning than alluvion:

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

Other hyponyms of alluvion include debacle, flashflood, Noachian deluge, Noah's flood, Noah and the Flood, the Flood.

alluvion (noun)	flash flood (noun)
clay or silt or gravel carried by rushing streams and deposited where the stream slows down	a sudden local flood of great volume and short duration
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