alluvion vs flashflood

alluvion and flashflood both are nouns.

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

As nouns, flashflood is a hyponym of alluvion; that is, flashflood 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
flashflood: a sudden local flood of great volume and short duration

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

alluvion (noun)	flashflood (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