flashflood vs alluvion

flashflood and alluvion both are nouns.

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

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

flashflood: 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 flashflood include deluge, flood, inundation.

flashflood (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