floodplain vs champaign

floodplain and champaign both are nouns.

Word	Noun	Adjective	Verb	Adverb
floodplain	Yes	No	No	No
champaign	Yes	No	No	No

As nouns, champaign is a hypernym of floodplain; that is, champaign is a word with a broader meaning than floodplain:

floodplain: a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding
champaign: extensive tract of level open land

Other hypernyms of floodplain include field, plain.

floodplain (noun)	champaign (noun)
a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding	extensive tract of level open land