stormwater drain vs storm sewer

stormwater drain and storm sewer both are nouns.

Word	Noun	Adjective	Verb	Adverb
stormwater drain	Yes	No	No	No
storm sewer	Yes	No	No	No

As nouns, stormwater drain and storm sewer are synonyms defined as:

stormwater drain and storm sewer: a system of watercourses or drains for carrying off excess water

Other synonyms of stormwater drain include drain system, drainage well, storm drain.

stormwater drain (noun)	storm sewer (noun)
a system of watercourses or drains for carrying off excess water	a system of watercourses or drains for carrying off excess water