ebb vs ebb down

ebb is a noun but ebb down is not a noun.

Word	Noun	Adjective	Verb	Adverb
ebb	Yes	No	Yes	No
ebb down	No	No	Yes	No

As verbs, ebb and ebb down are synonyms defined as:

Other synonyms of ebb include ebb away, ebb off, ebb out.

ebb (noun)	ebb down (noun)
the outward flow of the tide
a gradual decline (in size or strength or power or number)

ebb (verb)	ebb down (verb)
fall away or decline	flow back or recede
hem in fish with stakes and nets so as to prevent them from going back into the sea with the ebb
flow back or recede