I see it this way. [This]......... [knocks].. [the turtle]..............[down [several feet]] [subject].....[verb]...... [object]...................[adverbial phrase] [pronoun].. [verb]...... [[article]..[noun]].....[[preposition]...[[adjective] [noun]]] So "several feel" is a noun phrase consisting of an adjective and a noun. It is the object of the preposition "down". More importantly, it is part of the adverbial phrase "down several feet".

The only thing 'several feet' is influencing is the verb, therefore it isn't a noun, adjective, article, conjunction, preposition, numeral, pronoun or interjection. Since it is acting like an adverb and there isn't another class to fit it into, it definitely is one.