"Tsunami waves may appear only a foot or so high." Does "a foot" function as an adverbial phrase or subject complement?
The main verb in the sentence is the word appear. This verb can take adjective phrases as Complement. In this sentence the adjective phrase is:
- a foot or so high
The adjective high is the main word in the phrase. In other words, it is the Head of the adjective phrase. This word high is being modified by the phrase a foot or so. This phrase is a noun phrase. It is a special type of noun phrase called a MEASURE PHRASE. We often use measure phrases like this to give further information about an adjective:
- ten years old
- five minutes long
- six feet tall
- a foot high
The Original Poster's Question
The grammatical function of foot is Head of the noun phrase "a foot or so".
The phrase a foot or so is a Modifier in the adjective phrase "a foot or so high".
The adjective phrase a foot or so high is a Predicative Complement of the verb "appear". Because it describes the Subject of the sentence, some writers call it a Subject Complement. Books like A student's introduction to English grammar, by Huddleston & Pullum (2005) call it an Subjective Complement.
"foot" is part of the phrase "only a foot or so high", which Is a subject complement.
Specifically, it is a predicate adjective . - http://college.cengage.com/devenglish/broughton/focus_florida/1e/students/diagramming/exercise_6.pdf
(The other type of subject complement is a predicate nominative)