(adjective)

Definition: of or resembling a serpent (as in form or movement); having many bends and turns

source: http://www.merriam-webster.com/dictionary/serpentine

from the article “Waterfront: A Walk Around Manhattan”:
“the haunting shadow of the bridge on water; the serpentine plunge of the span’s railing, like a roller coaster descending into the high-rise city.”

I understand the vantage point from the author, which the bridge is like the shape of serpent, with many bends and cross the water.