Fifteen thousand feet adds a lot more water.
So what? When you're in the miracle business, what difference does that make? In for a dime, in for a dollar.
Unless Ararat, that very active mountain, rose up that far after the flood.
Which would decrease the likelihood that a wooden boat would survive, but I'm skeptical that there is anything on that mountain that would interest anyone but a tourist shark. G-d doesn't tend to leave footprints.
Shalom.