ROCKAWAY PENINSULA, N.Y. — Beachgoers in this New York City oasis can now flip-flop along a fully rebuilt boardwalk, one that reflects a coastal reimagination underway along the Mid-Atlantic and that heralds the staggering costs ahead of adapting to a changing climate.

Rockaway Beach, where a holiday getaway at the southern edge of Queens long ago transformed into a dense neighborhood, had its wooden boardwalk shredded by Hurricane Sandy. The homes behind it were crushed by a storm surge and inundated with floodwaters. Eight died here.

The new boardwalk along Rockaway Beach incorporates coastal protec