Turbulent seas long have created problems for Cape Hatteras, a long barrier island off the North Carolina coast that is home to the country's first national seashore. Pounding waves and storm surge related to hurricanes and nor'easters shifts beaches around, and even erase them. At Cape Hatteras, over the decades that has meant dune erosion and almost annual repairs to the highway that runs down the island. Against that, a plan has been approved to allow state and local governments to haul in sand from the seabed, when needed, to buttress the island against storms.