Just as waves and water build the island, so they also are constantly moving it around. In addition to daily tidal movements, there are seasonal movements as well. Gentle summer waves bring sand from offshore sandbars, forming wide beaches. In winter, waves pack more energy and the pounding on the beaches makes them more narrow as sand is carried back out and deposited on the sandbars. Major storms can bring in heavier grains of black sand made up of minerals that make the beach look dirty.
Time is required to restore the smaller and lighter grains of white sand that vacationers desire.
The natural movement of barrier islands poses no problem until humans decide to put hotels, swimming pools, and highways on them. Then, somewhere down the road, it becomes necessary to build sea walls, to shore up dunes in front of buildings by pumping in sand from offshore, or to build rock jetties to keep the island from continuing to transport itself somewhere else. What engineers have found, though, is that for every mechanical thing done to make the island stay put, there is a reaction somewhere else. For instance, the rock jetties built on the southern end of the next barrier island to the north the one that holds Ocean City have caused wide beaches to pile up there. But, in turn, the adjacent northern end of Assateague is eroding severely and migrating toward the mainland at a much faster speed than it once did.