Sand carried by longshore currents is a major source of replenishment for the beach. This give and take results in a net loss of sand. Still, if allowed to advance naturally, the shoreline is always bordered by a sandy strip of beach. An unimpeded beach stays about the same size, advancing landward. Buildings in the path of this advance are threatened.
Sand accumulates on upcurrent side of seawall.
Neighbor builds a seawall. Sand depletion and scouring of beach continue. When the Cape Hatteras lighthouse was built, in 1870, it was nearly half a mile from shore. Its location on one of the most exposed, wave-intensive shorelines in the state has put in the path of a rapidly retreating shoreline. Without the seawall at its base, a temporary measure, the lighthouse would have already been claimed by the sea.