The presence of groins can drastically alter the shape of an island. These groins at Cape May, N.J., have created pockets of sand along the beach. A Chain Reaction 1. Waves stir up sand. Part of the sand is moved offshore, and part is carried by longshore currents.
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.