The interruption of this flow robs downcurrent beaches of sand, which is retained by the jetty. This inlet, at Tavira in southern Portugal, shows a downcurrent island starved by an upcurrent jetty. 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.