Ever hear of the term http://en.wikipedia.org/wiki/Longshore_drift "littoral drift". It is the effect that tide, waves and prevailing winds have on the direction of water moving against a beach and the movement of the sand. Pot a jetty in and the up current side will rretain sand while the down current side will form a cresent shaped like beach.
Where I live, on the south shore of New York's Long Island, our 100 some odd miles of south facing beaches from Montauk Point to NYC, are known for having an east to west current sweep causing rip currents and cresent shaped beaches where ever the current sweep meets a jetty.
All beaches are effected the same way
