In California I took a fascinating class on Marine Biology. This class consisted of five weekends at various beaches studying a variety of things. One week end was devoted to surveying a beach. We also collected vials of sand showing the different layers and documented what critters lived at each level.
This teacher had been doing this beach with each class for years and had the results posted around the room above the chalk boards.
What it proved was that the ocean washes sand in and the oceans takes sand out. This happens on a very predictable schedule. If I remember correctly the winter storms take the beach away and the spring calmer seas brings the sand back giving us nice beaches for the summer. (But is should be noted you will freeze your butt off in the summer at Northern California beaches. The Spring and Fall are the times to go there.)
Marinas and sea walls interfere with this natural rotation of the beaches and sand spits build up causing all kinds of problems. I've been told that the caves under Ambergris Caye that were just north of the cut have now been filled in with sand because of the sea walls to the north.
AmbergrisCaye.com 
