Coast and Beach
The Sand Budget
Wave action moves incredible amounts of sand. As waves approach shallow water, they slow down because of friction with the bottom, then get steeper, and finally break. It is during this slowing and breaking that sand is transported. When waves reach the shore, the approach is almost straight on, so that the wave front is nearly parallel to the shore as it breaks.
When a breaking wave washes up onto the beach at a slight angle it moves sand on the beach with it. This movement is mostly towards shore, but also slightly down the beach. When the water sloshes back, it goes directly down the slope, without any oblique component. As a result, sand moves in a zigzag path with a net motion parallel to the beach. This is called "longshore drift." Although most easily observed and understood in the swash zone, the area of the beach that gets alternately wet and dry with each passing wave, longshore drift is active in any water shallow enough to slow waves down.
Many features of sandy coasts are the result of long-shore drift. Spits build out from projecting land masses, sometimes becoming hooked at their end, as sand moves parallel to the shore. At Cape Cod, Massachusetts, glacial debris deposited thousands of years ago is still being eroded and redistributed by wave action.
An artificial jetty or "groin" can trap sand on one side of it, broadening the beach there. On the other side, however, wave action will transport sand away. Because of the jetty it will not be replenished, and erosion of the beach will result.
The magnitude and direction of transport of long-shore drift depends on the strength and direction of approach of waves, and these may vary with the season. A beach with a very gentle slope, covered with fine sand every July may be a steep pebble beach in February.