Voyager: Why does sand move and how does it get from place to place?

Sand grains move along the shore and up and down beaches because of currents made by waves. Waves break when they reach shallow water, creating turbulence.

This area is called the surf zone. When waves break, some of the force is turned into currents. If you look toward the ocean and you see waves breaking to the right or left, it means the current will flow to the right or left. This is called a longshore current because it flows along the shore, parallel to the beach. Sometimes the waves make currents that flow perpendicular to the beach or cross-shore. These are called undertow and rip currents. The bigger the waves, the stronger and faster these currents are.

For sand grains on the ocean bottom in the surf zone, it’s like being in a washing machine. The turbulence kicks up the sand and then currents move it along the beach. During a big storm, a sand grain can move from the dry beach to depths of 30 feet offshore in a matter of minutes! Or, that tiny grain can move a few miles along the coast in a few hours. What a ride!

Sand comes to the beach from rivers during floods, from cliffs when big chunks fall off, and when people dig up land for coastal building projects or dredge out harbors like San Diego’s Mission Bay. Because of the building of dams and sea walls and other reasons, sand supply to the coast has decreased. Also, offshore breakwaters, like the one in Santa Monica, and harbor jetties like at Mission Bay, trap sand on one side. The beach there usually gets wider. But, the beach at the other side can get narrower unless people move the sand around the obstacle.

— Reinhard Flick, Research Associate, Integrative Oceanography Division