A beach is a sandy place where people go to soak in the sun and listen to the rhythm of the surf. But if you look closely, a beach can tell a story about the area’s natural history and the processes that shape its shoreline. Beach sediments may contain sand, shells, rocks and other materials that have been pulverized by waves, wind and other natural forces for thousands of years. This “sand chemistry” can provide scientists with valuable clues about the beach’s past conditions, which are important to understanding current wave and shore erosion conditions.
The size, shape and source of sand are key factors in determining beach characteristics. In general, sand that is close to the water tends to be coarser than sand farther from the water. The grain size composition of sand at a beach also depends on the slope of the beach. Steep beaches have larger sand grains than flat ones.
Most beach sand comes from eroded bits of rock, usually coral reefs, or from nearby cliffs. In addition, beach sand may be transported long distances by ocean currents and deposited in new locations. The sand that forms the outer bar on the coast of Tuvalu in the South Pacific, for example, was moved there by ocean currents from inland deserts and other beaches.
Beach sand also is continually changing, shifting and eroding, with the sand relocating as conditions change. Beach sediments can reflect the past, such as the shape of the coastline and seafloor substrate, or they can show the direction of ocean currents and winds.
Scientists can design surveys to characterize the sand on a particular beach, including its sand size, shape and source. The surveys help scientists understand the current beach conditions and predict how a beach might change in the future.
One of the most difficult aspects of analyzing beach sediments is that beach sands often have an unusual composition. They are made from a variety of materials, many of which are not easily characterized, such as silt, clay, sandstone and other metamorphic rocks.
A sandbar is a submerged or partially exposed ridge of sand that forms offshore from a beach. It is created when the swirling turbulence of waves breaking off a beach excavates a trough in the sandy bottom and deposits some of this material forward on the beach. Sand suspended in backwash and rip currents, along with sand moving shoreward from deeper water, also add to the bar. Eventually, the bar decays when energetic incident waves can no longer reach the beach and carry the sand offshore. In some cases, the bar may be regenerated as the result of a beach nourishment project, where sand is fed from an offshore sandbank to a beach to counter erosion. This is called “net bar migration”.