Forces that Shape Sandy Beaches

Coastal processes represent a careful balancing act between erosion and the deposition of sediment. A sandy beach is the often ephemeral record of a period when the process of deposition dominates. Sand, fine-grained sediment usually composed of quartz grains, is washed onto the beach by the action of waves and tides, and may even be reworked by onshore winds.

The first factor that shapes a sandy beach is the overall environment. A sandy beach can exist only where the land is subsiding relative to the water or is in equilibrium. Coastlines that are emergent – meaning that the land is rising relative to the water – are typically marked by cliffs or rocky shores. Classic examples include the sea cliffs of northern California and the rocky coves of Maine.  

The local shape of a coastline also controls beach deposition. Headlands, which project outward from the coastline, bear the brunt of wave action and are almost always sites of erosion. Relatively sheltered areas such as coves and other indentations in the coastline are more favorable to deposition of sandy beaches.

A sandy beach also requires a continuous supply of sand, which is usually transported parallel to the coastline by longshore currents.  Sand can be transported hundreds of miles before washing onto a beach: the sandy beaches and barrier islands on the Texas Gulf Coast are built from sand that entered the Gulf of Mexico at the mouth of the Mississippi River. Other beaches are composed of sand from sources close at hand. Dramatic examples include the black sand beaches of Hawaii, made up of bits of volcanic rock, and the beaches of Florida’s gulf coast. The characteristic white color of western Florida’s beaches comes from the presence of almost pure calcite (calcium carbonate) grains eroded from coral reefs and other sea life.

Sand, whatever its composition, washes onto the beach by wave action. The size and strength of the waves controls the amount of sand that is deposited or, when the waves are exceptionally powerful, eroded from the beach. Many beaches look dramatically different in winter, when powerful  storms carve away at the beach and move the sand back offshore. In summer, the more gentle and continuous action of the waves pushes the sand out of deeper water and up onto the strand. Major storms such as the hurricanes of the Atlantic basin can reshape a beach in a matter of hours. In an area of frequent or continuous heavy surf, the energy of the water can be sufficient to prevent deposition of sand at all, even where other factors are favorable. In such environments, beaches can be covered with gravel, coarser particles that require more energy to erode.

A sandy beach can form only where there is a suitable stretch of coastline – stable or subsiding, smooth or somewhat indented – and a sufficient supply of sand. The sand may be sourced locally or transported long distances, and may vary in composition from calcite to fragments of volcanic rock. Quartz sand is the most common material. Even when these requirements are met, the creation of a beach requires sufficient wave energy to transport sand onto a beach, but not enough energy to erode sand that is already in place. Once beach sand has dried, it is often transported away from the water’s edge to windblown deposits such as coastal dunes.  All of that sand can easily be reshaped, however, by a major storm.