Coastal Dynamics: How Waves, Currents, and Erosion Shape Our Shorelines

Coastlines are fascinating in mass and energy terms because it is here that the atmosphere, lithosphere, biosphere, and hydrosphere vigorously interact. They are also of great importance to humans because of our historic attraction to moderate climates, ocean trade, and fishing.

Water can erode shoreline material in several ways. The most important manner is by the tremendous hydraulic force provided by waves. Even modest waves can focus enough energy to break apart rock material and transport the loose remains. At the extreme, some coastlines are subject to large tsunamis generated by earthquakes. These are rare but can accomplish considerable erosion. The chemical action of water is considerable and able to break down rock by several means as part of weathering and mass wasting. Ice along shorelines is capable of considerable leverage in prying apart rocks.

Movement of materials along the shoreline is quite obvious by close observation. Sand moves in a rhythm shoreward and seaward with the break and ebb of the waves. These distances are, at best, a few yards. There is considerable work that is performed by the abrasion of small rock and sand pieces one against the other; they are lessened in mass and smoothed. Upon closer examination, sand and other small materials exhibit “odd” behavior as waves break along the shoreline. A small portion of the breaking wave water is absorbed into the wet beach while most of the broken wave flows down the beach slope in the direction of the ocean. Although this might seem obvious, the net motion of materials suspended in the waves is not. Usually, waves approach the shoreline from a direction other than straight on to the beach slope. The local beach has a single slope direction; however, wind can push waves from various directions. Thus, waves break onto a beach from directions other than that of the slope. The suspended materials travel with the waves onto the beach from the direction from which the waves originate. The water that has broken onto the beach travels directly down the beach slope via gravity. The motion is a zigzag transport of materials, causing net transport in parallel to the shoreline. This is known as beach drifting. Some of these materials are picked up in longshore currents and can be transported great distances.

The beach is a depositional form. Erosion is prominent along coasts, but beaches are, perhaps, the most obvious coastal landform. They represent depositional materials in “temporary storage” until such time as the materials can be resuspended. As such, they are continuously being built and/or eroded by water. Tides have surprisingly little net landscaping influence on beaches and are greatly overshadowed by events such as storms, tsunamis, or long-term rise and fall of sea level.

Beach sand is not always white and varies considerably in color, including reds and blacks as a result of the erosion of volcanic rocks. Though very common, sand is not the only material found along shorelines. Cobbles, rocks, pebbles, silts, and clays comprise beaches along various coastlines, and their occurrences are dictated by the particular combination of eroded materials and the energy of the water action at each location. In areas with plentiful shoreline sand and enough wind, dry sand can be suspended in the air and moved inland. Progressive wind erosion of the beach sand causes dunes inland.

Some coastal dunes rise many tens of yards, and their presence provides protection against higher water levels in major storms.

There are numerous other depositional forms. When sand piles on the sea bottom offshore, it is known as an offshore bar. Sometimes, offshore bars are deposited above sea level and become barrier islands. The Gulf and Southern Atlantic coasts of the United States are flanked by thousands of miles of barrier islands with shallow, protected lagoons lying between them and the mainland. As longshore currents encounter embayments, there is a tendency for them to drop some of their sand loads into deeper water. This causes elongated strips of sand known as spits to build out from the shore.

There are three types of coastlines delineated with respect to sea level. They are emergent, submergent, and neutral. Tectonic forces make land rise or fall, and sea level can rise and fall because of changes in ocean water volume. At this point in Earth’s history, we observe many submergent shorelines. This is because of the rise in sea level resulting from the melting of ice from the Pleistocene glaciers. The east coast of the United States is dominated by submergent coastlines, with Chesapeake Bay being a prime example of a river system that was drowned by the ingress of ocean water. Conversely, the Isle of Arran in western Scotland is home to King’s Caves, which were formed by erosion from water at sea level but are now several meters above sea level as a result of the isostatic uplift of land resulting from unloading of the mass of the Pleistocene ice.

Coastlines are not all inorganic. Coral coasts are common in shallow, tropical oceans. Coral polyps are simple, soft creatures a few millimeters tall and a few millimeters in circumference. They live as huge groups of genetically identical animals and excrete calcium carbonate, which makes up the hard substance commonly called coral. Besides providing support for the coral, the matrix of calcium carbonate frequently becomes a reef, providing habitat for a rich diversity of life.

Humankind has always been drawn to coasts. We have built prolifically along shorelines— to sometimes disastrous effects. For instance, the population of US coastal counties has more than doubled since World War II. The building of housing, towns, marinas, and golf courses has exponentially increased the chances for property damage during storms and played havoc with some parts of the natural system. By building on coastal sand, the local sand supply available to nature is effectively lessened, and beaches disappear because beach drifting far exceeds the local storage. In the modern era, with good knowledge of the physical geography of coastlines, it is unfortunate that we do not fully respect the power of nature along the coastline. Reuel R. Hanks.

FURTHER READING:Charlier, Roger Henri. 1998. Coastal Erosion: Response and Management. Berlin/New York: Springer.

Maanan, M. 2014. Sediment Fluxes in Coastal Areas (Coastal Research Library). Berlin/New York: Springer Ebooks.

McGuire, Chad J. 2013. Adapting to Sea Level Rise in the Coastal Zone: Law and Policy Considerations. Hoboken, NK: CRC Press.

Ramanathan, A. L., P Bhattacharya, T. Dittmar, M. Bala Krishna Prasad, and B. R. Neupane. 2010. Management and Sustainable Development of Coastal Zone Environments. Dordrecht, The Netherlands: Springer Netherlands.