Coastal Deposition

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Coastal deposition

Coastal deposition is a coastal process that involves the action of waves, and how they can often deposit sediments such as sand, rock, and pebbles. Gravity and friction play a key role in the transport and deposition of these materials as these are the driving forces that the waves experience. These sediments and materials are transported by waves through different methods, such as suspension, saltation, solution, and traction.

This coastal process is important as it allows for the displacement of different materials, and this displacement is usually the main cause for the development and formation of different depositional landforms such as beaches, lagoons, spits, and tombolos. Coastal deposition is the main reason behind as to why people notice materials that are particularly different from the surrounding environment around shorelines and beach areas. Most of the time these sediments and materials come from different parts of the ocean floor, and were carried away by the action of waves.

Before we discuss the different landforms formed by this coastal process, we must first gain an understanding of coastal deposition. The following are the different physical processes that influence the occurrence of coastal deposition.

What is Coastal Deposition?

Deposition is a geological process where soil, rocks, sand, and other sediments become added to an existing landscape. During the transportation of sediments and materials, waves may sometimes lose energy, and when they do, sediments may drop out and begin to settle down. This is what we refer to as the process of Coastal Deposition.

Deposition happens when the different forces of waves and wind struggle to overcome the impacts of gravity. This clashing of forces generates friction and subsequent transportation that eventually leads to deposition. In short, deposition occurs when water and wind cannot hold the different materials in the fluid, thus resulting in a subsequent build-up of sediment.

Waves play a major role in the transport of these sediments. Deposition usually happens when the swash, or the forward movement of waves, is stronger than the backwash, or the backward movement of waves. Because of this, coastal deposition is usually the product of constructive waves, where the swash is stronger than the backwash, but that does not mean that destructive waves cannot result in coastal deposition.

The longshore drift is the movement of sediments along the beach by the swash and backwash of waves that hit the shore obliquely. The swash of waves carries and deposits material up a beach at an angle, while the backwash flows back to the sea at 90º because of the influence of gravity. The angle at which waves hit the shore greatly affects how different materials are deposited.

The different sediments and materials that are transported and deposited may sometimes differ in grain size depending on the energy of the waves. For example, if wave energy is high, large sediments are usually transported and deposited. In contrast, if wave energy is low, usually small sediments such as fine-grained particles are transported and deposited.

Various other aspects play a key role in this process such as the rate of transportation, climate change, weather patterns, storm surges, and other geological processes. With these in mind, coastal deposition significantly impacts the biodiversity situated alongside these beach areas.

Deposition is likely to occur in areas where there is little wind, where there is a decent supply of sediments and materials, when waves enter an area of shallow water, and when waves enter sheltered areas.

What are the Landforms formed by Coastal Deposition?

Because of the action of waves and coastal deposition, the build-up of materials and sediments usually results in the development and formation of different geological structures and landforms along shorelines. The following are a few examples of depositional landforms.

Beaches

Beaches are the temporary deposition of sand and shingle along the coastline, and are the most common depositional landforms. In general, these structures protect the coast from the damaging effects of erosion.

Sandy beaches usually develop due to low energy, constructive waves, in sheltered bays. The action of these constructive waves usually results in gentle and flat profiles. On the other hand, exposed beaches usually have steep and rough profiles. Destructive waves along with their stronger backwashes make it so that pebbles are not able to move far up the beach, thus resulting in their steep profile. Lastly, storm beaches form due to aggressive weathers where waves hurl large materials such as boulders and large pebbles to the back of a beach.

Spits

Spits are another type of depositional landform that are unstable, narrow, long stretches of sand, that extend out into the sea. These usually form due to the action of longshore drifts, which move material obliquely along the coastline, and can sometimes result in these extended landforms. Sandy spits usually form due to constructive waves, while shingle spits usually form due to destructive waves. These structures are usually characterized by hooked ends due to the changing wind directions, and wave refraction which carries material into more sheltered areas.

Bars

Bars form when spits grow across bays, where the spit joins two headlands on either side of a bay. Like spits, these usually form due to the action of longshore drifts. Bars that enclose bays usually form lagoons, which are low energy regions that have high rates of deposition.

Tombolos

Tombolos are depositional landforms that develop when spits or bars connect the mainland to an island. Once attached, these islands are referred to as tied islands. Since these involve the formation of spits and bars, tombolos are also formed through the action of longshore drifts.

Sand Dunes

Sand dunes are another depositional landform which are characterized by small mounds and ridges of sand found at the top of beaches, above the reach of the waves. These landforms are formed due to the forces of wind that cause build-up of sand. Obstacles that limit sand movement help give sand dunes their mounds and hill-like characteristics.

Frequently Asked Questions

What do we mean by Coastal Deposition?  

Coastal deposition occurs when waves lose energy, thus resulting in the deposition and settlement of different sediments and materials that vary in size depending on how much energy is lost.

What are the key factors that affect Coastal Deposition?  

The forces of waves and wind that clash with the force of gravity are some of the key factors of coastal deposition. When gravity is much stronger than the forces of waves, deposition usually follows.

Other external factors that contribute to coastal deposition include climate, aggresive weather, rate of transportation, grain size, and other geological processes.

How frequent is Coastal Deposition?  

Coastal deposition is a continuous process. Depending on the materials and sediments being transported, some materials may take much longer to transport and deposit than other materials. Additionally, wave energy and other weather factors affect how fast and how much material is deposited along these coastlines.

References  

  1. https://sk.sagepub.com/reference/geography/n185.xml  
  2. Araya-Vergara J.F. (1982) Marine-deposition coasts. In: Beaches and Coastal Geology. Encyclopedia of Earth Science. Springer, Boston, MA  
  3. Wright, L. D., & Thom, B. G. (1977). Coastal depositional landforms: a morphodynamic approach. Progress in Physical Geography: Earth and Environment, 1(3), 412–459. https://doi.org/10.1177/030913337700100302
  4. https://www.nps.gov/articles/coastal-processes-sediment-transport-and-deposition.htm