Causes of Coastal Erosion

"The results of the erosion study are, in many ways, far more alarming than we thought. If current trends hold, more than 1500 homes a year will be lost to coastal erosion."
- James Witt, Director, FEMA

The coastline is a place where land and water, the lithosphere and the hydrosphere, come in contact with each other. This contact gives rise to several natural and geographical processes which, at times, can be extremely diverse in nature.

A coast can be both an area of peaceful resolution, as well as volatile confrontation. In other words, the geographical processes that may take place in this region, can have both positive and negative impact on the coastal features, as well as the life that thrives in these regions.

The geographical processes, taking place in the coastal regions or on the beaches, are determined by the actions of various natural forces which constantly tend to interact with each other, and also with the more stable geographical features. These natural forces are wind, waves, tides, currents, and so on.

The actions of these forces are responsible for shaping of coastal areas, as their collective energy forces the elements on land to move, inward and/or outward.

This wearing away of land and/or displacement of the shoreline towards land, caused by the actions of waves, tides, currents, also human intervention, is known as coastal erosion or beach erosion. Coastal erosion is a process of redistribution of land that is continuous in nature.

This is one of the major reasons why coastlines of the world are so dynamic. The process of coastal erosion continuously changes the shape of the beaches, dunes, and coastal cliffs.

According to Dr. Ken Rubin, from the School of Ocean and Earth Science and Technology, Hawaii, "a beach erodes because the supply of sand to the beach cannot keep up with the loss of sand to the sea." This is largely because the transportation of sand takes place many times through inland sources, such as rivers and streams.

However, if there are, say, dams constructed on the rivers, the supply of sand to the beaches would automatically be curbed, as the flow of the rivers would then be restricted.

There can be transportation of sand from one beach to the other, but this does not amount to the deposition of 'new' sand on the beach, as the sand that is already there on the coastline gets redistributed in such cases.

Coastal erosion is caused by two main factors viz.., the natural elements and human intervention/anthropogenic factors.

Waves - Oceanic waves can cause coastal erosion in three different ways viz.,

When strong, destructive waves collide with coastal cliffs, air enters the cracks of these cliffs with immense pressure. When the water subsides, all the trapped air is automatically released from the cliff cracks.

This sudden decrease in the air pressure within the cracks leads to the weakening of the cliff. Sometimes, parts of the cliff break apart to form caves. Over time, such action may also cause the cliff to collapse. Hydraulic action is also known as wave quarrying.

Abrasion or corrasion is caused when the materials that are picked by the waves - sand, stones, boulders, shingles, etc., have a sandpaper-like reaction on other rocks and coastal cliffs.

In other words, when these materials come in contact with the rocks and cliffs on the coast, their interaction causes them to grind away, and thus get eroded over a period of time. The collision of rocks picked up by the waves on coastal cliffs, with immense force, may also result in their chipping which, in the long run, may lead to extensive erosion.

When loose pieces of stones or rock debris collide with each other, due to wave action, it is known as attrition. Due to constant collision, they are ground and chipped off, subsequently becoming smaller and smaller in size. Moreover, this rock debris also tends to collide with cliff bases, thus causing abrasion.

Many coastal rocks can be corroded by the acid content in sea water. Carbonic acid in the sea, tends to dissolve limestone in case of limestone coasts. However, this also depends on the level of calcium carbonate in the water - the higher the level, lesser is the corrosion and vice versa.

Erosion that is caused due to the action of the wind is also known as aeolian erosion. Strong winds not only accelerate the force of waves, but also cause landward movement of coastal dunes, thus leading to the erosion of the coastline.

The tidal action in the ocean is also one of the major factors causing coastal erosion. During high tides, the water level is elevated, resulting in the formation of strong, destructive, and high-rising waves that collide with the shore and with the cliffs with tremendous force. This forceful landward movement of the sea, in turn, causes coastal erosion.

Ocean currents are generated by tidal action, and waves. These currents remove coarse sediments from the seabed and deposit them elsewhere, away from their original location. In other words, ocean currents move the sand up and down the coast (longshore movement).

They also facilitate onshore and offshore movements of sand, removing it from one place and depositing it on some other, thus contributing to coastal erosion.

Storms cause the water level to rise, and produce strong and highly energetic waves due to the wind action. These storms may cause coastal dunes and beaches to make a landward retreat, which can be several meters in just a few hours time. Added to this, they may also pose a challenge to the stability of the coastal cliffs.

Rise in the levels of sea water, due to global warming, is another factor that has been causing a considerable amount of coastal erosion lately. The level of global warming affects the rate at which the icebergs melt in the polar regions.

The melting of the icebergs leads to elevated seawater levels. This water then moves landward, and leads to coastal erosion.

Apart from the ones mentioned above, the occurrence of catastrophic events at sea, like cyclones and tsunamis, can also erode the coasts and lead to major coastal changes in a short time span.

While the erosion of coasts and beaches due to natural factors may be a slow process, and whose consequences are observed in the long run, erosion caused by the intervention of the humans can be a cause of alarm.

Human interference, primarily by way of urbanization and commercial activities along the coasts and/or in the vicinity of the coastal zones, is responsible for large-scale erosion and dwindling of the beaches and coasts.

• Destruction of mangroves

• Mining of seawater

Though coastal erosion may be advantageous, as it can give rise to new landforms, and can also increase the possibilities of land-sea interactions, it can also lead to major instabilities. Moreover, it can put the lives of people living in proximity of coasts at risk.