The coast divides into three types：coast, lake bank, and riverbank. It is a waterfront zone formed by the action of waves, tides, and currents at the contact between water surface and land. The bank formed by the accumulation of many sediments is called the beach. The coastline is the boundary between land and sea. The demarcation of the coastline is defined by the elevation of the submerged surface of the normal high tide level, that is, the submerged surface is the sea area and the dry-out zone is the land area.

The coastline is constantly changing. For example, Tianjin, China, was still a sea in BC. After more than 2000 years of evolution, the coastline has advanced tens of kilometers to the sea. About 1 million years ago, there have been two seawater intrusions here. When the seawater withdrew twice, the farthest coastline reached Miaodao islands in Bohai Bay. But after one million years of evolution, the modern coastline has advanced hundreds of kilometers to land.

The main reason for such a change in the coastline is the movement of the earth's crust. Due to the influence of crustal decline, seawater intrusion (transgression) or seawater retreat caused big changes in the coastline. Secondly, the changes of the coastline are greatly affected by glaciers. In the earth's Arctic and Antarctic regions, the land and mountains are covered with a huge number of glaciers. If the temperature rises, these glaciers in the world will melt, and the ice flows into the sea, which would cause the sea level to rise by more than ten meters. Meanwhile, the coastline will push greatly towards the land; On the contrary, if the temperature drops relatively, the glaciers will expand and thicken, the sea level will gradually decrease, and the coastline will advance to the ocean. Thirdly, the change of coastline is also affected by the sediment in the rivers entering the sea. When the river brings a large amount of sediment into the ocean, the sediment will accumulate near the coast, and deposit as land for many years. At this time, the coastline will move to the sea.

Through the analysis of the coastline of tiruvalur District, Jayakumar concluded that the main causes of coastal erosion and siltation in this area are human and natural factors, which have changed the coastal environment.

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References:

[1] https://en.wikipedia.org/wiki/Coast 

[2]K. Jayakumar. Analysis of shoreline changes along the coast of Tiruvallur district, Tamil Nadu, India. Journal of Geography and Cartagraphy 2021; 4(2): 12-20.