How nature and humankind alter the coasts
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Importance and characteristics of coastal zones
The coast is the interface between the land, ocean and atmosphere. There is no standard definition of what constitutes “the coast” because it depends largely on one’s perspective or the scientific question – the coastal zone can be considered more the sea, or more the land. Simply stated, the coastal zone encompasses that area where the land is significantly influenced by the sea, and the sea is notably influenced by the land. This is a complex space that is also strongly impacted by human activ-
ity. The coastal zones of the Earth are extremely diverse and tremendously important, not only for humankind.
- They cover around 20 per cent of the Earth’s surface;
- They are the site of vital transport routes and industrial facilities;
- They are prime recreation and tourist areas;
- They are a resource for minerals and geological products;
- They contain important ecosystems with large species diversity;
- They function as an important sediment trap that consolidates sediments from the rivers;
- They influence many global parameters in their role as a buffer between the land and sea;
- 75 per cent of the mega-cities with populations over ten million are located in coastal zones;
- 90 per cent of the global fishery activity occurs in coastal waters;
- They are the place where more than 45 per cent of the world’s population lives and works.
A large portion of the world population lives in flat coastal areas that can drastically change their shape within a short time. The populations in coastal zones are growing faster than in any other region on Earth. Coastal cities are expanding accordingly. People are claiming more and more land. At the same time they are making more intensive use of the coasts, for example by developing large wind-energy farms in the sea.
Sediments shape the coasts
The shape of a coast is influenced by many factors. One important factor is the shifting of sediments, such as mud, sand and gravel. The sediments are primarily transported by wind-induced waves and water currents – either tidal currents, or rivers that flow into the sea. Depending on the currents, sediments are either eroded, redistributed or deposited (accumulation). If sediments are carried away instead of being redeposited, the shape of the coast will change over time. One example of this is the East Frisian island Memmert, where currents caused so much erosion on the southwest side that the old lighthouse was standing in water until it was finally demolished. Another example is seen on the western beach of the Danish North Sea island Rømø. It is continually growing wider due to sediment input.
- 3.6 > Tidal currents on the coast of the Isle of Lewis, off the west coast of Scotland, have carried away the sand and left the stony ground exposed.
3.7 > The old lighthouse on the East Frisian island of Memmert once stood in the dunes, but was washed out by erosion over the course of decades.
- In principle, there are two main directions of sediment transport. One of these is parallel to, or along the coast. The other direction is either toward or away from the coast. The more sediment that is eroded or deposited, the more strongly the coastal form changes. The rate at which sediment can be eroded depends on its composition, as well as on the intensity and persistence of the wind and water currents. A strong storm tide can wash away immense amounts of sediment within a few hours. On hard, rocky coasts, which are more resistant to erosion than loosely deposited sand, the shape of the coastline changes comparatively slowly. A coast usually recedes as a result of erosion: More sediment is lost than is replaced by the currents. The developmental status of a coast, however, is not defined by its sediment balance alone. There are coastal areas that are stable over the long term because sediment is simply transported along them. In many coastal regions today, the natural input of sediments is hampered by construction projects such as dams. Only 20 per cent of the coastal regions worldwide are made up of loose materials such as sand, mud or gravel, but more than half of these coasts are suffering from erosion today. Of course, the loose-material coasts generally adapt quickly to changes because the sediments are redistributed relatively easily – material deficits at one place are balanced again by new sediment input. Yet whether the character of such a coast is preserved in individual cases is essentially dependent on the rate of sea-level rise, stability of the sediment, and sediment input. Even coastal protection measures do not only contribute to the preservation of the coasts. They can also alter the coasts. It is even very possible that by attempting to protect one segment of a coastline, another area is damaged. When one area is protected from erosion by construction of a breakwater, an adjacent, non-protected area may be deprived of its essential sediment input. Compared to sandy coasts, sea-level rise will have a less severe impact on steep and especially rocky coasts. Worldwide, steep and rocky coastlines make up about 80 per cent of all coasts.
Sediments cause the Earth’s crust to sink
It is clear that coastal areas subside under the weight of the glacial masses placed there in ice ages. But sediments can also accumulate in such thick layers that they press down on the lithosphere, the Earth’s crust. This subsides initially and then rises again later when the load is removed. In the case of glaciers, this happens when the ice melts at the end of an ice age. The rebound movement can last for several tens of thousands of years. An example of this can be seen on the Scandinavian land mass, which is still rising today by up to 9 millimetres a year. Sometimes the Earth’s crust rebounds unevenly, so that one part is still sinking while another is rising.
Thick sediment packages are often deposited in delta regions where the rivers transport enormous quantities of sediment to the sea. Because of the stacking of sediments, the lithosphere gradually yields to the pressure. The subsiding bedrock thus produces a rise in sea level relative to the land. In some cases this isostatic subsidence is compensated by the gradual upward increase in sediment thickness. In other cases, however, the sediment itself is compressed by the increasing load so that the land mass still sinks.
Humans can also accelerate the subsidence, for instance by extracting groundwater, oil or natural gas, as in the Niger Delta. There are regions where the land surface is falling by up to 5 cm per year due to the combined effects of these factors. Relative sea level rises there accordingly. >