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1 Living with the oceans. – A report on the state of the world's oceans

Altering the coasts

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Humans mould the face of the coasts

Over the past 8000 years, during the final phase of sea-level rise after the last ice age, sedimentation has con- tributed enormously to the development of the coasts. Coastal land areas grew by the deposition of transported material, and in some regions large river deltas were formed.
Rivers are very important transport paths for carrying sediments to the coasts.
The volume that they carry depends on several factors:
  • The size of the catchment basin from the source to the estuary;
  • The relief in the catchment basin (rivers in high-relief mountainous areas transport more sediment than rivers that flow through flatlands);
  • The rock and sediment characteristics (for example, grain size) or the amount of available sediments from weathering and mechanical erosion;
  • The climate in the catchment area and its impact on weathering;
  • The amount of surface water flowing downstream and the storage capacity of the soil (how much water flows also depends on the amount of precipitation, which is, in turn, dependent on climate).
Forest clearing, overgrazing and imprudent farming prac­­tices lead to severe soil erosion, especially in tropical regions. If the sediments are not blocked by dams, they are mostly deposited in coastal regions. This can have consequences. For one, the sediments can cloud the wa­ter, change the water quality, and thus severely impact living organisms in the water. Turbidity also decreases light penetration and thus lowers the primary productivity. On the other hand, high erosion could also lead to algal blooms because large amounts of nutrients in the rivers flow into the coastal waters with the sediments. When these algae die, they are broken down by microorganisms that consume oxygen. This can create lethal low-oxygen zones and consequently species diversity drops in these areas.
By contrast,
many regions are plagued by a paucity of sediments because the water is held back by dams.
Worldwide over 41,000 large dams are in operation. There are also many smaller dams and water reservoirs. Together, they block 14 per cent of the total global river flow, as well as enormous volumes of sediment. This is a severe loss for the replenishment of the coasts. Erosion increases. This sediment deficit is fatal in places where the basement is subsiding beneath the old, heavy sediment packages. New sediment is lacking that would normally be deposited to compensate for the subsidence. If the land sinks, saltwater gradually intrudes into the estuary and adds salt to the groundwater. The Nile is a good example of this. Before construction of the Aswan Dam, recurrent annual floods washed fertile sediments from the interior of the continent into the Nile Delta on the Mediterranean Sea. Not only were the sediments essential for the farmers on the banks of the Nile, they were also crucial to compensating for subsidence in the heavy delta region. After the dam was built in the 1960s, the flooding and transport of sediments came to a halt. This resulted in sustained harvest reduction and massive coastal erosion. Similar problems can be expected in China’s Yangtze Delta because of the recently completed Three Gorges Dam.
3.8 > With the beginning of operations in 1968, the Aswan Dam was celebrated as a masterpiece of technology. At that time nothing was known about environmental impacts such as land salination on the coasts. The dam was officially opened in 1971. Its construction took around eleven years.
3.8 > With the beginning of operations in 1968, the Aswan Dam was celebrated as a masterpiece of technology. At that time nothing was known about environmental impacts such as land salination on the coasts. The dam was officially opened in 1971. Its construction took around eleven years. © Keystone France/laif
Recent investigations on the North American Atlantic coast, based on interpretations of satellite photos and topographical maps covering a period of over 100 years, suggest that rising sea level also disturbs important sedimentation processes and will lead to changes on the coasts. It is believed that a sea-level rise of 1 metre will result in an average retreat of the coastline by about 150 metres. This presumes, according to the researchers, that the sediment balance (erosion and deposition) is in a state of equilibrium. The examples cited here, however, clearly show that that is not the case. For the calculations, therefore, one would at least have to consider the sediment transport along the coasts and changes in the sedimentation balance that sea-level rise would cause. So far that has not been done. The coastal retreat could therefore be even more drastic.
3.9 > At times of especially high water levels in the Lagoon of Venice some areas of the city, like the Piazza San Marco, are repeatedly flooded. Italians call the high water acqua alta. © www.bildagentur-online.com 3.9 > At times of especially high water levels in the Lagoon of Venice some areas of the city, like the Piazza San Marco, are repeatedly flooded. Italians call the high water acqua alta.

Hydrologic engineering impacts

In many estuarine regions there is an alternating in- and outflow of seawater in phase with the tides, and thus a mixing of saline water with the fresh water continuously flowing out from the rivers. Suspended sediment from the land and sea can be deposited when the currentenergy level drops. The sediment budget is subject to a very sensitive balance. The building of dams, deepening of channels, or other construction can severely disturb this balance. The impacts are often very serious. Among other activities, the deepening of channels is highly controversial today. 95 per cent of global commerce depends on shipping. For logistical reasons most of the world’s large harbours are located on river estuaries. The ever-increasing sizes of the ships used, with their corresponding deep draughts, require deeper channels. Additionally, the shipping routes are stabilized by structures on the banks, and current flow is optimized by control structures. The deepening of channels can release pollutants trapped in the sediments. In addition, the flow rates can increase, which also increases sediment redistribution. Due to the larger volume of water flowing in and out, the tidal range can also increase. This further affects the sediment budget, because more rapidly flowing water has more energy for moving sediments. Sea-level rise and high-water events amplify these effects. There is already concern about how this impacts the structural integrity of the river dams. The high volume of ship traffic exacerbates the situation because the ships’ wakes often erode the river banks. The
removal of sediment or sand,
which is a common procedure on the island of Sylt for beach restoration, changes the shape of the sea floor. In the long term this can definitely have an impact on the protection of the coasts. It is possible that deepening the seafloor could shift the wave-break zone to a position closer to shore. The removal of large quantities of sand would also change the habitats of marine organisms. This is similarly the case in the reverse situation when sand is discharged into the sea, for instance when dredged material is dumped onto the sea floor. 80 to 90 per cent of these sediments originate from operations related to channel deepening. Hundreds of millions of cubic metres of sediment are dumped annually worldwide. If they are not able to escape, marine organisms living in the dumping areas are covered up.
3.10 > If the coasts were not protected by dikes, sea-level rise of 2 metres would produce this picture. The red-coloured areas would be permanently inundated. According to current predictions sea level could rise by as much as 180 cm by the end of this century.
3.10 >  If the coasts were not protected by dikes, sea-level rise of 2 metres would produce this picture. The red-coloured areas would be permanently inundated. According to current predictions sea level could rise by as much as 180 cm by the end of this century. © maribus (after Brooks et al., 2006)

Coastal city growth

Human societies also damage the biotic environment through the tremendous growth rates of coastal cities. New land areas are often created in the sea to make room for the overflowing development. Many large projects worldwide, including the airport in Hong Kong, have been built in this way. For that project an area of over 9 square kilometres was filled. The harbour at Tianjin in China was even larger, appropriating around 30 square kilometres of marine area. These encroachments have significant impacts on the directly adjacent coastal zones. For example, fill material covering 180 hectares used in construction of the Nice airport triggered a disastrous landslide in 1979. This then caused a
tsunami
that took the lives of 23 people. >
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