Beach evolution
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The shoreline is where the land meets the sea and it is continually changing. Over the long term, the water is eroding the land. Beaches represent a special case, in that they exist where sand accumulated from the same processes that strip away rocky and sedimentary material. I.e., they can grow as well as erode. River deltas are another exception, in that silt that erodes up river can accrete at the river's outlet and extend ocean shorelines. Catastrophic events such as tsunamis, hurricanes and storm surges accelerate beach erosion, potentially carrying away the entire sand load. Human activities can be as catastrophic as hurricanes, albeit usually over a longer time interval.[citation needed]
Erosion and accretion
Extraordinary processes: tsunamis and hurricane-driven storm surges
Tsunamis, potentially enormous waves often caused by earthquakes, have great erosional and sediment-reworking potential. They may strip beaches of sand that may have taken years to accumulate and may destroy trees and other coastal vegetation. Tsunamis are also capable of flooding hundreds of meters inland past the typical high-water level and fast-moving water, associated with the inundating tsunami, can crush homes and other coastal structures.
A storm surge is an onshore gush of water associated with a low pressure weather system—storms. Storm surges can cause beach accretion and erosion.[1] Historically notable storm surges occurred during the North Sea Flood of 1953, Hurricane Katrina, and the 1970 Bhola cyclone.
Gradual processes
The gradual evolution of beaches often comes from the interaction of longshore drift, a wave-driven process by which sediments move along a beach shore, and other sources of erosion or accretion, such as nearby rivers.
Deltas
Deltas are nourished by alluvial systems and accumulate sand and silt, growing where the sediment flux from land is large enough to avoid complete removal by coastal currents, tides, or waves.
Most modern deltas formed during the last five thousand years, after the present sea-level high stand was attained. However, not all sediment remains permanently in place: in the short term (decades to centuries), exceptional river floods, storms or other energetic events may remove significant portions of delta sediment or change its lobe distribution and, on longer geological time scales, sea-level fluctuations lead to destruction of deltaic features.
Historical accretion of European beaches
In the Mediterranean sea, deltas have been continuously growing for the last several thousand years. Six to seven thousand years ago, the sea level stabilized, and continuous river systems, ephemeral torrents, and other factors began this steady accretion. Since intense human use of coastal areas is a relatively recent phenomenon (except in the Nile delta), beach contours were primarily shaped by natural forces until the last centuries.
In Barcelona, for example, the accretion of the coast was a natural process until the late Middle Ages, when harbor-building increased the rate of accretion.
The port of Ephesus, one of the great cities of the Ionian Greeks in Asia Minor, was filled with sediment due to accretion from a nearby river; it is now 5 kilometers (3.1 mi) from the sea. Likewise, Ostia, the once-important port near ancient Rome, is now several kilometers inland, the coastline having moved slowly seaward.
Bruges became a port during the early Middle Ages and was accessible by sea until around 1050. At that time, however, the natural link between Bruges and the sea silted up. In 1134, a storm flood opened a deep channel, the Zwin, linking the city to the sea until the fifteenth century via a canal from the Zwin to Bruges. Bruges had to use a number of outports, such as Damme and Sluis, for this purpose. In 1907, a new seaport was inaugurated in Zeebrugge.
Modern beach recession
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At the present time important segments of low coasts are in recession, losing sand and reducing beach dimensions. This loss can occur very rapidly. Examples of this are occurring at Sète, in California, in Poland, in Aveiro (Portugal), and in the Netherlands and elsewhere along the North Sea. In Europe, coastal erosion is widespread (at least 70%) and distributed very irregularly.
Relative sea level changes
Several geological events and the climate can change (progressively or suddenly) the relative height of the Earth's surface to the sea-level. These events or processes continuously change coastlines.
Volcanism and earthquakes
Volcanic activity can create new islands. The 800 meters (2,600 ft) in diameter Surtsey Island, Iceland, for example, was created between November 1963 and June 1967. The island has since partially eroded, but it is expected to last another 100 years.
Some earthquakes can create sudden variations of relative ground level and change the coastline dramatically. Structurally controlled coasts include the San Andreas fault zone in California and the seismic Mediterranean belt (from Gibraltar to Greece).
The Bay of Pozzuoli, in Pozzuoli, Italy experienced hundreds of tremors between August 1982 and December 1984. The tremors, which reached a peak on October 4, 1983, damaged 8,000 buildings in the city center and raised the sea bottom by almost 2 meters (6.6 ft). This rendered the Bay of Pozzuoli too shallow for large craft and required the reconstruction of the harbour with new quays. The photo at the upper right shows the harbor before the uplift while the one on the bottom right shows the new quay.
Gradual processes: subsidence and uplift
Subsidence is the motion of the Earth's surface downward relative to the sea level due to internal geodynamic causes. The opposite of subsidence is uplift, which increases elevation.
Venice is probably the best-known example of a subsiding location. It experiences periodic flooding when extreme high tides or surges arrive. This phenomenon is caused by the compaction of young sediments in the Po River delta area, magnified by subsurface water and gas exploitation. Man-made works to solve this progressive sinking have been unsuccessful.
Mälaren, the third-largest lake in Sweden, is an example of deglacial uplift. It was once a bay on which seagoing vessels were once able to sail far into the country's interior, but it ultimately became a lake. Its uplift was caused by deglaciation: the removal of the weight of ice-age glaciers caused rapid uplift of the depressed land. For 2,000 years as the ice was unloaded, uplift proceeded at about 7.5 centimeters (3.0 in)/year. Once deglaciation was complete, uplift slowed to about 2.5 centimeters (0.98 in) annually, and it decreased exponentially after that. Today, annual uplift rates are 1 centimeter (0.39 in) or less, and studies suggest that rebound will continue for about another 10,000 years. The total uplift from the end of deglaciation may be up to 400 meters (1,300 ft).
See also
- Coastal management
- Coastal erosion
- Longshore drift
- Coastal geography
- Strand plain
- Sand dune stabilization
