The shoreline is where the land meets the sea and it is continually changing. From a risk point of view, coastal erosion is the most widespread and continuous process. However, catastrophic events play a very significant role (tsunami, hurricane and storm surge), both for coastal erosion and human damage.
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. The most extreme storm surges result from extreme weather systems, such as tropical cyclones or hurricanes, but storm surges can also be produced by less powerful storms. Storm surges can cause beach accretion and erosion. Historically notable storm surges occurred during the North Sea Flood of 1953, Hurricane Katrina, and the 1970 Bhola cyclone.
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.
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.
In the Mediterranean sea, deltas have been continuously growing during 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. At that time, the initiation of harbour-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 km from the sea. Likewise, Ostia, the once-important port near ancient Rome, is now several kilometers inland, the coastline having moved slowly seaward.
Bruges, a port during the early Middle Ages, was accessible from the sea until around 1050. At that time, however, the natural link between Bruges and the sea silted up. In 1134, a storm flood caused a deep channel, the Zwin, to appear, and the city remained linked to the sea until the fifteenth century via a canal from the Zwin to Bruges. But 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.
At the present important segment of low coasts are in recession, losing sand and reducing the beaches' dimensions. This loss could occur very rapidly. Examples of this are occurring at Sète, in California, in Poland, in Aveiro (Portugal), and in Holland or elsewhere along the North Sea. In Europe, coastal erosion is widespread (at least 70%) and distributed very irregularly.
Several geological events and the climate can change (progressively or suddenly) the relative height of the Earth's surface to the sea-level. The coastline is continuously changing by these events or processes.
Volcanic activity can create new islands. Surtsey Island, Iceland, for example, was created between November 1963 and June 1967. The 800m-diameter island has since been partially eroded by waves, rain, and wind, 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).
A famous event occurred in the Bay of Pozzuoli, in Pozzuoli, Italy. The city 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 2m. 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.
Venice is probably the best-known example of a subsiding location. It experiences periodic flooding when tides or surges arrive. This phenomenon is caused by the compaction of young sediments in the Po River delta area, magnified by water and gas subsurface 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 interior of Sweden, 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 cm/year. Once deglaciation was complete, uplift slowed to about 2.5 cm/year, and it decreased exponentially after that. Today, typical uplift rates are 1 cm/year 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 m.