Sea level change, whether eustatic or isostatic, produces rapid changes in coastal processes and landforms. Sea level change is caused by global warming and greatly affects erosion, transportation and deposition which in turn then alter the shape of different landforms.
Sea level has changed in the past due to quaternary ice age and of plate movements. During times of maximum glaciation large volumes of water were stored on the land as ice, causing a eustatic fall in sea level (of 100 to 150m). As ice accumulated, its weight began to depress those parts of the crust lying beneath it, causing an isostatc change in sea level. This occurred in Canada, where the ice cap pushed the land down to increase sea level. As ice sheets on land are melting, the land can rise again, which is happening in the Baltic Sea, where it is now getting shallower.
Since the ice age, the world has been slowly warming due to global warming (release of greenhouse gases), causing sea levels to continue to rise. This is due to the thermal expansion of oceans (warmer water is less dense and so occupies a greater volume) and from melting ice (mainly alpine glaciers). Due to rising sea levels, the process of waves is going to change because there will be more violent storms, encouraging larger waves due to the hotter atmosphere. With a warmer climate, there are more storms, which bring bigger waves causing more erosion on coastal landforms.
Changes in sea level have affected: the shape of coastlines and the formation of new features by increased erosion / deposition, balance between erosion and deposition by rivers resulting in the drowning of lower sections of valleys or in the rejuvenation of rivers, and the migration of plants, animals and people. Estuaries are the tidal mouths of rivers, most of which have inherited the shape of the former river valley. In many cases, estuaries have resulted from the lower parts of the valleys being drowned by the post-glacial rise of sea level. An example of a drowned river valley is a ria, which is formed when valleys in a dissected upland are submerged. Fjords are formed by the draining of glacial troughs and are extremely deep and steep sided estuaries. These are both examples of a submerged coastline due to sea level change. These can be found in Canada and Southampton has also seen its lower course submerged. Another example of a submerged river valley is Poole Harbour. There was originally a bar separating Poole Harbour from the sea, but rising sea levels punctured a hole in the bar to form two spits and it was flooded by the sea. The entrance to Poole Harbour now has two spits, Sandbanks and Studland. The rising sea levels have therefore caused the shape of the coastline.
The process of sea level change has clearly affected the coastal landforms of Chesil Beach. 9000 years ago, The English Channel was dry land covered with glacial material. When the sea levels began to rise, the sea rolled up and transported the sediment towards the coast. This sediment was deposited by the coast to form a beach. The pebbles and shingle that make up the 25km stretch are round particles as they were rolled and bounced along by the rising sea levels. This landform was created by the rising sea level and now acts as a natural sea defence against large erosive waves to prevent flooding. However, as the sea level continues to rise, the sea defence is compromised more often, causing more coastal erosion. This can cut off the road to Portland, providing economic and physical problems. This beach is an historical feature and a serious problem is occurring. No more sediment is being added as there is no source or natural supply of sediment. As sea levels rise further, more and more floods will occur.
Furzy Cliff (east of Weymouth) has extremely weak geology (clay) and a very short beach. There is therefore very rapid coastal erosion. Subaerial erosion and rotational slipping cause cliff recession of 1m per year. Subaerial erosion softens the Oxford clay so that it doesn’t have sufficient physical coherence to sustain a very steep cliff face for long. Masses of begin to break away and slump towards the sea – a rotational slump. The main work of the sea is to remove debris produced mainly by the effects of groundwater. As the sea level rises, this process of erosion accelerates. The power of waves also increases, transferring more energy, causing faster coastal erosion. Both Chesil Beach and Furzy Cliff are too expensive to armour and are left for the rising sea levels to erode them away. This is also occurring at Barton-on-sea. The natural sea defence of the Barrier Reef in Australia is also under severe erosion by the rising sea levels which erode landforms much easier as the sea overrides the effect of the beach dissipating energy, as the waves break much closer to landforms.
Coastal flooding is also occurring at the Fens, the Thames Estuary (protected by Thames barrage) and salt marshes, which act as natural sea defences are being compromised, all due to rising sea levels. The final coastal landform influenced by rising sea levels is Lulworth Cove. It originated by a river cutting a valley and breaking the Portland stone. As the sea levels there rose, the sea flooded into the valley and gouged out the tourist attraction cove.
A rise of 1m over the next 100 years could inundate 25% of Bangladesh, affecting 60% of population and it could flood 30% of Egypt’s arable land. Some cities are trying to protect themselves, but at an astronomical cost. Several low-lying ocean states eg Maldives could be totally submerged.
However, it is not only rising sea levels influencing the coast. Landforms created as a result of land rising relative to the sea include erosion surfaces and raised beaches. In Cornwall and in South Wales, flat planation surfaces dominate the scenery. Where there general level is between 45 and 200m, the surfaces are thought to have been cut during the Pleistocene period when sea levels were higher – hence alternative name of marine platforms.
As the land rose, former wave cut platforms and their beaches were raised above the reach of the waves, forming raised beaches. Raised beaches are characteristic of the west coast of Scotland. Within the old cliff-line may be relict landforms such as wave-cut notches, caves, arches and stacks. The presence of small features indicates that isostative uplift could not have been constant. It has been estimated that it would have taken an unchanging sea level up to 2000 years to cut each wave-cut platform.
Overall, sea level change influences many coastal processes and landforms whether the sea rises or falls. Rapid changes can occur to the coastline by accelerated coastal processes such as erosion. This could erode cliff faces faster, or accelerate deposition to form bars and spits much quicker.