- Created by: lozzaqueen
- Created on: 31-05-17 12:00
Sea level change
Sea level changes over long periods of time, influences by eustatic, isostatic and tectonic changes. Two types:
Eustatic- Change in sea level (during ice age), thermal changes lead to ice melting/being stored as ice
Isostatic- Change in land level due to post glacial adjustment. When land is covered by ice it is heavy so pushes land down (isostatic subsidence) e.g. scotland, areas further away e.g. southern england would have been tilted up slightly. Once ice melts the land rebounds back and other areas sink slightly (isostatic adjustent). Southern coasts may experience accretion of sedimentary material due to the creation of low energy areas
Tectonic change- may have effect on the shape of coast. e.g. as plates move, some land may be pushed up e.g. Kerala coast of India. Other areas sink. Volcanic islands form new coastlines.
Sea level change has produced emergent and submerg
Emergent: Land has emerged from water, commonly result of isostatic rebound.
- Causes raised beaches (former beach above high tideline) with fossil cliffs (near vertical slope inland, evidence of erosion e.g. caves/wave cut platform may be visable)
Submergent: commonly result of sea level rise or isostatic sinking. Eustatic rise has created drowned landforms called:
- Rias: flooded river valley. Originally a V-shaped valley. Ice melted flooding rivers.
- Fjords: flooded glaciated valley. U-shape valley caused by glacier, glacier retreats and water from sea back fills as sea level rises
- Dalmation coasts: sea level floods valleys between mountain ridges parallel to coas
Humans have enhanced global warming through release of greenhouse gases into the atmosphere. Ice melts and sea level rises. A more significant factor is thermal expansion: seawater absorbs heat from atmosphere and its volume expands. If the icesheet on Greenland melted, sea levels would rise quickly by 7m. Low-lying lands such as the Maldives would disappear.
Human actions causing coastal retreat
As well as natural influences on coastal recession, humans can alter the natural processes in a sediment cell, increasing rates of retreat.
Coastal defences built at one location will limit the supply of sediment to a cell. The beach gets narrower, less able to absorb wave energy, wave hits backshore with more force e.g. Dunwich on Suffolk coast
Dredging: removes sand and gravel for construction purposes, deepen entrances to ports, or for beach nourishment. Deeper water means waves maintain energy closer inshore and have a more destructive impact.
- At the Great Barrier Reef, dredging removes bethnic species, and increases suspended sediment levels (turbidity) which can damage the coral reef
The Nile Delta: 240km coast, many holiday resorts, coastal defences, tourism, marine recreation and infrastructure. 95% Egypts population live at the Delta. Coastline is experiencing retreat due to climate change and sea level rise. 32.5% is highly vulnerable. If sea leels rise by 1m, 2 million hectares of fertile land will be lost and 6 million people displaced.
Subaerial processes and other factors affecting co
Subaerial processes e.g. weathering and mass movement also causes coastal retreat.
- Weathering: weakens rocks and increases erosion rates,
- Mass movement: moves sediment to base of slopes, wave action/longshore drift carries material away, exposing base of coastal slopes resulting in further cliff retreat.
Rates of recession are not constant and are influenced by different factors both short and longer term:
- Wind direction: determines wave direction, dominant wind produces largest waves, coasts experience maximum erosion. Longer fetch, more powerful the wave will be. Coasts with longer fetches may retreat faster
- Tides: Tides determine where the waves reach the shore. Four times a year the moon is at its closest to earth, creating higher tides, at higher tides waves reach the backshore and erode the land faster.
- Weather systems: there is a seasonal weather pattern based on a sequence of high-pressure (anticyclones) and low-pressure areas (depressions). High-prssure brings calmer conditions, low pressure has a mixture of air masses, air rises rapidly, with strong winds creating larger waves. Global warming intensified low pressure systems, increasing coastal erosion
Flood risk increases in coastal zones experiencing isostatic sinking (e.g. Essex- Has many estuaries and pasture land, to manage flood risk they breached the sea walls to flood areas, returning them to natural state so unexpected damage is limited), and land shrinkage due to drainage and reclamation (e.g. Netherlands- Half of the Netherlands is below sea level, so the country has protected its coasts with barriers.
Salt marshes and mangrove forests reduce flood risk, removal can increase flood risk significantly. Mangroves reduce wave heights, reducing wave erosion. They also stabilise sediments, keeping coastal land higher, and reduce storm surge levels by 0.5m every 1km of forest. In Java, Indonesia the coastline retreated due to coastal flooding when the mangroves were removed.
Global sea level rise further increases risk:
- Bangladesh: 70% of country consists of floodplains less than 6m above sea level. Up to 40 million people could be displaced. By 2050, local sea level could have risen by 1m
- Maldives: sea level rise of just 40cm would mean the Maldives would cease to exist and people will need to find new homes
Linked to low-pressure weather systems, tropical cyclones and depressions:
- When air pressure is low over the sea (depressions), air rises, reducing the weight on the water and creating a bulge as water is displaced under the depression.
- Lower the air pressure, the higher the dome
- Winds blow the depression towards the coast and onshore winds carry the bulge to shallower land causing magnified waves and storms, increasing risk of coastal flooding and erosion
- South East england 1953: 300 deaths in England, 1800 in Netherlands. £5 billion damage. Sea defences werent designed for such events, 50 dykes burst in Netherlands.
- Bangladesh: 40% storm surges occur in here. Killed 1.3mil since 1700s. In 2007, a category 4 tropical cyclone hit, reached 10m high in some places, $1.7bil damage, 3mil households affected, 2mil people lost jobs.
- Phillippines: Typhoon Haiyan 2013, category 5 (highest), 6200 killed, 28'000 injured. Tacloban one of the worst hit areas, flooding extended for 1km inland with 90% of the city destroyed. A year later, government only repaired 6 of the 43 damaged ports, 100 of the 14'500 promised homes had been built, and only 1% classrooms had been rebuilt.
Climate change and coastal flooding
Global warming (In atmosphere and oceans) means that depressions and tropical cyclones will have more energy and be stronger, with faster winds creating larger storm waves and lower air pressure causing higher domes of sea water. (intensity predicted to increase 11% by 2100)
Protection and preparation is improving in many areas, however more people are living in lowland coasts, increasing risks. The IPCC reported that between 1993 and 2010 sea levels have risen by 3.2mm per year, and it is predicted to rise between 0.43m-0.74m by 2100.
There is however a lot of uncertainty for the future threats and it needs mitigation and adaptation