Flashcards in How do coastal erosion and sea level change alter physical characteristics of coastlines and increase risk? Deck (25)
what is isostatic and eustatic in reference to sea level change?
local rise and fall in land level - isostatic
the global change in the volume of water in all the worlds seas and oceans - eustatic
describe reasons for isostatic change
isostatic fall in sea level- during the build up of ice sheets on land, the weight of ice causes the Earth's crust to sag. When the ice sheets melt, the land resurfaces slowly upward over thousands of years.
isostatic rise in sea level- land can 'sink' at the coast because of the deposition of sediment from large river deltas etc.
describe reasons for eustatic change
eustatic fall in sea level- During the glacial periods, when ice sheets form on land in high latitudes, water evaporated from the sea is locked up on land as ice, leading to global fall in sea levels.
eustatic rise in sea level- at the end of a glacial period, melting ice sheets return water to the sea and sea level rises globally.
-Global temps increase, causing the volume of ocean water to increase (thermal expansion) leading to sea level rise.
tectonic movement: can alter the volume of the ocean basins and therefor the sea level e.g sea floor spreading increases volume of basin and decreases the sea level.
(sometimes called post-glacial rebound) refers to the uplift experienced by land following the removal of the weight of ice sheets.
what are submergent coastlines?
coastlines that aren't effected by glacial ice cover do not experience post-glacial adjustment. instead, they were submerged when post glacial sea level rose.
they can be found in southern England and on the east coast of America
what is a Ria?
drowned river valleys in un-glaciated areas, caused by sea level rise flooding up the river valley, making it much wider than would be expected based on the river flowing into it.
what are barrier islands?
offshore sediment bars, usually sand-dune covered but, unlike spits, they are not attached to the coast. They are found between 500m and 30 km offshore and can be tens of kilometres long.
what is the rate of annual sea level rise?
what group is part of the United Nations and is a committee of scientists who periodically review the evidence for global warming?
IPCC- Inter-Governmental panel on climate change
what are the factors involved in rising sea level
thermal expansion- depends on how high global temps climb to
melting of mountain glaciers in the alps, Himalayas and other mountain ranges will increase ocean water volume
melting of major ice sheets (Greenland/ Antarctica) could dramatically increase global sea level
what is meant by thermal expansion?
the main driver for sea level rise occurs because the volume of ocean water increases as global temps increase
rapidly eroding coastlines have what physical features in common?
-long wave fetch, large destructive ocean waves
-cliffs with structural weaknesses such as Seaward rock drip and faults
-cliffs which are vulnerable to mass movement and weathering, as well as marine erosion
-strong LSD, so eroded debris is quickly removed exposing the cliff base to further erosion.
in what way does human activity make coastal retreat worse?
give an example
construction of major dams on rivers can trap river sediment behind the dam wall which then starves the coast of sediment source.
Aswan high dam on the river nile reduced sediment volume from 130mill to 15mill, meaning erosion rates jumped from 20-25 to 200 meters a year as the delta was starved of sediment
what is dredging?
scooping or sucking sediment up from the sea bed or river bed.
what is wave recession influenced by?
wind direction and fetch, seasonal changes to weather systems, and the occurrence of storms.
why are there variations in the rate of erosion on the Holderness coast in Yorkshire?
-coastal defences at Hornsea, Mappleton and Withernsea have stopped erosion.
-these defences have starved places further south of sediment as groins have interrupted LSD
-erosion rate therefor generally decreases from north to south
-some boulder clay are more vulnerable to erosion than others
-some cliffs are more susceptible to mass movement.
how does erosion vary over time at the holderness coast?
-during winter 2-6m of erosion in common when storms, combined with spring tides increase erosion rates
-summer erosion, during periods when constructive waves dominate, is much lower
-northeasterly storms cause most erosion, because of the long wave fetch of 1500km from the North Norwegian coast.
what does dissipation refer to?
how the energy of waves is decreased by friction with beach material during the wave swash up the beach. A wide beach slows waves down and saps their energy so when they break, most energy has gone
why is low lying land often densely populated?
-coastlines are popular with tourists, especially when access to beaches and the sea is easy
-deltas and estuaries are ideal locations for trade between up-river places and places along the coast or across the sea
-Deltas and coastal planes are especially fertile and ideal for farming.
coastal flooding in river deltas are made worse by processes such as...
-sediment trapped by upstream dams reduces new deposition on delta surfaces.
-storm surges from tropical cyclones flood the delta more frequently
-groundwater extraction causes subsidence and accelerates sinking
-river straightening for navigation means faster river flow and sediment being propelled too far offshore
-destruction of mangrove forests for wood and charcoal exposes the coast to erosion
-sea level rise increases flood risk and erosion
give an example of a low lying coastal plane at risk form sea level rise
-population of 340,000 spread out across 1200 islands.
-the highest point in the country is 2.3m above sea level.
-if sea level rises by the expected 50cm by 2100, we would see 77% of the Maldives disappear into the sea.
what Is the most common cause of coastal flooding and how does this come about?
storm surges: a localised, short-term rise in sea level caused by air pressure change: a 1 millibar fall in air pressure leads to a 1cm rise in sea level
-a depression (low pressure weather system) in mid latitudes (the UK)
-a tropical cyclone (hurricane, typhoon) in areas just north and south of the equator.
give an example of how coastal typography can have an effect on the intensity of storm surges
funnel shaped topography meaning the storm surges are concentrated and thus more destructive
why is Bangladesh especially vulnerable to the impacts of tropical cyclone storm surges?
-much of the country is a low-lying river delta, only 1-3m above sea level
-incoming storm surges meet out-flowing river discharge from the Gnages and Brahaputra rivers, meaning river flooding and coastal flooding combine
-intense rainfall from tropical cyclones contribute to flooding
-much of the coastline consists of unconsolidated delta sediment, which is very susceptible to erosion
-deforestation of coastal mangroves forests has removed vegetation that once stabilised coastal swaps and dissipated wave energy during tropical cyclones.
-the triangular shape of the Bay of Bengal concentrates a cyclone storm surge as it moves north, increasing its height when it makes landfall.