EQ3- How do coastal erosion + sea level change alter the physical characteristics of coastlines + increase risks?

Eustatic vs Isostatic change

Eustatic: Global sea level change caused by a change in the volume of water in the ocean store.

Isostatic: Local sea level change caused by a change in the level of land relative to the sea.

Eustatic change

Ice melting: At the end of a glacial period, melting ice sheets return water to the sea + sea level rises.

Thermal change: Global temp increases and causes the volume of ocean water to increase (thermal expansion) leading to sea level rise.

Isostatic change

Post glacial adjustment: The rise of land masses after the removal of the huge weight of ice sheets during the last glacial period.

Accretion: Deposition builds up land, leading to a fall in sea level.

Subsidence: Weight of sediment deposition/ lowering of the water table causes land to move vertically down.

Tectonics: Folding of sedimentary rock by compressive forces at a destructive plate margin produces an isostatic fall in sea level. e.g. the Alpine folding of Eurasian-African plate caused a 60cm rise in Croatia

Types of emergent coastlines

Raised beaches: Areas of former wave-cut platforms and their beaches left at higher level than present sea level due to sea level change.

Fossil cliff: a steep slope found at the back of a raised beach formed through marine erosion but now above high tide level.

• may contain wave-cut notches, caves + arches formed from previous marine erosion.

e.g. Isle of Arran has a raised beach 5m above current sea level

Submergent coastlines

Rias: A drowned river valley

-rivers eroded steep-sided v shaped valleys into the frozen landscape giving the ria a V-shaped cross section when the valley flooded.

-have a curving plan profile reflecting the meandering river course.

e.g. Kingsbury estuary on the South Devon Coast is a 6m long ria.

Fjords: Drowned glacial valleys.

-have a relatively straight profile as glaciers truncate spurs to produce a direct downslope route.

-glacier erosion is often cut deep into the landscape, meaning fjords are often cut deeper than the adjacent sea.

e.g. in Western Norway.

Dalmatian: Composed of long, narrow islands running parallel to the coastline.

-sea level rise flooded valleys behind(synclines), and overtopped the mountains, leaving the peaks of the mountains(islands)(anticlines).

e.g. Croatia

Human impact of sea level rise on coastlines

Residential property

In the UK, 26million+ live in major urban areas in the coastal zone. Relocation is expensive.

Major road + railway links near the coast threatened

Increases coastal erosion, bringing a threat to property e.g. Dunwich

Power stations located on low lying land.

Agriculture

57% of the most productive agricultural land is below the 5m contour line(low lying land)

-significant loss of income if flooded

Tourism

Scenic value of coastlines affect tourist revenue: as tide becomes higher, beaches become smaller

More hard defences to reduce flooding is unattractive- economic losses of tourism e.g. Cornwall

Wildlife habitats

Coastal ecosystems e.g. sand dunes + salt marshes are at risk

-they’re important for coastal protection + are home to wildlife.

Coastal recession is caused by?

Geological factors: lithology, geological structure

Marine factors: LSD, destructive waves

Coastal recession can be influenced by human activity

Dam construction: Trap river sediment behind the dam wall. this then starves the coast of a sediment source, leading to high rates of coastal recession.

e.g. Aswan Dam on river Nile reduced sediment volume from 130m tonnes to 15m tonnes per year. Erosion rates 25m-200metres per year.

Dredging: the removal of sediment from a beach, sea or river. Starves coast of sediment, no protection of coast from waves.

-sediment used by construction industry/ maintain channel for ship transport.

e.g. dredging on California coastline has long term impacts on supply of sediment to Santa Barbara littoral cell.

Holderness case study: human + physical reasons for rapid rates of recession

PHYSICAL

Soft rock(boulder clay): unconsolidated (weak structure)- collapses easily, little resistance to erosion.

Destructive waves: Caused by large fetch of waves from 500-800km across North Sea create from 2nd most strongest prevailing wind in UK + currents with high energy due to large fetch, causing destructive waves.

HUMAN

Groynes, sea wall, rock armour: flow of beach material transported by LSD is stopped at Hornsea. Causes Mappleton to be starved of material, increasing erosion (2m/ year)

Subaerial processes causing coastal recession

(weathering + mass movement)

1. When a coastal cliff or slope is continuously eroded and weathered at its base, it becomes weak.

2. Base can’t support material above it, making cliff susceptible to mass movement.

3. This aided by rainwater, lubricates slope material/ cliff.

4. When mass movement causes material to move downhill, cliff recedes.

Long + short term factors influencing rate of recession.

Tides: Rapid recession rate during high tide when deeper water in foreshore zone allows waves to maintain a higher energy when they reach the backshore.

Wind direction+ fetch: When wind blows from prevailing wind direction, strong winds produce large destructive waves + rapid recession. where wind blows from direction where theres a large fetch, large destructive waves are built up.

Season: Recession rate is higher in winter where storms are more frequent, creating more destructive waves.

Weather systems: High pressure produce anticyclones- gentle winds and small waves + low rates of recession. Low pressure depressions produce strong winds + high rates of recession.

Storm occurrence: deep (very low pressure) depressions. Produce large, high energy destructive waves + fast rates of recession.

Local factors increasing flood risk - Bangladesh example

Low height of land: low-lying coastlines at risk of flooding.

Bangladesh: 60% of the country is less than 3m high above sea level.

-most of the pop. lives on the island of Tarawa where the max height above sea level is 3m.

Subsidence: Low lying coastlines in estuaries are subject to natural subsidence through the settling + compaction of recently deposited sediment (ground level falls)

Bangladesh: 50 large islands subsided by 1.5m since 1960.

Vegetation removal: Vegetation e.g.mangrove forests+ salt marshes reduce flood risk- stabilises existing sediment + traps new sediment, raising the height of the land above sea level.

-absorbs wave energy, reducing wave impact + erosion, and reduces the distance the waves travel onshore before the energy is exhausted.

Bangladesh: home of the largest mangrove forest in the world. 71% is experiencing some vegetation removal. Some parts eroding at 200 m p.a.

Global factor increasing flood risk: sea level rise

IPCC predicts a further 18-59cm sea level rise by 2100.

Bangladesh: a 40cm sea level rise would permanently submerge 11% of Bangladesh, creating 7-10million environmental refugees.

Delta in Bangladesh-home to megacity Dhaka of 14million people.

-low-lying deltas are extremely fertile+ideal for agriculture

-ideal for trade

Storm surge

Types of storm surge events

A temporary rise in local sea level produced when a depression, storm, or tropical cyclone reaches the coast.

-low air pressure allows ocean surface to dome upwards, raising local sea level.

Depression: areas of low pressure generating surface winds that spiral into the centre of low pressure in an anti-clockwise direction.

Tropical cyclones: areas of very low surface air pressure(deeper depressions) generating very strong winds(118+ kph)

Effect of storm surges on coastal flooding- Bangladesh example

Force of onshore current of storm surge water can cause rapid coastal erosion.

Impact of storm surge increased by large destructive waves whipped up by strong storm winds on top of the already higher sea level causes coastal erosion.

Impacts-Bangladesh tropical cyclone 2007:

Deaths: Hypothermia(homes destroyed) + water-borne diseases(sewer systems + freshwater pipe destroyed) 15 000 people killed

No access to water + electricity: 700km electricity transmission lines + 900 freshwater tube wells destroyed

Homes + businesses destroyed: Agricultural land contaminated, crop harvest lost. Crops destroyed on 600 000 ha of land

How can climate change increase coastal flood risk + evaluation

Climate change warming of atmosphere + oceans predicted to increase intensity of atmospheric circulation.

Frequency + magnitude of storms: Tropical cyclones predicted to increase in strength by 2-11% by 2100 + rainfall by 20%.

Cyclone intensity would increase due to warmer ocean surface temperature + warmer atmosphere holding more moisture.

-this would cause even lower surface air pressure producing larger temporary sea level rises as storm surges + increasing risk of flooding.

-no. + intensity of tropical cyclones is highly variable- no statistical long term trend

-no. of cyclones not predicted to increase as a combination of factors form them.

Sea level rise: sea level will rise by 18-59cm by 2100.

-however, pace + extent is uncertain due to population growth, economic development, political commitment to restrict GHG emissions.

-also affected by adaptations:

• 3m sea wall in Malé, Maldives

• restorations of Mangrove forests, e.g. Sri Lanka

• storm surge barriers across river mouths, e.g. Thames Barrier