coasts paper 1 Flashcards
2B.4a different wave types (constructive/destructive) influence beach morphology and sediment profiles, which vary at a variety of temporal scales from short term (daily) through to longer periods.
A wave is created through friction between wind and water, transferring energy from the wind into the water. This generates ripples, which grow into waves when the wind is sustained.
A wave is the transfer of energy from one water particle to its neighbour with individual water particles moving in a circular orbit
The size of the wave particle orbit decreases with depth
Wave height is the vertical distance from peak to trough, it’s determined by the energy transferred from the wind, and the water depth
Wave length is the horizontal distance from crest to crest (or trough to trough)
Wave frequency is the number of waves passing a particular point over a given period of time
Wave size depends upon: the strength of the wind the duration for which the wind blows water depth wave fetch - uninterrupted distance across water over which wind blows, and so the distance waves have to grow in size.
how a wave breaks:
Friction between the sea bed begins to distort the wave particle orbit from circular to elliptical, and slows down the wave. The wave has entered the offshore zone. The wave depth decreases further, and the wave velocity slows, wavelength shortens, and wave height increases. Waves ‘bunch’ together. The wave crest begins to move forwards much faster than the wave trough.
The wave breaks in the nearshore zone, and water flows up the beach as swash. wave then losses energy and gravity pulls the water back down the beach as backwash
Constructive Waves
Low energy waves, Low, flat wave height (<1m)
Long wavelength (up to 100 m), Low wave frequency (about 6-9 per minute)
A strong swash, weaker backwash so sediment deposited as a ridge of sediment (berm) at the top of beach
A backwash that percolates into the beach material
encouraged by a long, shallow nearshore, so friction slows down the wave and releases energy
Destructive (plunging) Waves
High energy waves, Large wave height (>1 m)
Short wavelength (about 20 m), High wave frequency (13-15 per minute)
They’re encouraged by a short, steep nearshore zone, quickly dropping away into deeper water, so that there is little energy loss through friction
strong backwash erodes material and weak swash due to the steep angle of impact. material deposited as a offshore ridge or berm.
Beach morphology = shape of beach.
A beach sediment profile is the pattern of distribution of different sized or shaped deposited material.
constructive waves create steep beach profile
destructive waves reduce beach gradient
Decadal Variation
Climate change is expected to produce more extreme weather events in the UK.
Winter profiles may be present for longer time over course of year
More frequent and more powerful destructive waves may reduce beach size, allowing high tides to reach further inland and increasing rate of coastal erosion in what was backshore zone.
Seasonal Variation in the UK
Destructive, high-energy waves dominate in the winter, lowering angle of beach profile and spreading shingle over the whole beach. Offshore ridges/bars formed by destructive wave erosion and subsequent deposition of sand and shingle offshore.
In summer, constructive, low-energy waves dominate, steepening beach angle and sorting particles by size, with larger shingle particles towards back of beach. In summer, constructive waves build berm ridges, typically of gravel/shingle at high tide mark
Low channels and runnels between berms
Monthly Variation
Tide height varies over course of lunar month, with highest high tide occurring twice a month at spring tide and two very low high tides (neap tides)
As month progresses from spring down to neap tide, successively lower high tides may produce a series of berms at lower and lower points down the beach.
Once neap tide passes and move towards next spring tide, berms successively destroyed as material pushed further up beach by rising swash reach.
Daily Variation
Storm events during summer will produce destructive waves that reshape beach profile in a few hours.
Calm anticyclonic conditions in winter can produce constructive waves that begin to rebuild beach, steepening profile for few days before storm.
Destructive waves change to constructive ones as the wind drops.
Storm beaches, high at the back of the beach, result from high energy deposition of very coarse sediment during the most severe storms
2B.4b importance of wave erosion processes (hydraulic action, corrosion, abrasion, attrition) and how they are influenced by wave type, size and lithology.
erosion = Hydraulic action, Corrosion, Abrasion, Attrition
How these are influences by wave type, size and lithology They are most effective during high energy storm events with large destructive waves. However, even coastlines composed of unconsolidated sediment (e.g. boulder clay of Holderness Coast in Yorkshire), experience little erosion under normal conditions. Most erosion (in the UK) occurs in the winter, in high energy storms. it's faster when the wind is blowing directly onshore and when the tide is high.
The effect of erosion
The boulder clay of the Holderness coast has retreated by 120 m in the last 100 years.The granite of Land’s End in Cornwall has retreated by only 10 cm in the last 100 years.
hydraulic action = force of water breaks rock, air in crack expands.
abrasion = wave picks up sediment and throws against a rock. he repeated impact chips rock face until fragments break away.
corrosion = water in waves dissolves rock minerals.
attrition = material eroded through collision with other rocks. breaks down sediment into smaller particles, and repeated collision blunts any of the particles’ sharp edges, making the sediment increasingly rounded.
2B.4c erosion creates distinctive coastal landforms (wave-cut notches, wave cut platforms, cliffs, the cave-arch-stack-stump sequence)
Wave cut notches and platforms (e.g. Kimmeridge Bay)
wave cut notch = curved indentation along cliff base. It forms between the high tide and low tide marks, where destructive waves impact against the cliff. eroded by hydraulic action and abrasion. the depth of the notch varies depending upon the resistance of the rock at different points.
wave cut platform = flat rock surface at low tide extending out to sea from the base of a cliff.
abrasion and hydraulic action forms a wave-cut notch along the length of the cliff base.notch deepens by further erosion until the overlying material collapses by mass movement due to gravity, forming a cliff. the process repeats, and cliff retreats (coastal recession). rock below low tide level is always submerged, it’s uneroded as it’s never exposed to wave impact.
As the overlying material is eroded, uneroded rock at low tide level is left as flat rock surface, the wave cut platform.
cliffs = unvegetated steep slopes. marine erosion of land between the high tide and low tide mark by hydraulic action and abrasion forms a wave cut notch. notch deepens until rock collapses by mass movement due to the force of gravity. exposed face forms a cliff.
cave-arch-stack-stump sequence
Rocks have joints, faults or vertically dipping bedding planes in their geological structure = eroded more rapidly (e.g. hydraulic action), and the deepening and widening of a weak point forms a sea cave. this is accentuated by how wave refraction concentrates energy on the sides of the headland, producing destructive waves with a very large wave height.
Where a line of weakness extends right through the headland caves form on both sides. erosion deepens the caves until they connect up, creating a complete tunnel through the headland and forming an arch.
Hydraulic action and abrasion attack the sides of the arch between low tide and high tide, forming wave cut notches.
The undercutting of the sides leads to the collapse of some overlying material by mass movement, widening the arch.
Weathering and other sub-aerial processes attack the arch roof.
Eventually, the roof of the arch will collapse by blockfall leaving the seaward end of the headland detached from the land as a tall vertical column called a stack.
erosion at the base of the stack will form a notch on all sides until the stack collapses by blockfall.
Remnants of the stack base form a stump, a small projection of rock, exposed only at low tide.
However, unconsolidated and soft sedimentary or metamorphic rock won’t undergo this because it isn’t competent enough. (e.g. boulder clay, clay, shale)
2B.5a what is sediment transportation influenced by?
traction - sediment rolls along
saltation - sediment bounces along
suspension - sediment carried along
solution - dissolved sediment carried
- ANGLE OF WAVE ATTACK
swash-aligned – wave crests approach parallel to coast so limited longshore drift. sediment doesn’t travel up the beach far.
drift-aligned – waves approach at a significant angle, longshore drift causes sediment to
travel far up beach. - LONGSHORE DRIFT - waves hit the beach at an angle because of a prevailing wind direction. waves push sediment in this direction and up the beach in the swash. due to gravity, the wave then carries sediment back down the beach in the backwash. moves sediment along beach in right angle shape
- TIDES & CURRENTS - flows of water in direction driven by water density, wind and temp. some are continuous.
2B.5b what landforms do transportation and deposition processes produce that can be stabilised by plant succession?
- BEACHES - form due to constructive waves depositing sand & shingle.
- RECURVED / DOUBLE SPITS - long narrow strip of land/beach that’s curved landwards out to sea
- OFFSHORE BARS - when a spit grows across a bay, lagoon behind
- BARRIER BEACHES & BARS - beach connecting 2 land areas with lagoon behind
- TOMBOLOS - when spit stretches across the mainland and joins an offshore island
CUSPATE FORELAND - by longshore drift transporting sediment in two directions where sand and shingle deposition extends outward from shore in a triangular shape. leads to the formation of two spits which meet and this results in the trapping of sediment until eventually new land is formed.
these landforms made from unconsolidated sediment so vulnerable to change e.g if trampled or overgrazing vegetation. vegetation stabilises it. e.g sand dunes
2B.5c sediment cell concept - closed system
- helps understanding the coasts dynamic equilibrium with negative and positive feedback
SOURCES- subaerial processes, erosional processes (breaking down cliffs), marine organisms, currents, sediments brought to the coastline by rivers.
TRANSFERS- longshore drift, onshore and offshore winds and tides, Swash, Backwash
Tidal currents, Sea/ocean currents
SINKS - sinks are depositional landforms (spits, bars, beaches, barrier islands, sand dunes
e.g Flanborough head - source region
Holderness coast - transfer zone
Spurn head - sink region
sediment cell are dynamic because the sediment is constantly generated in the source region, transported through the transfer region and deposited in the sink region.
in dynamic equilibrium. erosion in source will be balanced by deposition in sink.
if theres change in source/ sink, positive or negative feedback can change the state of the cell.
negative feedback returns system to normal.
E.g. when erosion leads to rockfall mass movement. The debris acts as a barrier protecting the cliff base, slowing erosion for some time.
E.g. major erosion of sand dunes could lead to excessive deposition offshore, creating offshore bar that reduces energy, allowing dunes time to recover.
positive feedback changes system further until it settles at new equilibrium. human influence also changes system.
E.g. When wind erosion of a dune section during high velocity storms may removing stabilising vegetation. Further wind erosion now occurs in later low velocity wind conditions, increasing the depletion of dune sand.
changing system: climate change creating more frequent storms or erosion of the cliff line to a more resistant rock type. The system's equilibrium may be interrupted (e.g. during a storm event) but they tend to return to balance on average over time due to negative feedback. Seasonal change (e.g. storms and strong winds during winter) will change the dynamic equilibrium.
Coastal management in the source region may reduce sediment supply, e.g. sea walls preventing cliff erosion. Management in transport region may reduce or halt sediment supply to sink region, e.g. groynes trapping sediment to encourage beach outbuilding.
UK - 11 key sediment cells and sub-cells, each with a Shoreline Management Plan (SMP).
portland bill to selsey bill
2B.6a weathering (mechanical, chemical, biological) is important in sediment production and influences rates of recession.
Weathering is the breakdown of rock in situ at / near surface of Earth
Weathering and mass movement = subaerial processes.
Weathering attacks the backshore and foreshore parts of the littoral zone. it creates rock fragments that form sediment.
physical (mechanical) weathering = applying force to physically break rock into smaller pieces.
e.g
1. freeze-thaw weathering - water into cracks in rocks and freezes expands in volume by 9% and widens the rock.
Thawing allows more water to enter the crack, and the process repeats until cracks are forced open large, angular pebble, cobble, or boulder-sized fragments are loosened off.
rocks with cracks and fissures are vulnerable to it, especially high on cliffs away from sea spray. Freezing is uncommon on UK coasts.
2. wetting and drying - rocks containing clay minerals e.g clays and shales. at high tide minerals on the rock surface are soaked with sea water and expand in volume. at low tide, minerals dry and shrink. repeated cycles of expansion and contraction cause rock to fragment and crumble.
chemical Weathering = chemical reactions attack minerals in the rock, breaking bonds and producing new chemical compounds.
e.g
1. carbonation - attacks calcium carbonate in limestones.
Rainwater mixed with carbon dioxide from the air to form weak carbonic acid (pH 5.6). acidic rain mixes with calcium carbonate to form soluble calcium bicarbonate solution. Rock disappears as new minerals dissolve into the solution.
- hydrolysis - breakdown of minerals to form new clay minerals, and materials in solution, due to the effect of water and dissolved carbon dioxide. rocks vulnerable to it are: igneous and metamorphic rocks containing feldspar and other silicate minerals
This attacks the feldspar (pinky) minerals in granite. - oxidation - addition of oxygen to minerals esp iron compounds, which produces iron oxides and increases volume = breakdown
attacks iron minerals in haematite cements, e.g. Devonian sandstone. Change in iron compound breaks cement bonds releasing previously cemented clasts as sediment
Rocks that are vulnerable to it are: sandstones, siltstones and shales that often contain iron compounds which can be oxidised
It’s much more effective in seawater or water with impurities than in pure water.
biological weathering = break down of rock in situ by living or once-living organisms. speeds up mechanical or chemical weathering through the actions of plants, bacteria or animals.
Weathering increases rate of recession, it weakens rocks so more vulnerable to mass movement and cliff retreat
in some more vulnerable strataa wave-cut notch forms and deepens more rapidly in weathered rock, leading to faster recession through undercutting and mass movement collapse
Rates of weathering are very slow
Even in a hot, wet climate, basalt (igneous rock) weathers at a rate of 1-2 mm every 1000 years.
A hot, wet climate encourages chemical and biological weathering. carbonation increases in winter because calcium bicarbonate is more soluble in cold conditions.
2B.6b mass movement (blockfall, rotational slumping, landslides) is important on some coasts with weak and/or complex geology.
Mass movement is the downslope movement of material (rock and soil) under the force of gravity. It is the umbrella term for a wide range of specific movements including landslide, rotational slumping and blockfall. occurs when the downslope gravity exceeds the resisting forces of friction and internal rock cohesion.
The type of mass movement depends upon lithology: unconsolidated material (like boulder clay) - slumping consolidated rock (like carboniferous limestone and granite) - sliding
rockfall = occurs on slopes >40’, where a rock fragment breaks away and either drops vertically (so it isn’t in contact with the cliff) or bounces downslope.
It’s initiated:
By mechanical weathering - freeze-thaw, salt crystal growth
which break the cohesive bonds in the rock
By marine erosion - Hydraulic action, Abrasion
Undercutting cliff by creating a wave-cut notch - Notch removed supporting material that supplied the resistive force holding up the rock
Cliffs prone to blockfall have:
a geological structure with many joints, faults or bedding planes
steep, near vertical dip of strata. they’re often also in an earthquake-prone area
rockall is very rapid, taking only a few seconds to occur.
They may involve the detachment of single fragments or of a whole section of cliff that breaks up as it descends (which occurs by the undercutting of a wave-cut notch).
Rotational Slumping = involves rock failure and movement along a curved rock plane. slumping material usually moves intact as a single mass, without any internal deformation of material.
It’s slower than rockfall, often occurring in ‘slow motion’, and may take minutes, hours, days, or even years (for huge masses) to occur.
Rotational slumping occurs in:
weak rocks, e.g. clays and shales
unconsolidated material, e.g. boulder clay, sands, gravels
in rocks with complex geology, e.g. where permeable rock strata overlie impermeable beds
Slumping is facilitated by the presence of water, which adds weight (increasing the gravitational force) as well as lubricating it, reducing friction.
In dry weather, soil above sand cracks, funnelling water into permeable sand.
Increased pore water pressure along lines of percolation form lines of weakness in the sand.
Water accumulates in the lower sand as it is unable to to percolate into the impermeable clay. Pore water pressure lubricating the bedding plane encourages the movement of sand.
The weight of the water adds to the downslope gravitational force, while wave erosion created a notch at the cliff foot, removing support.
Eventually slumping occurs.
Landslides - downslope movement of rock down a flat/linear slip plane, maintaining contact with the cliff surface throughout.
The discrete blocks are released by mechanical weathering of well jointed rocks, (e.g. carboniferous limestone). Gravity then pulls the loosened block down the relatively flat slip plane of the joint or bedding plane, to the cliff foot.
Landslides can also be caused by erosion of a cliff foot undercutting blocks weakened by jointing. The removal of support allows gravity to release the block, resulting in sliding.
Rainstorm events can encourage a landslide, lubricating the slip plane, reducing the resistance.
Landslides occur in consolidated rocks with joints or bedding planes sloping seawards.
2B.6c what landforms does mass movement create?
- rotational scars - the scar left behind due to rotation slump
- talus scree slopes - steep fan shaped slope with large boulders in centre & small material on outside
- terraced cliff profiles - where cliff profile is stepped due to lithology and fractures in rock. caused by slumping
2B.7a longer-term sea level changes result from a complex interplay of factors eustatic (ice formation/melting, thermal changes) and isostatic (post glacial adjustment, subsistence, accretion) and tectonics.
Sea level changes constantly due to tides, variations in surface air pressure, winds pushing on the water surface, creating temporary bulges of higher sea level. long term sea level changes occur over thousands of years.
Eustatic change is a change in global sea level, usually due to a change in the volume of water in the oceans.
Eustatic fall in sea level - During 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 level.
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 temperature increases and causes the volume of ocean water to increase (thermal expansion) leading to sea level rise.
An isostatic change is a change in local land level.
Rises in local land level causes a fall in local sea level. This may be due to:
post-glacial adjustment
accretion = sink regions in the sediment cell are experiencing net deposition, land is built up, leading to a fall in sea level (in delta regions accretion -> subsidence -> accretion and so on)
tectonics
A fall in local land level produces a rise in local sea level. This may be due to:
post-glacial adjustment
subsidence = of land produces rise in sea level
the deposition of sediment, the weight of sediment deposition overcomes threshold and leads to very slow ‘crustal sag’ and delta subsidence, e.g. Nile, Mississippi, Amazon
can also be caused by the lowering of the water table (from increased evaporation from climate change or human abstraction) can lead to settling of overlying sediment and land subsidence as pore water pressure is removed
or by heavy buildings
Post-Glacial Adjustment = During glacial periods, weight of ice depresses the crust in areas below ice sheets. The solid lithosphere is forced down into the plastic asthenosphere. The rigid nature of the solid crust means that when sections of the crust are depressed by ice and forced down, adjacent areas are uplifted in a see-saw effect. The melting of ice causes previously ice-covered crust to slowly rebound upwards whilst adjacent areas subside.
At the end of the last ice age 12,000 years ago, the UK was covered in ice as far down as Birmingham. Northern Britain is experiencing a isostatic fall in sea level as land is uplifted by 1.5 mm per annum. Southern Britain is experiencing an isostatic rise in sea level as land is lowered by 1 mm per annum.
The UK is pivoting, with the south sinking and the north rising.
Tectonics
Eustatic - Rising magma at a constructive plate margin/hot spots lifts the overlying crust, reducing the capacity of the ocean and producing eustatic sea level rise. Uplift of crustal plate reduced Indian Ocean capacity causing 0.1 mm eustatic rise in global sea levels.
Isostatic - Folding of sedimentary rock by compressive forces at a destructive plate margin produces an isostatic fall in sea level for anticlines and a fall for synclines. Lava or ash from volcanic activity produces an isostatic fall, e.g. Hawaiian hot spot island chain
Sea floor spreading - carries volcanic islands away from the uplifted crust at mid-ocean ridge. Colder, more dense crust subsides and sea levels rise, e.g. Tonga, Fiji, Kiribati.
FAULTING can uplift HORST blocks of crust producing
isostatic rise in land & fall in local sea level.
Subsidence of crust blocks by faulting form GRABEN
experiencing isostatic fall in land level & rise in local sea
level.
During 2004 Boxing Day Tsunami in Indian Ocean
extension of crustal plate caused isostatic fall in land on
island of Sumatra by 20 cm in Banda Aceh region.
2B.7b sea level change has produced emergent coastlines (raised beaches with fossil cliffs) and submergent coastlines (rias, fjords and Dalmatian)
Emergent Coastlines
At the start of Holocene Interglacial (10,000 years BP) led to a rapid 100 m eustatic rise in global sea levels, as 3,000 years of ice sheets and glaciers shrank. This happened over about 1000 years, (very rapid) and submerged coastlines. However, the post-glacial adjustment of ice-covered land was much slower.
Emergent coastlines are being produced by post-glacial adjustment. These are parts of the littoral zone where a fall in sea level exposed land once part of the sea bed. They have landforms reflecting the previous sea levels.
Raised beach
A relict beach now above high tide level
A flat surface covered by sand or rounded pebbles/boulders.
Usually vegetated by plant succession (though further succession prevented due to grazing)
Fossil cliff
a steep slope found at the back of a raised beach exhibiting evidence of formation through marine erosion but now above high tide level.
they may contain wave-cut notches, caves and arches providing evidence of formation by marine erosion
episodic nature of isostatic recovery allows marine processes to erode cliffs and deposit beaches when sea levels are stable. Relatively rapid drop in sea level then leaves relict coastline abandoned above high tide and some distance inland.
raised stumps
Submergent coastlines are sections of the littoral zone where sea level rise inundated areas that were previously part of terrestrial land. They are found in southern England and the east coast of America.
- Ria
drowned river valley - a section of river valley flooded by the sea, making it much wider than would be expected based on the river flowing into it.
most common coastal landform.
They are common in periglacial areas that were adjacent to land covered by ice.
Rivers eroded steep-sided V-shaped valleys into the frozen landscape giving the ria a V-shaped cross section when the valley flooded.
Rias are a type of estuarine coastline. - Fjords
drowned glacial valleys - a section of a glacially eroded valley flooded by the sea.
have a relatively straight profile as glaciers truncate spurs to produce a direct downslope route.
Glacier erosion is often cut deep into the landscape, often tens of metres lower than the adjacent unglaciated land - meaning that fjords are often deeper than the adjacent sea.
Fjords often have a shallow entrance where there is a submerged ‘lip’ formed by the ridge of a terminal moraine.
Many fjords are shallowing by a few milimetres per year due to isostatic adjustment. - Dalmatian Coast
long, narrow islands running parallel to the coastline and separated from the coast by narrow sea channels called sounds.
produced by sea level rise flooding the coastline with the geological structure of folds aligned parallel to the coast.
Sea overtops low points forming straits linking straits linking sounds and separating sections of anticline ridge into narrow islands parallel to the coast.
e.g the Dalmatian region of Croatia has limestone coastline with 1,240 islands running parallel to the coast.
Barrier islands?
east coast of the USA has barrier island landforms. may have formed as lines of coastal sand dunes attached to the shore
later sea level rise flooded the land behind the dunes forming a lagoon, but the dunes themselves were not eroded and formed islands
as sea levels continued to rise, the dune systems slowly migrated landwards
rivers and tidal flows maintain open water between islands
barrier islands supplied with sediment from longshore drift
2B.7c contemporary sea level change from global warming or tectonic activity is a risk to some coastlines.
Climatic warming leads to eustatic sea level rise. Warming leads to the melting of mountain glaciers (Himalaya) and polar ice sheets increasing amount of water in ocean store.
sea ice melting has no effect on sea level as the ice was already displacing the equivalent water volume to that produced by melting.
IPCC attributes 50% of sea level rise 1990-2010 to ice sheets melting
Warming also leads to the thermal expansion of existing ocean water as its temperature rises. 94% of increased heat energy in the climate system is transferred to oceans.
Tectonic activity caused the other 10% of sea level rise
Emission of geothermal heat into oceans by underwater volcanic activity can cause thermal expansion of ocean water
Rising magma at constructive plate boundaries produces a doming upwards of crust along mid-ocean ridges reducing the ocean basin volume
At destructive margins:
folding of plates increases ocean basin volume lowering sea levels
earthquakes along boundary can allow rebound of non-subducting margin - uplift of sea floor reduces ocean volume raising sea levels
2004 Boxing Day tsunami with moment magnitude 9.3 lifted sections of the Indian Ocean bed raising sea levels by 0.1 mm
It can also cause isostatic change:
faulting can uplift sections of crust, lowering sea levels (or vice versa)
Sea floor spreading transports volcanic islands away from the uplifted crustal zone along constructive boundaries / hotspots - to places where the ocean floor is colder, denser and lower lying - islands sink
Past Change = Sea levels have risen by 125 m since the Devensian Glacial. Sea level rise was on average 10 mm p.a. in the early Holocene Interglacial (18,000 - 6,000 BP)
Since 1870 = rate of sea level rise has increased. IPCC attributes this to global warming due to anthropogenic forcing through greenhouse gas emissions. Contemporary sea level rise has accelerated since 1940
Future =
IPCC predicts sea level rise of 18-59 cm by 2100. (28-98 in 2013?)
US National Research Council predicts 56-200 cm
This wide variation in prediction is due to:
uncertainties in science of relationship between GHG increase and climatic warming due to complex feedback effects
uncertainties in science of relationship between climatic warming and rate of ice melting
uncertainties about rate of population growth and economic growth impacting on rate of GHG emission
uncertainty about future political commitment to introducing new measures to reduce GHG emissions
Complete melting of Greenland ice sheet would raise global sea levels by 7 m
Complete melting of Antarctic ice sheet would raise sea levels by 50 m
However complete melting of ice sheets would take many centuries even by the most rapid estimates
coastlines at risk
low lying ones - coastal flooding through marine trangression
low lying volcanic islands or coral atolls set atop submerged volcanic guyots e.g. Maldives & Kiribati Islands - at risk of complete disappearance
Volcanic islands at risk from both global warming and tectonic activity
2B.9a local factors increase flood risk on some low lying and estuarine coastline (height, degree of subsidence, vegetation removal); global sea level rise further increases risk.
Sea level rise affects a disproportionate number of people because:
Many low lying coastlines are densely populated as beaches and the sea attract a large number of tourists
Low lying deltas are extremely fertile and ideal for agriculture
Estuaries and deltas are ideal for trade with good navigable access inland up rivers
Many river deltas support megacities, e.g. Shanghai, Yangtze Delta China - 24 million people. Dhaka, Bangladesh, Ganges-Brahmaputra delta - 14 million people. Karachi, Pakistan Indus delta - 23.5 million people
Local Factors
- Height
- Low lying coastlines are only 1-2 m high above (high tide) sea level so at risk from flooding
- Temporary flood risk from storm surges, permanent flooding from global sea level rise
- The Maldives archipelago in the Indian Ocean has a population of 340,000 spread across 1,200 islands. The highest point in the Maldives is only 2.3 m above sea level. Malé, the main island and capital, is protected by a 3 m sea wall
- Bangladesh occupies the Ganges-Brahmaputra delta, 60% of the country is less than 3 m high above sea level
- The Kiribati archipelago in the Pacific Ocean is composed of 33 coral atolls. Most of the population lives on the island of Tarawa where the maximum height above sea level is 3 - Subsidence
- Low lying coastlines in estauries, deltas or outbuilding zones are subject to natural subsidence through the settling and compaction of recently deposited sediment
- However, subsidence is usually outpaced by fresh deposition and the bioaccretion of organic matter
- Deltas experience periodic isostatic subsidence when the weight of the delta sediment reaches the threshold sufficient to cause the crust to depress - leading to marine transgression and flooding
- human activity can also cause local subsidence: Drainage of saturated sediment/soil for agriculture reduces sediment volume and causes subsidence
Weight of cities and built environment can also compress sediment, leading to subsidence
Land reclaimed from the sea is subject to subsidence due to water abstraction via evapo-transpiration by agricultural crops.
- Volcanic islands and coral atolls - seafloor spreading away from hotspot or mid-ocean ridge
- Isostatic readjustment after ice sheet retreat (southern England)
- Bangladesh: 50 large islands in the Ganges-Brahmaputra delta subsided by 1.5 m since 1960. Partly due to isostatic crustal depression and partly due to water abstraction by occupying populations, partly due to natural settling of sediment while the earth bund flood protection prevents compensation fresh sediment deposition
- Vegetation Removal: salt marshes and mangrove forest, reduces flood risk, stabilises existing sediment and traps new sediment, raising the height of the land above sea level, absorbs wave energy, reducing wave impact and erosion. Bangladesh contains the 180 km Sundarbans, the largest mangrove forest in the world. However, 71% is experiencing some vegetation removal
Global Sea Level Rise increases the risk of flooding in low lying coastlines (duh)
global sea level rose by 20cm in the 1900. 50% of the Netherland and large areas of the East Anglian Fens are now below sea level, but protected by coastal defences
IPCC predicts a further 18-59 cm rise in sea level by 2100
Bangladesh - a 40 cm sea level rise would permanently submerge 11% of Bangladesh, creating 7-10 million environmental refugees
Maldives - 50 cm sea level rise would permanently flood 77% of the Maldive Islands’ land area.
2B.9b storm surge events can cause severe coastal flooding with dramatic short term impacts (depressions, tropical cyclones)
Depressions are areas of low air pressure generating surface winds that spiral into the centre of low pressure in an anti-clockwise direction. They occur in mid-latitutes, like the UK. Storms are depression, areas of low surface pressure that generate strong winds. They occur in areas just north and south of the equator. Tropical cyclones are areas of very low surface air pressure (deeper depressions) generating very strong winds
A storm surge = temporary rise in local sea level produced when a depression, storm, tropical cyclone, reaches the coast.
The rise in sea level during the storm surge is accentuated:
At high tide, particularly spring tide, Shape of coastline funnels into increasingly narrow space, Sea bed shallows towards coast
Storm surges can produce severe coastal flooding on low-lying coastlines. Force of storm surge water cause rapid coastal erosion.
Short term impacts:
Deaths and injuries to people immediately through drowning or collapsing buildings
Subsequent deaths from hypothermia (homes destroyed), water borne diseases (sewer systems and freshwater pipe destroyed), natural causes (transport routes to medical care cut)
Destruction of infrastructure - roads, railways, ports, and airports flooded or destroyed
Damaged water pipes, electricity transmission lines and sewage systems - no power or water.
Homes destoyed - older houses worse standards, cheap in poor areas - homes on marginal low lying land (slums and shanty towns) most vulnerable - reconstruction may take several years, richer (insurance) likely to be rehoused first
Businesses destroyed - factories, offices - loss of power, interruption of raw material delivery, workers killed/injured/can’t get there - agricultural land contaminated - crop harvest lost
Bangladesh, Tropical Cyclone Sidr 2007
Category 4 cyclone, air pressure 944 mb, 240 kmph and 6 m storm surges
Impact worsened by: funnel shape of the Bay of Bengal focussing water on Bangladesh at the bay apex. Out flowing discharge from the Ganges and Brahmaputra rivers combine with coastal flooding. Intense rainfall from cyclone increases flooding.
Coastline from unconsolidated delta sediment - easily eroded.
Deforestation of mangrove swamps.
60% of Bangladesh low lying, less than 3 m above sea level
15,000 people killed and 55,000 injured. 1.6 million homes destroyed. 8,000 km of roads, 700 km of electricity transmission lines and 900 fresh water tube wells destroyed. Crops destroyed on 600,000 ha of agricultural land. Total damage estimated at $1.7 billion
However, the impacts of deaths were much lower than in the 1970 Bhola Cyclone where 300,000 were killed. ($90 million economic loss) Improved warnings, embankments and cyclone shelter network saves many lives.
United Kingdom, Storm Xavier, December 2013
80+ mph winds , Storm coincided with spring tide
In the North Sea the coastline narrows into a funnel shape for a storm approaching from the north - storm surge funnelled - sea shallows towards coast - severe coastal flooding.
Average 3 m storm surge in East Anglia, but 6 m at Blakeney in North Norfolk
2 people killed. 18,000 evacuated. Coastal defences breached in Yorkshire and Kent and 1,400 homes flooded.
In Norfolk sand dunes were eroded, seven houses and a lifeboat station destroyed. East coast rail services suspended for one day, economic loss estimated at $100 million
Impacts much lower than the 1953 storm surge when 307 people were killed, 65,000 ha of farmland flooded and there were an economic loss equivalent to $1.2 billion today
This was prevented by: improved flood defences in 2013, including the Thames Barrier (raised during the storm), which protected 800,000 homes according to Environment Agency estimates. Improved forecasting and efficient evacuation also saved lives and mitigated in areas where flooding or erosion still occurred.
2B.9c climate change may increase coastal flood risk (frequency and magnitude of storms, sea level rise) but the pace and magnitude of this threat is uncertain.
High Confidence
Sea level will rise by 18-59 cm by 2100, however the pace and extent of sea level within the predicted range is uncertain due to population growth, economic development, natural positive and negative feedback, political commitment to restrict GHG emissions.
it is also affected by adaption:
building sea walls, e.g. on the North Norfolk coast, 3 m sea wall on Malé.
new artificial island, Hulhamalé created by the reclamation of sediment from the sea bed between 1997-2002, which is 4 m above sea level and cost $32 million to construct.
building earth embankments, like the bunds in Bangladesh
storm surge barriers across river mouths - Thames Barrier, Eastern Scheldt Barrier in the Netherlands (part of the 2.5 billion euro project begun after the 1953 storm surge)
restoration of mangrove forest - protection belts, e.g. Sri Lanka replanting after the 2004 Indian Ocean tsunami killed 6,000 people in one coastal village where mangroves were cleared, but only 2 deaths in an adjacent village protected by a mangrove forest
mitigation - efforts to reduce magnitude of event. Reducing GHG emissions to limit level of global warming would mitigate sea level rise and cyclone intensity.
Delta flooding - The area of the world’s major deltas at risk from coastal flooding is likely to increase by 50%
Medium Confidence
Wind and waves - Some evidence of increase wind speeds and large waves
Coastal erosion - Erosion will generally increase because of the combined effects of changes to weather systems and sea level
Low Confidence - evidence weak and uncertainty high
Tropical cyclones - The frequency is likely to remain unchanged, but there could be more larger storms. Predicted to increase in strength by 2-11% by 2100. Associated rainfall will increase by 20%. Cyclone intensity would increase due to warmer ocean surface temperature and warmed atmosphere holding more moisture.
Number of tropical cyclones not predicted to increase - combination of factors form them, and a warm ocean temperature is only one of them.
However, this is only low confidence - no observed increase in maximum intensity in the Pacific and Indian Oceans over the last 20 years of monitoring. The number and intensity of tropical cyclones is highly variable each year and decade - no statistically significant long term trend.
Storm surges - These are linked to depressions that are likely to become more common. More intense tropical cyclones will exhibit even lower surface air pressure producing larger temporary sea level rises as storm surges and increasing the risk of coastal flooding
Depressions - Polar front jet streams will accelerate, possibly increasing the number and intensity of depressions and storm surges in mid-latitudes
The magnitude and timing of all these changes is uncertain.
