aqa coasts

Inputs

Inputs

sediment can be brought into the system in various ways. Energy inputs come from wind, waves, tides and currents.

Outputs

e.g. sediment can be washed out to sea or deposited further along the shore.

Flows/Transfers

e.g. processes such as erosion, weathering, transportation and deposition can move sediment within the system.

Stores/Components

landforms such as beaches, dunes and spits

Negative Feedback

when the effects of an action are cancelled out by its subsequent knock-on effects.

Positive Feedback

when the effects of an action are amplified or multiplied by subsequent knock-on effects (a loop/cycle).

Sources of energy

• Wind

• Wave

• Tidal

• Currents

Wave Energy

• created by the frictional drag of the wind over the water.

• effect of wave depends on height. height is determined by wind speed and fetch of the wind.

• waves break as they approach the shore. Friction with the sea bed slows the bottom of the waves.

Size of wave depends on 3 factors:

• Distance wave has travelled

• Time wind has been blowing

• Strength of the wind

Wave height

height difference between a wave crest and the neighbouring trough

Wavelength

distance between successive crests

Wave frequency

time between one crest and the following crest passing a fixed point

Constructive waves

• swash greater then backwash

• weak backwash, low-energy deposition

• form beaches

• long, not very high max 1m

• frequency 6-9/minute

• form in calm conditions with light winds

• leads to formation of ridges (berms)

Destructive waves

• backwash greater than swash

• remove material from the beach

• erode the coastline

• 2-3m in height and steep

• frequency of 11-15/minute

• form in stormy conditions

• may form a ridge called a storm beach

Wave refraction

Slowing and bending of progressive waves in shallow water.

Energy dissipates in deeper waters, waves are a lot smaller and don't slow down as much at the deeper bays.

Energy of waves is concentrated at the headland, waves are bigger and erosion more likely in this shallow water.

Wind energy

• winds are created by air moving from areas of high pressure to areas of low pressure. During events such as storms, the jump from one to the other is large.

• strong winds produce powerful waves.

Most coastlines will have a prevailing wind direction. The wind will generally reach the coast from one direction.

This therefore controls:

1. the direction that waves approach.

2. the direction material is transported.

Prevailing wind

the dominant wind direction in a particular location.

Tides

The periodic rise and fall of the ocean surface, caused by the gravitational pull of the moon and the sun.

They affect the position at which waves break on the beach e.g. at higher tides, waves break higher up the beach.

Sea currents

Current is the general flow of water in one direction - it can't be caused by wind or by variations in water temperature and salinity.

They move material along the coast.

Termohaline circulation

currents driven by the difference in water's density which is controlled by temperature and salinity.

High energy coasts

• typical landforms: headlands, cliffs, wave-cut platforms

• coastlines where strong, steady prevailing winds create high energy waves

• rate of erosion greater than rate of deposition

• e.g. exposed Atlantic coasts of northern Europe + north America. north Cornish coast of south-west England.

Low energy coasts

• typical landforms: beaches, spits

• coastlines where wave energy is low

• rate of deposition often exceeds rate of erosion of sediment

• e.g. many estuaries, inlets and sheltered bays. The Baltic sea, sheltered waters + low tidal range.

Inputs of sediment at the coast

• rivers, streams, river estuaries

• sea level rise floods river valleys forming estuaries which input sediment

• cliff erosion

• biological material e.g. shells

• offshore sand banks

• wind

• glaciers

Sediment budget

the difference between the amount of sediment that enters the system and the amount that leaves.

Positive sediment budget: more sediment enters.

Negative sediment budget: more sediment leaves.

Sediment cell

• the coast is divided into sediment cells, or littoral cells.

These cells are self-contained, sediment doesn't move between the cells.

Processes in one cell don't affect any other cell. Each cell is a closed coastal system.

Erosional processes: CORRASION (ABRASION)

Sediment dragged up and down/across shoreline, erodes and smooths rocky surfaces. Created wave-cut platform.

Erosional processes: CAVITATION

Air bubbles forming in waves implode under high pressure, generating tiny jets of water which erode rock over time.

Erosional processes: WAVE QUARRYING

• wave exerts considerable energy as tonnes of water hit the rock face

• this high pressure is compressed between wave and cliff

• if air is trapped, pressure may loosen blocks of rocks

Erosional processes: SOLUTION (CORROSION)

Weak acids in seawater dissolve alkaline rock e.g. limestone.

Erosional processes: ATTRITION

Bits of rock in the water smash against each other and break into smaller bits.

Transportation processes: TRACTION

rolling of coarse sediment along the sea bed that is too heavy to be picked up and carried by the sea

Transportation processes: SALTATION

sediment 'bounced' along the seabed, light enough to be picked up/dislodged but too heavy to remain within the flow of the water.

Transportation processes: SUSPENSION

smaller (lighter) sediment picked up and carried within the flow of the water.

Transportation processes: SOLUTION

chemicals dissolved in the water, transported and precipitated elsewhere. This form of transportation plays an important role in the carbon cycle, transferring and redepositing carbon in oceans.

Factors affecting transportation

• velocity (energy)

• particle size (mass)

e.g.

• high energy environments: large particles can be transported

• low energy environments: small particles can be transported

Littoral drift

• most waves approach a beach at an angle - usually same direction a prevailing wind

• backwash pulls material down the beach at right angles to the shore (due to force of gravity)

• net effect of the zigzag movement of sediment up and down the beach

Marine deposition

sediment carried by seawater is dropped.

Aeolian deposition

sediment carried by wind is dropped.

Weathering

breakdown or disintegration of rock in situ. Active at the coast where rock faces are exposed to the elements.

Mechanical weathering: FROST SHATTERING

• Water expands by 10% when it freezes

• It enters a crack, freezes, and the expansion exerts pressure on the rock.

Mechanical weathering: SALT CRYSTALLISATION

• Salt water leaves behind salt crystals when it evaporates

• Salt can corrode rock

Mechanical weathering: WETTING AND DRYING

• Rocks rich in clay expand when wet and contract when dry

• Over time, they crack and break up

Biological weathering: PLANT ROOTS

grow into cracks, widened as they grow

Biological weathering: BIRDS

animals dig burrows into cliffs

Biological weathering: MARINE ORGANISMS

burrow into rocks or secrete acids (e.g. limpets)

Chemical weathering: CARBONATION

• Rainwater absorbs CO2, forms weak carbonic acid - Reacts with calcium carbonate in rocks e.g. limestone, dissolves them - More CO2 absorbed in winter months when it's cold

Chemical weathering: OXIDATION

rock minerals react with oxygen to form rusty red powder, more vulnerable to weathering

Chemical weathering: SOLUTION

dissolving of rock minerals

Mass movement

the downhill movement of rock and soil under the influence of gravity.

Types of mass movement

• creep

• flow

• slide

Factors affecting type of mass movement

• angle of slope/cliff

• rock type

• rock structure

• vegetation cover

• how wet ground is

Sub-aerial processes

weathering and mass movement

Mass movement: SOIL CREEP

• Extremely slow

• movement of individual soil particles

• particles rise to ground surface due to wetting/freezing, return vertically to the surface in response to gravity as soil dries out/thaws

• implied by formation of terracettes

• build-up of soil on the upslope side of walls and bending of tree trunks

Mass movement: MUDFLOWS

• mud flowing downhill over unconsolidated or weak bedrock e.g. clay

• often after heavy rainfall

• water gets trapped within rock, increased water pressure forcing rock particles apart

• pore water pressure = form of energy within slope system Extremely important factor in determining slope instability.

• sudden + fast-flowing

• represent a significant natural hazard

Mass movement: LANDSLIDES

• block of rock moving very rapidly downhill along a planar surface (slide plane)

• bedding plane is roughly parallel to the ground

• moving block remains largely intact

• frequently triggered by earthquakes or very heavy rainfall

• slip surface becomes lubricated and friction is reduced

• very rapid, considerable threat to people and property.

Mass movement: ROCKFALL

• sudden collapse or breaking away of individual rock fragments (or block)

• steep/vertical cliffs in heavily jointed and often quite resistant rock

• often triggered by mechanical weathering (particularly freeze-thaw) or an earthquake

• rocks fall/bounce down slope to form scree

• scree forms a temporary store, also an input as it is removed and transported

Mass movement: SLUMP

• differs from landslides but slide surface is curved rather than flat

• in weak, unconsolidated clays and sands

• often where permeable rock overlies impermeable rock, build-up of pore water pressure

• characterised by a sharp break of slope and formation of a scar

• can result in a terraced appearance

Mass movement: RUNOFF

• when overland flow occurs down a slope/cliff face, small particles are moved downslope

• potentially forms input to sediment cell

• transfers water and sediment from one store to another (rock face to beach/sea)

• toxic chemicals can contaminate storm water = threat to coastal ecosystems

Mass movement: SOLIFLUCTION

• similar to soil creep but specific to cold periglacial environments

• in summer, surface layer of soil thaws out and becomes extremely saturated as it lies on top of impermeable frozen ground (permafrost)

• sodden soil with blanket of vegetation slowly moves downhill by combo of heave and flow

• characteristically form solifluction lobes

Cliff profile

the shape of a cliff (e.g. steepness)

Steep cliff profiles - where rock is more resistant, where there is no beach and an exposed orientation with high-energy waves

Gentle cliff profiles - less resistant/unconsolidated rocks that are prone to slumping. In sheltered locations with low-energy waves.

Factors affecting rate of cliff retreat

• rate of weathering + mass movement

• rock type

• wave energy

Erosional features: WAVE-CUT NOTCHES

erosion concentrated at high-tide line undercutting the cliff face.

Erosional features: WAVE-CUT PLATFORMS

• as cliff retreats it leaves gently sloping platform

• only completely exposed at low tide

• force waves to break earlier, reducing rate of erosion of cliff face

Erosional features: HEADLANDS AND BAYS

• form where there are bands of alternating hard rock and soft rock at right angles to the shoreline.

• soft rock eroded quickly, forming a bay. harder rock eroded less quickly forming a headland.

Erosional features: CAVES, ARCHES, STACKS

Depositional landforms: BEACHES

• beaches form when constructive waves deposit sediment on the shore.

• act as a store in the coastal system.

• are mainly composed of sand and shingle.

Sand: gentle gradient beach as sand particles compact when wet. Little energy lost to friction and material is carried down beach. Leads to development of ridges.

Shingle: may make up whole/just upper part of beach. Water percolates through shingle so backwash is limited in transporting material.

Depositional landforms: BEACHES - Storm Beach

ridge composed of biggest boulders thrown by largest waves.

Depositional landforms: BEACHES - Berms

built up by constructive waves during successive lower high-tides.

Depositional landforms: BEACHES - Cusps

semi-circular shaped depressions formed when waves break directly on beach with strong swash + backwash. junction of shingle and sand.

Depositional landforms: SPIT

• form where the coastline suddenly changes direction.

• LSD continues to deposit sediment across the river mouth, leaving a long narrow feature extending from land into sea.

• changes to dominant wind + wave direction can curve the end of the spit.

• area behind the spit is sheltered from waves, often developing into mudflats and saltmarshes.

Depositional landforms: OFFSHORE BARS + TOMBOLOS

• bars form when a spit joins two headlands.

• across a bay or river mouth

• act as a sediment sink + input stores.

• tombolos connect land to separate island.

• absorb wave energy, reducing impact of waves on coastline.

• lagoon forms behind bar

Depositional landforms: BARRIER ISLANDS

• where a beach/spit extends across a bay to join two headlands.

• form where there's a good supply of sediment, a gentle slope offshore, fairly powerful waves and a small tidal range.

• LSD adds more sediment.

• can trap water behind them to from lagoons.

Depositional landforms: SAND DUNES

• form on the dry backshore of a flat sandy beach due to winds blowing sand onto the land. Over time vegetation will grow on the sand.

• form when sand deposited by LSD is moved up the beach by the wind.

• sand trapped by berms is colonised by plants/grasses

• vegetation stabilises sand, encouraging more sand to accumulate

Depositional landforms: SAND DUNES - Factors needed for sand dunes to form (psammosere)

• large supply of sand

• onshore wind (wind blowing from the sea)

• large tidal range (to give sand time to dry)

• obstacles for the sand to build up against

Depositional landforms: MUDFLATS + SALTMARSHES

• form in sheltered, low-energy environments eg. behind spits

• rising tides push water into estuary from the sea, slowing river velocity = deposition

• most sediment deposited is mud, forming mudflats and over time salt marshes

Depositional landforms: MUDFLATS + SALTMARSHES - salt marsh formation (halosere)

Factors needed:

1. Sheltered areas where deposition occurs

2. Where salt and freshwater meet

3. Where there are no strong tides or currents to prevent sediment deposition accumulation

exist in intertidal zone, covered at high, exposed at low.

• mud deposited at high tide line

• pioneer plants colonise intertidal zone

• more mud is trapped by plants = more land for veg to grow

• soil becomes more stable, more veg grows leading to trees colonising

Sea level

The relative position of the sea as it comes into contact with the land

Eustatic sea level change

caused by the change in the volume of the water in the sea, or by a change in the shape of the ocean basins

Types of eustatic sea level change

WATER AS ICE

• ice sheets melt after glacial period or freeze during glacial period, causing global change in water volume in oceans

• when locked in ice, there's less liquid in seas so level falls

THERMAL EXPANSION

• higher ocean temp causes molecules to vibrate more due to kinetic energy, causing water to expand and take up more room = sea level rise.

TECTONIC MOVEMENTS

• can alter shape of ocean basin, affecting sea level

Isostatic sea level change

caused by vertical movements of the land relative to the sea, changes in height of the land.

Types of isostatic sea level change

ISOSTATIC SUBSIDENCE

• ice on land is heavy, during glacial period land may sink due to weight

• when ice melts, weight is removed, causing land to rise + recover = relative fall in sea level.

TECTONIC PROCESSES

• can cause land to rise/fall

• e.g. 2004 earthquake, crust sank, sea level rose 0.1mm permanently.

SEDIMENT LOADING

• sediment pouring into an estuary from a river can be deposited, causing extra weight to push down on the land.

Sea level change in past 10,000 years

• at maximum, sea level was 130m lower than present

• has been rising since 1930

Climate change and sea level

• 1900-2016; rise of 1.08 degrees.

• climate change a result of human activities: deforestation, fossil fuel burning

• increased GHGs = global warming = melted ice sheets + thermal expansion = sea level rise

• rising 2mm per year

Impacts of climate change + sea level rise on coastal areas

1. Storms more frequent + more intense. Causes damage to coastal ecosystems and settlements.

2. Sea level rise has major impacts on coastal areas: • More severe coastal flooding. • Submergence of low-lying islands. • Changes in the coastline. • Contamination of water sources and farmland.

3. Increased coastal erosion, putting ecosystems, homes and businesses at risk.

Emergent coastlines

areas of coast which have risen above the present sea level.

Emergent coastlines: RAISED BEACHES

• waves erode cliff

• as cliff is undercut, it loses stability + breaks

• cliff starts to retreat to form wave-cut platform

• material is transported away, and platform rises as weight of material is now removed

• high-tide level is now lower than it previous was

e.g King's Cave, Scotland

Submergent coastlines

areas of coast which have fallen below the present sea level

Submergent coastlines: RIAS

• submerged river valleys

• v-shaped

• gentle + long cross-profile

• wide and deep at mouth, get shallower further inland

e.g. Kingsbridge, Devon

Submergent coastlines: FJORDS

• submerged glacial valleys

• u-shaped

• created as glacier carves its way through a river valley

• water up to 1000m deep

• deeper as you go inland

e.g. Hardangerfjord, Norway

Submergent coastlines: DALMATIAN COASTLINE

• where topography of land runs parallel to the coastline and becomes flooded by sea level rise

• leaves behind islands which are parallel to the coast

e.g. Dalmatian Coast, Croatia

Hard engineering

involves a physical change to the landscape using resistant materials e.g. concrete, boulders, wood and metal.

Soft engineering

uses natural systems for coastal defence, such as beaches, dunes, salt marshes, which can absorb and adjust to wave and tide energy.

4 approached/options for coastal management

1. Hold the line - maintaining existing coastal defences.

2. Advance the line - build new coastal defences further out to sea.

3. Do nothing - build no coastal defences at all.

4. Managed realignment - allow shoreline to move, but manage retreat to cause least damage.

Hard engineering: GROYNES

structures of wood capture sediment being carried by long shore drift. help build up the beach, to dissipate wave energy

Cost: quite cheap

Pros: work with natural processes, increased tourist potential, not too expensive.

Cons: prevents input down coast, increasing erosion down coast, unnatural.

Hard engineering: SEA WALLS

stone/concrete walls at foot of clidd or top of cliff reflect waves back into the sea.

Cost: expensive to build + maintain

Pros: effective, promenade for people to walk along.

Cons: reflect energy rather than absorbing it, intrusive + unnatural, very expensive.

Hard engineering: RIP RAP

large rocks placed at foot of cliff or top of beach. permeable barrier to sea, breaking up waves.

Cost: fairly cheap

Pros: cheap, easy to construct, used for recreation.

Cons: intrusive, not local rocks so out of place, dangerous as people can climb on them.

Hard engineering: REVETMENTS

sloping wooden concrete.rock structures at foot of cliff or top of beach. they break up wave's energy.

Cost: expensive to build, cheap to maintain

Pros: inexpensive to build.

Cons: intrusive + unnatural, high level of maintenance.

Hard engineering: OFFSHORE BREAKWATER

partly submerged rock barrier, designed to break up waves before they reach the coast.

Cost: expensive

Pros: effective.

Cons: intrusive + unnatural, high level of maintenance as damaged in storms.

Soft engineering: BEACH NOURISHMENT

adding sand/pebbles to a beach to make it higher/wider.

Pros: cheap, easy to maintain, natural, increased tourist potential.

Cons: constant maintenance.

Soft engineering: CLIFF REGRADING + DRAINAGE

reducing angle of the cliff to help stabilise it. drainage removes water to help prevent landslides/slumping.

Pros: effective on clay/loose rock, cost-effective.

Cons: regrading causes cliff to retreat and drainage can dry cliff out = collapse.

Soft engineering: DUNE STABILISATION

marram grass planted to stabilise dunes. areas fenced off so people don't enter.

Pros: natural environment, habitat for wildlife, cheap, sustainable.

Cons: time consuming, negative response as areas fenced off.