Coasts

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  • Created by: Fran99
  • Created on: 11-01-16 12:48

Coasts

Coasts are constantly being reshaped by:

  • Waves
  • Tides
  • Ocean currents
  • Effects of the weather

When rock structures are more resistant or sheltered from prevailing wind and waves, changes occur slowly.

When rock structures are less resistant and are open to storm conditions and heavy rainfall, sudden and dramatic changes can occur.

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Waves

A wave is a long body of water which curls into an arch form and then breaks on the shore. Waves are created by the wind blowing on the surface, this then creates friction proceding a swell. Energy from the wind causes water particles to rotate inside the swell created, this then rotates which causes the wave to move forward.

  • As waves approach the shore they break, friction with the sea bed slows down the bottom of the waves and makes their motion more elliptical, the crest of the wave rises up and then collapses.
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Littoral Cell System

A littoral cell is a length of coastline that is an independant system as far as the movement of sand or shingle is concerned. They are important because a change in flood defence in one sub-cell will have an impacr in another, but not an adjacent littoral cell.

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California Littoral Cell

Inputs

  • River deposits
  • Sediment from cliff
  • Materials for beach replenishment.

Processes

  • Rip current 
  • Offshore currents
  • Longshore drift
  • Erosion.

Outputs

  • Movement out to sea by currents
  • Fine silts and sands are trapped in the harbour and needed to be dreged.
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Marine Processes

Attrition: When rocks crash into each other causing them to be worn down, into smaller and more rounded pieces.

Abrasion: This is when sand, shingle and boulders are hurled against a cliff, causing it to wear away the cliff.

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Marine Processes

Hydraulic Action: A breaking wave traps air as it hits a clif face. The force of water compresses this air into any gap in the rock face, creating pressure within the joint. As the water pulls back there is an explosive effect of air under pressure being released.

Solution: Dissolving of calcium-based rocks by the chemicals in the sea water and the evaporation of salts from water in the rocks.

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Constructive Waves

  • Builds up beaches due to a strong swash.
  • Long wavelength.
  • Low frequency (about 6-8 waves per minute).
  • Sediment thrown up by the breaking waves accumulates in ridges or 'berms'.
  • Backwash has little power to  move the sediment back towards the sea.
  • Low energy and gradient.
  • Also called swell or surging waves.
  • Depostional.
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Destructive Waves

  • Higher frequency (10-14 waves per minute).
  • Short wavelenght.
  • Steep gradient.
  • Caused by local wings or storms.
  • Also called surfing, storm or plunging waves.
  • High energy.
  • Backwash is stronger then swash.
  • Ealrosion.
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Tides

The effects of a wave on a coastline is only between high and low tide. they are regualr rising and falling movement of the surface of the sea. Tides are caused by the gravitional pull mainly from the moon, but can also be from the sun. There are two high tides in one 24 hour day.

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Neap and Spring Tide

The moon takes 29 days to orbit the earth, its position in relation to the earth and sun will depend whether there is a spring tide or a neap tide. When the moon is in the middle of the earth and the sun, the moon and the sun combined their gravitional pull to create the biggest bulge of water and the highest tide known as a spring tide. A neap tide occurs when the earth, moon and the sun form a right angle, causing the gravitional pull to interfer with each other; creating the lowest tide.

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Storm Surge

Storm Surges occur when the following factors collide:

  • High tide.
  • Strong onshore winds creating high levels of wave energy.
  • Low pressure weather systems allowing the sea to expand.

The shape of the landscape can add to the intensity of a storm surge:

  • Where the sea is 'pushed' into a narrow area between two land masses, it is forced to rise causing flooding in coastal areas.
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Sub-Aerial Processes

Sub-aerial processes describes coastal processes that are not linked to the action of the sea, it included weathering and mass movment.

  • Weathering: The wearing away of the rock by weather. this happen everywhere not just at the coast.
  • Mass Movement: The movememt of material down a slope by the force of gravity.
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Factors that affect the rate of Marine Processes

Wave Types:Steeper waves have more energy meaning they have a greater erosion power. Waves that break at the foot of the cliff release more energy than those who break some distance away.

Fetch: How far a wave has travelled determines the amount of energy it has collected.

Human Activity:People may remove protective materials from beachesleading to more erosion. Or they may reduce erosion by putting in flood defence, sea defences in one place may lead to an increase of erosion further down the coast.

Coastal Configuration:Headlands attract wave energy through refraction.

Beach Preence:The beach absorbs wave energy and therfore acts as protection against erosion. Steep, narrow beaches dissipate energy from flatter waves. Flattish, wide beaches spread out the energy and disspate high and rapid energy inputs.

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Factors that affect the rate of Marine Processes

Geology:This affects the rate of erosion in different ways e.g. some rocks are more resistant to erosion than others, for example granite is more resistant than clay.

Lithology:This refers to the characteristics of rocks, mainly their resistance to erosion and permeability.

Concordant:When rocks run parallel to the coast. If a harder rock isthe first line it protects the coast from erosio as it is more resistance.

Discordant:When rocks run at a right angle to the coast. This allows the sea to erode weaker rocks creating large bays.

Angle Dip:The steepest cliffs tend to form in rocks that have horizontal strata or which dip gently inland. rocks that dip towards the coast tend to produce much more gentle sloping features.

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Weathering

Freeze Thaw:When water repeatedly freezes and melts within cracks in a rock. Water expands when it freeze, and creates pressure on the coast. With repeatd fluction in pressure, fragments of the rock may break off.

Biological Weathering:Weathering resulting from organic agents, such as tree roots growing inot and physically widening the joints.

Chemical Weathering:Occurs when there is alternative wetting and drying towards the bottom of slopes where material and moisture often accumulate.

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Mass Movement

Soil Creep: Slowest form of mass movement. Raindrop impact, in intense storms may cause a splash of soil particles. The particles that fall on the downslope fall further. Wet periods add additional moisture to soil particles which swell upoon wetting, then they expand and fall due to gravity in drier periods.

Soilfluction:Occurs mainly in tundra areas where the soil is often frozen for most of the year and there is little vegetation. When the topsoil thraw in brief summer months, the additional water and lack of vegetation to hold the saturated soil together causes a flowing active layer.

Earthflows: Faster movement than soil creep and occur on steeper slopes, which have become become saturated, produce bulging lobes of soil.

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Mass Movement

Slides/Slumps: Slides largely retain their internal structure and move as a large mass. Slumps occur when the movement seems to have a rotational element and may produce a curved rupture surface. Cliffs formed of relatively weak or impermeable rocks are susceptible to rotational slumping after prolonged rainfall as the rising of the water table undergroud reduces the internal friction of particles and facililates failure. 

Rockfall:Relatively rare movement. May result from from extreme of physical or chemical weathering. They produce deloris slopes beneath the cliff as the material from the rockfall disintegrates at the cliff foot.

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Landforms of Erosion

Geos: Where erosion digs in enugh material along a joint or plane of weakness, a steep-sided inlet may form.

Cave: If the inlet orginally created (geos) continues to extend into the cliff, a cave will form.

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Landforms of Erosion

Blow Hole: Where the cave keeps developing, and the overlaying (roof) rock may collapse due to gravity, opening the cave up to the sky.

Wave-Cut Platform: When the wave begins to retreat due to erosion at the bottom of the cliff, it leaves a gentle sloping base. 

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Landforms of Erosion

Cove/Bay: This happens where there are areas of alternating high and low resistant rocks. the less resistant rocks experience the most erosion, forming bays, while the more resistant rocks form headlands. Due to refracturing the headlands recieve the highest-energy waves and are more vulnerable to the forces of erosion than the bays are. The bays experience low-energy waves that allow sediment to accumulate and form beaches. Thses act to protect that part of the coastline. When the rocks run parallel with the coast, it is possible for continued erosion to break through the more resistant rocks to the coast and begin to attack the weaker rock behind it. if that happens, a cove will form which will be enlargedby erosion into a bay. 

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Landforms of Erosion

Cliffs: Formed due to weathering breaking bits of the rock and erosion breaks of bits as well.

Wave Cut Notch: When high steep waves break at the foot of a cliff they concentrate their erosion capabilities into the only small area of the rock face. This concentration eventually leads to the cliff being undercut.

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Landforms of Erosion

Arch, Stocks and Stumps: Once a geo is formed the gap will normally keep eroding backward, once it has fully eroded to the other side of the rock, it is known as an arch. Eventually the top of the arch will collape due to gravity leaving a stack. the bottom of the stack will be eroded causing it to collapse, leaving a stump.

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Coastal Deposition

What causes Deposition to Occur?

Deposition happens when there is a lack of energy to transport the sediment. This can be beacuse of wave refraction, where all the energy is concentrated on to the headlands, meaning the waves going into the bay have little energy and what energy there is, is spread out. Deposition can also occur when the sea meets a physical barrier such as a river.

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Wave Refraction

As waves approach the coastline, they leave deep water and are increasingly affected by frictional drag resulting from contact with the seabed. this causes them to gradually ralign to become more parallel to the line of the coast. Where the coastline is irregular some parts of the wave will be slowed doen by frictional contact and some will remain largely unaffected and may move faster. Thus there is a concentration of wave energy on the headlands and dissipation of energy in the bays.

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Landforms of Deposition

Beaches

  • Bars: This is when sand accumulates at the lower edge of a beach, the material had probably been combed from the beach by destructive waves.
  • Runnels: They seperate pools of standing water at low tide. They run parallel to the shoreline at the low water mark. The runnels are distrupted by channels that help to drain water dow the beach
  • Storm Beach: Built up by strong swash during spring tide and consists of the largest calibre materail thrown up by the largest waves. The storm beaches remain largely unmoved because the tides cannot reach them.
  • Berms: These are ridges below the storm beach, and they mark the successively lower high tides as the cycle goes from spring tide to neap tides.
  • Cusps: These form where there is a junction of sand and shingle. They are semicircular depressions which form when waves break directly onto the beach and the swash and backwash are strong. Once the curving shape is created, swah is concentrated in the small bay in the centre of the cusp, which produces a stronger backwash.
  • Ripples: Develop on the sand by wave action. 
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Landforms of Deposition

Salt Marshes

Salt marshes only form in low energy environments where there is shelter from the wind and waves. Depositional landforms such as spit can help provide this shelter. Salt marshes require a large input of sediment which can arrive from the sea and rivers. The most likely place along a coastline where you’ll find this sort of sediment input is near a tidal flat. The low gradient of a tidal flat means that any rivers that flow into it will very quickly deposit any sediment they’re transporting. At the same time, the periodic flooding of the tidal flat by the tides will deposit even more sediment.

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Landforms of Deposition

Salt Marshes

Over time, sediment accumulates and the elevation of the tidal flat increases in a process known as coastal accretion. This reduces the duration of tidal flooding allowing a small selection of plants to grow on the now developing salt marsh. These plants are halophytic—they love salt—and are capable of surviving underwater for several hours a day. They’re often called pioneer species because of their hardy nature and, well, pioneering growth on salt marshes. These plants, which include species of cordgrass (Spartina) and glasswort (Salicornia), have several adaptations that not only help them thrive in saline environments.

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Landforms of Deposition

Salt Marshes

Long blades of cordgrass trap sediment that is too fine to settle out of water in a salt marsh, building up a muddy substrate. At the same time, the roots of the cordgrass plant (that are long to tap into the water table) help stabilise already deposited sediment, aiding coastal accretion. Pioneer species such as Spartina alterniflora (a species of cordgrass) are invasive plants that spread rapidly. Once these plants are introduced to a salt marsh, coastal accretion takes place quickly and the elevation of the salt marsh increases greatly. This creates new environments that are submerged by the tide for shorter periods of time, allowing even more species of plants and animals to colonise the salt marsh.

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Landforms of Deposition

Sand Dunes

Sand Dunes are concentrations of mound-like landforms composed of sand that has been blown off the beach by onshore winds.

There are several conditions that need to be met for sand dunes to develop. First, a large supply of sediment is needed. The best place to get this is from a large tidal flat. An area with a large tidal range (a big difference between the high and low tide) will result in a lot of sand being exposed to the wind, ready to be transported. A (relatively) strong and continuous wind is needed to move sand grains and transport them inland via saltation. The best place to find strong winds that don’t change direction is in areas that face the prevailing wind direction.

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Landforms of Deposition

Sand Dunes

Initially sand may become trapped by deloris towards the back of the beach, for example on a high berm or storm beach. the sand has a high pH (carbonate from seashells) and is not moisture retentive, so only very hardy pioneer plants such as lyme grass can colonise at this stage.

The first dunes to develop are known as embryo dunes. Grasses such as sea couch, lyme and marram survive here because they grow upwards through accumulating wind-blown sand, stabilising the surface. These robust species have advantages such as the ability to spread underground, succulent leaves to store water and dep tap roots to reach down for water. All these plants add organic matter to the dunes, aiding water etention and making the environment more hospitable for later plants.

As the embryo dunes grows they develop into bigger foredunes (yellow dunes), which are initially yello but darken to grey as decaying plants add humus to the soil over time. 

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Landforms of Deposition

Sand Dunes

Further colonisation by new species allows the dunes inland (grey dunes and dune ridges) to become more fixed. As the soil recieves more organic material, the nutrient supply and water retention improve and the microclimate is more congenial to plants that could not tolerate the early dune conditions. For example, lichens, mosses and flowering plants begin to appear along with red fescue grass, creeping will and dewberry.

Depressions between the dune ridges may develop into dune slacks, which are damper areas where the water table is closest to the surface. Here mosses, rushes and willows will thrive if there is enough moisture. 

Behind the yellow and grey dunes, very little new sand is added from the beach, so these wasting dunes and dune health exhibit smaller dune features. Plants such as heather, gorse, broom and buckthrone can thrive here as the soil is more acidic, humus rich and water retentive. In time the rear of the dune system may be colonised by pine birch and oak, hich are the first steps to the development of the type of woodland.

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Landforms of Deposition

Spits

A spit is a narrow and long piece of land that is joined to the mainland at one end and the  projects out to sea. 

they begin to form when there is a sudden change of direction in the coatline. Longshore drift normally transports material along the coastline,however if there is a sudden change in direction in the coastline, longshore drift won't follow it, but will carry on transporting material in the direction it was already going in, this causes material to be transported out to sea. Over time there becomes a build up of material in the more sheltered area, by the headlands. As the material begins to project out to sea, storms will add to the developing spit. Increasingly though, the end of the spit will begin to curve round as wave refraction transports material round into the more sheltered water

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Landforms of Deposition

Bar

The process of how a bar is formed is similar to that of a spit. However both ends will be connected to a headland blocking off a bay behind it. Both ends are connected to a headland because there isn't a strong flow of water coming from the in land side.

Tombolo

Again the process of a tombolo is similar to a spit, however a tombolo connects the mainland to an island.

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Succession

  • The changing structure of a plant community over time.
  • Each stage is known as a seral stage.
  • Begins with pioneer species, which make conditions more hospitable for other species through adding more organic content to and stabilising the soil.
  • the final stage is known as a climatic climax- a community of plants that are fully adapted top their local climate and environment.
  • If humans interfere in this process, then the community of plants is known as a plagioclimax.
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Zonation

  • This refers to the way in which ecosystems develop to cover an area of ground - it is the spatial equivalent of succession.
  • In the case of dunes, this means a series of 'strips' of vegetation running parallel to the shore.
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Key Terms

  • Halosere: A salty environment in which succession occurs.
  • Halophyte: A species of plant that is salt tolerant. They have a number of methods of dealing with regular changes in levels of salinity and water cover. For example they have deep roots to anchor them in the mud, and can extract nitrogen from the air.
  • Bioconstruction: The porcess whereby plants modify their environment,stabilising soils through the presence of roots and the contribution of organci content.
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Sea Levels Change

Eustatic Change: This is a global change in sea levels

Isostatic Change: This is the land in a certain area changing height, therfore sea levels change. (Local change)

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Long Term Sea Level Change

Stage One: As the climate begins to get colder marking the onset of a new glacial period, an increasing amount of precipitation falls as snow. Eventually, this snow turns into glacier ice. Snow and ice act as a store for water, so the hydrological cycle slows down - water cycled from the sea to the land does not return to the sea. As a consequence sea levels fall and this affects the whole planet.

Stage Two: The weight of ice causes the land surface to sink. this affects only some coastlines and then to a varying degree. Such movement is said to be isostatic and it moderates the eustatic sea level fall in some areas.

Stage Three: the climate begins to get warmer. Eventually the ice masses on the land begins to melt. This starts to replenish the main store and sea levels rise worldwide (eustatic). In many areas this floods the lowerparts of the land.

Stage Four: As the ice is removed from some land areas they begin to move back up to their previous levels (isostatic readjustment). If the isostatic movemtn is faster than the eustatic, then emergent features are formed such as raised beaches. Isostatic recovery is complicated as it affects different places in different ways. In some parts of the world it is still taking place as the land continues to adjust to having masses of ice removed.

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Emergent Coast

  • Caused when sea level is falling or land rising.
  • Raised Beaches: These are reremians of former coastlines e.g. Portland Dorset.
  • Relict Cliffs: An old Cliff life, which used to be on the coastla front e.g. Scotland
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Submergent Coastlines

  • Caused when sea level is rising or land sinks.
  • Rias: A deep sunken valley drowned by the sea.
  • Fjords: Drowned glacial valley e.g. norway
  • Fjards: Drowned glacial low lands e.g. Bantry Bay, Ireland.
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Coastal Protection and Management

Sea Walls: Hard Engineering

  • Protects coast from wave energy by shielding it.
  • Some have a recurved structure with a lip at the top to throw waves backwards.
  • Waves are retracted away from coast so it is not eroded.
  • Where waves rebound the energy is still available for erosion elsewhere.
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Coastal Protection and Management

Coastal Barrages: Hard Engineering

  • Partly submerged wall with sluice gates to control the flow of river water from the land.
  • Maintian constant water level.
  • Control flooding.
  • cause large changes in sediment pattern due to affecting river and tidal flow.
  • Remove the intertidal habitats of wading birds.
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Coastal Protection and Management

Rock Armour or Rip-Rap: Hard Engineering

  • Large boulders dumped in fron of a cliff or sea wall to take full force of the waves.
  • Absorb more wave energy.
  • Lower cost.
  • Not as long lasting as sea walls.
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Coastal Protection and Management

Groynes: Hard Engineering

  • Wooden or stell break waters built nearly at a right angle to the waves.
  • Slows down long shore drift by trapping sediment.
  • Builds up beach.
  • May have serious effecrs down the coast as a reduction of the supply of sediment can accelerate erosion.
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Coastal Protection and Management

Cliff Fixing: Hard Engineering 

  • Driving iron bars into the cliff face to stabilise it and absorb smoe wave power.
  • Metal mesh netting may also be added to try and stop lose rocks falling so easily.
  • Doesn't actually stop erosion.
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Coastal Protection and Management

Offshore Reefs: Hard Engineering

  • Encourage waves to break offshore, which reduces impact on the base of the cliff.
  • reduces erosion at the base of the cliff.
  • Generates surfing waves. 
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Coastal Protection and Management

Beaches, Dunes, Salt Marshes: Soft Engineering

  • Absorbs wave energy and tide energy.
  • Can adjust with time.
  • Lower cost.
  • More natural appearance.
  • Not as effective in events of storms.
  • Require regualr maintenance.
  • Shifting nature of defences.
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Coastal Protection and Management

Managed Retreat

Do nothing: There is no further active intervention. Without intervention the defences would eventually fail and areas currently protected from flooding would no longer be protected.

Hold the line: This involves maintianing the exisitng flood defences and control structures in their present position and increasing the standard of protection against flooding in some areas.

Managed Realignment: Place new flood defences landward of the exisitingflood defences or realign exisitng defences to higher ground. 

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