The Coastal System
Coastal System - the coast is a narrow zone where the land and the sea overlap. Coats are perhaps the most varied and rapidly changing of landforms, as this is the zone that witnesses the interaction of many processes.
- Inputs - Wind direction, Fetch and Strength
- Processes - Erosion, Transportation and Deposition
- Output - Coastal landforms
What process help shape the coastline?
Sediment Sources and Cells
Sediment Sources:- The sources of sediment at the coast include coastal erosion, the sediment transported by rivers and the wind, shell and marine deposits transported either along the coast by longshore drift or onshore by ttides and currents. Sediments may also be lost to the coastal system if they are blown inland, transported offshore or removed by human action.
Sediment Cell:- A length of coastline that is relatively self-contained as far as the movement of sand or shingle is concerned and where interruption to such movement should not have a significant effect on adjacent sediment cells.
There are 11 on the England and Wales coastline.
Waves:- are created by the action of wind on te surface of the sea.
- Locally generated waves are typified by steeper, shorter period and higher-energy waveforms.
- Swell (from distant storms) exhibit less steep, longer perios and lower energy waves.
Constructive and Destructive waves
Constructive Waves (beach)
- Strong swash pushes sediment up.
- Weak backwash soaks into beach, sediments not pulled back.
- Long wavelength.
- Lower energy waves.
Destrcutive Waves (cliff)
- Short wavelength.
- Steep wave faces and high wave height.
- Wave crashes downwards into through with little swash.
- Backwash is very strong and drags material back.
- Backwash interferes with swash of the next wave.
- Higher energy waves generate localised storm conditions.
Wave trough:- the lowest point of a wave
Wave frequecy:- the number of waves per minute.
Fetch:- distance over which the wind blew.
Wave steepness:- the ratio of the wave heoght to the wavelength.
Wind velocity:- wind speed.
Duration:- period of time during which the wind blew.
Wave period:- the time taken for a wave to travel between one wavelength.
Wave crest:- the highest point of a wave.
Wave energy:- proportional to the wavelength and wave height.
- When the coastline is irregular, some parts of the wave will be slowed down by frictional contact.
- Some will remain largely unimpeded and may move faster.
- The orthogonals (direction of the waves) converge on the headlands and represent higher energy waves but diverge in bays and represent lower energy waves.
Tides:- is the periodic rise and fall in the level of the sea.
- They are caused by the gravitational pull of the moon and the Sun, although the moon has much the greater influence because it is nearer.
- As the moon orbits the Earth, the high tides follow it.
- Twice in a lunar month, when the Moon, Sun and Earth are in a straight line.
- This produces the highest monthly tidal range - spring tide.
- When the moon and sun are positioned at 90o to each other in relation to the Earth.
- This alignment gives the lowest monthly tidal range - neap tide.
Case Study - Holderness Coast
- The Holderness Coast is one of Europe's fastest eroding coastlines.
- The rate of erosion is about 2 metres per year.
- Most of the coast is made up of soft boulder clay, which erodes rapidly.
- Many villages have been lost by the eroded coastline.
- Some defences have been attempted.
- At Flamborough, the chalk is exposed.
- This has caused caves, arches and stacks to form.
- Spurn Point provides evidence of longshore drift on the Holderness coast
- Around 3% of the material eroded from the Holderness Coast is deposited here each year.
Types of Erosion - Marine Erosion
- When the waves break it traps ai, when the waves hit the cliffs the air is forced into the cracks in the cliff.
- This puts them under pressure, when the water pulls back the pressure is realed and causes an explosive effect.
- The rocks in the sea, are slowly worn down into smaller and more rounded pieces.
- Sand, shingle and boulders in the sea are hurled against a cliff.
- This creates an intertial rock platform, which is caused by sediments grinding at the platform.
- The dissolving of calcium-based rocks by chemicals in the sea water.
- The evaporation of salts from water in the rocks produce crystals, that put stress upon rocks.
Sub-ariel Weathering - Mechanical
- Water gets into the cracks in the rocks and reapeatedly freezes.
- With repeated fluctuations in pressure, fragments of rock break off.
- When the rocks are porous and permeable, they soak up the rain and get heavy.
- Also, the ground can get satuated which can cause landslides.
- When jointed rocks are under pressure, it opens up the cracks and joints even more.
Sub-ariel Weathering - Biological
- Tree roots growing into rocks, physically widen joints.
- Also, animals burrowing can also widen joints.
- The secretions of molluscs, sponges and sea urchines can also weather exposed rocks in tidal zones.
Sub-ariel Weathering - Chemcial
- Rocks are exposed to oxygen in air and water they react.
- Rocks that may include salts absorb water and swell.
- This makes them more prone to decomposition.
- The H+ and OH+ ions combine with ions, this results in producing weaker clay.
- Carbonic acid in rainwater attacks limestone and creates cracks.
- The dissloving action of water on rocks.
- Humic acids (from plant decay), cause a process called chelation.
- Carbon dioxide in the atmosphere produce acid rain, which accelerates weathering of limestone.
Types of mass movement - Rock Fall
- Steeper slopes.
- More than 40o
- Very rare.
- Little warning.
- Large amount of material.
- E.g. Beachy Head
Types of mass movement - Landslide
- Most severe.
- Little warning.
- After periods of heavy rain.
- Weathered material can also slip along the bedding plane.
Types of mass movement - Soil Creep
- Slowest type of downhill movement.
- Less than 1cm a year.
- Occurs at times a heavy rain, it becomes heavy and moves down the hill.
- When soil dries it contacts and moves back up the hill.
Types of mass movement - Rotational Slumping
- Fast movements.
- Along bedding plane which is curves
- Where rock has reacted with water to dissolve such as limestone and chalk.
- Fine sediment carried within the water.
- The rolling effect of large heavy boulders.
- The hopping or bouncing effect of the pebbles, which are too heavy to be suspended.
- When waves approach he shore at an angle, material is pushed up the beach by the swash in the same direction.
- As the wave aproach, water runs back down the beach the backwash drags material down the steepest gradient.
- Which is generally at right angles, this creates a zigzag movement.
- Obstacles such as groynes. piers or headlands entrap each material
Case Study - Holbeck Hotel
- In the South of Scarborough, Holbeck Hotel was destryed between the night of 3rd June and 5th June 1993.
- This was caused by a rotational slip, which cut back the cliff by 70m.
- The cause of the landslide were:
- Rainfall of 140mm in 2 months.
- Issues related to the drainage of the slope.
- Water pressure build up in the slope.
- The first signs were cracks i n the footpath running across the cliff.
- At 6am on 4th June, 55m of the garden had disappeared.
- By 5th June, the east wing of the hotel had collapsed.
- The landslide was a rotational landslide, which degraded to a mudflow, which covered the beach with rocks.
The Beach Profile
Backshore:- the area between the high water mark and the landward limit of marine activity.
Foreshore:- is the area lying between the high water mark and low water mark.
Inshore:- is the area between the low water mark and the pint where waves cease to have any influence on the land beneath them.
Offshore:- the area beyond the point where waves cease to impact upon he seabed and in which activity is limited to deposition of sediments.
Factors That Effect a Coastline
There are a number of factors that determine the shape, form and appearance of coastline:
- Wave size, frequency, type, energy produced and direction.
- Local sea currents.
- Longshore drift.
- Depth of water offshore.
- Type and amount of sediments off shore.
- Rock type and structure.
- Sub-aerial processes.
- Land-based agents of erosion.
- Climate and weather.
- Long-term sea level change.
- Coastal ecosystems.
- The presence of coral.
- Human activity.
Formation of bays and headlands - discordant
- Where there are alternating resistant and less resistant rocks, bays and headlands are formed.
- Because of refraction, the headlands receive the highest energy waves and are more vulnerable to forces of erosion.
- The bays experience low energy waves, which allows sediment to accumuate and form beaches.
- Dorset coastline is an example.
Formation of bays and headlands - concordant
- Where the rocks run parallel with the coast, it is possible for continued erosion to break through the more resistant rocks.
- Which then begin to attack the weaker rock behind.
- A cove forms which will be enlarged by erosion into a bay.
Deposition Landform Features
- This is where a spit or bar connects the mainalnd to an island.
- This is bevause the island obstructs the waves and wind, which the slows down longshore drift.
- E.g. Llandudno, North Wales.
- A ridge of sand and shingle, which has joined two headlands, cutting off a bay.
- Behind the bar, a lagoon is created.
- This is made due to longshore drift operating on a coastline from two different directions.
- The two sets of storm waves build up a series of ridges, creating the triangular feature.
- E.g. Dungeness, Sussex.
- Spits formed in shallow and sheltered water when there is a change in the direction of the coastline.
- The material is dropped due to the reduction in energy.
Deposition Landform Features Diagram
Sand Dune Succession - Psammosere
- Dunes are mobile
- PH problems
- Lack of nutrients
- The succession that develops is known as a halosere:- tolerant to salty conditions.
- Succession plants want to get to the Climatic Climax Community.
Saltmarsh - Pioneer Stage
- Halophytes:- Salt tolerant plants.
- Conditions for growth:- Sediment deposited by river normally found behind a spit.
- Needs to withstand the periodic tides.
- Seal Blite
- Accumulation of dead organic matter by 1-25mm per year:
- Marsh grass
- Sea lavender
- Sea thrift
- Eventually, the marsh rises above sea level and is not regularly inundated by seawater:
- When the marsh is developed tolerant trees arrive completing succession (Climatic Climax Community) :
Case Study - Barton-on-Sea
- The cliffs are famous for their fossil content.
- The fossils are exposed, as the sea erodes the cliffs about 1m a year.
- There have been major engineerig activities to try to stop the erosion.
- Parts of the cliff is made up of soft clay, but with water-bearing sand on top.
- But, there is porous and permeable gravel on top.
- This means the cliff is vulnerable to landslides.
- Pressure for coastal defence schemes, as there is popular residential area and there have been housing developments.
Case Study - North Sea Storm Surge 1953
- The height of the land above sea level.
- The tidal range.
- The incidence of storm surges.
- A deep depression moved from the Shetland to the Netherlands, the low-pressure air allowed the see to rise by 0.5m.
- Strong winds were drawn into the centre, waves over 6m high were created.
- The storm surge funeled water towards southern North Sea.
- The tides reaches 2 to 3m in East Anglia, the Thames Estuary and the Netherlands, causing widespread flooding.
- Over 260 people died in England.
- 1835 died in the Netherlands.
Case Study - North Sea Storm Surge 1953 continued
- The Netherlands have reclaimed land from the sea, also in East Anglia marshland was drained
- East Anglia has many trade links.
- Also, many people desire to have a holiday home near the sea, increasing East Anglia's coastal population.
- The defence scheme of 1953 were inadequate.
What did they do?
- A year after, £20 million had been spent on reinforcing thousand of miles of sea and tidal river defences.
- The Thames Barrier did not become operational until 1982, and have not been compromised since.
- However, sea levels are rising and more extreme storms are predicted in the next 25 years.
- The Environement Agency is currently budgeting £200 million over 12 years to be spent on extra defences.
Case Study - Hurricane Katrina 2005
- On 23rd August 2005 the huricane started off as a tropical depression.
- Then on the 24th it turned into a tropical storm.
- The wind speed then increase from 75mph to 125mph.
- The New Orleans is a low-lying area.
- The storm went from category 1 (74-95 mph) to category 5 (155+mph).
- New Orleans was the most damaged city.
- Storm surges reached over 6m in height.
- The levee's were unable to cope with the strength of Katrina.
- People found refuge in the Superdome Stadium, which became unhygenic and there was a food and water shortage.
- Looting continued happened.
- 1200 people drowned.
- Oil facilities were damaged.
- Petrol prices rose in the UK and USA.
Case Study - Hurricane Katrina 2005
Hurricane Katrina Continued
- The authorities were critised for their handling of the disaster.
- Many people were evacuated.
- The governement gave $50 billion in aid.
- The UK governemt sent food aid during the early stages of the recovery process.
- The National Guard was mobilised to restore and maintain law and order.
Sea Level Change
- Sea - levels are predicted to rise by 1m by 2100, because of:
- Thermal expansion
- Glaciers melting
Eustatic:- global-scale sea level change cuased by a change in the volume of water in the ocean store.
Isostatic:- local-scale sea level change caused by a change in the level of the land relative to the level of the sea.
Emergence:- the impact of a relative FALL in sea level.
Submergence:- the impact of a RISE in reltive sea level.
Sea Level Change - Eustatic and Isostatic
Eustatic (global) Changes
- A decrease in global temperature leads to more precipitation occuring in the form of snow.
- Eventually, the snow turns to ice meaning the water is stored on land.
- Consequently, there is a global FALL in sea level.
- When temperatures rise, glaciers retreat and ice melts causing a RISE in global sea level.
Isostatic (local) Changes
- During a glacial period, the weight of the ice adds weight to the earth's crust.
- This causes the crust to sink lower into the mantle rock beneath.
- This causes the sea level to appear to rise.
- At the end of the gracial period, the weight of the ice is reduced.
- The crust starts to slowly rise.
- The sea level appears to fall.
- E.g East coast of Scotland are rising at a rate of 7mm a year.
Sea Level Change - Landforms
Landforms of Submergence
- Fjards - submerged glacial lowlands.
- Dalmatian coats - submerged valleys running parallel to the coast.
Landforms of Emergence
- Raised/relic beaches
- Abandoned coastlines
Features of Sea Level Change
- Created by rising sea levels drowning river valleys.
- The floodplain of the river will vanish.
- This leaves the higher land dry and produces rjas.
- These are drowned glacial valleys.
- Found in Norway and New Zealand.
- Fjords have step valley sides are fairly straight and are narrow.
Features of Sea Level Change
- There is evidence of relatively higher sea-levels in the form of raised beaches.
- They are deposits of sand and pebbles on gently sloping platforms.
- Cliffs that used to be active (eroded), when the sea reached the raised beach beneath it.
- But now the cliffs are not being eroded.
- Sea walls - protect the coast from wave energy by shieling it with a facade of stone, steel adn concrete.
- Rock armour - consists of large boulders put infront of a cliff or sea wall to take the full force of the waves.
- Groynes - used as a buffer between the waves and the cliff line.
- Gabions - a cage filled with rocks, concreye or sand that takes the impact of the waves.
- Revetments - sloping structures made of stone or cement to absorb the energy of waves.
- Coastal barrages - partly submerged walls with sluice gates to control the flow of river water and variations in tidal flow.
- Cliff fixing - driving iron bars into the cliff face to stabilise it.
- Offshore reefs - encourages the waves to break offshore, which reduces their impact on the base of cliffs.
- Beach nourishment - where sediments that is lost through longshore drift are replaces, to create a wider beach.
- Stabilising sand dunes - planting vegetation in the dune it reduces the impact of wind and water, also the vegetation stops the sand moving so it takes the impact of the waves.
- Managed retreat - where no protection is put into place; they just check how erosion has occured over the years.
Case Study - Chesil Beach
- Chesil beach is a tombolo in Dorset.
- This connects the Isle of Portland to the mainland of the Dorset coast
- Chesil beach stretches for 18 miles.
- Lagoons have formed behind the stretch of beach.
Case Study - Hurst Castle Spit
- Hurst Castle Spit is a shingle bank in Hampshire.
- The spit extends 2.5 km from the end of Milford Beach out into the Solent.
- It is an important coastal defence - protecting the Western Solent from flooding.
- It also shelters the designated area of saltmarsh to the north.
- The spit has been declining in volume - since the coastal protection of Christchurch Bay.
- This interrupted the movement of the shingle that maintains its stability.
- Behind the spit, a salt marsh has formed - Key haven saltmarsh.
Case Study - Keyhaven Marshes
- Keyhaven marshes are locaste on the South coast of England.
- They were formed behind Hurst Castle Spit, which has formed because of longshore drift from the West.
- The spit has sheltered the land, for sediment accumulation and for eel grass to accumulate.
- The eel gradd helps stabilise the area - pioneer plant.
- Halophytes accumulate in the mud flats - glass wort and sea blite.
- Eventually the saltmarsh will grow further back - vegetation succession