The Coastal Zone
The coastal zone is the zone between the land and sea, coasts are always changing.
Advantages of living by the coast include - fishing, good agricultural land found next to the coast, access for trade, tourism opportunities and recreation.
Hard rock coast - e.g. Granite at Land's End, Cornwall
Shape of cliff - high, steep and rugged.
On the cliff face - cliff face often bare with no vegetation and little lose rock,
At the foot of the cliff face - a few boulders and rocks have fallen from the cliffs.
Soft rock coast - e.g. Scarborough, Yorkshire
Shape of cliff - may be high but are less rugged and not so steep,
On the cliff face - may be piles of mud and clay which have slipped down the cliff face,
At the foot of the cliff face - very few rocks, some mud and sand.
Types of Erosion
Corrasion: The material in the wave eroding the coastline. Waves carry debris (such as sand, shingle and pebbles) which is hurled against the coastline. The debris rubs against the rock and eventually causes pieces to break off. These broken fragments are in turn thrown back at the rock face by the waves and the process repeats itself. This causes dents in the coast which gradually grow.
Abrasion: This occurs when waves cause loose pieces of debris (also known as scree) to collide. The collision of the scree causes grinding and chipping which in turn makes the debris particles smaller, smoother and rounder.
Solution: Also known as Corrosion occurs when the pH level of the sea is below the neutral 7.0. The acidic sea water slowly dissolves and corrodes cliff faces with rocks such as limestone particularly affected as it has such a high pH level.
Hydraulic action: This occurs when the waves break against cliffs and enter the cracks in the rock. The air is then compressed and puts rock under pressure causing chips and pieces of the rock to break away, after a while, this cam result in a cave which eventually collapses.
Wave pounding: This is when huge waves break at the base of the cliff, shaking and weakening it. This enables the other erosion processes to work more effectively.
How landforms are formed
Wave cut north/platform
- The sea attacks the base of the cliff forming a wave-cut notch.
- The notch increases in size causing the cliff to collapse.
- The process repeats and the cliff continues to retreat.
- A wave cut platform is left at the bottom, exposed at low tides.
Erosion of a headland
- Weak areas are attacked by waves and opened to form a cave.
- The cave is widened and deepened by erosion to form an arch.
- As the roof of the arch is continually undercut it eventually collapses leaving an isolated stack.
- Stack is continually eroded eventually forming a stump.
Rock structure - the way rock types are arranged, usually in layers (strata).
Discordant coastline has different layers of rock at right angles to the coast.
Concordant coastline has the same rock type the whole way along the coastline.
How sediment is transported and deposited of coast
Longshore Drift - the particles of sand or shingle are transported along the beach in a zig zag movement, carried by the swash and backwash. The LSD is often in the same direction as the prevailing wind.
Spits - At a corner in the coastline LSD continues to deposit out to sea forming a neck of sand and shingle. The end is curved round by the wind and waves. Salt marsh forms in the centre of the spit.
A tombolo is where the sediment joins the mainland to an island. A bar joins two sections of mainland.
Sand dune plants (e.g Marram grass) are plants that grow on sand dunes - they need to be tough and so have adaptations such as:
Long roots to hold them in place during strong winds,
Tough, waxy leaves,
They can survive being sprayed with salt water.
Concordant coast - Lulworth Cove
Lulworth cove is a natural harbour formed by the combined action of river and sea:
Formation began after the last ice age. A river swollen by melt water flowed overland to the sea. The river cut a valley and breached the Portland Stone.
The rising sea flooded into the valley and started to exploit joints and weaknesses in the Purbeck beds.
Erosion occurred on the soft Wealden Clay, which had already been scoped out by the river.
The cove formed as a pan shaped inlet.
The cove continued to enlarge, eroding through the greensand rock.
Erosion was contained and slowed down by the semi hard chalk beds to produce the semi landlocked cove of today.
Discordant Coast - Headlands and bays
The soft rock erodes much faster than the hard rock - creating bays. The more resistant rock is left sticking out as headlands. Weaknesses in the rock can be small natural cracks - joints or larger cracks caused by past tectonic plate movements - faults.
The difference between erosion, weathering and mas
Weathering/sub aerial processes occur in three main ways:
1) Mechanical weathering - salt crystal growth as sea water lands on rocks, the water is evaporated leaving salt behind. The salt crystals grow and leave stresses in the rock.
2) Chemical weathering - All rain is slightly acidic, this causes rocks to decay.
3) Biological weathering - Roots of vegetation can grow in cracks in a rock and split the rock apart.
Rock fall - occurs when fragments of rock weathered from a cliff face fall under gravity and collect at the base.
Slumping - occurs when the bottom of a cliff is eroded by the waves making the slope steeper. The cliff can slide downwards due to rain saturation which lubricates the rocks and makes them heavier.
The wind blows across the sea, the friction between the wind and the water created waves. Wave size depends on:
The strength of the wind,
How long the wind blows for,
The length of the water the wind blows over (the fetch).
Wavelength is the distance from trough to trough or crest to crest. Wave amplitude (power) is the vertical high from the trough to the crest.
How do waves break?
out at sea when the wind creates a wave shape,
within a wave each water particle moves in a circular motion and returns to the start,
it is only energy and not the water itself that is moving forward,
when the wave reaches shallow water the wave is distorted from a circular shape to an ellipse shape until it becomes so top heave that it 'breaks',
it is now not only energy but also water that moves forward.
Swash is when the waves come up the beach in the direction of the prevailing wind.
Backwash is when the waves go back down the beach at right angles due to gravity.
Constructive waves occur in summer, they are small with long wavelengths and low amplitudes. They have a strong swash so transport sand up the beach and deposit it - this builds up the beach.
Destructive waves occur in winter, they have a larger amplitude so are taller, they have a smaller wavelength and are plunging waves - this means they are dangerous because they are so quick the backwash has to flow under the incoming waves creating a rip current which is dangerous to swimmers as it can drag them out to sea. They have a strong backwash which erodes sand from the beach and carries it out to sea where it is deposited and a steep beach is formed.
How some coasts are threatened by rapid erosion an
With increasing sea levels due to thermal expansion and the melting of the ice sheets, low lying coast lines such as Bangladesh, Essex and the Pacific Islands are at risk.
The gravity of the moon creates tides - twice a day we have high tides which gives raised sea levels.
A few times a year we have 'spring tides' which are very high.
If a spring tide and low air pressure coincide a storm surge occurs - huge waves flooding the coast.
Global warming could make depressions more powerful and therefore storm surges more common. Higher sea levels and more stormed would speed up erosion rates. The sea defences currently installed would be useless and a lot of money would have to be spent on new ones.
Management options for the coast
Hard engineering - traditional, building structures, costly, ugly.
Sea walls reflect the waves back out to sea, they are expensive, make it hard to access the beach and have high maintenance costs as the wall itself erodes.
Groynes trap and stop the longshore drift from moving along. This builds up a nice big sandy beach. This is the best form of protection against erosion - the waves move around every grain of sand, taking lots of energy out of the wave. Groynes are good for tourism but have a negative impact down the coast which is starved of sediment - here the beach becomes small and offers less protection so erosion rates increase gently, this causes conflict.
Rip rap are big boulders placed at the base of the cliff to dissipate the energy of the waves, they look natural. However, they make access to the beach difficult and can be hard to transport.
Management options for the coast - continued
Soft engineering - working with nature, cheaper, less intrusive.
Plating vegetation - makes the cliffs more stable.
Beach nourishment - pump sand onto beaches, having dredged it from under the sea to make nice big sandy beaches. This has to be maintained as LSD moves the sediment down the coast all the time.
Off shore breakwaters - forces the waves to break before reaching the beach.
In cliff drainage to prevent saturation.
Managed retreat - people and activities are gradually moved back from the vulnerable areas of the coastline.
Cliff regrading - making the cliff face longer so it is less steep.
Some places are too expensive to protect, or the value of the land/buildings doesn't justify the cost. Defences may cause erosion down coast and some places may be impossible to protect due to rising sea levels. In some places, defences are being abandoned and nature let take its course. At the moment the government thinks it is too expensive to protect isolated houses and farmland. It is hard to convince people who have lived there all their lives that protecting their property is not sustainable. We don't know the impact that rising sea levels will have so planning new defences is difficult. CONFLICT.
Holistic management - managing the whole stretch of coast and not just one place. Holistic management takes into account:
The needs of different groups and people, economic costs and benefits, the environment of land and sea.
ICZM - intergrated coastal zone management.
SMP - Shoreline management plans (for long stretches of coast).
Councils have 4 choices on how to manage the coast.
1) Hold the line - use defences to stop erosion and keep the coast where it is, expensive.
2) Advance the line - move the coast further into the sea, very expensive.
3) Strategic retreat - gradually let the coast erode and move people/business away as necessary. Compensation must be paid.
4) Do nothing and let nature take it's course.
Councils would undertake a cost-benefit analysis to indentify whether the option is economically viable - if nothing is done there will be loss of houses. Building defences against the sea vs. people not losing their farms and land.
Rates of erosion in both Swanage Bay and Durlston Bay are around 40-50cm per year.
Methods of protection include a sea wall built in the 1920's - this provided a barrier to wave attack and a promenade.
Cliff regrading and a series of steps made into the cliff to lower the slope angle.
A series of timber groynes in the 1930's to reduce LSD and make sure the beach absorbed the energy of the breaking waves.
Beach replenishment - 90 000m3 of sand dredged from Studland bay and pumped onto the beach. Cost of recent new groynes and beach replenishment was £2.2 mil.
Durlston bay - cliff regraded, slope brought forward to make it longer and less steep. Installing drainage removed excess moisture so slope wasn't too heavy. Rip rap to resist wave attack.
Reasons for choice in:
Swanage - Erosion occurs along a large distance of the cliff, not just one point. Cliff top has houses and hotels which were in danger of collapsing. Area is a tourist destination and therefore economically viable to protect.
Durlston - Erosion occurs mainly in one particular point where there are weaknesses in the cliff. Cliff top has houses and apartments. Economically viable to protect that particular point due to the price of the houses on the top of the cliff.