Coasts

The systems approach is a way of analysing the relationship within a unit, for example a coast. It consists of a number of components and linkages; for example, inputs into the system can inlcude energy from wind, waves, tides and sea currents, components can come in the form of erosional or depositional landforms and landscapes, finally outputs from the system would include the dipasstion of wave energy or accumilation of sediment. 

The coast is an open system - it has inputs (in the form of waves, wind, tides and currents), stores/components, flows/transfers and outputs of both energy and matter, which cross the noundary of the system to the surrounding environment. The combination of all these factors form distinctive landscapes, which are made up of a range of erosional and depositional landscapes. 

Coasts are dynamic places. Therefore, the system is in a state of dynamic equilibirum when there is a balance between inputs and outputs. Changes occur to upset the balance of the system, e.g. landslides, storms or human activity. The system adjusts by a process of feedback which can either be positive or negative. 

Wind is the primary source of energy for a range of other processes: 

• Length of fetch determines the size of energy of waves. 
• It is an agent of erosion and transport.
• It is a primary source of energy for other processes, e.g. waves. 

Waves are often a source of high energy. The South West of England and Wales can receive massive waves. Waves are the movement of water moleculres within the ocean, and are restricted to the surface layers. They are created by the transfer of energy from the wind blowing over the surface of the sea. Wave energy can result from wind velocity, depth of water, duration of wind, direction of the wind or the fetch. 

Key features of waves - 

• Height - The difference between the crest and the trough of the wave
• Length - The distance between crests. 
• Frequency - The time lapse between crusts.

There are two types of wave; constructive and destructive. Constructive waves will move material slowly up the beach forming berms. Destructive beaches will move very little material up the beach and will form storm beaches. 

Wave refraction - When the waves approach a coastline that isnt a regular shape, they are refracted and become increasingly parallel to the coastline. As each wave approaches the coast it drags in the shallow water, this increases wave heigh, steepness and length. The part of the wave in deeper water moves forward faster, causing the wave to bend. 

The term currents refers to the permanent or seasonal movement of surface water in the seas and oceans. 

Longshore currents - Occur as most waves don't hit the coastline 'head on' but approach at an angle to the shoreline. This generates a flow of water parallel to the coastline. 

Rip currents - Strong currents moving away from the shoreline. They develop when seawater is piled up along the coastline by oncoming waves. 

Upwelling - Movement of cold water from deep in the ocean towards the surface

Tides are the periodic rise and fall in the level of the sea. They are caused by gravitational pull of the sun and moon, although the moon has a greater influence because its nearer. 

As the moon orbits the Earth, the high tides follow it. Twice a lunar month, when the moon, sun and Earth are in a straight line, the tide-raising force is strongest. This produces the highest monthly tidal range or spring tide. 

Also, twice a month, the moon and sun are positioned perpendicular to the Earth. This alignment gives the lowest monthly tidal range, or neap tides. 

Low energy coasts - 

• Low wave energy 
• Waves spread out, energy is dissipated. 
• Low waves
• Deposition is greater than erosion
• Low tidal range

Landforms; beaches, spits etc. 

High energy coasts - 

• Predominantly strong wave power
• High waves
• Strong prevailing wind
• Erosion is greater than deposition

Landforms; wave-cut platforms, e.g. the west coast of Britian

The crustal sediments that form depositional features such as beaches and mudflats originate from the following sources: 

• Seabed - Rising sea levels have meant that sediment from continental shelf areas have been swept towards the shoreline. 
• Rivers - Rivers account for 90% of coastal sediment. 
• Cliff erosion - Sediment from erosion contributes only 5% or less to coastal system. 
• Biological origin - for example, shells and corals. 

Sediment movement occurs in distinct areas called cells. If part of a larger cell they are called sub-cells. For example, the Flamborough Head-Humber Estuary sub-cell is part of the larger Flamborough Head-The Wash cell, 

Sediment budget is the balance between sediment being added to and removed from the coastal system. If more material is added than removed there is a positive budget, this means that the shoreline will build towards the sea. If more material is removed than added then there is a negative budget, this means that the shoreline will recede landwards. Human activity can interfere with the processes operating in a sediment cell by distrupting the supply of the sediment. For example, groynes can block the movement of sediment causing beach erosion further down drift. 

Material can be transported by the seawater in a number of ways. These include: 

• Traction - Large stones and boulders are olled and slid along the beach by moving water. This happens in high energy environments. 
• Saltation - Small stones bounce or leadfrog along the seabed and beach. 
• Suspension - Very small particles of sand and silt are carried along by moving water.
• Solution - Dissolved materials are transported within the mass of moving water. 
• Longshore drift - Waves approach the shore at an angle and swash moved matieral up the beach in a diagonal direction. Backwash then pulls the material straight down the beach, the result is that material is transported in a zig-zag fashion. 

Physical weathering - 

• Freeze-thaw, where water percolates into cracks, joints and pores in a cliff. When temperatures drop to 0 the water freezes and expands, this exerts pressure on the surrounding rock causing the cracks to widen. This pressure is released and the ice thaws. It is common in countries within mid-latitudes and deserts where temperatures drop to below 0. 
• Pressure release, when rock is newly exposed, the pressure of overlying rock is suddenly released. This can cause rocks to crack. Thermal expansion then occurs, this is when the rock expands when it gets heated and contracts when cooled. 

Biological weathering - 

• Includes processes that lead to the breakdown of rocks by the action of vegetation and coastal organisms. Biological weathering is quite common on coastlines. Some marine organisms, such as paddock, have specially adapted shells that enable them to produce sponge-like rock pitted with holes. Animals such as birds or rabbits can also weaken cliffs as they burrow or dig into them. 
• Seaweed attaches itself to rocks and the action of the sea can be enough to cause swaying seaweed to prise away any loose rocks from the sea floor. 

Chemical weathering - 

• Oxidation - Where iron minerals in rocks are exposed to oxygen in the air. The oxygen oxidizes the iron forming rust which crumbles. 
• Hydrolysis - Where hydrogen and hydroxide ions in water combine with mineral ions in the rock. This causes the rocks to breakdown. 
• Hydration - Where rocks containing mineral salt absorb water and swell.
• Carbonation - Where rainwater absorbs Co2 from the atmosphere to form weak carbonic acid. 

Landslides - Occur mainly on cliffs made from softer rocks or deposited materials which slip down when lubricted. It is the rapid movement of detached slabs of rock down a slide plane. 

Rockfall - Where individual rock fragments fall of a cliff face, often due to weathering and particularly freeze-thaw. 

Mud flows - Saturated soil flow downhill, often triggered by heavy rain. 

Soil creep - Very slow downhill movement of individual rock particles. 

Rotational slip/slumping - Where softer materials overlie much more resistant materials after heavy rainfall. Whole sections of cliff face may move downward with a slide plane that is concave. 

Wave-cut platforms - 

• When waves break against the foot of a cliff, erosion tends to be concentrated close to the high-tide line. This creates a feature called a wave cut notch and begins to undercut the cliff. This weakens the base of the cliff and means that some of the cliff face will erode and fall off. This leads to cliffs retreating. Eventually the cliff becomes unstable and the upper part collapses. The repetition of these processes over hundreds of years cause the cliff to retreat inland. At is base it leaves behind a smooth and rocky wave-cut platform.

Caves - 

• Forms due to hydraulic action/wave quarrying. 

Arches - 

• The sea had worn through the cave and formed an arch. 

Stacks and Stumps - 

• Weathering and erosion will attack the arch and eventually a stack/stump will be formed. Examples of stacks include Old Harry and old Harry's wife. A stump is a smaller/collapsed version of a stack. 

Geo -

• Forms when the sea attacks a crack made out of weak rock. 

Beaches - 

• A zone of deposition between high and low tidal limits. Normally are found at the head of the bay, the bay becomes a natural sediment trap protected by the headlands. They are mainly constructed from sand and shingle. Berms (ridges), beach cusps and storm beaches can all occur on a beach.

Salt Marshes -

• Behind the spit a low-energy environment forms. Waves and rivers enter the sheltered area and deposit the finer materials such as silt and mud. This creates an area of alt marsh which is then colonised by vegetation. 

Bars - 

• Form when a spit develops across a bay and there is no strong flow of water from the landward side, in other words it links to the two headlands. They can also occur when materials have been progressively carried onshore from offshore as sea levels rose following the last ice age. 

Tombolos - 

• A spit that extends offshore to join with an offshore island, e.g. Chesil Beach. 

Sand Dunes - 

• Sand Dunes are accumulations of sand shaped into moulds by the wind. 

Sand dunes are accumulations of sand shaped into moulds by the wind. They are a dynamic lanform because they are always changing due to factors of the wind, e.g. strength and direction. The main processes that form sand dunes are saltation and creep. 

Sand dunes require the following conditions to develop: 

• Plentiful supply of sand.
• Shallow offshore zone that allows large areas of sand to dry out
• A wide backshore for sand to accumulate

When sand dries out on a beach the wind blows the sand inland to form dunes. Intially sand accumulates around peices of detritus, such as litter or drift wood. As the process of dune morphology takes place, vegetation such as marram grass anchors the dunes. A typical sequence of sand dune development would be - pioneer species - embryo dunes - foredunes - fixed dunes - wasting dunes. 

Key features are: 

• Ridges - sequences of dunes parallel to the coastline. 
• Slacks - depressions which reach down to the water table and separate dunes. 
• Blowouts - these forms where fragile sand dunes have their vegetation cover destroyed by grazing animals, usually through trampling, or by human activity. 

Eustatic sea level change - Is a global change is sea level relative to the land. This can be a fall or rise due to the changes in the mass of water, e.g. changes in the amount of water stored on the land compared to the oceans. Or a rise or fall due to changes in the volume of water, e.g. as ocean water heats up its volume increases - this is called thermal expansion. 

Since 1993, global sea level has risen by 3mm per year, it is thought that half of this rise is due to thermal expansion of the ocean. When molecules in the sea warm up they spread out and expand, which increases the volume of the oceans. For every one degree in temperature, the sea level will rise by 0.8m. 

Isostatic sea level change - Occurs on a local level and caused by movement of land. Relative change can be positive or negative. Movement could be the result of changes in the weight of ice masses on land. E.g. as ice melts, the loss of weight causese the land surface to re-adjust and 'bounce up' to its original position, this is called isostatic re-adjustment. Plate movement at subduction zones may also causae the land to rise relative to the sea. The transition in and out of glacial periods is one of the mani causes for isostatic sea level change. 

Eustatic sea level change is quicker than isostatic sea level change. 

Geology - 

• Coastal configuration - headlands attract energy due to wave refraction.
• Rock characteristics - resistant rocks such as chalk erode more slowly than weak rocks such as clay. 

Climate - 

• In countries with lower temperatures freeze-thaw is more likely to occur.

Nature of tides and waves - 

• Wave steepness = high energy

Human activity and coastal managment.