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Stage 1: Before you start

Stage 1: Waves | Coastal erosion | Sub-aerial processes | Landforms | Questions to investigate

What are high energy coasts?

• stretches of the coastline where waves are powerful for a significant part of the year
• often the rate of erosion exceeds the rate of deposition
• landforms include headlands, cliffs and wave-cut platforms

Waves

Waves are created by the action of wind blowing over the surface of the sea. Wave energy depends on

• wind strength
• wind duration (how long the wind is blowing)
• water depth
• the fetch of the wave

Fetch is the maximum distance of open sea a wave can travel over.

The highest part of a wave is the crest and the lowest point is the trough. The difference between crest and trough is the wave height.

Out at sea, water moves in a circular motion as each wave passes. The water does not move across the ocean. Only the energy of the wave moves. But when a wave approaches land, friction with the sea bed makes the base of the wave travel more slowly than the top of the wave. The top of the wave will eventually topple over, and the wave will break.

When a wave breaks, water washes forward onto the sea-shore. This part of the wave is called the swash. The swash transfers energy up the beach. The water that returns down the beach is called the backwash. The backwash returns energy down the beach.

Constructive waves and destructive waves

There are two types of wave: constructive waves and destructive waves.

• Constructive waves have limited energy. Most of this is used by the backwash to transport material up the beach. Constructive waves are lower and less frequent (under 13 waves per minute). They have a strong swash that pushes material up the beach.
• Destructive waves have much more energy. Most of this is used by the backwash to transport material back down the beach. Destructive waves are higher and more frequent (about 15 waves per minute). They have a strong backwash that scours the beach and pulls material down the beach.

Beach angle affects the strength of the backwash. If the beach is gentle, little water percolates into the sand. Instead it will wash down the beach and destroy the swash from the next wave. If the beach is steep, more water percolates into the sand and so the backwash is weak.

Wave refraction

The direction in which a wave moves may be altered by the shape of the coastline. Waves travel faster in deeper water. If, for example, a wave is approaching a coast at an angle, it will bend round, as the side of the wave nearer to the coast travels more slowly. This is because the side nearer the coast loses more energy to friction, as the sea is shallower. The shape of the waves is also affected by headlands and bays.

Wave refraction where waves are diagonal to the coast	Wave refraction in an indented coastline	Waves can be refracted if they pass through a narrow channel

Coastal erosion

The processes of erosion, transport and deposition at the coast are similar to the processes in fluvial environments. There are five types of coastal erosion.

• Hydraulic action - air present in joints is trapped and compressed by the pressure of incoming sea-water. Over a period of time, this increase in pressure weakens and breaks off the rock. The rate of hydraulic action is high on coasts where waves are powerful and the coastline is made up of a densely jointed rock.
• Corrasion (abrasion) - sand, shingle and boulders, carried by the sea, rub against the surface of cliffs and wear it down. It is the fastest form of coastal erosion
• Attrition - the movement of waves makes rocks and pebbles crash together, so that sharp edges are broken down, and particles become smaller and more rounded. It affects boulders and stones that have already been eroded from the coast.
• Corrosion (solution) - occurs when rocks are dissolved by sea water.
• Biological action - boring organisms (e.g. limpets) can drill into the rock and create small depressions. Seaweed attaches itself to rocks and the action of the waves can be enough to cause the swaying seaweed to prise away loose material from the sea bed.

Factors affecting the rate of coastal erosion

The rate of erosion is affected by the force of the waves (erosivity) and the resistance of the coast to erosion (erodibility).

What determines the force of the waves?

• Breaking point of the wave - when a wave breaks it releases a great deal of energy. A wave which breaks at the foot of a cliff releases the most energy and causes fastest erosion, particularly corrasion. A wave which breaks offshore will have lost most of its energy as it travels up a beach.
• Type of wave - steep destructive waves have more energy, and power to erode, than shallow constructive waves.
• Fetch of the wave - waves tend to become higher and more erosive as their fetch increases.
• Shape of coastline - refraction makes waves stronger and more erosive on headlands rather than bays.
• Gradient of the sea-bed - the steeper the gradient of sea-bed, the more likely it is that the wave will break closer to the shore. Less of the wave's energy is used in overcoming friction with the sea-bed, so there is more energy to erode.

What determines the resistance of the coast to erosion?

• Mechanical strength of rocks - some rocks (e.g. granite) are stronger and more resistant to erosion than others (e.g. unconsolidated sediments such as volcanic ash). Rocks which can become saturated with water can collapse (e.g. fuller's earth).
• Jointing - densely jointed or faulted rocks are susceptible to hydraulic action. Faults, joints, cracks and bedding planes can all act as points of weakness.
• Chemical composition of rock - some rocks are soluble in water (e.g. chalk is soluble in acidified water) and can be eroded by corrosion.
• Vegetation - the foliage and roots of vegetation bind soil and rocks together and reduce the rate of erosion.
• Human protection - in many locations, physical structures (e.g. sea walls) have been installed to absorb the energy of waves and so reduce the rate of erosion.

Sub-aerial processes

Sub-aerial processes are those processes which operate at the coast but do not involve direct contact with the sea. Material is loosened and made more vulnerable by sub-aerial weathering and mass movement.

• Salt weathering - sea spray enters cracks. Later the water evaporates to leave crystals of salt. Further evaporation enlarges the crystals. The growing crystal exerts force on the rock. The rate of salt weathering is most rapid in well-jointed rocks.
  
• Frost shattering - rain-water or sea-water enters cracks. Later the water freezes to ice and expands. This exerts extra pressure on the rocks and makes cracks become larger. Thawing of the ice allows the water to trickle into the new cracks. The rate of frost shattering is most rapid in well-jointed rocks. Frost shattering is slower than inland because sea-water freezes at a lower temperature than freshwater. Furthermore, frost is less likely at the coast than inland.
  
• Wetting and drying - water enters sediments and causes expansion. The sediment contracts when it dries out. Repeated wetting and drying causes stress fractures in some rocks, such as clay and shale.
  
• Other processes of weathering - hydration, hydrolysis, carbonation and biological weathering may also occur at the coast.

Mass movement - mass movement is particularly active at the coast because undercutting of rocks by the sea makes them unstable. There are two basic types.

• Rockfalls occur when the waves undercut the cliffs and weathering loosens pieces of rocks on the cliff face. Rockfalls are most common on cliffed coastlines with resistant rocks such as chalk or limestone
• Landslips occur when rocks become saturated with water. The slip is triggered by undercutting by the waves or by loading by heavy rain. The saturated material flows out from the base of the cliff to form a tongue of mud

Landforms

(a) Headland features

Headlands form a focus point for waves, as they refract into them. They tend to be formed from resistant rock which limits the speed of erosion in this high energy environment, but over time landforms are created through erosion. The headland is more exposed than the bay to the wind and waves. Wave refraction may make waves more erosiove.

Green Bridge of Wales: an arch	Elegug rock: a stack

Although headlands consist of resistant rock, they are still likely to contain areas of weakness. Zones of weakness, such as joints, bedding planes and cracks, will be the first to be eroded by the sea. First the sea attacks the foot of the cliff and erodes the areas of weakness. It makes a crack become larger to form a wave-cut notch . The crack develops into a small cave. Hydraulic action is most severe at the end of the cave, and the roof may collapse to leave a blowhole, through which sea spray emerges.

The cave is widened and deepened until it cuts through the headland to form an arch . Further undercutting by the waves causes the arch to collapse. This leaves part of the cliff detached as a stack . Further undercutting causes the stack to collapse. This leaves a stump, which is covered by the sea at high tide.

(b) Wave-cut platforms

Wave-cut platforms (sometimes called shore platforms) are created by a mixture of weathering and erosion.

A large (480m wide), dissected wave-cut platform which is still active and being worn away by the sea. (Gorah Rocks, South Devon)

Originally land sloped down to the sea. Corrasion and hydraulic action at the base creates a cliff. The cliff is undercut to form a wave-cut notch. As the notch gets larger, the cliff above it becomes increasingly unsupported and in time will collapse. As this process is repeated the cliff will slowly retreat and increase in height. A gently sloping area of rock is left at the foot of the retreating cliff. This is called the shore platform.

Downward corrasion on the shore creates potholes and rockpools. Wave-cut platforms are exposed at low tide but covered by water at high tide. Rockpools remain filled with water at low tide.

The shape of the cliff depends mainly on rock type. Higher and steeper cliffs are found on more resistant rocks. The steepest cliffs are usually found where the rock's structure is horizontal or vertical. Lower and weaker cliffs are found on less resistant rocks, or where the rock dips upwards from the sea.

Wave-cut platforms take a great deal of time to form. Until 10% of the rocky shoreline is uniform and smooth, the feature is not considered to be a wave-cut platform. Platforms can be divided into two groups:

• Incipient Platforms, which have a number of irregularities such as stumps are younger, meaning the feature has not been ‘smoothed’ out.
• Dissected platforms, which are much smoother and even, apart from the gullies created by streams running over them at low tide, and are older.

Relic wave-cut platforms indicate the height of previous sea-levels, and when the sea level changes wave-cut platforms are abandoned by the sea.

(c) Evidence of sea-level change

In a world facing the challenges of global warming, rising sea levels are an increasing threat and as such are a focus of increasing study and research. Clues of previous sea level change are scattered far and wide around the UK, and a close examination of many stretches of the coast will yield evidence of both higher and lower previous sea levels.

Reasons for sea level change are numerous but two key categories are often referred to:

• Isostatic change. This is change in the height of the land itself. For example if an ice sheet melts over a landmass the reduced weight pressing down on land means it is likely to slowly rise.
• Eustatic change. Absolute sea level change, where the height of the water itself is changing rather than the land moving up or down. For example if the sea warms up, because of climatic changes, the water is likely to expand leading to eustatic sea level rise.

Some clues to look out for when you are out in the field include:

Questions to investigate

Investigations at high energy coasts tend to be more descriptive than investigations at low energy coasts. Here are some possibilitities

• How and why do cliff profiles vary?
• What factors affect the distribution of wave-cut platforms?
• An investigation into the development of headland features along coastline x.

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