Coastal Studies

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 platform

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

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