Marine erosion.
Cliffs:
A coast is exposed to wave action which occurs between high tide and low tide.
The sea forms cliffs by a process called hydraulic action. This is the action of
breaking waves on a coastline. Some waves are termed destructive because they
are driven on by strong winds or they have travelled over long distances(fetch).
These waves tend to be high and steep.
Hydraulic action is the sheer impact of the water which shatters rocks as the
waves pound the coastline. Cliffs of boulder clay are more easily eroded as
loosened rocks and soil are washed away. During storms waves can exert
pressure of up to 30 tonnes per square metre. Cracks are widened and masses of
material are broken away.
Compressed air occurs when waves crash into the coast and trap air in cracks.
This puts pressure on the rocks. When the wave retreats the air is released and
expands which has an explosive effect on the rocks and enlarges fissures. The
structure of the rock is gradually weakened and made more susceptible to other
forms of erosion.
Using these 2 processes, the sea erodes a wave cut notch in the coastline near
high water level. As the wave cut notch increases in size, this continued
undercutting causes tension and stress above the notch. In time the unsupported
overhang collapses and forms a cliff. This is called slumping.
As these processes are repeated, the cliff retreats, leaving at it’s base a gently
sloping(slope angle of less than 4 ) wave cut platform. This is a level stretch of
rock in the inter tidal zone with occasional pools of water.
As the cliff retreats, the wave cut platform becomes wider and the energy of the
waves is dissipated before they reach the cliffs. Erosion slows down and the cliff
becomes inactive.
e.g. The Cliffs of Moher Co. Clare.
Bays and headlands:
A stretch of coastline is composed of different types of rocks, some very resistant
to erosion, others not so resistant. Differential erosion occurs.
Destructive waves crash into the coastline and begin to attack the softer rocks
and erode them more rapidly by means of abrasion. This is where boulders,
pebbles and sand are hurled by the waves against the coastline. This material
crashes off the coast and breaks off pieces of rock. Abrasion is most effective
during storms and at high tide, when breaking waves throw water and
suspended rock material high up the cliff face.
Solution is another process at work in forming bays and headlands. Sea water
contains chemicals such as carbonic acid which can dissolve certain types of rock
such as limestone and chalk. Salt from sea water and spray is also capable of
corroding several rock types. As the structure of the rock breaks down, it easily
erodes when attacked by other erosive processes.
These 2 processes erode the soft rock back to form a bay while the more resistant
rocks stand out as headlands.
e.g. Dublin Bay and Howth Head.
Caves, arches and stacks:
These are features of marine erosion. When a headland has been formed,
incoming waves around the headland. This is known as wave refraction. This is
because sometimes the water is shallower in front of the headland than in the
bay. Wave refraction increases the erosion around the headland.
Destructive waves crash into the coastline and begin to attack lines of weakness
in the headland such as faults or joints. The sea begins to undercut the headland
to form a cave by means of abrasion. This is where boulders, pebbles and sand
are hurled by the waves against the headland. This material crashes off the
headland and breaks off pieces of rock. Abrasion is most effective during storms
and at high tide, when breaking waves throw suspended material against the
roof and sides of the cave. The cave is enlarged and deepened to such an extent
that the cave may break through the headland to the other side to form an arch.
Compressed air is also a very common process in forming arches and stacks.
Air is trapped by an incoming wave in a cave and compressed. As the wave
retreats and the air expands, it blasts a hole in the roof of the cave. Over time a
blow hole may develop which resembles a chimney with an opening on the
surface of the headland.
Solution is another process at work in forming these features. Sea water
contains chemicals such as carbonic acid which can dissolve certain types of rock
such as limestone and chalk. Salt from sea water and spray is also capable of
corroding several rock types. As the structure of the rock breaks down, it easily
erodes when attacked by other erosive processes.
As these 3 processes combine to widen the arch, the roof becomes too heavy and
finally collapses into the sea to form a sea stack. This is a column of rock cut off
from the headland and surrounded by water.
e.g. caves and stacks are found along the Cliffs of Moher
Sea Spit:
This is a feature of marine deposition.
A spit is a ridge of sand and shingle, always above sea level and attached to the
land at one end. Many spits curve at the end due to wave refraction.
Spits tend to develop where the coastline undergoes a change of direction e.g. a
river mouth, a sheltered bay or between an island and the shore where longshore
is interrupted.
Spits are formed by the processes of constructive waves and longshore drift.
Constructive waves have a powerful swash which carries much material onto
the beach and a weak backwash, as the water percolates through the beach
depositing much material.
Longshore drift occurs where the waves break onto the shore at an angle of
less than 90. The swash moves the material along the beach and as the backwash
returns some of this material, the next swash moves the material further down
the beach. Thus sand and pebbles are moved diagonally along the beach and
begin to pile up where the coastline changes direction.
Longshore drift is slowed and some of the larger particles are deposited in the
slacker water in the lee of the headland (A) or river mouth. As the spit continues
to grow into a ridge, storm waves throw larger material above the high water
mark (B) making the feature more permanent; while under normal conditions,
the finer sand is carried towards the end of the spit at (C). Waves swing around
the end of the spit causing it to curve towards the land. Low energy waves enter
the sheltered area behind the spit depositing fine silt and mud, creating an area
of salt marsh.
e.g. Bull Island in Dublin Bay
or
Wexford Harbour.
Beaches: This is a feature of marine deposition.
A beach is a build up of loose material such as sand , pebbles, boulders, shingle
mud and shells. It forms between low water mark and the highest point reached
by storm waves.
These materials are deposited by means of constructive waves.
Beaches are common where the coastline is broken by headlands and coves.
Incoming waves are slowed down by the calm water in the sheltered cove.
Deposition occurs and constructive waves carry mainly finer material such as
sand into the shore to form bay head beaches. When these waves break on the
shore, their powerful swash sweeps material up the beach. As the wave retreats,
the backwash is weak because much of the water percolates through the sand
and shingle and so is unable to bring much material back out to sea. The beach
begins to grow.
Shingle beaches have a steeper gradient than sandy beaches. This is because
water will pass through coarse grained shingle more rapidly than through fine
grained sand.
Beaches can be divided into 2 sections – the backshore and the foreshore.
Storm beaches occur in front of cliffs or sand dunes in the backshore. Large
boulders are thrown here during storms by waves driven on by strong winds,
high above the level of a normal high tide. The storm waves percolate back down
the beach without moving the stones. All these boulders are smooth and rounded
due to attrition(the load is constantly bashing off itself).
A berm will form at the spring high tide level. This is a long ridge of large
shingle running parallel to the shore. Several smaller ridges may be found below
this, which mark the height of successively lower high tides which follow the
maximum spring tide.
Beach cusps also occur in the backshore. These are sandy areas surrounded by
ridges of shingle.
The foreshore is composed of sand and shell particles. It has a gentle gradient
and is covered by the sea every day. Constructive waves lead to the formation of
ridges and runnels. Ridges are broad, gently sloping lines of sand. Pools of water
are trapped in shallow depressions called runnels between the ridges as the tide
goes out. Both these features are parallel to the shore.
e.g. Brittas Bay Co. Wicklow.
Human interference with natural processes in the sea.
Case study –Rosslare.
Man has altered the process of Longshore Drift in Rosslare Harbour.
Rosslare is an ideal location for a ferry port as it is relatively close to the U. K.
and Europe. As tourist numbers increased in recent years, the port facilities and
the surrounding infrastructure also improved. 2 national primary roads link the
port with the North and the West for easier movement of tourists.
New sea walls were built to improve the port facilities. Longshore Drift was
moving material, eroded from the cliffs south of Rosslare Harbour, to the beach
at Rosslare Strand to the north. This material began to build up behind the new
walls depriving the beach at Rosslare Strand of it’s supply of sand. A wide sandy
beach is the best protection against coastal erosion. Rosslare Strand was being
eroded especially during storms. This was having a negative impact on locals and
tourists as the amenity was in danger of being washed away, an estimate of 2
mts. in depth of sand after every storm.
Groynes were built at right angles to the beach to trap sand. These were initially
wooden walls which were unsightly but as they rotted quickly they have since
used walls of stone. They have been very successful.