Transcript 9. The Sea - Moyle Park College

The Sea
Creator and Destroyer
The action of the sea
• The sea is constantly changing the shape
of our coastline.
• This is happening because waves, like
rivers, erode, transport, and deposit
material.
• Waves move material along our coastline.
Why do waves break on the Shoreline?
Waves
• Waves are made because of friction
between the wind and the surface of the
sea.
• The size of the wave depends on 2 things.
1. The strength of the wind.
2. The length of the sea (fetch).
Waves – Swash & Backwash
• Swash – As the wave reaches the shore it
breaks. The white foamy water that flows
up the beach is called the Swash.
• Backwash – When the water runs out of
speed it stops and it flows back down the
beach into the sea. This is called the
backwash.
Constructive waves
• They have a strong Swash which transports and
deposits material on the beach.
• They have a weak Backwash which does not
carry much material back to the sea.
• As a result they construct/add material to the
beach.
Constructive Waves
Destructive waves
• They have a weak Swash.
• They have a very strong Backwash which
wears away and carries away material
form the beach.
• Therefore, there is a lot of erosion and
they destroy/destruct the coastline.
Destructive Waves
Sea Erosion
• Sea erosion is caused by Destructive
waves. They erode the coast in 4 ways.
1.
2.
3.
4.
Hydraulic Action
Air Compression
Abrasion
Attrition
Sea Erosion
1. Hydraulic Action – this is the power of
the water hitting the coastline.
2. Air Compression – The waves trap air in
crack in the rocks. As the waves retreat
the trapped air expand quickly and can
crack the rock.
Sea Erosion
3. Abrasion – this is erosion caused by small
rocks carried in the sea water hitting
against the coastline.
4. Attrition – this is erosion caused by the
small stones hitting each other breaking
themselves into smaller pieces.
Landforms of Sea Erosion
• Like rivers, the sea creates many features or
landforms as a result of erosion. These
include,
1.
2.
3.
4.
Cliffs
Bays and Headlands
Sea caves, sea arch, sea stack, sea stump
Blowhole
Cliffs
• As the waves break onto the coast they cut a
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notch into the rock. When the notch gets deeper
the overhanging rock collapses.
This process continues and the coastline retreats
inland.
The material that falls into the sea form the cliff
is called a wave cut platform.
E.g. Cliffs of Moher, Co. Clare.
Bays and Headlands
• A bay is wide deep opening into the coastline.
• A headland is a piece of land jutting out into the
sea.
• Bays and headlands form when there are different
types of rocks occur along the coastline.
• The softer rocks are eroded quickly creating a bay.
• The harder rocks are eroded slowly and so they
stick out into the sea
• E.g. Dublin Bay & Howth Head
Bays and Headlands
Caves, Arches, Stacks & Blowholes
Sea Cave
• A sea cave is a tunnel or passage at the
base of a cliff.
• The waves erode a weak point (soft rock
or an area with cracks/joints) in the cliffs.
This forms a passage called a cave.
Sea Arch
• A sea arch is a passage that runs
completely through the headland.
• This can happen when a cave is eroded
deep into headland, or if two caves on
either side of the headland are eroded and
join up to form a complete passage.
Sea Stack and sea stumps
• A sea stack is a pillar of rock standing in
the water near the coast.
• When sea arches are eroded further they
become wider.
• Their roof collapses leaving a piece of land
isolated from the coast.
• This is a sea stack. It will be further
eroded by the sea to form a sea stump.
Blowhole
• A blowhole is a passage from a cave up to the
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surface of a cliff top.
Compress air is trapped in the cave by strong
waves.
The pressure created cracks and loosens rock in
the roof of the cave. This rock collapses over
time forming a passageway called a blowhole.
Sea spray spurts out of the opening during
stormy weather.
E.g. The Two Pistols, Co. Donegal.
Caves, Arches, Stacks & Blowholes
Transport by the Sea
• We have already learned that rivers
transport material (sand, silt, clay). This is
called the rivers load.
• The sea also has a load which it transports
along. The load is made up of sand,
pebbles, stones, and mud.
Transport by the Sea
• Waves are responsible for transporting the
load. They do this in two ways.
• 1. Up and down the shore by the Swash
and the Backwash.
• 2. Along the shore by a process known as
Longshore Drift.
Swash and Backwash
• We have already looked at the processes
of swash and backwash.
• The swash will transport material up onto
the beach.
• The backwash will transport material back
down the beach towards the sea.
Longshore Drift
• Longshore drift moves material along the beach.
• This occurs when waves approach the beach at
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an angle.
The swash brings material up onto the beach,
then the backwash brings it back down to the
sea in a straight line. The swash, again moving
in at an angle will move the material further
across the beach.
This process continues and the load is moved
along in a zig zag pattern.
Deposition by the Sea
• The sea deposits (drops off) some of its
load in sheltered places where the waves
have less energy.
• This may occur in a bay, or very gently
sloping shoreline.
• Like river deposition which we have
already studied, sea deposition also
creates features or landforms.
Landforms of Sea Deposition
• Sea deposition creates the following features
along our coastlines.
1.
2.
3.
4.
5.
Beaches
Sand Dunes
Sand spits
Tombolo
Lagoons
Beaches
Beach
• A beach is made of sand and shingle (small stones) which
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are deposited by the waves.
The swash carries material up onto the beach. Some of
this material, especially the heavier material, stays on the
beach because the weak backwash cannot carry it back to
the sea. The backwash carries only lighter sand particles
back down towards the beach.
For this reason we say that beach material is sorted, with
the heavier materials furthest from the water, and the
lighter sand close to the water.
During storms, when the swash is very powerful, it can
force heavy stones high up onto the beach where they
remain to form a storm beach.
E.g. sandy beach, Curracloe, Co. Wexford.
Sand Dunes
• Sand dunes are hills of sand which form
above the high tide level.
• Wind dries sand on the beach making it
lighter. The sand is then blown inland until
it is trapped by a wall or vegetation.
• The sand builds up to form hills of sand
which are sand dunes.
• E.g. Portmarnock, Co. Dublin.
Spits, Bars, Tombolo, Lagoon
Sand Spit
• A sand spit is a ridge of sand or shingle which
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extends out into the sea. It is connected to the
land at one end.
Longshore drift moves material along the beach.
When it reaches a shallow bay the waves have
less energy and they deposit some of the load
on the sea bed.
This material builds up over time and rise above
the sea water.
E.g. Tramore, Co. Waterford.
Tombolo
• A tombolo is a ridge of sand or shingle
which connects an island to the coastline.
• A tombolo is formed when a sand spit
occurs linking an island to the coastline
rather than stretching across a bay.
• E.g. At Howth, Co. Dublin.
Lagoon
• A lagoon is a body of water cut off from the sea
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by a sand spit.
When a sand spit is formed it stretches out
across a bay. It may eventually reach the other
side of the bay closing off the water behind it.
This water forms a saltwater lake called a
lagoon.
E.g. Lough Gill, Co. Kerry.
Spits, Bars, Tombolo, Lagoon
People and Coastlines
• The sea plays an important part in our
lives. Many people live by the coast, and
most of the worlds largest cities are in
coastal regions.
• The sea provides us with energy (oil, gas,
wave power) as well as leisure activities,
transport routes, and food.
People and Coastlines
• However, the sea can also be destructive.
People have tried to control this
destructive power of the sea using,
• 1. Sea Walls
• 2. Boulders
• 3. Groynes
People and Coastlines
• Sea Walls – Sea walls are concrete walls with a
curved top on them which helps to deflect the
waves back out to sea.
• Boulders – Large rocks are placed at the base of
cliffs or in front of sand dunes. It prevents erosion
by reducing the power of the waves.
• Groynes – Groynes are low walls built of wood.
They are built at right angles to the shore to help
prevent sand being moved along the beach by
Longshore Drift.
Protecting the Coastline using Groynes
Protecting the Coastline using Boulders
Tourism and our Coastline