Geography: The Sea

Waves

Waves are formed by wind blowing over the sea surface. The distance a wave travels before reaching land is called the fetch. The longer the fetch, the stronger the wave.

When a wave breaks, the water that rushes up the beach is called the swash. The water that returns back down the beach is known as the backwash.

There are two types of waves:

1. Constructive waves= Waves in which the swash is greater than the backwash. Constructive waves deposit materials and there are less then 10 waves per minute.
2. Destructive waves= Waves in which the backwash is greater than the swash. Destructive waves erode material and carry it away and there are more than 10 waves per minute hitting the shore.

Processes of coastal erosion

How the sea erodes the land

1. Hydraulic Action= The physical force of the waves breaks material off the coastline. During a storm, waves are stronger and their ability to erode increases.
2. Abrasion= Loose material is thrown against the coastline by waves and this action breaks more material off the coastline.
3. Compression (Compressed Air) =When waves crash against a cliff, air gets trapped in the cracks on the cliff face and becomes compressed. When the waves retreat, the pressure is suddenly released, and this creates a mini shockwave. This compression and release happens repeatedly until the rock eventually shatters.
4. Solution= Some rocks, such as limestone and chalk, are dissolved by acids in the sea water.
5. Attrition= The stones that are carried in the waves are constantly hitting against each other. Over time, they become smooth and rounded. This can eventually lead to the formation of sand.

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Bays and Headlands

A bay is a curved opening into the coastline where waves have eroded softer rock. A headland is a narrow finger of rock sticking out into the sea. Bays and headlands are formed when the coastline is made up of alternating bands of hard and soft rock.

Formation:

• As waves crash against the coastline, soft rock, such as limestone, is eroded faster than hard rock, such as basalt.
• The force of waves hitting against the coast (hydraulic action) wears away and breaks off bits of the softer rock, faster than the hard rock.
• Water fills in the place where the soft rock used to be, to form a bay.
• The sea can also use the stones it carries to hit off and wear down the coastline by abrasion.
• The harder rock remains resistant to erosion and stands out on either side of the bay to form headlands.
• Sometimes beaches can form at the back of the bay.

Example of bay: Bantry Bay

Example of headland: Missen Head

Sea cliffs

A sea cliff is a vertical or steep slope on the coastline.

Formation:

• Waves erode a hole on the coastline between the high-water mark and the low-water mark. Over time, this hole grows larger due to the hydraulic action of the waves.

• Undercutting (the cutting of the waves into the rock base) then occurs.

• The rock above the hole is called the overhang. It is left without support and eventually collapses.

• As the sea erodes further and the rock it erodes gets higher, a cliff is formed.

• The process of hydraulic action and abrasion constantly erode the cliff so that it retreats into the coastline.

• The rock that is left at the former base of the cliff is called a wave-cut platform. Material eroded from the new cliff face is then deposited on top of this platform to form a wave-built terrace.

  Example of cliffs: Cliffs of Moher in Co. Clare

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Sea caves, arches, stacks and stumps

^^Sea Caves^^

A sea caves is a tunnel at the base of a cliff.

Formation:

• The hydraulic action of the waves erodes a weak spot on the cliff face.

• This creates an opening in the cliff.

• Compression and abrasion make this opening wider and deeper until a cave is formed

  Example of sea cave: The caves of Ballybunion in Co. Kerry

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^^Sea Arches^^

A sea arch is an arch-shaped tunnel that stretches through a headland.

Formation:

• A sea arch is formed through the processes of hydraulic action, compression and abrasion.

• It forms when two caves develop back-to-back and eventually meet, or when erosion causes one cave to reach the other side to the headland.

  Example of sea arch: The Bull Rock at Dursey Island in Co. Cork

^^Sea stacks and Stumps^^

A sea stack is a pillar of rock cut off from the cliff or headland and left standing on its own.

Formation:

• A sea stack is formed when a sea arch collapses.

• Over time, the sea stack is eroded, and it collapses, and a sea stump is formed.

• These sea stumps can only be seen at low tide.

  Examples of sea stacks: Cnoc na Mara in Co. Donegal

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Blowholes

A blowhole is a passage that links the surface of the cliff top with the roof of a sea cave. They can be seen on O.S. maps as small blue circles near the coast.

Formation:

• When waves crash into a sea cave, air is trapped and compressed inside, causing the rock in the roof of the sea cave to shatter.
• The rock eventually collapses, forming a blowhole.
• During stormy weather conditions, sea spray can be seen coming out of the blowhole.

Examples of blowholes: The Two Pistols and McSwyne’s Gun on the Donegal coast, Garrets Town Co. Cork

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Longshore drift

The sea transports sand and sediment in a zig-zag pattern down along the shore by a process called longshore drift.

The material transported by waves, such as sand, shingle, mud and eroded materials, is called the load.

Longshore drift involves to steps

1. Waves approach and move up the shore at an angle (matching the direction of the prevailing winds). When a wave breaks, the swash moves the load up the shore at an angle and deposits some material there, and so the material is moved along the shore.

2. The backwash carries material back down the shore. This movement follows the slope of the beach and is usually in a straight line.

   The process is repeated over and over, resulting in the gradual movement of material along the shore in a zigzag fashion.

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Beaches

A beach: is a build-up of ==sand and shingle== that have been deposited by constructive waves ==between the high-water and low-water mark==.

Formation:

• When waves break, they lose energy and are unable to carry their load. The swash carries material up the shore and deposits it.

• The waves are unable to carry all the material back because the back wash is weaker than the swash.

• The heaviest material is dropped first, at the top of the beach and the finer material is carried back down, closer to the shoreline. Over time, this material builds up to form a beach.

• During storms, waves are stronger and are able to carry heavier material such as boulders and rocks. This heavy material is then deposited above the high-water line. This creates a storm beach.

  Examples of a beach: Rosslare in Co. Wexford

• Examples of a storm beach: Kilkee Co. Clare

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Sand spits

A sand spit is a long ridge of sand or shingle that stretches across a bay and is connected to one side of the bay.

Formation:

• Longshore drift loses its energy and deposits the material it is carrying when it is interrupted by a sheltered bay.
• The material builds up over time and gradually extends across the bay. A sandspit is formed.
• As the sand spit increases in size, vegetation grows on it and a beach can form on the side facing out to the sea. The end of the spit is often curved due to the current.

Examples: Rossbeigh Strand near Glenbeigh, Co. Kerry.

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Lagoons

A lagoon is an area of water that has been cut off from the sea by a sandbar.

Formation:

• A sandbar forms when a sand spit continues to grow across a bay until it connects with the other side.
• The bay is then cut off from the sea, and a lake, called a lagoon, is formed.

Example of a lagoon: Lady’s Island Lake in Co. Wexford.

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Tombolo’s

A tombolo is a ridge of sand that connects the mainland to an island.

A tombolo is formed when longshore drift creates a sand spit in the sheltered waters between the mainland and an island.

Example of a tombolo: Howth in Co. Dublin was once an island but is now connected to the mainland by a tombolo. Nearby Sutton is built on this tombolo, 5000 people are housed there.

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Ways that people use the sea

1.Food Supply

People have used the sea as a source of food. Many people in coastal areas rely on incomes earned from fishing. The fishing industry provides jobs on fishing trawlers and in onshore fish-processing plants. Killybegs in Co. Donegal is an example of such a coastal community. In recent decades, fish farms on the Irish coast have provided another source of food and income. E.g., Salmon farms and Mussel lines. Factories in Castletown Bere, Bantry and Killybegs

2.Transport

Ferries from Rosslare, Cork and Dublin transport passengers to and from the UK and France, which helps our tourist industry. Huge cargo ships are used to transport heavy raw materials, such as oil, iron, steel and other minerals by the sea. However, pollution can be a negative side-effect. Oil spillages are a constant threat and can be damaging to coastal environments. Ferries from Rosslare transport passenger to and from the UK and France.

3.Recreation

The sea and beaches provide a wide range of recreational activities such as surfing, kayaking and sun-bathing. Coastal towns benefit when tourists are attracted to these areas. Tourism brings increased income and employment to areas, as well as improvements in roads and services. The west coast of Ireland is a prime example of an area that has benefitted from tourism

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