Chapter 12

The Sea: How it shapes our coast

Learning outcomes

In this presentation you will learn:

• How waves erode, transport and deposit

material along the coast

• Why erosion occurs in some places and

deposition in others

• About features associated with coastal

erosion and deposition

• How human interaction affects coastlines

Waves

• Waves erode, transport and deposit material

along the coast

• Formed by wind moving across surface of the

sea

• Fetch – distance of open sea over which wind

blows

• Greater fetch – stronger waves

• Waves break when reach shallow water

Exam hint: You need to know how waves are formed.

Waves

• Swash – water that rushes up beach when wave breaks

• Backwash – water that returns back down the beach

• Constructive wave – swash is greater than backwash so material deposited

• Destructive wave – backwash greater than swash so materials eroded

Waves

Identify which is a

constructive wave and

which is a destructive

wave.

Exam hint: You need to know what constructive and destructive waves are.

Processes of coastal erosion

Hydraulic action Physical force of waves breaks off material

Abrasion Loose material thrown by waves against coast,

breaks off more material

Compressed air Air gets trapped in cliff joints by waves, becomes

compressed, waves retreat, pressure suddenly

released, rocks eventually shatter

Solution Some rocks dissolved by water – e.g. limestone and

chalk

Attrition Stones carried in water hit off each other, become

worn and smooth, can lead to formation of sand

Exam hint: You need to know how the sea erodes the coast.

Features of coastal erosion

The features of coastal erosion are:

• Bays and headlands

• Sea cliffs

• Sea caves

• Sea arches

• Sea stacks and stumps

• Blowholes

[Insert 12.07.01]

Exam hint: You must be able to recognise the features of coastal erosion

and describe how one feature was formed.

Bays and headlands

• Bay – curved area

• Headland – hard rock jutting out

• Coastline – made up of hard rock and soft rock

• Softer rock is eroded faster than hard rock

• Hydraulic action, abrasion, compression and solution erode softer rock to form bay

• Harder rock stands out on sides of bay to form headlands

Bays and headlands

Use the diagram

above to explain

how bays and

headlands are

formed.

Sea cliffs

• Sea cliff – a vertical slope on the coast

• Waves erode a notch between high-water mark and low-water mark

• Hydraulic action – notch grows larger, causes undercutting

• Rock above eventually collapses forming cliff

• Air gets trapped in cliff joints, becomes compressed

• When waves retreat, pressure is released

• Happens repeatedly until rock shatters

• Cliff is constantly eroded – retreating

Sea cliffs

• Wave-cut platform – rock left at base of cliff

• Wave-built terrace – materials eroded are deposited on top of platform

• Examples of cliffs – Cliffs of Moher, Co. Clare and Croaghaun, Achill Island, Co. Mayo

Features of erosion

Identify the following

features on the

diagram:

• Sea cave

• Sea arch

• Sea stack

• Sea stump

• Blowhole

Sea caves

• A tunnel into a cliff, usually at base

• Hydraulic action erodes weak spot

• Creates an opening

• Compression and abrasion make opening

wider and deeper

• Example: Caves at Ballybunion, Co. Kerry

Sea arches

• Arch-shaped tunnel, stretches through headland

• Formed when:

Two caves develop back to back and meet

Continued erosion causes a cave to reach other side of headland

• Processes: hydraulic action, compression and abrasion

• Example: Bull Rock, Co. Cork

Sea stacks and stumps

Sea stack

• Pillar of rock cut off

from cliff

• Left standing on own

when arch collapses

• Example: Dún Briste,

Downpatrick Head,

Co. Mayo

Sea stump

• Sea stack eroded

over time

• Collapses leaving a

stump

• Can be seen at low

tide

Blowholes

• Passage linking cliff top with roof of cave

• Formed when waves crash into cave

• Air is trapped and compressed inside

• Rock in cave roof shatters

• Eventually collapses forming blowhole

• Stormy weather – sea spray exits through

blowhole

• Examples: The Two Pistols and McSwyne’s

Gun, Co. Donegal

Longshore drift

• Movement of load along coast

• Two steps: Waves move up

shore at angle. When wave breaks, swash deposits material. Material moved along shore at angle.

Backwash carries material back down shore in straight line.

Features of coastal deposition

• Sea deposits load in sheltered areas such as bays when

no longer able to carry it

• Features:

Beaches

Sand dunes

Sand spits

Lagoons

Tombolos

Exam hint: You need to be able to name features of coastal deposition

and describe how one feature was formed.

Beaches

• Build-up of sand and shingle

• Deposited by constructive waves

• Waves break and loose energy – unable to carry

load

• Swash carries material up shore and deposits it

• Weaker backwash unable to carry material back

• Heaviest material dropped first, finer material

near shore

• Builds up to form beach

Beaches

• During storms –

stronger waves

deposit material

above high-water

mark

• Creates storm beach

• Example of beach:

Tramore, Co.

Waterford

Sand dunes

• Dry sand blown inland by

onshore winds

• Gets trapped in

vegetation

• Builds up to form hills or

dunes

• Marram grass planted to

prevent sand blowing

further

• Deep roots, anchor sand

• Example: Castlegregory,

Co. Kerry

Sand spits

• Long ridge of sand or

shingle

• Stretches across bay,

connected to one side

• Longshore drift – loses

energy at sheltered bay

• Material is deposited and

builds up

• Extends across bay

• Example: Rossbeigh

Strand, Co. Kerry

Lagoons

• Area of water cut off from sea

• Sand bar – sand spit grows across bay

and connects with other side

• Bay is cut off from sea

• Lake, called lagoon, is formed

• Example: Lady’s Island Lake, Co. Wexford

Tombolos

• Ridge of sand

• Connects island to

mainland

• Longshore drift

• Creates a sand spit in

sheltered water

• Builds up between island

and mainland

• Example: Howth, Co.

Dublin, was once an Island

Features of coastal deposition

Identify the features of

coastal deposition

shown at numbers 1,

2, 3 and 4.

Describe how each

was formed.

Exam hint: You need to be able to recognise features of coastal deposition.

People and the sea

Positive interactions Negative interactions

Food supply – source of food,

income from fishing, fishing industry

Example, Killybegs, Co. Donegal

Overfishing – depleted Irish fish

stocks, fish farms can be an eyesore

Transport – heavy raw materials

transported by sea, ferries transport

passengers, tourist industry

Oil spillages – damage coastal

environments, constant threat

Recreation – coastal towns,

activities such as surfing, kayaking,

sunbathing

Pollution – littering and pollution

from sewage, damage to sand

dunes

Exam hint: You may be asked to discuss the ways that people use

the sea and the ways they can cause it damage.

Coastal protection

Can you name these four types of coastal protection?

Coastal protection

Sea walls Break power of waves, curved walls push water back

out to sea

Example: Salthill, Co. Galway

Rock

armour

Large boulders at base of cliff or in front of dunes,

waves hit rocks and lose energy

Example: Tramore, Co. Waterford

Groynes Concrete or wooden structures at right angles to

sea, trap sediment carried by longshore drift,

increases erosion further down coast

Example: Rosslare Strand, Co. Wexford

Gabions Wire cages filled with stones, in front of beaches or

dunes, break power of waves and reduce erosion

Example: Lahinch, Co. Clare

Exam hint: You need to be able to describe ways to protect the coast.

Rosslare Strand and Rosslare

Harbour, Co. Wexford

• Sea plays large role in

economic activities of Rosslare

Rosslare Harbour:

famous port, passenger

ferries, cargo ships,

transport hub to UK and

France

Rosslare Strand: five

miles of Blue Flag

beaches, important tourist

resort

Exam hint: You need to be able to describe a coastal area you have studied.

Rosslare Strand and Rosslare Harbour,

Co. Wexford

• Sea wall at Rosslare Harbour to provide

shelter and prevent build up of sediment

Build up of material south of Rosslare Harbour

Reduction of sand on beach and dunes

• Gabions, rock armour and groynes built

• Sand dredged from seabed to build beach

• Example of how human interaction in one

area can affect another area

Sample answer

(i) Name and briefly explain one way that people use coastal areas.

Answer: People use coastal areas for recreational activities such as surfing, swimming and sunbathing. Coastal towns attract tourists to the area.

(ii) Name and briefly explain one way that people pollute coastal areas.

Oil spillages can pollute coastal areas. They are a constant threat. They damage the marine and coastal environment.

(JC, HL, 2008, Q2C)