Coasts Chapter 13

8/6/2019 Coasts Chapter 13

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Coastal Systemand Processes

Refer to Pages 254 to 271 of theTextbook

Monday, May 24, 2010


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Learning Objectives

Describe the different agents of coastal processes

Explain the different coastal processes (erosion,deposition and transportation)

Describe and explain the formation of erosional

and depositional landforms

Illustrate the formation of these landforms withwell-labelled diagrams

Assess the effectiveness of the various coastalprotection measures (natural/soft engineeringmethods and artificial/hard engineering methods)



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What is a Coast? (p. 254)

The zone where the land meets and is shaped bythe sea.

A coast is dynamic, ever changing as it is alteredby the coastal processes of erosion, deposition andtransportation: a result of waves, tides and

currents.



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Agents of Coastal Processes:

Waves, Tides and Currents



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What are Waves?

Waves are agents of erosion, deposition and

transportation.

Waves: Generated by wind (small-scale).As wind blows over the surface of the ocean, wind

energy is transferred from the wind to the water,causing the water on the surface to rise and fall,

leading to the formation of waves.



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What are Waves?



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What are Waves?

Wave Energy: Energy produced by the movementof waves.

Three factors affecting wave energy:1. wind energy

2. duration of fetch3. fetch (definition: distance of sea over which

wind blows to generate waves)



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Factors that Determine the Size

and Energy of Waves

Factor Size and Energy of Waves

Wind Energy

The greater the difference in pressure

between 2 regions, the faster the windspeed, the greater the wind energy.

Duration ofWind

The longer the wind blows, the largerthe waves.

FetchThe greater the fetch, the more energythe waves have



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How do Waves Move?

Waves move in a series of circular loop-likemotions.

Waves slow down as they approach the coast dueto friction (the shallower waters and sea floor),

causing the waves to rise and bend forward,eventually collapsing into foaming water as itbreaks onto the beach.

Forward movement of waves up the shore: swash(carries material up the shore)

Flow back to the sea (due to gravity): backwash(carries material back away from the shore)



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Types of Waves

Two types: destructive and constructive

Destructive Waves:- Occur on steep sloping coasts, breaking violently

with high energy.

- Steep gradient causes waves to break and plungedirectly back down the coast.- Near vertical breaking of waves resulting in

weak swash (depositional) and strong backwash

(erosional).



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Types of WavesConstructive Waves:

- Occur on gentle sloping coasts during calmweather, breaking gently with little energy.

- Gentle gradient allows waves to surge a greaterdistance up the coast and carry sediments up the

beach, depositing them.- Strong swash (depositional) and weak backwash

(erosional).- Over time, the beach is built up by the deposited

sediments.



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Whatre the Differences?Constructive Waves Destructive Waves

Low wave height High wave height

Long wave length Short wave length

Wave frequency of between6 and 8 per minute

Wave frequency of 10 and14 per minute

Spilling breakers Plunging breakers

Swash more powerful thanbackwash

Backwash more powerfulthan swash

Occur on gentle slopes andsheltered coasts

Occur on steep slopes andopen coasts

Deposition more prominent Erosion more prominentMonday, May 24, 2010


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What are Tides?Daily alternate rising and falling sea level seen

along the coasts.

Caused primarily by the gravitational pull of theMoon and Sun.

Tides affect the part of the coast that can bereached by waves.

At high tide, waves attack, erode and transportaway a larger part of the coast than at low tide.

Areas located between low and high tides willhave more erosion than areas constantlysubmerged by sea water as it experiences

continuous wetting and drying.Monday, May 24, 2010


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What are Currents?

Large-scale and persistent movements of water inthe ocean, driven by prevailing winds.

E.g. Longshore currents- Flow parallel to the coast

- Formed by waves that approach the coast at anoblique angle



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Coastal

Processes:

Erosion,Transportation

and Deposition



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Coastal Transportation

Longshore drift:- When waves approach the beach at an angle, theswash (depositional) carries the materials up the

beach at an oblique angle while the backwash(erosional) carries the materials perpendicularly

down the beach because of the pull of gravity.- This results in a zigzag movement of materials

along the beach.- Capable of moving large amount of sediments

along the beach.



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Longshore drift:



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Factor affecting direction of longshore drift:wind direction.

For example: If the wind is moving from aSoutheast direction, the direction of the longshore

drift will be from East to West.



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Coastal ErosionProcesses of coastal erosion:

1. Hydraulic action

- Direct impact of waves against coast, exertinggreat pressure on the coast.

- Air in the cracks in the rocks along the coastsmay be compressed by the entry of water into the

cracks, exerting pressure, widening the cracksand causing it to break down.



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Coastal Erosion



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Coastal ErosionProcesses of coastal erosion:

2. Abrasion:

- Impact of materials carried by waves scrapingagainst the coast.

- Destructive waves are capable of lifting largepieces of rock from the sea bed and hurling them

against the coast.



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Coastal Erosion

Processes of coastal erosion:

3. Attrition

- Rocks break down into smaller pieces as they arecarried by waves and rubbed and hit against each

other.- Rounded edges of rocks over time.



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Coastal Erosion

Processes of coastal erosion:

4. Solution

- Soluble minerals in the rocks react chemicallywith the waves, resulting in the formation of a

chemical solution.- Rock is weakened and breaks down over time.

- E.g. limestone.



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3 Factors Affecting Coastal Erosion

1. Types of waves:- Destructive waves VS constructive waves

2. Structure and composition of coastal rocks- Lines of weaknesses

- Soft rock VS hard rock- Chemical composition of rocks

3. Position of the coast- Protected (natural or artificial) coasts VS open

coasts



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4 Factors Affecting Coastal

Deposition

1. Energy of waves:- Weak wave energy

2. Increase in load:- Insufficient energy to transport load

3. Gradient of slope:- On gentle slopes, wave energy is spread out andreduced due to friction with shore and

gravitational pull, resulting in constructive waves

4. Position of coast:- Protected coasts allow for calm coastal

conditions and thus, deposition of sedimentsMonday, May 24, 2010


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Landforms Created by Coastal

Processes (p. 259-265)

Erosional Landforms:1. Cliff and Wave-cut/Shore Platforms2. Headland and Bays

Depositional Landforms:1. Beaches2. Spits and Tombolos



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Erosional Landforms: Cliff and Wave-

cut/Shore Platforms (p. 260-1)

Cliff:- Steep rock surface, tilts forward or backwards.- Produced by the action of waves undercutting asteep rocky coast.

- Hydraulic action and abrasion causes lines ofweaknesses to form on the rocks surface, erodes

and enlarges it to form a notch.- Notch may be further eroded to form a cave.

- Overhanging cliff collapses over time.- Some materials may be washed out to sea while

others are picked up by the waves for furthererosion of the base of the cliff.



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Wave-cut/Shore Platform:- Formed after the cliff retreats inland.- Gently sloping platform at the base where the cliff used

to be.

- Usually submerged during high tide and undergoes

continuous erosion.- At low tide, sediments are deposited on the wave-cut/

shore platform because it lies exposed above the water

and its gentle slopes causes waves to lose their energy

due to friction.

Good examples of cliffs and wave-cut platforms can befound at Hunstanton (North Norfolk) and Flamborough

Head (Yorkshire)



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Erosional Landforms:

Headlands and Bays (p. 262-3)

Characterised by coasts with bands of alternatingresistance to erosion.

Results in an indented coast/uneven coastline

Less resistant soft rock is eroded more quickly(resulting in the formation of bays) than the moreresistant hard rock (resulting in the formation of

headlands which extends into the sea).

E.g. South coast of United Kingdom.



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Gives rise to wave refraction (bending of waves anduneven wave energy).- Energy of waves concentrated on headlands due to waverefraction, resulting in erosion.- Other waves continue, unaffected and its energy spreads

out in the bays, resulting in deposition.

Wave refraction determines when and where erosion,transportation and deposition take place.

Indented/uneven coastline becomes more prominent overtime.

The Dorset coast has excellent examples of Headlands andBaysE.g. Swanage Bay and the Foreland (a headland)



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Headlands can be further eroded to form caves,arches, stacks and stumps (not in syllabus).



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Depositional Landforms:

Beaches (p. 264)

An accumulation of sediments on the coast. Sand,broken coral pieces, small stones and blackvolcanic sand.

Different materials ---> Different gradient.

Coarser materials ---> SteeperFiner materials ---> Gentler



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Beaches (p. 264)

Size and composition of material on beach variesand changes over time according to changes inweather conditions, wind direction and ocean

currents.

- Waves and winds cause materials to be sorted bysize.* Finer materials are typically deposited near the

sea and coarser materials further inland if the

wind is strong and there are destructive waves.* Fine materials are deposited further up thecoast and coarser materials near the sea when the

weather conditions are calm and there are

constructive waves.Monday, May 24, 2010


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Beaches (p. 264)

Bondi Beach in Australia



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Spits and Tombolos (p. 264)

Created by longshore drift:- Longshore currents encounter a bay with shallowsheltered water, causing materials to be deposited in thedirection of longshore drift.

Spit: One end is connected to the mainland while the otherend projects out into the sea.Examples of apits:- Spurn Head - Holderness Coast- Orford Ness - Suffolk

Tombolo: When the spit is joined to a neighbouring island,forming a bridge.Example of tombolo: Chesil Beach (joining S Dorset Coastto Isle of Portland)



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l df


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Why Manage Coasts?

60% of the worlds population live near coasts.Tourism revenue.

Erosion and water pollution affects coasts and

settlements along it.


2 W t M C t


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2 Ways to Manage Coasts

(p. 266-271)

1. Hard engineering methods: Construction ofphysical structures.E.g. Seawalls, breakwaters, groynes and gabions.


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2 Ways to Manage Coasts

(p. 266-271)

2. Soft engineering methods:- Planning and management so that both coastalareas and property will not be damaged by

erosion.

- Aims to change individual behaviour or attitudestowards coastal protection by encouragingminimal human interference and allowing nature

to take its course.

- Added advantage of improving natural habitats.E.g. Beach nourishment, relocation of property,planting of mangroves, stabilising dunes and

growth of coral reefs.



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HEM 1: Seawalls (p. 266)

Made of concrete, rocks or wood: Absorbs theenergy of the waves and protects the coast againststrong waves.

Limitations:- May not protect the coast from erosion in thelong run.- Do not prevent the powerful backwash of waves

from eroding the base of the seawall. Undermines

the base of seawalls and leads to their collapse.- Must be carefully maintained.- Expensive.



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HEM 2: Breakwaters (p. 267)Made of granite.

Protects the coast and harbour by reducing theforce of high energy waves before they reach the

shore.

Can either be built with one end attached to thecoast or built away from the coast.

Create a zone of shallow water between itself andthe coast so that waves will break against it before

reaching the coast.

Sediments can then be deposited and built up inthis zone to form beaches.



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HEM 2: Breakwaters (p. 267)Limitation:

- Unable to provide complete protection as theystill leave areas of the coast unprotected (eroded

away due to lack of new supply of materials).



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HEM 3: Groynes (p. 267)

Low wall built at right angles to prevent materialsfrom being transported by the longshore drift andaccumulated on the side of the groyne facing the

longshore drift.

Limitations:

- Beach will not be replenished by materialscarried by the longshore drift on the other side of

the groyne, resulting in the erosion of the beachfurther down the coast.

- Eyesore



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G ( 267)


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4 G b ( 268)


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HEM 4: Gabions (p. 268)Wire cages containing small rocks prevent

erosion.

Protect other coastal protection structures likeseawalls.

Limitations:- Easily destroyed by powerful waves duringstorms.

- Short life span of less than 15 years as compared

to others of 30 years.- Eyesore.


HEM 4 G bi ( 268)


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HEM 4: Gabions (p. 268)



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SEM 1: Stabilising Coastal Dunes (p. 269)

Coastal dune: ridge of sand piled up by wind onthe coast.

Advantages of coastal dunes:- Barriers along the coast that protect humanproperty against coastal erosion and flooding.

- Provides habitat for animals.

Plant vegetation to trap and bind sand together toprevent it from being blown inland.

Limitation:- Limits the development of lucrative propertyand recreational establishments.



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SEM 1: Stabilising Coastal Dunes (p. 269)



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SEM 2: Planting Mangroves(p. 270)Propped and kneed roots of mangroves bind loose

soil and protects from erosion.

Success: Coastal areas of Bangladesh during 2004Indian Ocean Tsunami.

Limitation:- Characteristic of young mangroves as fragilerequires cooperation of local people in the area for

mangrove planting to be successful.


S ( 270)


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SEM 3: Beach Nourishment (p. 270)

Add large amounts of sand to eroded beach.

Limitations:- Short-term effectiveness. 10 years.- Expensive and requires regular maintenance.



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SEM 4: Encouraging Growth of Coral Reefs (p. 271)

Coral reefs protect against coastal erosion byreducing speed of approaching waves (erosionpotential).

However, human activities have destroyed coralreefs and the polluted water prevents the growth

of coral reefs.

Success: Malaysia.

Limitations:- Short-term effectiveness. 10 years.

- Expensive and requires regular maintenance.


SEM 5 R l ti f P t ( 271)


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SEM 5: Relocation of Property (p. 271)

Role of coastal planners in settlement planning.

Allow nature to reclaim beach in its own time.

E.g. Green line policy in parts of the East coast ofEngland where buildings are discouraged beyond

a certain line.

Limitation:- Opposed by people who have considerable

investments in the coastal areas.

Coasts Chapter 13

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