Chapter_16 Coast and Coral Reefs

7/25/2019 Chapter_16 Coast and Coral Reefs

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Chapter 16

The Coast and

Coral Reefs


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Coast

Zone of interaction between the sea and the land

Where waves, sea currents and winds act on the land

Chapter 16: The Coast and Coral Reefs


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Zonation of the coast

1. High tide shoreline

Level reached during high tide

2. Low tide shoreline

Level reached during the lowest tide

3. Coastline

Highest level reached by storm waves

Beyond the high tide shoreline

4. Offshore

Zone submerged below the low tide shoreline


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5. Foreshore

Zone between the low tide and high tide shorelines

6. Backshore

Zone between the high tide shoreline and the coastline

7. Beach Feature formed by deposition of sand, gravel and pebbles on

a wave-cut platform

. Ber!

aised part of the beach on which vegetation often grows


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Agents of coastal change

1. "inds

!ain agent of coastal change

"ransport sand and deposit it along the coast

#enerate waves as they blow across the water surface

2. C#rrents Bodies of water moving through the sea in a certain direction

either hori$ontally or vertically

%urrents formed when waves approach the coast at an angle

and brea& obli'uely against the coast are called longshore

currents


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3. $wash

(dvance of sea water up a beach after the brea&ing of a

wave

4. Backwash

eturn flow of sea water down the beach following the swash



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Parts of a wave

%rest ) "he highest part of a wave

"rough ) "he lowest part of a wave

Wave height ) "he vertical distance between the crest

and the trough

Wave length ) "he distance between two consecutivewave crests



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Wave energy

*epends on the si$e of the wave

"he si$e of the wave increases as the speed of the

wind increases

"he greater the e+panse of water over which the wind

blows termed fetch./, the larger the wave



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Wave action

Water particles move in a circular path within a wave

(s a wave approaches the shallow waters near the

shore, the wave path becomes more oval-shaped and

the wave length decreases

*ue to friction between the wave and the sea bed, the

wave slows down

"he waves behind move at a faster speed and push

against the preceding wave

(s a result, the wave height increases while the wave

length decreases

"he waves eventually brea&



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When a wave brea&s, the crest is thrown forward and

crashes against the shore

"he water rushes up the shore as swash, carrying withit sediments which may be deposited on the shore

#ravity pulls the swash bac& to the sea as bac&wash

!aterials are carried bac& to the sea with the bac&wash


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%onstructive waves

Waves that result in deposition of materials

When the swash is stronger than the bac&wash

0ccur on gently-sloping beaches

When they brea&, they are called spilling brea&ers



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*estructive waves

Waves that encourage erosion

!aterials on the beach are carried into the sea by the strongerbac&wash

0ccur on beaches with steep slopes

Waves brea& violently as plunging brea&ers



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Constructive Waves Destructive Waves

Long wa%e length $hort wa%e length

Low wa%e height High wa%e height

$&ill o%er when 'reaking (l#nge o%er when 'reaking

Co!!on on gentl)*slo&ing shores Co!!on on stee& coastal slo&es

+e&osit on the coast ,rode the coast

Less than ten wa%es 'reaking&er !in#te

-ore than ten wa%es 'reaking&er !in#te

Characteristics of constructive and destructive waves


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Processes of wave erosion

1. H)dra#lic action

epeated crashing of waves against the coast

"he roc& structure wea&ens and the roc&s brea& down

2. Ca%itation

Brea&ing waves enter the crac&s and 1oints in the roc&s "he water traps and compresses the air within the 1oints

"he compressed air e+erts pressure on the crac&s and 1oints

When the waves return to the sea, the pressure is released

and the air e+pands

epeated contraction and e+pansion of the air enlarges the

crac&s and 1oints

"he roc&s eventually brea& into smaller fragments


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3. $ol#tion corrosion/

oc&s may contain water-soluble minerals such as calcium

carbonate When these minerals dissolve upon contact with sea water,

pores are left in the roc&s

0ver time, the roc&s wea&en and disintegrate

4. 0'rasion corrasion/

oc& fragments carried by the water are thrown against the

coast, brea&ing up the coastal roc&s

5. 0ttrition oc& particles carried by the water collide with each other,

becoming smaller, smoother and rounder particles


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actors affecting !arine erosion

1. Hardness of the rocks

Less resistant roc&s are eroded faster

2. $tr#ct#re of the rocks

oc&s with more lines of wea&ness such as 1oints are eroded more

rapidly

3. "a%e energ)

2tormy weather causes more erosion as the waves are bigger due

to the strong winds

Larger waves usually have stronger bac&wash and more erosive

energy


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4. H#!an action

3n some places, seawalls

and brea&waters havebeen built to slow down

coastal erosion

5. i!e

0lder roc&s are more eroded since they have been e+posed

to wave action longer than more recent roc&s4

"he duration of a storm affect the amount of erosion

Structures to slow down

sea erosion


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"andfor!s caused #y !arine erosion

1. otches cliffs and wa%e*c#t &latfor!s

Waves act on a line of wea&ness on the roc& surface through

the processes of hydraulic action and abrasion

"his line of wea&ness enlarges to become a notch

Further erosion enlarges the notch into a cave


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"he roof of the cave eventually collapses and a steep cliff is

formed

(t the cliff base is a flat terrace called a wave-cut platform "he wave-cut platform will e+tend farther inland as the cliff

retreats


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2. Blowholes and sea inlets Waves pounding on a sea cave trap air in the crac&s and

1oints of the roc&s

"he compressed air e+erts pressure on the crac&s and 1oints

When the waves retreat, the air e+pands

0ver time, the roc&s are bro&en down and an opening called

a blowhole is formed at the roof of the cave

"he blowhole may

enlarge until the cavecollapses, resulting in a

deep, long and narrow

inlet called a geo


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3. Headlands and 'a)s

*evelop along coasts with alternate bands of resistant and

less resistant roc&s "he resistant roc&s are eroded more slowly and protrude into

the sea to form headlands

"he less resistant roc&s form bays between the headlands

%an also develop when destructive waves erode along lines

of wea&ness in roc&s to form bays


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Headland

and bay


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(t the bays, waves curve

out, resulting in wave

energy being dispersed

and thus encouraging

deposition

*eposition in the

bay results in a

straighter shoreline

Wave refraction occurs in areas where

there are headlands and bays Waves concentrate their energy on the

headlands by curving in on them


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4. Ca%es arches stacks and st#!&s

Waves attac& lines of wea&ness in roc&s along the base of

the headland cliff to form notches 0ver time, the notches enlarge to become caves

%ontinued erosion of caves on two sides of the same

headland cuts through the headland, creating an arch

A $ C


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An arch


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"he arch widens and the roof eventually collapses

"his leaves an isolated pillar &nown as a stac&

"he stac& is gradually eroded down into a stump

Arch collapses

%tac&

'otch

%tu!p


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A coast in Bathsheba, Barbados, showing typical coastal

features like stacks, stumps and cliff


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Wave transportation and deposition 0ccurs mainly through longshore drift along the coast

When waves approach the shore at an angle, the swash rushes up

the shore diagonally, carrying sediment up the shore


"he bac&wash brings

sediment bac& into the

sea (s a result, sediment

is moved in a $ig-$ag

manner along the

shore

"ongshore drift

%wash

(one

$rea&er

(one

%wash

$ac&wash

)ove!ent of !aterial #y longshore

current

Direction of wind

$each


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"andfor!s caused #y wave deposition

1. Beaches

(ccumulation of roc& debris and sediment on or along a wave-cut platform

%onstructive waves deposit materials on the coast %oarser materials

are deposited

farther inland

while finer

materials are

found nearer

the sea

A cobblestone beach at

Georgetown, St. incent


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2. $&its

Long, narrow low-lying strips of sand and shingle

pro1ecting from the shore towards the sea

or!ation of a spit


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3. Bars

5arrow ridges of deposited material lying away from and

parallel to the coast off-shore bar/ ( spit may grow across an estuary, a lagoon or a bay to

become a bar bay-bar/

4. o!'olos

Formed when a spit or

a bar e+tends to 1oin

an offshore island

!he Cocal Spit in "ayaro,

!rinidad

Recurved spit

'ariva River

Ch t 16 Th C t d C l R f


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Causes of !arine erosion

1. "a%e action and longshore drift

6rosion occurs when the amount of materials deposited is

less than the amount carried away

Large storm waves cause coastal erosion which can lead to

long-term loss of sediments or temporary redistribution of

sediments

*estructive waves erode the beach by carrying materials out

to sea, whereas constructive waves increase the si$e of the

beach by depositing materials on it

Longshore drift moves sediments from one part of the coast

to another part farther down the coastline



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2. ock str#ct#re and strength oc&s with 1oints and fractures are eroded faster

Less resistant roc&s such as limestone are more vulnerable to erosion

3. at#ral haards Hurricanes, volcanic eruptions and earth'ua&es can cause dramatic

changes in coastlines

"hey can destroy coastal features

Changes to Coconut Beach #$ominica% during the &''( hurricane season



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4. 0t!os&heric &rocesses

0nshore winds pic& up sediments and move them up the beach to form sand dunes

ain helps to carry sediments down to the beach

5. H#!an actions and interference *am construction and river channelisation reduce the amount of sand that reaches

the shore

Humans sometimes remove beach sand as raw material for the construction industry

Building groynes is one way

humans can interfere in

coastal formation

http://upload.wikimedia.org/wikipedia/en/9/95/Bourngroynes.jpg


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Coastal !anage!ent

1. ro)nes

2tructures built out from the shore and into the sea

%onstructed at a right angle to the sea

6ffective in preventing longshore drift from moving sediments

from one point to another farther along the coastline

While they protect one part of the coast from erosion, theycontribute to erosion of the beach behind them by cutting

off the supply of sediments to the beach

2. e&lenishing the 'each 2and is sometimes added artificially to badly eroded

beaches

"he sand is ta&en from e+ternal sources



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3. $eawalls

Walls constructed on the inland part of the coast to deflect

oncoming waves Built parallel to the coast

7sually made of hard roc&s or concrete

%an be sloping or vertical

!ay cause erosion in the long run

"he energy of the bac&wash is reflected from the wall and

erodes the beach materials beneath and in front of the

wall

2couring occurs at the base of the seawall, wea&ening it



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4. Breakwaters

2tructures built either offshore or pro1ecting out into the sea

from the shore to dissipate the energy of oncoming waves !ade of roc&s or concrete

%an be fi+ed or floating

"he erosive energy of oncoming waves is concentrated on

the brea&water

!aterials are deposited

behind the brea&water

"he nearby unprotected

section of the coast stops

receiving fresh supplies

of depositional materials

and becomes more

vulnerable to erosion

A breakwater protecting the coast



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Case study: $eacherosion in $ar#ados

"he beaches are an importantsource of income

"hey are protected by coral reefssurrounding most parts ofBarbados

But the reefs are fast

disappearing Beach erosion is aggravated byhurricanes

"he government has ta&en stepsto protect the beaches

Beach enhancement and

stabilisation wor&s were carriedout between 8998 and 899: onoc&ley %hrist %hurch/ andWeston 2t ;ames/


* + 1*&!



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Case study: $each erosion in $ar#ados


Coral limestone being undercut at )istins, Barbados



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,ow to descri#e coastal

scenery "rend or direction, e4g4 5-2

2hape, e4g4 straight, gentlyundulating or deeply indented


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,ow to descri#e coastal

scenery "ransportation as evidenced by longshoredrift accumulating sediment on one side of

groyne

*eposition features such as spits, bars and

deltas 0ffshore features such as cays and coral reefs


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Corals

!ade up of the limestone s&eletons of tiny marine

organisms called coral polyps

Conditions for the growth of coral polyps

2ea temperature between >?% and @?%

2hallow sea water less than A?m deep

%lear salt water

olyps thrive on the seaward side of coral reefs where

waves and currents bring an abundant supply of

o+ygen and food

6+tensive coral formations develop between latitudes

@?5 and @?2, on the eastern side of land masses

where there are warm currents




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-rigins of Corals

2everal theories proposed

2ubsidence of islands proposed by %harles *arwin

%oral growth e+tends to surface as island

subsides

Wider and deeper lagoon results %ontinued subsidence results in island being

completely submerged

(toll a horseshoe-shaped reef/ remains

3sland subsidence caused by volcanoes becominge+tinct



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Types of coral reefs

1. Fringing reef

( narrow coral platform separated from the coast by a

shallow lagoon

2. Barrier reef

( coral platform separated from the coast by a deep widelagoon

3. 0toll

( circular coral reef which

encloses a lagoon



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Coral reef structure

!ost reefs are fairly narrow

"he tops lie near to low tide level

"hey are steep on the seaward side


0n the landward

side, sand is

deposited by the

brea&ing waves lants readily

inhabit these

sand deposits

Cross*section of a coral

reef



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Coral reefs in the Cari##ean

%oral reefs have declined significantly

%oral cover decreased from more than :?C in 89AA to

8?C in >??8, a total loss of D?C

eefs at is& pro1ect >??E/ found that EC of

%aribbean reefs were being threatened by high levels

of human activities

"he reefs in eastern and southern %aribbean, the

#reater (ntilles, Florida Geys, ucatan and the

!esoamerican Barrier eef are under threat

"hough natural factors also influence reef development,humans are the main culprits responsible for destroying

the coral reefs of the wider %aribbean




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5atural factors

Hurricanes destroy coral reefs and the organisms that

live there 5atural predators such as the crown-of-thorns starfish

reduce the population of coral polyps

7pwelling of warm water may raise sea water

temperatures and inhibit reef development


A Crown*of*!horns starfish in

the midst of corals



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Human factors

1. (oll#tion

Land-based sources of pollution and sediments threaten@:C of the reefs

Waste materials from factories and holiday resorts pollute the

sea water

esticides washed off farms contaminate coral colonies

ollution from ships threatens 8:C of the reefs

(reas under threat are ;amaica, Hispaniola, uerto ico, the

high islands of the Lesser (ntilles, Beli$e, %osta ica and

anama



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p

2. O%er*fishing

(s the world.s population increases, so does the demand for

food, including seafood 0ver-fishing in reefs may result in algae blooms which inhibit

the growth of corals

3. Coastal de%elo&!ents and acti%ities

%oastal developments disrupt currents and cause sedimentdamage to the fragile corals

"hese include the reclamation of reef areas to build airports

and the development of marinas, groynes and causeways

ecreational activities such as boating, windsurfing,

waters&iing and diving in reef areas also damage corals by

stirring up sediments, thus bloc&ing out sunlight

Chapter_16 Coast and Coral Reefs

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